Public supply key fact	Statistics
Number of water companies	24
Population supplied	59.74
Water supplied (L/day)	14.48bn
Treatment works	989
Service reservoirs	3626
Water supply zones	2111
Length of mains pipe (km)	322.38k
Surface sources	1,184
Groundwater sources	976
Mixed water sources	241

Private supply key fact	Statistics
Population supplied	818,213
Water supplied (M³/day)	330,970
Number of supplies	34,102
Number of local authorities with private supplies	219
Sources without a risk assessment	3,119

Company code	Water supply zones	Volume supplied (l/day)	Population supplied
ALB	2	168,000	3,000
ALE	1	40,000	500
ESP	39	1,699,000	13,000
ICW	132	5,150,000	89,568
IWN	212	43,924,434	318,293
LNW	99	13,402,000	103,000
MTX	1	74,000	454
MUA	13	18,079	411
VWP	1	5,900,000	17,292
Total	500	70,775,513	545,518

Parameter group	% Compliance (2dp)
Chemical parameters	99.95%
Indicator parameters	99.97%
Microbiological parameters All	99.98%
Microbiological parameters (Reservoirs)	99.96%
Microbiological parameters (Treatment works)	99.98%
Pesticides	100.00%
Overall	99.97%

Company code	Number of tests per company	Target number of tests	Number of sample shortfalls	Shortfall
AWI	1698	1,730	32	1.85%
AFW	189,963	190,139	176	0.09%
ALB	963	963	0	0%
ANH	384,310	384,591	281	0.07%
BRL	90,027	90,127	100	0.11%
CAM	29,664	30,193	529	1.75%
ESP	10,661	10,661	0	0%
IWN	38,221	41,189	2968	7.21%
LMW	17,537	17,867	330	1.85%
LNW	19,509	19,511	2	0.01%
NES	265,978	266,146	168	0.06%
PRT	48,870	49,440	570	1.15%
SES	12,582	12,730	148	1.16%
SEW	208,580	209,223	643	0.31%
SRN	225,207	225,305	98	0.04%
SST	65,670	65,680	10	0.02%
SVT	452,105	452,290	185	0.04%
SWB	214,966	215,456	490	0.23%
TMS	436,206	436,304	98	0.02%
UUT	385,624	386,805	1,181	0.31%
VWP	2,660	2,660	0	0%
WSX	179,001	179,632	631	0.35%
YKS	388,919	389,491	572	0.15%
Total	3,698,820	3,666,843	6,244	0.17%

Learning from microbiological failures

The following tables show all the compliance failures within the year, the full list is published within Annex B.

Parameter	Standard	Total number of tests	Number of tests not meeting the standard	Percentage compliance (3 DP)
Clostridium Perfringens (sulphite-reducing clostridia) (confirmed)	0 number/100 ml	24,840	8	99.968%
Turbidity – indicator	1 NTU	171,438	19	99.989%
E. coli (confirmed)	0 number/100 ml	171,591	3	99.998%
Total coliforms (confirmed)	0 number/100 ml	171,598	80	99.953%
Total		539,467	110	99.980%

Table 6 – Failures at treatment works in 2025

Microbiological failures

E. coli at treatment works

E. coli is an indicator of faecal contamination, and robust contingency plans must be in place to mitigate the impact of any detections and to protect consumers from water that may be considered unwholesome or unfit for consumption.

In 2025, there were three detections of E. coli in samples taken from treatment works out of 171,550 tests (Table 7). A further four detections were identified from 182,499 tests at service reservoirs, and 26 detections from 156,949 tests at consumer properties.

In total, there were 33 E. coli detections in 2025. This represents a continued year-on-year reduction from over 40 detections in 2024 and 44 in 2023, indicating consistently high and improving compliance for this parameter across the industry.

Company code	Site name	DWI action
BRL	Shipton Moyne	Satisfactory investigation, did not identify cause
NES	Coldfair Green	Recommendations made
TMS	Lullingstone	Recommendations made

Table 7 – treatment works with E. coli detections

Bristol Water – Shipton Moyne

In July 2025, a single E. coli detection was reported in a regulatory sample taken at Bristol Water’s Shipton Moyne works. The detection was also reported as an event.

The company’s investigation established that the raw water quality remained within normal operating parameters and they identified no issues with the works treatment processes. Company checks confirmed that the disinfection systems, both UV and chlorination, were operating correctly at the time of the detection. No deficiencies were identified in the sampling arrangements at the works, nor in sampling or laboratory practices. All samples taken in response to the breach returned satisfactory results.

Due to elevated seasonal demand, Bristol Water reported a short delay in removing the works from supply to allow the inspection of the treated water tanks. To enable isolation of the site, a temporary pumping station was installed which was operational one week after the initial E. coli detection. Inspection of the treated water tanks found no structural defects, no evidence of ingress, and no conditions that could account for the detection.

As an interim risk mitigation measure, pending removal of the works from supply, Bristol Water increased the chorine setpoint and undertook enhanced monitoring at the works. This continued once the works was returned to supply with no further detections reported providing assurance on the quality of the supply.

The company conducted a comprehensive investigation, the root cause of the E. coli detection could not be determined.

Northumbrian, Essex and Suffolk Water – Coldfair Green

Coldfair Green works is a pristine groundwater site located in Suffolk. The works had an E. coli (and coliform) detection in a final water sample on 30 June. The company’s investigation sampling found turbidity measured in a raw water sample on 2 July was 43 NTU, which was significantly elevated compared with historical data for this borehole; raw water sampling data from 2022 indicated a maximum previously recorded turbidity of 27 NTU, with typical values considerably lower. In addition, Clostridium perfringens was detected in an investigation sample collected on 2 July from the downstream asset, Leiston tower, two days after the E. coli detection from the works. The company’s investigation neither addressed nor linked the two results and a root cause analysis found no cause for either microbiological detection. Contact tank inspections conducted subsequently did not identify any ingress or any water quality impacting issues and all site process trends presented as satisfactory.

The Inspectorate recommended the company undertake timeseries sampling at all individual raw water boreholes supplying Coldfair Green works to assess incoming water quality and review the capability of the treatment process to manage elevated raw water challenges, in accordance with regulation 18. The company did not complete this exercise and instead advised they would enhance the frequency of raw water samples taken for bacteriological parameters and turbidity commencing in 2026. The company’s explanation for the elevated turbidity was that it was likely due to oxidation of iron in the sample, although there was no additional iron analysis carried out to evidence this working theory.

The Inspectorate expects that all instances of microbiological contamination, especially by faecal indicators, are appropriately investigated in order to understand and mitigate the root cause as per the requirements of regulations 4, 18 and 27.

Thames Water – Lullingstone treatment works

In July 2025 Thames Water detected E. coli in a compliance sample from Lullingstone works. The detection was also reported as an event. A second compliance sample collected at the works later the same day, and a compliance sample collected the next morning, were satisfactory for microbiological parameters. The works was removed from supply on notification of the E.coli detection. Investigational in-process samples through the works were satisfactory for coliform bacteria including E. coli. Samples from the downstream service reservoir and at consumers’ properties were satisfactory for a wider range of microbiological tests.

The detection was ultimately attributed to the compliance sampling kiosk environment and failure to follow the sampling procedure. A new-style plastic sampling kiosk had been installed. However, the kiosk had limited ventilation which led to a significant build-up of condensation on the sampling pipework. The company subsequently installed additional meshed ventilation at this site and amended the company asset standard for sampling kiosks.

The compliance sample tap at Lullingstone works runs constantly. The sampler had attempted to disinfect the sample tap with water running – which was allowed in the company sampling procedure manual. A recommendation had previously been made by the Inspectorate to amend the sampling procedure manual. Following this detection, the company issued guidance to the sampling teams to turn off the tap and flame sterilise the tap before sampling. Subsequently the company updated and reissued the sampling procedures manual.

E. coli at reservoirs

South East Water – Broader lane

Analysis of a compliance sample collected in May 2025 from South East Water’s Broader Lane No. 1 Service Reservoir returned a result of one E.coli and one total coliform bacterium per 100 ml. A compliance sample collected approximately 20 minutes earlier from the adjacent Broader Lane No. 2 Service Reservoir was satisfactory.

The detection of E. coli indicated the potential for faecal contamination within the supply system. The company removed Broader Lane No. 1 Service Reservoir from service approximately three hours after the result was reported by the laboratory.

The company undertook an investigation to identify the source and extent of contamination. Sampling was carried out at both Broader Lane service reservoirs, upstream and downstream assets and at downstream consumer properties. All investigative samples were satisfactory and did not indicate evidence of wider contamination within the associated supply system.

Meteorological conditions in the week preceding the failure were mixed, comprising two days of rainfall interspersed with predominantly dry conditions. No clear linkage between the weather conditions and the bacteriological failure was established.

An external inspection of the service reservoir was undertaken which did not identify any deficiencies that could reasonably explain the failure. Access hatches were found to be dry and secure, and the sample tap was reported to be in good condition.

Further investigation was undertaken by the use of a remotely operated vehicle while Broader Lane No. 1 Service Reservoir was being drained to waste. This inspection identified no obvious pathway or source of contamination and found the internal condition of the reservoir to be generally good. A subsequent full internal inspection and clean were completed again without identifying any defect or ingress route that could be positively attributed to the failure. The reservoir had also undergone a full inspection in April 2025, at which time no significant deficiencies were identified.

Despite the company carrying out a comprehensive investigation, no definitive root cause for the presence of E. coli was established. Following completion of the inspection and cleaning programme the company returned Broader Lane No. 1 Service Reservoir to service in July 2025.

Following return to supply, the company undertook additional large-volume sampling at Broader Lane No. 1 Service Reservoir for a period of three months, all following enhanced monitoring samples were satisfactory.

Yorkshire Water – Somerset Avenue

A regulatory sample taken at Somerset Avenue Service Reservoir, Baildon, in May 2025 contained 19 E. coli per 100 mL. Subsequent investigatory samples from the downstream distribution system and upstream treatment works were satisfactory, and an external inspection of the structure found no immediate operational deficiencies. As a precautionary measure, Yorkshire Water isolated both reservoir compartments, supplied the area directly from Graincliffe Water Treatment Works, issued a boil water notice to affected consumers, and undertook a detailed investigation. Internal reservoir inspections found no defects that could explain the contamination, while a root cause analysis was unable to identify a definitive source of the E. coli detection.

The company’s mitigation actions included isolating the reservoir, supplying customers directly from the treatment works, carrying out extensive sampling, inspecting reservoir infrastructure, checking upstream assets, flushing the distribution network, and maintaining the restriction of use until three consecutive satisfactory sample sets had been obtained; the boil water notice was withdrawn on 26 May 2025 and the reservoir returned to service on 2 June 2025. Key lessons included improving investigation and asset management processes: confirming adequate vermin protection on service reservoir overflows and washouts, strengthening root cause analysis templates to explicitly consider all contamination pathways, ensuring air valves are maintained and recorded within inspection programmes, considering chemical dosing assets during investigations, and reducing contamination risks around regulatory sampling points.

Yorkshire Water – Rudston

E. coli was detected in a compliance sample taken at Rudston Service Reservoir in August 2025, with a result of 2 cfu per 100 mL. The sample was taken from the reservoir tank outlet and had an adequate chlorine residual at the time of sampling (0.50 mg/L free chlorine and 0.54 mg/L total chlorine). A desktop root cause investigation involving operational and public health teams was undertaken, but no likely cause for the detection could be identified. Subsequent investigations, including an internal reservoir inspection under flood conditions, air valve inspections on the inlet main, and pressure monitoring, found no water quality related defects or operational issues that could reasonably explain the positive sample result.

To mitigate the potential risk and investigate the sample result, Yorkshire Water drained and inspected the service reservoir, inspected all inlet air valves, deployed pressure logging on the inlet main, and undertook enhanced bacteriological monitoring following the reservoir’s return to service in September 2025. Pressure monitoring demonstrated that the inlet main remained under positive pressure throughout operation, reducing the likelihood of contaminant ingress. In addition to routine compliance sampling, the company implemented enhanced monitoring, taking 1-litre bacteriological samples. No further detections of coliform bacteria have been identified in any subsequent samples, and the company’s investigations concluded that there was insufficient evidence to determine a definitive cause for the original E. coli detection.

Yorkshire Water – Drighlington

E. coli was detected (44 per 100 mL) in a routine compliance sample from Drighlington Service Reservoir in October 2025. Yorkshire Water carried out a comprehensive investigation, including repeat sampling, Cryptosporidium monitoring, reservoir inspections and checks of upstream treatment processes. All follow-up samples were satisfactory and no defects or evidence of contamination were found, meaning no root cause could be identified. The Inspectorate concluded that potentially unwholesome water may have been supplied, but there was no further evidence of contamination within the reservoir or distribution network.

The company immediately isolated and drained one reservoir compartment, undertook inspections and introduced enhanced monitoring. However, the second compartment remained in supply for a prolonged period because operational constraints prevented full isolation of the reservoir. The Inspectorate suggested that the company review and improve their reservoir bypass and resilience arrangements to enable rapid isolation during future events and recommended that the company ensure operational decisions are informed by contemporaneous water quality data, considering additional public health protections when full isolation is not possible.

Coliforms at reservoirs

Testing for coliform bacteria gives reassurance that water entering supply was treated adequately to remove bacterial and viral pathogens. Repeated occurrences of coliform bacteria in samples from the same works in one year are thus of concern and require action to be taken. There were 75 coliform failures from 182,576 tests undertaken at reservoirs. In 2025 three reservoirs failed the 95% wholesomeness test with repeated coliform detections.

All companies are expected to complete internal inspections as part of a risk based process, where the history of ingress and the age of the asset are taken into consideration to ensure public health remains protected.

Company	Reservoir name	Parameter	Samples taken	Samples failed	Percentage compliance (2 DP)
YKS	Scar House Sct	Total coliforms (confirmed)	51	3	94.12%
SEW	Cuckfields reservoir	Total coliforms (confirmed)	6	1	83.33%
AFW	Gunters Hill tower	Total coliforms (confirmed)	13	1	92.31%

Table 8 – reservoirs failing the 95% wholesomeness requirement

Yorkshire Water – Scar House suction tank

Coliform bacteria were initially detected in separate regulatory samples collected from the Scar House suction tank in January and June, and were appropriately notified to the Inspectorate as events and compliance failures. Subsequent sampling identified further coliform detections, both in regulatory samples and in investigatory large-volume samples, as well as at consumer properties.

Between August and October, additional coliform detections were identified upstream of the Scar House suction tank, including within the network downstream of Irongate Bridge Water pumping station, at Darley Low Green service reservoir, and at consumer taps. During this period, there were also isolated detections of E. coli and Clostridium perfringens, indicating a broader microbiological risk within the system.

In response to the initial detections, the company implemented a comprehensive sampling programme and, where appropriate, issued health protection advice during the June, August, September, and October events. A large number of water fittings inspections were also undertaken at rural properties to assess backflow protection arrangements and the influence of private supplies within the area.

The investigation identified several contributing factors, including long residence times within a sparsely populated rural network, the historic use of chloramination with low and inconsistently maintained disinfectant residuals, biofilm development within the system, and evidence of ingress at Darley Low Green service reservoir. To address these risks, a secondary chlorination booster system was installed at Eagle Hall service reservoir inlet, upstream of the Scar House suction tank, to improve chlorine residuals and control suspected biofilm growth.

Attempts to increase chlorine residuals within the network led to elevated chlorate concentrations, which the company attributed to a combination of dosing practices and potential backflow from a large consumer site in the area. This resulted in the issue of additional health protection advice in the form of a do not drink notice.

The Inspectorate’s assessment of these events identified concerns relating to the company’s chlorine management strategy, pressure resilience within the rural network, and the management of chlorate. Deficiencies were also noted in consumer communications during the incidents, although the timeliness of issuing initial health protection advice was recognised.

Yorkshire Water acknowledged the seriousness of the events and implemented a range of mitigating actions. These included ceasing chloramination in the affected network to improve chlorine residual stability, revising incident management guidance for senior scientists, and strengthening chlorate management through enhanced training, tighter stock control, improved monitoring, and the establishment of new chemical storage hubs. The company also committed to a wider review of similar low-turnover, chloraminated networks and improvements to customer communication during incidents.

Lessons for the wider water industry highlight that the use of chloramination in low-turnover rural systems may increase the risk of biofilm development and intermittent coliform detections. Interventions to increase free chlorine to manage microbiological risks must be carefully balanced, as they can lead to elevated disinfection by-products, such as trihalomethanes (THMs) and chlorate, and may obscure the underlying root cause. These competing risks should be anticipated, formally assessed, and managed proactively rather than reactively during incidents.

Public health risk assessments must draw on all available and emerging data, including uncertainties, to ensure that appropriate and timely health protection advice is provided. Additionally, even where prior inspections and safeguards (such as air gaps) are in place, large third-party premises with private supplies and on-site treatment must be re-evaluated during incidents. Finally, consumer communication must evolve in line with the incident, ensuring that accurate, address-level information and clear explanations of potential impacts are provided, particularly to vulnerable customers.

South East Water – Cuckfields reservoir

A compliance sample taken from South East Water’s Cuckfield number 1 reservoir on 15 July 2025 detected coliform bacteria. Samples collected from the neighbouring Cuckfield number 2 reservoir and from an upstream asset on the same day were satisfactory, as were investigatory samples collected in response to the failure. Cuckfield number 1 was removed from supply on 24 July 2025 for inspection.

Before this detection, Cuckfield number 1 had last been inspected in December 2024. That inspection identified points of ingress. Following remedial works, successful flood testing and reinspection, the reservoir was returned to supply in June 2025.

In response to the Inspectorate’s assessment of the compliance failure, the company confirmed that a further inspection planned for July 2025 had been postponed while it assessed whether additional works were required at the site. The company identified that, under the current configuration of the two reservoirs, pipework modifications were required to Cuckfield number 2. Inspection of Cuckfield number 1 was planned for completion by the end of November 2025, with return to supply in early December. Cuckfield number 2 would then be removed from supply to allow the required modifications to be completed.

On 4 December 2025, an operational sample collected from Cuckfield number 2 detected both coliform bacteria and Enterococci. The reservoir was removed from supply on 6 December following receipt of the presumptive Enterococci result. As Cuckfield number 1 had not yet been returned to supply following the July detection, the area was supplied using an alternative network configuration. Cuckfield number 1 was returned to supply on 25 March 2026. Cuckfield number 2 remains out of supply until the company completes the required pipework modifications.

Affinity Water Gunters Hill tower

In January 2025, Affinity Water detected a coliform in a regulatory sample collected from Gunters Hill water tower. All investigational samples subsequently collected from the reservoir, the supplying works, and consumer properties returned satisfactory results. The supplying works was confirmed to be operating satisfactorily in the period prior to the detection.

Upon notification of the presumptive coliform result, the company took precautionary action and isolated the tower from supply. The subsequent investigation identified ingress into the reservoir as the most likely root cause of the detection, alongside other contributory issues. The asset remained out of service until the necessary remedial works were completed.

A previous inspection conducted in November 2015 had also identified ingress at the tower. Although the interval between inspections remained within the maximum permissible period, greater application of risk-based methodology by taking into account the asset’s age (constructed in 1967) and its history of ingress may have prompted earlier proactive inspection and intervention ahead of the standard ten-year cycle.

All companies are expected to undertake internal inspections as part of a risk-based approach, considering factors such as asset condition, age, and prior defects, including ingress. This is essential to ensure that risks are appropriately managed and that public health protection is maintained.

Learning from turbidity failures

Turbidity is a measure of the cloudiness of water caused by suspended particles. At treatment works, turbidity is an important operational and regulatory control because it provides evidence of whether raw water has been adequately treated before disinfection and supply. The treatment works standard for turbidity is 1 NTU. In 2025, there were 12 failures of this standard from 171,515 tests undertaken at treatment works in England.

There were also five zonal turbidity failures, where the standard is 4 NTU. The case studies below illustrate the importance of effective process control, maintenance planning, timely investigation and appropriate operational response.

Anglian Water – Pitsford treatment works

Pitsford works is a surface water treatment works located in Northamptonshire. In February 2025, a regulatory final water compliance sample recorded turbidity of 1.48 NTU, exceeding the treatment works standard of 1 NTU. Further analysis of the same sample identified iron at 335 µg/L, exceeding the iron standard of 200 µg/L.

The company’s investigation concluded that the most likely cause of the turbidity and iron exceedances was a transient hydraulic disturbance within the Boughton pumping main. This was associated with the return to service of Boughton high lift pump No. 1 following planned maintenance. The final water compliance sampling point at Pitsford works is located on the Boughton pumping main, on the delivery side of the high lift pumps. The pump was flushed and returned to service at 14:13, shortly before the regulatory sample was collected at 14:24.

The company had not undertaken an adequate risk assessment for returning the pump to service. A short length of cast-iron pipework containing stagnant water upstream of the pump was not flushed before the pump was returned to supply. This allowed iron-rich stagnant water to be mobilised, resulting in the turbidity and iron exceedances following the maintenance activity. The Inspectorate recommended that the company update the site risk assessment and method statement for the Boughton pump set at Pitsford works to include appropriate flushing arrangements following pump maintenance, ensuring that stagnant water is removed before the asset is returned to service.

The company’s investigation did not include interstage turbidity sampling at Pitsford works, on the basis that the treatment process was operating normally at the time. Although several routine indicators supported this conclusion, the absence of interstage turbidity monitoring limited the evidence available to demonstrate conclusively that treatment performance had not been affected. Interstage sampling would have provided stronger assurance and supported the company’s investigatory obligations under regulation 18. The Inspectorate required the company to ensure that learning from this event was effectively disseminated to relevant operational, maintenance and remedial teams, to reduce the likelihood of similar failures occurring elsewhere across the supply system. The event demonstrates the importance of fully assessing water quality risks associated with planned maintenance, ensuring that return-to-service procedures are supported by clear flushing requirements and appropriate verification.

Anglian Water – West Pinchbeck treatment works

West Pinchbeck treatment works, located near Grantham, treats groundwater from five artesian borehole sources. On 11 January 2025, a final water sample collected at the works recorded a turbidity result of 1.10 NTU, exceeding the treatment works standard of 1 NTU. The sample was not analysed until two days after collection, meaning the company was not aware of the result until the late afternoon of 13 January.

Delays in sample analysis have been identified previously as a recurring issue for the company. The Inspectorate has already recommended that the company review analytical turnaround times to ensure that timely, appropriate and meaningful investigations can be undertaken. Delayed analysis can compromise the ability to establish the cause and extent of a failure promptly and may also limit the effectiveness of any immediate protective action.

This is not consistent with the expectations of regulation 18, which requires companies to take immediate steps to investigate and establish the cause of any potential risk to water quality, and to take appropriate action to protect public health.

This was the second turbidity exceedance at West Pinchbeck works within a 20-month period. The first, in August 2023, was attributed to elevated iron concentrations. During that event, the works remained in supply following a site shutdown due to artesian pressure from the boreholes.

The Inspectorate had considered enforcement action requiring the company to implement a process to divert excess flow to waste during site shutdown. However, within three days of the 2023 breach, the company modified the shutdown control philosophy by programming an additional run-to-waste function into the Programmable Logic Controller (PLC), intended to prevent forward flow beyond the filtration stage during shutdown conditions.

Those controls were designed to ensure that, in the event of a shutdown, artesian flows from the boreholes would be diverted to waste before reaching the contact tank, thereby protecting treated water quality. The company also identified a further 35 groundwater sites with artesian flow and undertook checks to confirm that similar forward flow would not occur at those sites during shutdown scenarios.

Following the January 2025 exceedance, the company’s investigation found that the turbidity failure at West Pinchbeck works was associated with elevated iron and manganese concentrations above the relevant standards. A faulty ultrasonic level sensor incorrectly indicated a satisfactory level in one of the balance tanks while the actual water level was low during drain-down. The low level disturbed sediment at the base of the balance tanks. Additional artesian flow through the works then caused the run-to-waste valve to close once the high-flow shutoff threshold had been exceeded, allowing forward flow to occur once the balance tank level recovered. This chain of events demonstrated that the PLC control changes introduced following the 2023 exceedance were not sufficiently robust.

In response to the 2025 event, the company implemented further PLC changes to prevent forward flow through the works when either the pre-contact turbidity monitor or final water turbidity monitor entered a state of alarm. Under these arrangements, borehole water is diverted to either the pre-filter or post-filter run-to-waste route during a turbidity shutdown. These changes are intended to prevent forward flow through the treatment process during relevant shutdown conditions; however, they also mean that a representative sample cannot be collected from the final water tap while the works is in this shutdown state.

The Inspectorate required the company to take several actions in response to the 2025 event. These included reviewing PLC controls at all other works affected by artesian flow to determine whether additional safeguards were required. The Inspectorate also made three key recommendations: first, to improve turbidity sample analysis timeframes to support compliance with regulation 18; second, to undertake CCTV surveys of all five supplying boreholes, which had last been surveyed between 10 and 30 years previously despite the company’s risk-assessed survey frequency of 10 years; and third, to carry out full internal inspections of all four contact and balance tanks on site, which had last been inspected between seven and nine years previously, exceeding the company’s risk-assessed maximum inspection interval of seven years for works tanks. These actions are necessary to support ongoing compliance with regulations 4 and 27.

Learning from chemical, taste and odour failures

The table below shows all the chemical failures during 2025, the full list of all chemical parameters sampled in included within Annex B.

Parameter name	Standard	Total number of tests	Tests not meeting the standard	Tests not meeting the standard per company	Percentage compliance (3dp)
Trichloroethene & Tetrachloroethene – sum of 2 substances	10 µg/L	11,156	1	ESP (1)	99.991%
Benzo[a]Pyrene (total)	0.01 µg/L	13,644	1	SVT (1)	99.993%
Polycyclic aromatic hydrocarbons (total by calculation)	0.1 µg/L	13,886	3	IWN(2) SRN(1)	99.978%
Aluminium	200 µg/L Al	52,398	10	BRL (1), NES (4), UUT (5)	99.981%
Bromate	10 µg/L BrO₃	10,026	1	ESP (1)	99.990%
Iron	200 µg/L Fe	53,039	98	SRN(3) SWB(7) IWN(1) WSX(1) TMS(6) SVT(15) AFW(1) NES(9) BRL(2) SST(1) SEW(7) YKS(20) ANH(6) UUT(18) LNW(1)	99.815%
Manganese	50 µg/L Mn	52,918	18	NES(3) WSX(1) SVT(2) UUT(7) SWB(3) LNW(1) BRL(1)	99.966%
Nickel	20 µg/L Ni	14,081	72	WSX(3) BRL(4) YKS(7) NES(2) TMS(20) AWI(2) IWN(3) SES(2) UUT(3) LNW(5) SST(1) ANH(3) ESP(3) SVT(6) MUA(1) AFW(3) SEW(1) SRN(3)	99.489%
Lead	10 µg/L Pb	14,120	60	YKS(6) SST(3) VWP(1) SWB(1) AFW(6) NES(5) SRN(3) SEW(3) IWN(2) AWI(2) UUT(7) WSX(1) ANH(2) ICW(1) SVT(4) TMS(13)	99.575%
Gross alpha	0.1 Bq/L	1,257	15	ESP(4) LNW(1) IWN(10)	98.807%
Odour	Acceptable to consumers and no abnormal change	57,660	43	ANH(4) SVT(5) ESP(1) TMS(1) ICW(1) AFW(1) YKS(3) SWB(4) BRL(2) NES(10) UUT(3) SRN(6) SST(2)	99.925%
Taste	Acceptable to consumers and no abnormal change	57,502	65	UUT(5) ESP(1) BRL(2) SVT(5) NES(6) SWB(8) ANH(13) YKS(5) SRN(6) IWN(6) WSX(1) PRT(1) TMS(6)	99.887%
Copper	2 mg/L Cu	13,785	2	ANH(1) WSX(1)	99.985%
Sodium	200 mg/L Na	14,071	1	NES (1)	99.993%
Nitrite – consumers taps	0.5 mg/L NO₂	24,557	5	TMS(4) NES(1)	99.980%
Nitrate	50 mg/L NO₃	24,518	2	ANH(1) IWN(1)	99.992%

–
Table 9 – Chemical parameter failures

Nitrate

Nitrate in drinking water is most commonly associated with diffuse agricultural pollution, particularly the use of nitrogen fertilisers and organic manures within drinking water catchments. It can persist in groundwater for long periods and may also affect surface water sources following rainfall and runoff. Under the Regulations, nitrate has a prescribed concentration value of 50 mg/L as nitrate at consumers’ taps.

Nitrate exceedances are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the source and extent of elevated nitrate, assess any risk to consumers, review the effectiveness of blending or treatment controls, and take corrective action to restore compliance and prevent recurrence.

In 2025, two nitrate exceedances were reported from 24,518 tests. These were reported by Anglian Water and Independent Water Networks. Regulation 27 risk assessments should identify sources vulnerable to nitrate pollution and set out appropriate control measures, including catchment management, source blending, treatment, operational monitoring and contingency arrangements where required. The Inspectorate expects companies to manage nitrate risks proactively, recognising that catchment pressures and groundwater lag times can make deterioration slow to reverse. Monitoring data should be used to identify upward trends before standards are exceeded, and companies should maintain clear evidence that control measures are effective and resilient under seasonal and operational variation.

Lead

Lead is a toxic metal that can dissolve into the drinking water supply when water comes into contact with lead pipes. The source of lead in drinking water is mainly communication pipes owned by water companies and supply pipes and fittings owned by consumers within their property which are made from lead. There were 60 compliance sample failures for lead during 2025 out of 14,122 samples analysed for lead. The highest concentration of lead found was 562 µg/L in Blackheath zone during April where the supply pipe is over 100m in length and the owner is reluctant to change it. The company changed their communication pipe.

Consumers are, to a large extent, protected from exposure to lead by the practice of phosphate dosing at water treatment works to reduce plumbosolvency. To further reduce exposure to consumers from lead, national policy and company strategy is required to mitigate the risk. Lead contributed to 57 events reported to the Inspectorate in 2025. The combined ERI score for these events was 0.043.

The Inspectorate collects data on lead via the compliance process which sets out how many compliance samples must be taken each year from water quality zones. This number varies by population but as a maximum this is eight samples per year per zone as stipulated within the Regulations. Therefore, the size of the data set held for lead is small compared to the amount of data held on other parameters. Companies, however, do collect far more data than the compliance samples we require them to take. The Inspectorate has been in discussions with the industry lead working group to see whether all lead sample data can be collated on behalf of the industry to help the Inspectorate and form an overall picture of the lead risk within the country, with the assistance on the lead working group this will form part of the data return from 2027 onwards. Holding this data set would enable the Inspectorate to work better with Defra and central government to be able to advocate for changes to the governmental policy for lead management within water.

Lead is a problem which to a large extent is confined to within the boundaries of homes and therefore is under the control of the homeowners to rectify. It is however a relatively invisible problem and one which is tough to compel homeowners to spend the often large amount of money required to remove lead from their properties. Large-scale company-led removal schemes have been successful in discrete areas, but scaling up to national levels is not currently feasible.

Of note is a lead event from Advanced Water Infrastructure Networks who are a new appointment and variant company, NAVs predominantly supply new build housing estates given that lead has been banned since the 1970s. However, this failure of 11.7 µg/L taken from a show home was believed to be due to the low turnover and the properties fittings. The company received two recommendations related to their investigation and follow up sampling which was not timely.

The issue is also compounded by the fact that some plumbers still use lead solder despite it being banned for use on cold water systems. Using lead solder on hot water systems is still possible and lead solder can still be purchased from plumbing and DIY shops should the homeowner be willing to carry out their own modifications which, unlike the gas industry, is still legal to do so. There is an approved plumber scheme in place, but there is no legal requirement to use this service. The World Health Organization (WHO) has declared there is no safe level of lead in drinking water. Many lead pipes still exist within company distribution networks, particularly on the final sections of pipe supplying consumer properties. The Inspectorate set out its expectations in respect of lead in its long-term planning guidance. Water companies have collectively declared an ambition to be lead pipe free by 2050.

Iron, manganese and aluminium

Iron, manganese and aluminium are commonly detected in drinking water and are associated with source water quality, treatment performance, asset condition and distribution system operation. Under the Regulations, these parameters are subject to indicator standards. Although they are not normally health-based limits at the concentrations typically encountered, they are important indicators of treatment control, network stability and consumer acceptability.

Elevated iron and manganese concentrations are most often associated with groundwater sources or the mobilisation of deposits within the distribution system, particularly following changes in flow, pressure or system operation. Aluminium exceedances are more commonly linked to treatment performance, especially where aluminium-based coagulants are used and where coagulation, filtration, pH control or residual management is not sufficiently effective.

Exceedances of these standards are compliance failures and must be investigated in accordance with regulation 18. They may indicate weaknesses in treatment control, asset maintenance, network operation or operational response, including inadequate flushing, insufficient optimisation of treatment processes, or poor understanding of deposits within the distribution system. Failures may also be associated with turbidity, discolouration and consumer rejection of water.

Companies are expected to maintain effective process control, use monitoring data to identify deteriorating trends and manage treatment and network risks proactively. Regulation 27 risk assessments should identify where iron, manganese or aluminium may present a risk to wholesomeness, consumer acceptability or treatment performance, and should set out appropriate control measures. Where failures are persistent, repeated or linked to wider asset or operational deficiencies, companies must take corrective action to restore compliance and prevent recurrence.

In samples taken from water supply zones, iron, manganese and aluminium accounted for 29% of all chemical failures in 2025. These parameters are significant because they can cause visible discolouration and affect the acceptability of drinking water, leading consumers to reject the supply even where the primary concern is aesthetic rather than toxicological. Each failure is assessed by the Inspectorate to determine whether the cause lies in source water quality, treatment performance, asset condition or distribution system operation. Where appropriate, the Inspectorate makes recommendations or takes enforcement action to secure improvement and protect consumers.

Trichloroethene and tetrachloroethene

Trichloroethene (TCE) and tetrachloroethene (PCE) are chlorinated hydrocarbons included in the Regulations under the combined parameter for the sum of trichloroethene and tetrachloroethene. Their presence in drinking water is most commonly associated with groundwater sources affected by historic industrial contamination, including metal degreasing and dry-cleaning activities, because these compounds can persist in aquifers for long periods, they represent a long-term source water risk which must be identified and controlled through company risk assessments.

The parametric value applies to the combined concentration of TCE and PCE in water supplied for human consumption. Any exceedance is a compliance failure and must be investigated in accordance with regulation 18. Companies are expected to establish the cause, assess any risk to human health, determine the extent of the issue, and take appropriate action to restore compliance and prevent recurrence.

In 2025, the reported TCE and PCE failure did not reflect deterioration in water quality. The investigation established that the failure arose from a transcription error by the company when transferring laboratory data, which resulted in an erroneous compliance failure being recorded.

Although the supplied water did not exceed the regulatory standard in this instance, the error demonstrates the importance of robust data governance. Companies must maintain effective validation, verification and reporting controls to ensure that compliance data submitted under the Regulations is accurate, reliable and traceable. Errors in regulatory reporting can misrepresent the quality of water supplied, lead to unnecessary investigation, and undermine confidence in published compliance information.

Bromate

A bromate exceedance of 200 µg/L was reported by ESP Water Limited and subsequently confirmed to be incorrect. The investigation showed that the true laboratory result was 0.2 µg/L, which is well below the legal limit of 10 µg/L. The error occurred during the manual data entry, where an incorrect multiplier was applied to the analytical result. This led to an inaccurate value being reported. No additional sampling was required, as the original laboratory data was valid.

To prevent a similar error in the future, the incorrect multiplier was removed from the company’s reporting spreadsheet. Previous compliance returns were reviewed, and no other errors of this type were identified. The company also improved its reporting arrangements to reduce reliance on manual processes, including additional automated checks to ensure that any genuine exceedances are identified promptly before data are submitted.

Data integrity and reporting errors

Although the supplied water did not exceed the bromate standard, the incident demonstrates the importance of robust data governance in regulatory reporting. Companies must ensure that laboratory results are transferred, calculated, validated and submitted accurately. Manual data handling, unsupported calculation steps, or weak validation controls can lead to erroneous compliance reporting, unnecessary investigation and inaccurate public information.

In response, the company removed the incorrect multiplier from its reporting system and reviewed previous compliance returns. That review did not identify further errors of this type. Additional automated validation checks were also introduced to reduce reliance on manual processes and to improve the accuracy of future compliance reporting.

The TCE/PCE and bromate as did the heptachlor epoxide cases arose from deficiencies in data handling rather than from deterioration in water quality. In addition laboratory errors account for the PAH failures for example. Such errors are not acceptable. Companies are required to maintain effective data management, validation and verification arrangements so that compliance data submitted under the Regulations is accurate, reliable and traceable.

The Inspectorate expects companies to minimise manual handling of compliance data wherever practicable, maintain clear audit trails, apply automated validation checks, and investigate anomalies before submission. Where errors are identified, companies must correct the data, assess whether previous returns may have been affected, and implement controls to prevent recurrence.

These incidents reinforce that strong data governance is a fundamental component of regulatory compliance and public confidence in drinking water quality. Accurate reporting is essential to ensure that regulatory scrutiny is directed to genuine risks and that published compliance information properly reflects the quality of water supplied to consumers.

Nickel

Nickel is a naturally occurring metal that may be present in drinking water from source waters or through leaching from metallic materials, particularly within distribution systems and domestic plumbing. Under the Regulations, nickel is a prescribed parameter with a parametric value of 20 µg/L at the consumer’s tap, reflecting the potential contribution from premise plumbing and fittings.

Exceedances of the nickel standard are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to take prompt steps to establish whether the source lies within company assets, supply conditions or private plumbing, assess any risk to consumers, and identify appropriate corrective action. Elevated concentrations are most commonly associated with new or recently installed fittings, stagnation within internal plumbing, or water quality conditions that increase metal dissolution.

Companies should use investigation outcomes to understand whether failures are isolated to individual premises or indicate a wider risk associated with water chemistry, materials in contact with drinking water, or local distribution conditions. Regulation 27 risk assessments should identify where nickel may present a risk to wholesomeness and set out proportionate control measures, including advice to consumers where failures arise from premise plumbing.

In 2025, nickel represented nearly one fifth of all chemical failures for the second consecutive year, accounting for 19% of the total. Most failures were associated with consumer distribution systems and were commonly linked to new taps or fittings. The number of failures reported by new appointment and variation companies was notable relative to the size of the populations served, including Independent Water Networks, Last Mile Water, Albion Water and ESP Water.

Where nickel failures occur, companies are expected to provide clear and appropriate advice to affected consumers, including steps to reduce exposure where stagnation or premise plumbing is implicated. This may include flushing taps after periods of non-use and ensuring that any replacement fittings are suitable for contact with drinking water and installed correctly. Companies should also ensure that learning from individual failures is reflected in sampling strategies, risk assessments and engagement with developers, installers and consumers where appropriate.

Nitrite

In 2025, Thames Water reported four nitrite exceedances in four separate water supply zones. Each affected zone was supplied from treatment works using chloramination as part of the disinfection strategy.

Nitrite in drinking water is closely associated with chloraminated supply systems. Under the Regulations, nitrite is a prescribed parameter with a parametric value of 0.5 mg/L at consumers’ taps. The standard reflects potential health impacts, particularly for bottle-fed infants. In chloraminated systems, nitrite can form through nitrification, a biological process in which ammonia-oxidising bacteria convert ammonia to nitrite, particularly where disinfectant residuals are low, water residence times are long, or temperatures are elevated.

Nitrite exceedances are compliance failures and must be investigated in accordance with regulation 18. Where chloramination is used, companies are expected to maintain effective operational control to prevent nitrification, including the management of disinfectant residuals, ammonia and nitrite monitoring, and adequate turnover within the network. Elevated nitrite may indicate a loss of system control and requires prompt corrective action to ensure that supplies remain wholesome and safe for consumption.

Thames Water’s investigations indicated that there were no associated treatment issues at the supplying works at the time of the exceedances. Two of the four failures were associated with the internal distribution systems of the sampled properties. At one property, the investigation was inconclusive because resamples from the original property and neighbouring properties were below the limit of quantification. At the remaining property, the consumer refused access for resampling, although samples from neighbouring properties were also below the limit of quantification. The company provided appropriate flushing advice to affected consumers. These findings demonstrate the importance of considering both company distribution systems and premise plumbing when investigating nitrite failures.

Northumbrian, Essex and Suffolk Water reported a nitrite concentration of 0.51 mg/L in a sample from a consumer property in July 2025. Investigation samples from neighbouring upstream and downstream properties also recorded nitrite concentrations above the standard, at 0.52 mg/L. Subsequent resamples from the original property and one upstream neighbouring property were satisfactory. Further sampling identified nitrite concentrations below the regulatory standard but above the company’s internal trigger level of 0.35 mg/L.

The affected properties were located in a zone supplied from works using chloramination to maintain a residual disinfectant in the downstream network. The company’s investigation confirmed that there were no nitrite issues at the supplying works and that the chloramination dosing process was operating normally. Network flushing was undertaken, and the local network configuration was verified using pressure loggers and valve checks. The company concluded that elevated water temperatures, long residence times and reduced turnover in the local distribution network were probable contributory factors.

The company presented its nitrate and nitrite management strategy to the Inspectorate. This includes enhanced nitrite monitoring at relevant service reservoirs, with an internal trigger level of 0.35 mg/L requiring investigation and flushing to reduce concentrations. The Inspectorate expects companies operating chloraminated systems to use monitoring data, operational knowledge and risk assessments to identify locations vulnerable to nitrification and to take proactive action before regulatory standards are exceeded.

Copper

Copper in drinking water is primarily associated with leaching from internal plumbing systems, including pipework, fittings and storage tanks, rather than from source waters or treatment processes. Under the Regulations, copper is a prescribed parameter with a parametric value of 2 mg/L at the consumer’s tap, reflecting its predominant origin within premise plumbing. Exceedances are compliance failures and must be investigated in accordance with regulation 18 to establish whether the cause lies within company assets, water chemistry, local distribution conditions or private plumbing. In 2025, two copper exceedances were reported, which is lower than in previous years. Companies are expected to assess the effectiveness of corrosion control measures, consider any wider implications for regulation 27 risk assessments, and provide appropriate advice to affected consumers where failures arise from premise plumbing or fittings.

Wessex Water – Copper

In October a sample collected from a Wessex Water’s consumer’s property in the Shepherds Shore zone contained a pre-flush copper concentration of 2.1mg/l. A flushed sample collected at the same visit contained copper at a concentration of 0.04mg/l. As part of the company’s investigations a resample from the original property was satisfactory as were samples collected from a neighbouring property. A water fittings inspection identified that new copper pipework had been fitted during a kitchen refurbishment earlier in the year. Wessex Water wrote to the consumer and advised them to flush their tap before drinking for a period of time and offered to return to the property at a later date to take further samples to demonstrate the risk from the new copper pipework has reduced once the internal pipework has become conditioned with a build up of scale.

Anglian Water – Copper

A pre-flush sample taken from a motor garage in August contained copper at 2.995mg/L, and also contained a petrol/solvent odour. The results of the flushed sample were satisfactory for copper. Anglian Water’s investigation found organic compounds typical of fuel in samples at the premises, while samples from neighbouring properties were satisfactory. Advice not to drink the water was provided and work was carried out to replace the communication pipe with a barrier pipe which prevents migration of hydrocarbons from the soil, and the boundary box was also replaced with one suitable for contaminated land. The consumer’s supply pipe was found to be made of copper, and advice was provided to replace it to prevent further copper exceedances.

Sodium

Sodium in drinking water may arise from source water characteristics, treatment processes, or domestic water softening systems. Under the Regulations, sodium has a prescribed concentration value of 200 mg/L. Exceedances at consumer taps must be investigated in accordance with regulation 18 to establish whether the result is representative of the public supply or has been influenced by premise plumbing, including ion-exchange water softeners.

Domestic water softeners can significantly increase sodium concentrations in softened water and may also affect the corrosivity of water in premise plumbing. Regulatory samples should therefore be collected from points that are representative of the mains supply. Where a softening system is present, companies should confirm the sampling location, avoid softened outlets where possible, and ensure that sampling procedures include appropriate checks to prevent unrepresentative results.

In 2025, Northumbrian, Essex and Suffolk Water reported a sodium concentration of 290 mg/L from a consumer tap sampled in February. The investigation attributed the exceedance to a domestic water softener connected to the sampled tap. The zone is supplied by a bulk Anglian Water supply with elevated hardness, and previous sodium exceedances in this zone have also been associated with domestic softener installations. In response, the company instructed samplers not to sample from properties with domestic water softeners where this would compromise representativeness, introduced hardness checks at properties before sampling, and updated its sampling procedure. The Inspectorate also advised the company to consider proactive communication with consumers in the affected zone on the importance of ensuring that any water softener is correctly installed and does not supply taps used for drinking or cooking unless appropriate controls are in place.

Benzo(a)pyrene

Benzo(a)pyrene is a polycyclic aromatic hydrocarbon (PAH) and is regulated separately from the combined PAH parameter because of its toxicological significance. PAHs in drinking water are most commonly associated with the deterioration or disturbance of coal-tar lined cast iron mains. Exceedances of benzo(a)pyrene or the combined PAH standard are compliance failures and must be investigated in accordance with regulation 18 to establish the cause, extent and recurrence risk. Regulation 27 risk assessments should identify where coal-tar lined mains or other relevant materials present a risk and set out appropriate monitoring, control and replacement strategies.

Independent Water Networks reported a benzo(a)pyrene exceedance in the Tendring underground zone in January 2025. Repeat samples from the original property and investigational samples from neighbouring properties did not detect benzo(a)pyrene above the limit of quantification. The company concluded that the likely source was deterioration of coal-tar lining in older cast iron mains within the upstream network used to provide the bulk supply. The bulk supplier confirmed the presence of older mains upstream of the company’s water supply zone. Fluoranthene, another PAH, was detected in all samples collected as part of the investigation. On this basis, the Inspectorate did not agree with the company’s conclusion that recurrence was unlikely. Recommendations were made to strengthen monitoring protocols and to ensure that the PAH risk assessment for the zone was reviewed in a timely manner.

In July 2025, Thames Water reported an exceedance of the benzo(a)pyrene standard in the Epping Forest zone. The company attributed the failure to temporary mobilisation of mains material following a local burst main. Investigatory samples confirmed elevated PAH concentrations close to the burst location. Coal-tar lined mains are present in the area, and this risk had already been identified by the company and was subject to enforcement action by the Inspectorate. Following repair of the burst main and return to the usual supply arrangements, sample results returned below the regulatory standard. The event demonstrates the importance of understanding where legacy mains materials remain in service and ensuring that operational disturbances, including bursts and flow changes, are considered within company risk assessments and response procedures.

Severn Trent Water reported a benzo(a)pyrene result of 0.02496 µg/L. Although this individual result exceeded the benzo(a)pyrene standard, the combined PAH concentration remained below the regulatory limit. The company investigated the local distribution network, including additional sampling from nearby properties, key network points and the original property. All follow-up samples were satisfactory, with no further detections of benzo(a)pyrene or related compounds. Reviews of network operation, recent maintenance activity, flow and pressure conditions did not identify a likely cause, and there was no evidence of network materials commonly associated with PAH failures. Targeted flushing and boundary box sampling were undertaken as a precaution and confirmed satisfactory water quality.

Taste and Odour

Taste and odour are important indicators of consumer acceptability and confidence in drinking water. Under the Regulations, taste and odour are indicator parameters and water must be acceptable to consumers with no abnormal change. Although failures are not usually associated with direct health effects at the levels typically encountered, they can indicate deterioration in source water quality, treatment performance, distribution system condition or operational control.

Unacceptable taste or odour may arise from algal activity in source waters, naturally occurring organic compounds, treatment inefficiencies, changes in disinfection practice, or mobilisation of deposits and biofilm within the distribution system. Chlorine and chlorinous tastes may also occur where dosing is elevated, poorly controlled, or where residual management is affected by long residence times or changes in demand.

Taste and odour failures are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the cause and extent of the issue, assess whether there is any wider risk to water quality or consumer acceptability, and take appropriate corrective action. Persistent or widespread complaints may indicate weaknesses in treatment optimisation, asset performance, network management or customer response arrangements.

Companies are expected to manage taste and odour proactively through effective source water monitoring, treatment process control, maintenance of distribution systems and timely response to customer contacts. Regulation 27 risk assessments should identify where taste and odour risks may arise and set out appropriate control measures so that water remains not only compliant, but acceptable to consumers.

Anglian Water

For instance, a failure for taste and odour as well as copper in Billingborough zone of Anglian water in August was from a petrol station. Following this failure the company made contact with the property and issued a do not drink. The following day they replaced the communication pipe with barrier pipe and also replaced the boundary box for one suitable on contaminated land. The property were issued with further advice about replacing taps, and the company will resample once this work is complete from the customers side to remove the restriction of use.

United Utilities

A quantitative taste and odour exceedance was identified in a sample taken in June 2025 in the Ashton West supply zone, supplied by Buckton Castle works. The Inspectorate noted that between January 2020 and June 2025 the company had reported 13 quantitative odour and 11 quantitative taste exceedances from four supply zones supplied from Buckton Castle works. Despite repeated incidents, no root cause had been identified. 2,4,6-trichloroanisole (TCA), linked to musty odours, was detected four times in 2022 and 2025 from within the treatment process, at the final water, from service reservoirs, and at consumer taps. Following the June 2025 exceedance confirming persistent taste and odour issues, enforcement was initiated requiring the company to carry out a full risk review and investigation. This Notice requires the company to carry our a full review of the taste and odour risk assessment and mitigation at Buckton Castle works.

South West and Bournemouth Water

A routine compliance sample collected by South West Water from a property on the Isle of Tresco in February was tested by laboratory panellists who reported salty, oily and astringent descriptors. South West Water carried out an investigation following notification of the result. Operational checks confirmed that the supplying Vane Hill Works was operating normally and no consumer complaints relating to taste or odour had been received. Follow-up sampling from the affected property, the supplying service reservoir, the treatment works and elsewhere within the distribution network did not identify any further failures for the taste parameter.

The company investigation concluded that the most likely cause of the taste detection was elevated sodium and chloride concentrations resulting from saline intrusion into the Isle of Tresco’s groundwater sources. These conditions are long-standing and reflect the limitations of the existing treatment process on the island, which does not remove sodium or chloride. Historical monitoring shows these parameters have remained relatively stable over recent years, with no evidence of deterioration in raw water quality or the quality of water supplied to consumers.

South West Water has legal instruments in place for each of the Isle’s of Scilly habited islands to secure long term upgrades to the legacy systems. The Isle of Tresco is subject to Notice ISC-2021-00009, which includes actions to mitigate taste, sodium and chloride parameters.

Portsmouth Water

In February Portsmouth Water collected a random compliance sample from a hotel which contained a salty taste. Subsequent samples from the original and neighbouring properties returned satisfactory results with no taste detected.

A water fittings inspection completed as part of Portsmouth Water’s investigation confirmed the presence of a water softener system, which was not identified at the time of sampling. This was despite the use of hardness test strips being a requirement in the company procedure to alert the sampler to the potential for the presence of a water softener, which can impart a salty taste. The sampling team has since been re-briefed on the importance of using hardness test strips and evidence of this was provided by the company.

The hotel owner was advised to maintain a mains-fed unsoftened supply for drinking water, however the Inspectorate noted that sodium was not tested in the during the original investigation, preventing adequate risk assessment for hotel guests and staff. The company received a recommendation to further investigate sodium concentrations at the hotel and consider whether further action was needed, including under Section 75(2) of the Water Industry Act. The response to the recommendation included details of the sodium concentration investigation which did not highlight any concerns. A suggestion was also made to Portsmouth to strengthen the company sampler auditing process.

Thames Water

An unusual onion/indole odour was detected from a supermarket within Thames Water’s Keston and Forestdale water supply zone in August. The sample was collected from a tap in the staff canteen which was not routinely used as a drinking water outlet.

Following notification of the failure, the company undertook an extensive investigation that included repeat sampling from the original tap, an alternative tap within the supermarket, neighbouring properties and the upstream service reservoir. Laboratory assessments continued to identify a low-level onion odour in samples collected from both taps within the affected premises. In contrast, all samples collected from neighbouring properties and the upstream supplying asset showed no evidence of the unusual taste or odour. GCMS analysis did not identify hydrocarbons or other contaminants of concern.

A water fittings inspection undertaken at the affected property identified several non-compliances requiring remedial action, including deficiencies in backflow protection arrangements associated with appliances in the staff canteen and bakery areas. The company considered these findings, together with the persistence of the odour within the affected premises and the absence of evidence elsewhere in the supply system, to indicate that the issue was isolated to the property. The water fittings breaches were promptly rectified.

ESP Utilities

In May, ESP Utilities detected a petrol/chemical odour in a sample taken from a show home on a newly constructed housing estate in Bromsgrove that was still under development at the time of sampling.

The company investigated the failure, however resamples from the affected show home and two neighbouring occupied properties were satisfactory for taste, odour and hydrocarbons. Site inspections, discussions with site personnel, and a water fittings inspection did not pinpoint any specific issue and no evidence of fuel spills, construction-related contamination or operational issues affecting the supply was found. The bulk supplier, Severn Trent Water, also confirmed that there were no upstream operational issues at the time of the failure.

Whilst no evidence of contamination or wider network impact was identified, the Inspectorate considers that petrochemical tastes or odours warrant a precautionary approach. The Inspectorate therefore recommended that consumers should be provided with appropriate precautionary do not drink advice pending the outcome of investigations into petrochemical-related failures.

Learning from pesticide failures

Pesticide name	Standard	Total samples	Total failures	Comments
Propyzamide	0.1 µg/L	5,927	2	ICW (1), NES (1)
Dieldrin	0.1 µg/L	4,790	1	LNW (1)

Table 10 – Pesticide failures

Pesticides in drinking water are tightly regulated because of their potential risk to human health and their widespread use in agriculture and land management. Under the Regulations, individual pesticides are subject to a parametric value of 0.1 µg/L and total pesticides to a parametric value of 0.5 µg/L. These standards reflect a precautionary approach rather than substance-specific toxicological thresholds.

Pesticides are most commonly associated with surface water and shallow groundwater sources, where they may enter supplies through agricultural runoff, spray drift or leaching into aquifers. Their occurrence is closely linked to catchment activities, seasonal use patterns, rainfall and the condition and effectiveness of treatment barriers.

Pesticide exceedances are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the source and extent of contamination, assess any risk to consumers, review the adequacy of treatment and catchment controls, and take corrective action to restore compliance and prevent recurrence. Detections below the parametric value also remain important because they may indicate increasing catchment pressure or deterioration in source water quality.

Companies are expected to apply a preventative, risk-based approach through regulation 27 risk assessments, catchment management, engagement with land users and appropriate treatment processes, including granular activated carbon where required. Effective monitoring, timely laboratory analysis and early operational intervention are essential to maintain compliance and ensure that water supplied remains wholesome and safe for consumers.

During 2025, three pesticide failures were reported from 20,831 samples, giving an overall compliance rate of 99.986%. Failures of the pesticide standard have reduced year on year since 2014. This improvement reflects the effect of catchment management initiatives, regulatory undertakings and continuing treatment improvements, including the use and maintenance of granular activated carbon.

Some pesticides remain difficult and costly to remove using available treatment technology. Companies must therefore continue to work with catchment stakeholders to reduce pesticide inputs at source, while maintaining effective treatment and operational controls. Small changes in catchment conditions, rainfall or raw water blending can materially affect pesticide concentrations in drinking water sources, and companies must remain vigilant in areas where these risks are known.

Propyzamide

Last Mile Water reported an exceedance of the individual pesticide standard after propyzamide was detected at 0.111 µg/L in a sample collected in November 2025. The company receives a bulk supply from Northumbrian, Essex and Suffolk Water, which had recorded a propyzamide exceedance at Horsley works a few days earlier. Northumbrian, Essex and Suffolk Water altered the raw water source blend, and subsequent samples showed concentrations reducing to below the regulatory standard. Further samples collected within the inset zone in December 2025 were below the limit of quantification.

A regulatory sample taken from Northumbrian, Essex and Suffolk Water’s Horsley works in November contained 0.14 µg/L propyzamide. This was the third recent propyzamide exceedance at the works, following detections above the standard in December 2022 and January 2023. Two previous propyzamide exceedances had also been recorded at Whittle Dene works in December 2020 and November 2022. Horsley works and Whittle Dene works are both subject to a notice requiring the company to deliver improvements to propyzamide removal, including replacement of granular activated carbon media and implementation of an effective ongoing maintenance strategy.

Both works abstract from the same raw water source, within a catchment containing significant arable land where propyzamide is used on crops. Significant rainfall in the weeks preceding the detections increased raw water turbidity and colour. At Horsley works, the clarifier stream was treating reservoir water at the time of the detection, while the other two streams were supplied by the River Tyne.

The company’s root cause analysis concluded that increased propyzamide concentrations in the raw water system, management of the raw water blend and the condition of the granular activated carbon media were contributory factors. The Inspectorate also made a recommendation relating to laboratory turnaround times, as delayed analysis can reduce the opportunity for timely operational response and source management.

The company confirmed that improvements had been made to raw water source operation and to its root cause analysis process following pesticide detections. It also updated its catchment risk assessment and provided details of the refurbishment of granular activated carbon cells required by the notice. The Inspectorate expects the company to manage and optimise current treatment performance while longer-term improvements are delivered, ensuring that catchment risk, treatment capacity and monitoring data are used together to prevent recurrence.

Dieldrin

Leep Networks Water reported a dieldrin exceedance after a result was reported as less than 0.036 µg/L, above the prescribed concentration value of 0.03 µg/L. The result was automatically flagged as a failure because the laboratory limit of quantification had been raised above the regulatory limit.

Further investigation confirmed that the reported result arose from an analytical issue rather than deterioration in water quality. A quality control standard failed during analysis, resulting in a “less than” value being reported above the regulatory limit. Follow-up samples from nearby properties were satisfactory for dieldrin and related pesticides, and monitoring data from the surrounding supply area showed stable and low pesticide concentrations.

The investigation concluded that the exceedance was not caused by the public water supply and did not present a risk to public health. However, the incident reinforces the importance of robust laboratory quality assurance and clear reporting of analytical limitations. The company’s laboratory service provider strengthened its procedures so that analytical anomalies and raised limits of quantification are clearly flagged and promptly reported to the company before regulatory data is submitted.

Asset health and service reservoir integrity

Asset health remains central to the protection of drinking water quality. During 2025, there were 49 coliform compliance failures at treatment works and 65 at service reservoirs. There were also three E. coli detections at treatment works and four at service reservoirs from a total of 3,632,669 microbiological tests. Ingress into contact tanks, treated water tanks and service reservoirs remains a recurrent risk, demonstrating the importance of maintaining the integrity of treated water storage and post-treatment assets.

In addition to these compliance failures, 47 water quality events were attributed directly to poor asset health or plant failure. These included 30 microbiological contamination events and 17 structural failures affecting treatment works, treated water tanks or service reservoirs. A further 24 events were associated with treatment works plant failure. These figures demonstrate that asset condition, inspection quality and timely remediation remain critical to preventing events and protecting consumers.

Companies are expected to understand and maintain their assets through risk-based inspection and maintenance programmes. Physical internal inspections remain important because they provide greater assurance than remotely operated vehicle inspections alone and allow walls, roofs, floors and internal structures to be cleaned and assessed in detail. While the Principles of Water Supply Hygiene identify ten years as the maximum advised interval for treated water storage inspections, the Inspectorate expects companies to adopt shorter, risk-based frequencies where asset age, condition, previous defects, microbiological history or operational importance indicate a higher risk.

Over the past six years, the Inspectorate has increased scrutiny of service reservoirs and tanks through the service reservoir and tanks data return. This has improved visibility of inspection frequency, inspection quality and the robustness of remedial works. The 2025 dataset identified 195 tanks that had not been inspected within the maximum advised interval, representing 3.1% of tanks. This is unchanged from 2024 but remains an improvement compared with 4.9% in 2023 and 6.0% in 2022. Although this indicates progress, overdue inspections continue to present a residual risk that companies must manage and reduce.

Company	Number of tanks within maximum inspection frequency	Number of tanks overdue	Overdue (%)
ANH	732	52	6.63%
BRL	181	0	–
CAM	37	0	–
NES	518	7	1.33%
PRT	34	3	8.11%
SES	68	4	5.56%
SEW	310	35	10.14%
SRN	330	5	1.49%
SVT	797	37	4.44%
SWB	483	16	3.21%
TMS	644	18	2.72%
UUT	639	5	0.78%
WSX	560	5	0.88%
Total	5333	187	3.39%

Table 11 – Reservoir inspection data

For this analysis, reservoirs identified as abandoned or out of service were manually removed from the dataset. From 2026, the Information Direction will include a specific field to capture asset status more consistently. This will improve the quality of the dataset, reduce the need for manual adjustment and support more reliable scrutiny of inspection performance.

Portsmouth Water made notable progress during 2025, reducing the proportion of reservoirs overdue for inspection from 16% to 8.82%. Although this remains the second highest rate in the industry, the improvement demonstrates that focused action can reduce overdue inspections and improve assurance over treated water storage assets. The Inspectorate expects companies with overdue inspections to maintain clear, risk-based programmes for completing inspections and any associated remedial works.

Figure 7 – Reservoir inspection data

Figure 8 – assets not inspected within past 10 years

Events relevant to asset health in 2025

Anglian Water – Stonewall reservoir 2A

Stonewall Reservoir 2A was last internally inspected in October 2017. Following repeat coliform detections, the company undertook a further inspection approximately eight years later, which identified 18 separate roof ingress points. The findings confirmed that the integrity of the treated water storage asset had been compromised and that there was a credible pathway for contamination.

The Inspectorate had taken enforcement action in early 2023 requiring Anglian Water to complete internal inspections of all treated water tanks that had not been inspected within 10 years, with agreed milestones to mitigate microbiological risk. The notice was revised on 24 February 2023 to strengthen procedural requirements. The Inspectorate concluded that the company had not established and implemented an appropriate risk-based inspection frequency, as required by the notice.

Despite a final completion deadline of 30 April 2025, the company later advised that it would be unable to inspect 23 tanks within this period. Separate regulation 28(4) notices were therefore issued with extended deadlines, many of which were subsequently delayed further. The Inspectorate considered these delays to constitute a failure to comply with regulation 28(4) and section 68(1) of the Water Industry Act 1991, raising significant concerns about asset resilience and protection of drinking water quality. As the contravention was not trivial, further enforcement is ongoing and will be reported in the 2026 report.

Consumer contacts

Consumer contacts provide an important measure of the acceptability of drinking water and of consumer confidence in the public supply. Acceptability can be affected by appearance, taste or odour, including discolouration, aeration, particles, chlorine, earthy or musty tastes, or other changes noticed by consumers.

Most appearance-related contacts are associated with black, brown or orange discolouration, commonly linked to the mobilisation of iron, manganese or aluminium deposits within the distribution system. These metals are rarely present at concentrations that pose a direct risk to health, but they can cause consumers to reject the supply. The Inspectorate therefore treats discolouration and other acceptability issues seriously, particularly where contacts are repeated, widespread or associated with operational events. Several companies have legal instruments in place to address the root causes of discolouration and improve the acceptability of supplies.

In February 2024, the Inspectorate issued Information Letter 01/2024, Annual Provision of Information on Consumer Contacts. This introduced more detailed reporting requirements so that contact data could be used more effectively to assess performance, identify repeat issues and link consumer experience to notifiable events.

A unique reference for each contact
Details of the district metered area the contact is located in
The date and time of the contact
The location of the contact
The mode of contact
Whether the contact is a repeat within a 12-month rolling period (that is a new occurrence or ‘case’ of a similar issue from the same consumer)
If the contact is associated with a notifiable event, and the associated event reference number

The revised return requires companies to provide a unique reference for each contact, the district metered area, date and time, location, mode of contact, whether the contact is a repeat within a rolling 12-month period, whether it is associated with a notifiable event, and the relevant event reference where applicable. Companies must also record where a single consumer contact raises more than one drinking water quality concern.

The data submitted in 2025, covering contacts received in 2024, was the first return to include the additional fields. To maintain continuity with previous reporting and allow long-term performance trends to be monitored, companies continue to identify primary and secondary descriptors where multiple concerns are reported. Primary contacts are used to assess ongoing industry performance.

In England in 2025, companies wholly or mainly operating in England reported 57,759 consumer contacts relating to the appearance, taste or odour of drinking water. This equates to 0.97 contacts per 1,000 population. The table below shows the number of contacts by category. Brown, black and orange discolouration remained the most common reason for contact.

Category	Consumer contacts	England contact rate	Industry contact rate
Appearance – Brown black orange	24,810
Taste Odour Other	7,716
DWQ Concern – Lead and analysis	7,714
Appearance – White Air	7,474
Taste Odour Chlorine	5,618
Appearance – Particles	4,123
Appearance – General Conditions	4,088
Illness – Gastroenteritis	3,606
Taste Odour – Earthy Musty	1,888
Illness – Skin	1,469
Appearance – Blue Green	995
DWQ Concern – Incident Related	680
Appearance – White Chalk	497
DWQ Concern – Lifestyle	416
Taste Odour Petrol Diesel	402
Illness – Oral	280
DWQ Concern – Campaign	278
Illness – Medical Opinion	264
Appearance – Animalcules	148
DWQ Concern – Pets Animals	104

Table 12 – Number of consumer contacts in 2025

Discoloured water

Discoloured water is most commonly caused by the mobilisation of deposits within distribution mains following changes in flow, pressure or direction. These changes may occur during planned operations, valve movements, mains repairs, bursts, high demand, shutdowns, restarts or other network disturbances. Companies are expected to understand where deposits may accumulate, manage operational changes carefully and take timely action to minimise consumer impact. In 2025, around 63 events were reported in England where brown, black or orange discolouration was experienced by more than one consumer at the same time.

The Inspectorate reviews consumer contact data for discoloured water each year. Companies performing worse than the industry average, or showing deterioration over time, are subject to further scrutiny and enforcement action where necessary. Legal instruments are in place for companies where discolouration risk requires sustained improvement:

Northumbrian Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/northumbrian-water-improvement-programmes/nne-2018-00003/

Figure 9 – Company discolouration trend

Severn Trent Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/severn-trent-water-improvement-programmes/

Figure 10 – Company discolouration trend

Southern Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/southern-water-improvement-programmes/srn-2024-00001-v1/

Figure 11 – Company discolouration trend

South East Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/south-east-water-improvement-programmes/sew-2023-00001/

Figure 12 – Company discolouration trend

South West Water (including Bristol Water)

https://www.dwi.gov.uk/water-companies/improvement-programmes/south-west-water-improvement-programmes/swb-2023-00015-2/

Figure 13 – Company discolouration trend

United Utilities

https://www.dwi.gov.uk/water-companies/improvement-programmes/united-utilities-water-improvement-programmes/uut-2020-00005v4/

Figure 14 – Company discolouration trend

Wessex Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/wessex-water-improvement-programmes/wsx-2021-00002/

Figure 15 – Company discolouration trend

Yorkshire Water

https://www.dwi.gov.uk/water-companies/improvement-programmes/yorkshire-water-improvement-programmes/yks-2023-00008/

Figure 16 – Company discolouration trend

Company performance

Company-wide discolouration performance is considered alongside water supply zone performance when deciding whether legal instruments are required. These instruments are intended to address underlying causes, including deposit accumulation, network operation, asset condition and flushing or conditioning programmes, aiming for sustained improvements in consumer acceptability to be delivered.

Across England and Wales, the number of contacts reporting brown, black or orange discoloured water had reduced over recent years, but this improvement has now stalled. The industry rate deteriorated to 0.49 contacts per 1,000 population in 2024 and showed little improvement in 2025, at 0.48. This equated to 29,959 contacts in 2025 across the industry. In England, the rate followed a similar pattern, increasing from 0.40 contacts per 1,000 population in 2023 to 0.42 in both 2024 and 2025.

Figure 17 – England vs industry BBO contact rate

The England company ranking is shown below. South West Water remained the poorest performer in England, followed by United Utilities. These two companies have ranked worst and second worst in England since 2017. Both have nevertheless achieved material improvements over the longer term, with South West Water reducing its rate by around half over the past decade. Continued regulatory focus remains necessary to ensure that these improvements are sustained and that further reductions in consumer contacts are delivered.

Figure 18 – all companies BBO contact rate

More recent performance indicates that some company discolouration rates have deteriorated or plateaued. This suggests that current interventions may not be sufficient in all areas, or that additional investment and more targeted network management may be required. Companies with persistent or worsening performance are expected to use consumer contact data, event investigations, network modelling, flushing records and asset information together to identify root causes and prioritise action.

Taste and odour

Taste and odour contacts provide a direct measure of consumer experience and confidence in drinking water. As with discoloured water contacts, the industry rate had reduced over recent years, but this improvement has not been sustained. The industry rate deteriorated in 2024 and remained at 0.27 contacts per 1,000 population in 2025. In England, the rate was slightly lower, at 0.26 contacts per 1,000 population in 2025, but this also represents deterioration from the low point of 0.21 recorded in both 2022 and 2023.

Figure 19 – Taste & Odour contact rate

The compliance section includes examples of investigations into taste and odour failures. These failures do not always result in consumer contacts, and consumer contacts are not always linked to a compliance failure. Contact data therefore provides an important additional perspective on the acceptability of drinking water as experienced by consumers. General taste and odour contacts and chlorine-related contacts have featured prominently in recent years. While it is unrealistic to expect all contacts to be eliminated, companies should use the data to understand recurring causes, including the composition of the “other” category, and to identify opportunities for improved source control, treatment optimisation, residual management and customer response.

Drinking water quality events

Water companies must notify the Inspectorate of any event that has, or is likely to have, an adverse effect on the quality or sufficiency of water supplied to consumers, or that may cause concern to consumers. Event notification is an important regulatory control because it enables timely scrutiny of company actions to protect public health, maintain supplies and restore consumer confidence. In 2025, the Inspectorate was notified of 586 water quality events in England.

Events may arise at any point in the supply system, from source to consumer tap. The figure below summarises the types of events reported to the Inspectorate in 2025 and their point of impact across the supply chain. Some events were outside the direct control of water companies; for example, 217 events, representing 37% of all notifications, were attributed to consumers’ distribution systems.

Ground contamination, including spillages of petrol or other chemicals affecting pipes, accounted for a further 8.4% of events. Mains problems or damage accounted for 11%. These categories demonstrate the importance of effective asset management, rapid investigation, cross-sector communication and clear operational controls to prevent, contain and resolve events that could affect consumers.

Figure 30 – Water quality events in 2025

Figure 31 – Number of water quality events

The Event Risk Index (ERI) is used by the Inspectorate to assess the relative risk and consumer impact of notified events. It considers both the nature of the event and the adequacy and timeliness of the company’s response, including actions taken to protect consumers, investigate the cause, communicate effectively and prevent recurrence. ERI supports risk-based regulation by directing scrutiny and mitigation towards companies, sites and event types presenting the greatest relative risk.

Figure 32 – ERI since 2021, this is a sum of the raw score divided by the total population of each company

Event risk index by company

The ERI enables the Inspectorate to identify companies where events have had the greatest relative impact on consumers and where further regulatory scrutiny or mitigation may be required. The figure below shows company ERI performance in 2025. Four companies were above the industry ERI: South Staffordshire Water, United Utilities, SES Water and the Isles of Scilly. The Inspectorate uses this information alongside event investigations, compliance data and company risk assessments to determine whether additional action is required to protect consumers and reduce recurrence.

Figure 33 – ERI by company

Highest Event Risk Index (ERI) scoring events

ERI also supports identification of the individual events requiring the greatest regulatory attention. The highest-scoring events in 2025 were generally associated with asset health, treatment process control, prolonged consumer impact or weaknesses in company response. Although most of the 586 notifications were short in duration and companies generally took appropriate action to safeguard consumers, 403 notifications were assessed as either not an event, not significant or minor, resulting in low ERI scores. A further 168 events were significant and seven were serious. No major events were reported in 2025.

Figure 34 – Top 10 ERI scoring events – this might need an update as it doesnt match the database download –

Specific events – Top 10 ERI

Affinity Water – Installation and use of ZeeWeed 500D (440) membrane modules unapproved under regulation 31

The Inspectorate was notified that ultrafiltration ZeeWeed 500D (440) membrane modules had been installed and used at Affinity Water’s Amersham and Northmoor treatment works and Anglian Water’s Heigham works without the required approval under regulation 31. The membranes, supplied by Veolia Water Technologies and Solutions, had been in use since 2021 at the Affinity Water sites and since 2019 at Heigham works.

Veolia subsequently informed the Inspectorate that the membrane modules supplied had been manufactured using Evatane 28-800 adhesive, whereas the regulation 31 approval specified the use of Hot Melt 9402 or Licocene PP-1602 adhesives. This discrepancy was identified during an internal review of Veolia’s certification management processes.

Following this discovery, Veolia notified both companies that 996 membranes at Northmoor works, 832 at Amersham works and 252 at Heigham works did not conform to the conditions of the regulation 31 approval.

In response, Anglian Water removed all unapproved membrane modules from supply shortly after notification. Affinity Water undertook a risk assessment and concluded that the risk to water quality was low, because any potential risks, including microbial growth on membranes in contact with raw water, would be managed by the ultrafiltration process and downstream disinfection. The ultrafiltration plant at Northmoor works was subsequently bypassed in July 2025, while the plant at Amersham works remained in operation.

The Inspectorate concluded that offences had been committed under regulation 31, although the use of the non-compliant membranes was not intentional. Both companies were required to review and strengthen their regulation 31 approval and sign-off procedures to prevent recurrence. Affinity Water was also expected to update its risk assessment to reflect fully the risks associated with the use of non-approved membrane materials, decommission the affected membranes at the earliest opportunity, and implement enhanced monitoring to ensure that no unwholesome water was supplied while the modules remained in use.

The ZeeWeed 500D (440) module and cassette were subsequently removed from the list of approved products on 20 February 2026. The event demonstrates the importance of robust supplier assurance, product approval checks and change-control arrangements for all materials and products in contact with drinking water.

South Staffs Water – Elevated Geosmin at Hampton Loade treatment works

South Staffordshire Water reported elevated geosmin concentrations in treated water supplied from Hampton Loade treatment works between 20 April and 18 June 2025, with a maximum concentration of 6.6 ng/L. This exceeded the operational Suggested No Adverse Response Levels (SNARLs) of 1–4 ng/L. No taste or odour compliance failures were attributed to the event, and there was no corresponding increase in consumer complaints.

During this period, the powdered activated carbon (PAC) dosing system, which is intended to remove geosmin, was operated at maximum output. The company also attempted to increase contact time by reducing flows through the works, but this could not be sustained because of demand requirements. The investigation identified wider operational constraints, including limitations in the management of granular activated carbon (GAC) filters. In particular, decisions to restrict GAC replacement and regeneration to manage manganese risk may have reduced the effectiveness of organic removal, including geosmin.

In July 2025, the company reported an Enterococci detection in a treated water sample, followed six days later by a further detection in a sample taken from the outlet of a surge vessel at the works. These detections were consistent with a pattern of intermittent microbiological failures downstream of disinfection that has been observed over several years.

The company maintained that effective pathogen removal is achieved through the recently installed ceramic filtration process and attributed the most recent detections to biofilm accumulation within pipework associated with the UV system. In response, the company undertook targeted cleaning of the affected pipework and began decommissioning the UV disinfection system.

Prior to these events, Hampton Loade works had experienced ongoing challenges, including elevated geosmin, disinfection by-product formation and microbiological detections. While improvements have been delivered through the introduction of ceramic filtration, the 2025 events highlight the breadth and interdependence of water quality risks at the site. The works supplies, in varying proportions, more than 690,000 consumers and is subject to a regulatory notice relating to taste and odour control.

Following review of the company’s response and its wider assessment of operational performance, the Inspectorate remained concerned that the proposed actions required further strengthening to ensure that risks were adequately controlled. The event demonstrates the importance of adopting a holistic and integrated approach to managing multiple and potentially competing water quality challenges at treatment works, ensuring that control measures are balanced, effective and supported by robust monitoring and timely operational decision-making.

Southern Water – Tenants Hill service reservoir potential ingress

Tenants Hill is a twin-celled treated water storage reservoir separated by a partial-height dividing wall. An inspection in November 2024 identified ingress associated with deterioration of the North cell roof. Repairs were completed, the cell was flood-tested and it was subsequently returned to service.

The overall condition of the roof membrane meant that a full replacement was scheduled. Before this work was undertaken, a remotely operated vehicle inspection identified a crack with associated dampness in the soffit of the South cell roof. The defect was repaired in May 2025, and a new membrane was installed over both cells in June 2025. Enhanced monitoring and routine remotely operated vehicle inspections were implemented as interim risk control measures.

The South cell was removed from service in September 2025 to allow a full internal inspection and repair. The event attracted a significant Event Risk Index score because of the requirements of the legal instrument in place for treated water storage assets and the time taken to remove the South cell from supply after potential ingress had been identified.

Under the conditions of the legal instrument, Southern Water is required to notify the Inspectorate where a service reservoir with identified ingress cannot be taken out of supply immediately. The circumstances of this event demonstrate the importance of timely isolation, clear notification and effective interim controls where treated water storage integrity may be compromised. Companies must ensure that temporary mitigations are proportionate, verified and supported by a clear programme for permanent repair.

Thames Water – Bromate exceedance at Ashford Common works

Elevated bromate concentrations were detected in samples taken from Thames Water’s Ashford Common treatment works in July 2025. The maximum reported concentration was 56.9 µg/L, exceeding the prescribed concentration value of 10 µg/L. A subsequent result of 17.0 µg/L was recorded in the downstream Enfield South water supply zone. Ashford Common works supplies, in combination with other sources, a population of more than 4.2 million consumers.

The company attributed the exceedances to a fault in the ozonation process, which resulted in unstable and excessive ozone dosing between 10 and 15 July 2025. Bromate can form where ozone is applied to water containing bromide, and effective control of ozone dose is therefore essential to prevent disinfection by-product formation. The issue was resolved on 15 July, after which bromate concentrations returned to expected levels.

Thames Water validated the laboratory results and reviewed raw water bromide concentrations, which were found to be within normal ranges. A detailed investigation of treatment performance identified two periods of unstable ozone dosing linked to mechanical and electronic faults within the ozone control system. Although an initial valve issue was addressed by maintenance contractors, a subsequent electronic control failure was not identified promptly by site operators.

The investigation identified significant deficiencies in process control and monitoring. Online instruments intended to identify ozone dosing variability were not operational, and no effective alarms were triggered during the event. The company had introduced daily manual ozone residual checks as a supplementary control; however, these checks were not completed consistently, delaying identification of the fault.

Following the event, the company implemented improvements to strengthen control of the ozonation process. These included enhanced online monitoring with visibility of applied ozone dose trends and revised alarm settings. The event was also escalated to Thames Water’s Ozone Working Group to assess whether similar risks existed at other treatment works and to inform wider improvements across ozonation sites.

Although water exceeding the bromate standard was supplied during the event, concentrations remained below short-term health-based guidance values. Nevertheless, the failure to maintain effective treatment control constituted a significant regulatory concern. The absence of functional online monitoring, ineffective alarm arrangements and inconsistent completion of manual checks meant that the company did not identify or respond to the issue in a timely manner.

The Inspectorate expects companies to ensure that critical treatment processes, particularly those associated with disinfection and disinfection by-product formation, are supported by robust monitoring, alarms, verification and escalation arrangements. Following this event, key expectations included restoration and enhancement of online monitoring capability, clear and effective alarm settings for ozone dosing, consistent completion of verification checks, and a time-bound action plan to address comparable risks across other ozonation sites.

Thames Water – Cryptosporidium detection at Kempton Park works

In February 2025, Thames Water reported the detection of single Cryptosporidium oocysts in three separate samples taken at Kempton Park treatment works. The works directly supplies approximately 230,000 consumers across four water supply zones and contributes, in varying proportions, to supplies serving around 4.25 million consumers across north and south London.

During January and February 2025, output from the works was higher than typical for the time of year, with flows ranging between 140 and 180 ML/d and frequently sustained at 165–170 ML/d. This increased output was used to support a wider outage programme across the London supply area.

During the period of detections, the works experienced several operational challenges, including significant variations in flow at site level and across individual slow sand filters. On 28 January 2025, a power interruption resulted in a full works shutdown. On 12 February 2025, elevated turbidity was encountered during a raw water source change. These factors introduced instability in process performance, particularly affecting the operation of the slow sand filtration stage.

The Inspectorate identified several areas for improvement in the operation and control of the slow sand filters, particularly in managing flow changes, maintaining stable treatment conditions and ensuring that operational changes do not compromise Cryptosporidium risk control.

In response, the company implemented enhanced monitoring, and all subsequent samples were clear for Cryptosporidium. Improvements were also made to procedures for returning individual slow sand filters to service, including requirements to keep associated flow changes within defined operational limits.

The company also took proactive steps to strengthen monitoring capability ahead of the 2025–26 winter period. A programme of work was initiated to assess potential performance improvements, including the relining or remediation of one slow sand filter. This work remained ongoing at the end of 2025.

The event demonstrates the importance of maintaining stable treatment performance during periods of increased demand, outage management and operational disruption. Where slow sand filtration forms a key barrier to Cryptosporidium risk, companies must ensure that flow changes, source changes, shutdowns and returns to service are carefully controlled, monitored and verified so that supplies remain protected.

Yorkshire Water – Microbiology failures at Chellow Heights treatment works

The Inspectorate assessed two bacteriological events at Chellow Heights treatment works in May and September 2025. Both involved detections of E. coli in large-volume treated final water samples. The May event was classified as significant, with evidence identifying a credible route of ingress through defects at the roof-to-wall interface of one of the clean water tanks.

Although downstream distribution samples remained satisfactory and disinfection criteria were met, the Inspectorate concluded that unwholesome water may have entered supply. The investigation identified structural deficiencies, limitations in inspection effectiveness and the need for enhanced monitoring. This included appropriate monitoring downstream of any suspected ingress into final water storage assets, including Cryptosporidium sampling where relevant.

The September detection occurred in the alternate clean water tank while the first tank remained out of service. No definitive root cause was established, although minor roof defects and environmental factors were identified as potential contributors. The recurrence of microbiological detections across separate treated water storage assets demonstrated the need for a broader assessment of asset integrity and inspection arrangements at the works.

In response, Yorkshire Water undertook remedial repairs, strengthened inspection and monitoring practices, and committed to increasing the frequency and scope of internal inspections. The company also set out longer-term plans for significant asset refurbishment and renewal over the next five to ten years within AMP8 and AMP9.

These events highlight the critical importance of maintaining treated water storage integrity as a key barrier to microbiological risk, regardless of otherwise satisfactory treatment and disinfection performance. Inspection and maintenance regimes must be risk-based, responsive to asset condition and capable of identifying latent ingress pathways, including through flood testing and sufficiently detailed internal inspection scopes. The event also reinforces that reliance on ageing infrastructure with limited resilience increases water quality risk. Proactive, strategic investment in refurbishment or replacement, supported by transparent engagement with the Inspectorate, is essential to maintain compliance and ensure the continued protection of public health.

Southern Water – Fairlight Old service reservoir ingress

A remotely operated vehicle inspection at Fairlight Old service reservoir in January 2025 identified significant root ingress through the west wall, together with further evidence of root and water ingress. These findings indicated that the integrity of the treated water storage asset had been compromised and that a credible pathway for contamination existed.

As an interim risk control, the company increased the distribution chlorine dose from 0.6 mg/L to 0.7 mg/L and implemented enhanced sampling to provide additional assurance of water quality while the reservoir remained in supply.

Removal of the reservoir from supply was delayed because redundant pipework had to be recommissioned to maintain continuity of supply. The event attracted a high Event Risk Index score, reflecting both the requirements of the legal instrument in place for treated water storage assets and the extended period taken to remove the reservoir from supply after ingress had been identified.

The event demonstrates the importance of timely isolation where treated water storage integrity is compromised. Where immediate removal from supply is not possible, companies must notify the Inspectorate, maintain verified interim controls and progress permanent remediation without avoidable delay. Reliance on enhanced disinfection and monitoring should not be treated as a substitute for addressing confirmed ingress pathways.

Southern Water – Darland Chatham West South service reservoir ingress

Darland Chatham service reservoir comprises North and South cells separated by a partial-height dividing wall. The West North cell was removed from supply in October 2025 for a planned inspection and clean. That inspection identified multiple defects, including points of ingress, loose and blown render, surface cracking and the need for recoating.

During subsequent roof flood testing of the West North cell, ingress was observed into the adjacent live West South cell. This demonstrated that the integrity of the shared structure had been compromised and that a credible pathway for contamination existed while one cell remained in supply.

The company advised that immediate removal of the West South cell from supply was not possible because of network resilience constraints and ongoing improvement works at the supplying treatment works. As an interim mitigation measure, a temporary roof membrane was installed across both cells to reduce the risk of further ingress.

Following completion of remedial works, the West North cell was returned to service on 3 December 2025. The West South cell was subsequently removed from supply on 10 December 2025 and returned to service on 10 February 2026 with the temporary membrane remaining in place. Both cells have been prioritised within the company’s inspection and maintenance programme for permanent remediation during 2026.

A second phase of works is planned, involving sequential isolation of each cell to undertake comprehensive internal and external repairs. This includes removal of the temporary membrane and overburden, repair of the exposed roof structure, installation of a new membrane and drainage layer, reinstatement of overburden and return to service.

The event attracted a high Event Risk Index score, reflecting both the requirements of the legal instrument governing treated water storage assets and the duration taken to fully resolve the identified risks. It reinforces the need for companies to isolate affected treated water storage assets promptly where ingress is confirmed, and to ensure that temporary mitigations are verified, time-bound and not used as a substitute for permanent repair.

Anglian Water – Hardness at Winterton Holmes treatment works

Winterton Holmes treatment works was subject to an improvement notice following a pesticide contamination event in 2012. The agreed mitigation was a catchment-based solution, which was implemented by March 2018. The works returned to supply on 29 March 2018, having been out of service since November 2012.

In June 2019, a pesticide application incident within the Winterton catchment resulted in the spillage of approximately 2,000 litres of diluted fungicide containing tebuconazole and prothioconazole. As a precautionary measure, Winterton Holmes works and its associated boreholes were removed from supply on 10 July 2019. The works remained out of service until 27 August 2024.

During this period, approximately 27,000 consumers in the Winterton and Burton upon Stather zones were supplied from Elsham works. Following the return of Winterton Holmes works to service in August 2024, the company received customer contacts relating to increased hardness. A total of 154 contacts were recorded between 28 August 2024 and 13 February 2025, with average hardness increasing by approximately 130 mg/L in the affected zones.

Despite the scale and persistence of customer contacts, the issue was not initially notified to the Inspectorate as an event. Proactive communication with affected consumers was also not undertaken. The company relied instead on passive website messaging advising customers to adjust domestic appliance settings.

The investigation identified that the works had been returned to supply without reinstating phosphate dosing. This contributed to changes in water chemistry, including pH, and exacerbated customer acceptability issues in the affected zones. This event highlights the importance of timely notification where significant changes in water quality parameters result in widespread or sustained customer impact. It also demonstrates the need for proactive consumer communication and for ensuring that all relevant treatment processes are fully reinstated, operational and optimised before assets are returned to supply. The Inspectorate expects companies to assess consumer acceptability risks as part of return-to-service planning and to maintain clear escalation arrangements where customer contacts indicate a material change in supplied water quality.

Northumbrian, Essex and Suffolk Water – Wear Valley network discolouration

A discolouration event occurred in March 2025 following a control system fault and the associated shutdown and restart of Wear Valley treatment works. During the restart, flow changes and partial system drain-down mobilised historic deposits within the downstream distribution network. This resulted in widespread consumer contacts reporting discoloured water and regulatory exceedances for aluminium, iron and manganese. The discolouration was transient, and there were no concurrent turbidity or microbiological failures.

The company’s investigation identified weaknesses in operational control and verification during restart. In particular, there was a delay in undertaking manual confirmatory sampling after anomalous online turbidity and iron readings were observed. Instrument performance was also affected by air entrainment, reducing confidence in the online monitoring data available to operators at the time.

The company acknowledged that its response was affected by operational pressures and committed to strengthening restart procedures. Revised protocols now require prompt manual sampling where there is uncertainty about the reliability of online data. These changes were communicated to operational staff, and a review of monitoring equipment suitability was initiated.

The risks associated with discolouration in downstream networks, particularly in gravity-fed systems with known sediment accumulation, are well understood. Shutdown and restart activities must therefore be planned and controlled to minimise disturbance within the network. The event also demonstrates the importance of timely manual verification where online monitoring may be unreliable, and of ensuring that instruments are fit for purpose, including consideration of known issues such as air entrainment and installation of appropriate mitigation measures, such as debubblers, where required.

More broadly, the event reinforces that even where treatment performance is satisfactory, consumer impact can arise if distribution system risks are not effectively controlled. Companies must manage treatment and distribution risks as a single system, ensuring that operational changes are risk assessed, verified and communicated so that compliance is maintained and consumer confidence is protected.

Southern Water – Ditchling service reservoir bacteriological detection

Ditchling Road service reservoir comprises North and South cells separated by a dividing wall extending to approximately 80% of the cell height. In December 2025, coliform bacteria were detected in a compliance sample taken from the combined outlet sampling point. Four further coliform detections were recorded in repeat samples later that month.

Investigatory dip samples were collected from each cell to determine whether the contamination was associated with a specific part of the reservoir. The North cell was clear, while samples from the South cell detected coliform bacteria. The investigation also identified site defects, including a leaking washout valve and standing water within the sampling chamber below the tapping point, both of which were subsequently rectified.

The South cell was drained and internally inspected in January 2026. No structural defects were identified. The North cell was drained in February 2026, during which repairs were completed to the washout valve and cable ducts were sealed to remove potential ingress pathways.

A legal instrument is in place between Southern Water and the Inspectorate covering service reservoir integrity and management. The presence of this instrument, together with the duration of the incident and the need to investigate potential ingress pathways within a treated water storage asset, contributed to a high Event Risk Index score.

Serious events

South East Water – Sevenoaks loss of supplies/media interest

In January 2025, a significant loss of supply event affected consumers in and around Sevenoaks, Kent. Approximately 74,860 consumers experienced loss of supply or low pressure, with a smaller number also reporting discolouration and the presence of air in their water.

The event was triggered by Storm Éowyn, which caused flooding and elevated turbidity at Tonbridge treatment works, resulting in its shutdown. The following day, a power failure occurred at Cramptons Road works. The standby generator failed to start as intended because of issues associated with the uninterruptible power supply. As a result, a critical valve remained open and untreated artesian borehole water entered the contact tank.

Once the issue was identified, the company acted to protect water quality by partially draining the contact tank, super-chlorinating the remaining water and allowing sufficient contact time before returning the works to supply. These actions were complicated by flooding within the works, caused by the draining of the contact tank and continuing artesian flow, together with blockages in hypochlorite dosing equipment that prolonged the outage.

With both treatment works compromised, downstream service reservoir levels fell rapidly, leading to widespread supply interruptions. The company declared an incident and implemented emergency response arrangements, including mobilisation of water tankers to replenish reservoirs, establishment of bottled water stations and direct deliveries to Priority Services Register customers. In total, more than 146,000 litres of bottled water were distributed. Consumers were kept informed through multiple communication channels, including alerts, traditional media and social media.

During recovery, air entrainment and discolouration were reported in parts of the distribution network. The company responded with targeted flushing and comprehensive water quality sampling at treatment works, service reservoirs and consumer properties. All regulatory samples returned satisfactory results.

The Inspectorate concluded that there was evidence of potential regulatory non-compliance because untreated water had entered the contact tank, although no evidence was identified to confirm that unwholesome water entered supply. The primary cause was inadequate maintenance and testing of backup power systems at Cramptons Road works, with the concurrent outage at Tonbridge works acting as a significant exacerbating factor.

The Inspectorate required the company to take corrective action, including improvements to the maintenance and testing of standby power systems, installation of failsafe controls to prevent uncontrolled flows, review of comparable risks at other sites, and improvements to the accessibility and resilience of alternative water supply arrangements. Some bottled water distribution centres were not available overnight, limiting consumer access during critical periods. This event highlights the importance of resilient asset design, robust maintenance and testing of emergency systems, and effective operational controls to safeguard both water quality and continuity of supply. It also reinforces the need for resilient incident response arrangements, including continuous availability of alternative water supplies and clear, timely communication with affected consumers.

Anglian Water – Enterococci detection at Welton treatment works

Welton works has experienced repeated microbiological detections over the past two years. An Inspectorate audit in 2024 identified deficiencies in rapid gravity filter performance and recommended enhanced investigation, including individual filter turbidity and head loss monitoring, and inspection of final water contact tanks and upstream storage assets.

Although Anglian Water committed to defined timescales for inspecting both on-site and upstream tanks, these commitments were not met. Inspections were repeatedly deferred into 2025 despite ongoing microbiological detections from 2023 onwards. This represented a failure to undertake timely and adequate investigation, resulting in enforcement action by the Inspectorate in May 2025.

Evidence subsequently provided by the company confirmed that all relevant tanks had been inspected and that ingress had been identified in most of them. This indicated an ongoing and unmitigated risk of contamination during the extended period between the initial detections and completion of inspections. In total, seven microbiological detections were recorded over the two-year period, with recurring issues associated with both filtration performance and treated water storage.

In response, the company installed ultraviolet treatment downstream of the disinfection process as a precautionary additional barrier. However, this treatment was located after the regulatory compliance sampling point. A subsequent Enterococci detection in February 2025 occurred at a sampling point downstream of the UV treatment, indicating the potential for an alternative route of ingress. The root cause of this exceedance was not confirmed, although both the disinfection and UV processes were operating satisfactorily at the time. The Inspectorate considers that the key learning from these events is the need for prompt, thorough and evidence-led investigation of potential contamination sources, with agreed corrective actions implemented without delay. Reliance on downstream “catch-all” treatment measures is not an acceptable substitute for identifying and addressing underlying deficiencies in treatment performance or asset condition. Effective risk management requires timely inspection, intervention and resolution to prevent prolonged exposure to potential contamination risks and to protect public health.

South West and Bournemouth Water – Loss of supply at Dousland treatment works

In April 2025, a control system failure occurred at South West Water’s Dousland treatment works. The failure closed the raw water inlet valve and stopped all chemical dosing, including chlorination. At the same time, the raw water main upstream of the inlet valve burst, although the raw water pumps continued to operate.

A limited volume of water remained in the treatment process and continued to pass through the filters, contact tank and final water tank before entering the distribution system. Flow through the works ceased only when levels fell below the outlet of the contact tank. During this period, a low chlorine residual was recorded in water entering the contact tank. The company responded by manually dosing the contact tank with sodium hypochlorite. Once the final water tank level had reduced to approximately 13%, the outlet valve was closed. The burst raw water main was subsequently identified and repaired.

During restart, the works was operated to waste up to the filtration stage to confirm satisfactory upstream water quality. The final water tank was refilled while isolated from supply, with manual turbidity monitoring undertaken. Although turbidity reduced during refilling, it remained above 1 NTU. The outlet valve was then opened against a closed downstream valve and the system was flushed through a hydrant until turbidity fell below 1 NTU. Normal supply was restored three days after the initial failure.

The event resulted in approximately 700 consumer contacts reporting loss of supply and a further 56 contacts reporting discolouration. The company implemented emergency response arrangements, including rezoning parts of the network through installation of a temporary main, using tankers to replenish service reservoirs and providing bottled water to affected consumers. Following restoration of supply, additional flushing was undertaken to address residual discolouration.

The most probable cause of the elevated turbidity during restart was disturbance of lime deposits that had accumulated within the final water tank. The Inspectorate had previously served an enforcement notice in relation to both the final water tank and contact tank, as neither had been internally inspected or cleaned for more than ten years. Both assets were surveyed using a remotely operated vehicle in 2024 and minor external repairs were completed to the contact tank. A formal programme remains in place requiring full drain-down and internal inspection by June 2028.

Following the event, and in response to Inspectorate recommendations, the company implemented improvements to the operation of the lime dosing system. These included modifications to the lime silo to prevent compaction and improve dosing consistency. The changes reduced the frequency of blockages and improved operational performance.

The company also revised and tested control system settings to ensure that, during a shutdown, valves within the treatment process fail safe and prevent forward flow. This review was extended to other similarly configured sites. This event highlights the importance of control system resilience, timely inspection and maintenance of treated water storage assets, and robust shutdown and restart procedures. It also reinforces the need to manage accumulated deposits within treated water storage, as operational disturbances can otherwise mobilise material and affect water quality during recovery.

Yorkshire Water – Woolley Edge reservoir Enterococci

Yorkshire Water detected Enterococci at 1 cfu per 100 mL in a sample taken from Woolley Edge service reservoir following a burst on the inlet main. Immediate and subsequent follow-up samples from the reservoir, upstream sources and downstream consumer taps were bacteriologically satisfactory, and no public health risk was identified.

An inspection of the reservoir identified structural defects, including evidence of ingress via roof hatches and upstands, which were considered the most probable source of contamination. In response, compartment 1 was isolated promptly, interim repairs were implemented to prevent further ingress, enhanced bacteriological sampling was undertaken and an internal inspection of the second compartment was scheduled.

This event provides a further example of the microbiological risks associated with ingress, even where defects appear limited. It reinforces the importance of proactive inspection, maintenance and effective vermin-proofing of treated water storage assets. The timely isolation of the affected compartment, implementation of interim controls and enhanced sampling limited the duration and impact of the event, demonstrating effective incident management and protection of water quality.

Other events of note

United Utilities – Coliforms at Denton treatment works

United Utilities undertook maintenance on an outlet pump within the contact tank at Denton treatment works in early April 2025. Following completion of the work, the pump chamber was covered using a weighted tarpaulin rather than being sealed with an engineered blanking plate. The works was subsequently returned to supply with this temporary arrangement in place, creating a sustained risk of ingress downstream of disinfection and close to the regulatory compliance sampling point.

In July 2025, two regulatory samples taken at the works contained coliform bacteria above the standard, confirming that unwholesome water had been supplied. Downstream consumer tap samples remained satisfactory. The temporary tarpaulin covering was identified only during the investigation and was subsequently replaced with a steel blanking plate. Heavy rainfall in the preceding days had resulted in pooling of surface water on the tarpaulin, providing a credible pathway for contamination.

Exterior view of a water supply installation area showing exposed pipework and fittings associated with distribution infrastructure. Large diameter green pipes with flanged connections are mounted above ground, with visible sections of older, corroded blue pipework. Adjacent to the pipework is an open or uncovered access chamber, temporarily sealed with a circular metal plate placed on a wooden pallet rather than being fixed in position. Electrical cables and equipment are present and loosely arranged across the area, with some cabling draped over the structure. A temporary barrier using red and white hazard tape is positioned around the installation, indicating restricted access, but the containment appears informal and not fully secured. A blue tarpaulin sheet is placed beneath part of the arrangement, possibly to protect the surface or collect debris. The surrounding area includes hardstanding and nearby buildings, with vehicles parked in the background. The overall condition suggests ongoing works or maintenance activity; however, the exposed access point, unsecured cover, and presence of loose materials may present risks of contamination ingress, physical hazards, or unauthorised access if not adequately controlled. The arrangement highlights the importance of secure sealing of infrastructure, proper site safeguarding, and management of potential contamination pathways within drinking water systems. — Figure 35 – Photograph of the pump chamber provided by the company. The photograph shows the weighted down tarpaulin cover used to cover the pump chamber.

Exterior view of a water supply installation following remedial works, showing a previously exposed access point now securely sealed with a fitted metal cover. The cover is fixed in place with large bolts at each corner and sealed around the perimeter, forming a continuous barrier between the access chamber and the surrounding environment. The plate sits flush within a concrete surround, reducing the potential for gaps or ingress pathways. Adjacent pipework remains visible, including large diameter green pipes with flanged connections, but the immediate area around the access point is now clear of loose materials and temporary coverings. Cables and equipment appear tidied and better managed compared to the previous condition. The surrounding hardstanding is intact, and the area is contained within a structured framework, with no reliance on temporary barriers. The completed arrangement demonstrates improved asset security and protection against contamination ingress, with the fixed, sealed cover providing a robust and permanent solution. The removal of loose materials and proper enclosure of the access point reduces both hygiene risks and physical hazards, supporting compliance with expectations for safeguarding drinking water infrastructure. — Figure 36 – Photograph of the pump chamber provided by the company. The photograph shows the steel blanking plate that was used to seal the pump chamber on 23 July 2025.

The Inspectorate concluded that there were breaches of multiple regulations, including regulations 4, 18, 27 and 28, with a potential breach of regulation 26(4). The event highlighted systemic weaknesses in the company’s control arrangements, including ineffective supervision and sign-off of contractor activities, failure to identify clear ingress risks during routine site inspections, delays in undertaking site-based investigations following microbiological detections, and deficiencies in the governance of staff competency and training records. The Inspectorate also raised concerns that learning from previous contractor-related water quality events had not been fully embedded.

This event underscores the critical importance of protecting post-disinfection assets and control points. Any work affecting contact tanks or downstream infrastructure must be treated as high risk, with robust controls maintained throughout planning, delivery and return to service. Temporary measures, such as tarpaulin coverings, are not acceptable substitutes for properly engineered and sealed solutions. Any deviation from agreed methods by contractors must trigger immediate escalation, reassessment and formal re-approval.

Companies retain full responsibility for contractor activities, and accountability for water quality cannot be delegated. Companies must also be able to demonstrate that all personnel, including contractors, are appropriately trained and competent for their roles. The event reinforces the importance of thorough physical site inspections as part of effective water quality investigation and assurance, particularly where failures occur at or downstream of disinfection barriers.

Yorkshire Water – Repeat coliform detections at High Bentham reservoir

High Bentham service reservoir is supplied from Embsay works via Cold Cotes service reservoir and forms part of a chloraminated supply system. A routine compliance sample collected on 8 May 2025 identified elevated total coliforms. Subsequent sampling confirmed ongoing contamination, principally associated with compartment 2 of the reservoir. Two of four downstream consumer properties sampled also detected total coliform bacteria.

Compartment 2 was removed from supply on 11 May 2025, and a precautionary boil water notice was issued. The company supported this with targeted consumer communications, bottled water provision and flushing within affected distribution management areas. Subsequent sampling was largely satisfactory, with the exception of a single Clostridium perfringens detection at a consumer property. The boil water notice was lifted on 14 May 2025 following satisfactory sample results. Inspection and cleaning of the reservoir did not identify obvious structural defects.

The investigation identified a credible potential contamination route associated with a 3.5 metre section of riser pipework connected to an upstream air valve chamber. There was uncertainty about whether this chamber had flooded before the event. If the inlet main depressurised during the refilling of compartment 1 on 1 May 2025, following cleaning and inspection, contaminated water or material may have been drawn into the system through this pathway.

Microbiological analysis identified mixed Serratia species. Low disinfectant residuals were considered a contributory factor because insufficient chlorine residual can allow coliform bacteria to persist and proliferate within biofilms and distribution pipework. At the time of the initial compliance sample, the free chlorine residual was 0.07 mg/L and total chlorine was 0.13 mg/L, with even lower values recorded in follow-up samples. These results were significantly below the company’s target total chlorine residual of 0.4 mg/L at service reservoir outlets. The investigation also noted that 10 of 26 reservoirs within the system were operating below this target.

In response, the company removed the identified air valve and undertook a wider inspection programme of upstream air valves to provide assurance. The event highlights the importance of effective inspection and maintenance of air valves, particularly where there is potential for network depressurisation. It also demonstrates the vulnerability of low-residual chloraminated systems to localised recontamination, reinforcing the need for robust residual management and proactive control of potential ingress pathways to maintain water quality and protect public health.

United Utilities – Kings Lane Cranage boil water notice

The event at Kings Lane, Cranage, Cheshire, involved the prolonged supply of unwholesome water to four hydraulically connected rural properties between September 2023 and September 2024. Consumers experienced repeated episodes of discoloured water, including yellow, brown, green and grey water, alongside iron and manganese concentrations above the prescribed values and intermittent detections of coliform bacteria and E. coli.

The company initially attributed the issues to the condition of, and low turnover within, the supplying cast iron and asbestos cement main. Microbiological detections were also initially considered to be associated with tap hygiene, despite repeated positive samples from neighbouring properties indicating a wider network issue.

Interior scene showing a domestic bathtub filled with discoloured water, appearing yellow to brown in colour. The bathtub is white enamel and partially filled, with the water exhibiting a cloudy and opaque quality rather than clear appearance. The discolouration is uniform across the bath, with a slightly lighter patch visible where water movement or reflection occurs near the centre. A metal drain fitting is visible at the end of the bath, and surrounding surfaces appear clean and intact. The colour and clarity of the water indicate a significant deterioration in aesthetic water quality, likely associated with elevated turbidity, iron or manganese presence, or disturbance within the distribution system. This condition would be unacceptable for normal consumption and use, and may result in consumer complaints relating to discolouration, taste, or confidence in the supply. The image illustrates a clear example of discoloured water reaching a consumer property, highlighting the importance of maintaining distribution system integrity and effective control of particulates within the network. — Figure 37 – Photograph of discoloured water experienced by consumer at a property during the event

A cross connection between the company’s distribution system and a private borehole at a nearby golf club was identified only in September 2024, following continued bacteriological failures and abnormal chemical indicators. Consumers were advised to boil their water on 14 September 2024, the borehole connection was disconnected on 20 September 2024, and the affected main was subsequently chlorinated.

The boil water advice was lifted in October 2024 following satisfactory sample results; however, persistent iron-related discolouration remained. The affected properties were supplied by tanker for approximately eight months while permanent rehabilitation of the main was undertaken. This work was completed in June 2025.

The Inspectorate concluded that the company had failed to carry out a timely, adequate and holistic investigation, as required under regulation 18. A formal warning letter was issued.

This event highlights the importance of early, evidence-led investigation where repeated discolouration and microbiological detections occur, particularly in rural networks where private supplies and cross connections may be present. Companies should not prematurely attribute issues to asset condition alone, but should consider microbiological, chemical, operational and risk assessment evidence together.

Repeated detections within the same network should trigger escalation, including early consideration of precautionary public health protection measures and water fittings inspections beyond immediately affected properties. Prolonged reliance on operational responses, such as flushing, is not an acceptable substitute for thorough root cause investigation and permanent risk control.

Companies must ensure that learning from previous incidents is embedded into operational practice. Drinking Water Safety Plans must remain live and accurate, with known hazards such as cross connections actively identified, controlled and prevented from recurring to protect public health.

Yorkshire Water – Lead exceedances at Keadby

This event involved prolonged consumer exposure to elevated lead concentrations associated with the deterioration of a company-owned galvanised steel main supplying four properties in Keadby. Lead exceedances above the prescribed concentration value of 10 µg/L were identified intermittently from April 2021 onwards, alongside elevated zinc concentrations indicative of galvanised steel pipe degradation.

The Inspectorate concluded that Yorkshire Water did not act with sufficient urgency and did not adopt an adequately precautionary approach once repeated lead exceedances and supporting evidence of asset deterioration had been identified. Delays occurred both in establishing the condition and material of the asset and in progressing replacement of the main. Given that lead is a health-based parameter, particularly relevant to infants and young children, evidence of repeated exceedance required earlier escalation, clearer public health consideration and more timely corrective action.

The Inspectorate took enforcement action, including service of a regulation 28 notice requiring replacement of approximately 1.4 km of galvanised steel main. A formal warning letter was also issued and forms part of the company’s enforcement record. The enforcement response reflected the seriousness of the delay, the health-based nature of the parameter and the fact that the implicated asset was within company ownership and control.

Section of a water pipe which was suspected of containing lead, the section is approximately 220 mm long and appears heavily corroded on the outside. — Figure 38 – A sample of the company’s excavated galvanised main

Following the event, Yorkshire Water implemented corrective and preventative measures. These included revising internal trigger points for public health protection advice, strengthening the prioritisation of lead-related investigations, and accelerating timescales for surveying and replacing suspected lead or otherwise high-risk communication pipework. Learning from the event was also incorporated into updated internal guidance and water quality bulletins used by senior scientists responsible for approving health protection advice.

Section of a water pipe which was suspected of containing lead, the section appears relatively small and appears heavily corroded on the outside. — Figure 39 – A sample of the company’s excavated galvanised main

For the wider industry, the event demonstrates the importance of precautionary decision-making where health-based parameters such as lead are exceeded. Companies must interpret chemical, asset, sampling and customer information together and should not delay action where repeated exceedances indicate a credible risk from company-owned assets. Ageing galvanised steel assets can present a significant lead risk and should be proactively identified, monitored and prioritised for replacement where deterioration is evident. The event also reinforces the need for accurate asset records and geographic information systems, robust and timely regulation 18 investigations, clear internal triggers for public health protection, and transparent engagement with UKHSA and regulators. Where evidence indicates a potential risk from lead, protection of public health must remain the primary consideration.

Wessex Water – Netherhampton suspected backflow

In November 2025, Wessex Water confirmed suspected backflow from a private water supply into the public distribution network at a non-household premises in Wiltshire.

The issue was first indicated in September 2025, when the site’s retailer reported a negative meter reading, suggesting potential reverse flow. When company inspectors attended the site in November 2025, the external stop tap was found to be closed. On briefly opening the valve, the meter ran in reverse, confirming that water was flowing from the premises into the public main. The site operated a private water supply, and analysis of historic meter data indicated that approximately 724 litres of water may have entered the distribution network between January and July 2025. Subsequent pressure testing confirmed that the private supply was operating at a higher pressure than the public network.

The company responded by removing the water meter and capping the pipework to physically disconnect the premises from the public supply. Water quality samples were collected from two upstream neighbouring properties, with all results returning satisfactory. The local authority was notified because of its responsibility for regulating private water supplies.

A previous inspection of the site in 2019 had identified a physical cross connection between the private supply and the public main. At that time, the company required installation of a compliant air gap to provide fluid category 5 protection, supported by a double check valve, and these measures were signed off as complete. On that basis, the site was placed on a ten-year inspection cycle. The 2025 investigation found that these protections had subsequently been removed by the site, allowing the private supply to reconnect directly to the public network without the company’s knowledge.

The Inspectorate concluded that there were weaknesses in the company’s control of backflow risk. In particular, reliance on customer-owned backflow protection was identified as a vulnerability, because such measures can be altered or removed without notification. Delays in escalation of anomalous meter readings between retailer and wholesaler were also noted, alongside the limited value of reactive sampling given the extended period over which backflow may have occurred.

The Inspectorate required the company to review its procedures to strengthen backflow risk control. This included considering company-owned backflow protection at high-risk non-household premises, reviewing inspection frequencies, improving arrangements for timely investigation of negative meter readings, and strengthening information sharing with local authorities where private water supplies are present.

This event reinforces the importance of robust and verifiable backflow protection, effective communication between retailers and wholesalers, and risk-based inspection regimes. The integrity of the public water supply depends on proactive control measures, particularly where there is potential for cross connection with private water supplies.

South West Water – Cryptosporidium detection in Brixham – 2024

In May 2024, a major drinking water quality event occurred in the Brixham area of Devon, affecting up to approximately 39,000 consumers. This was due to the detection of the protozoan parasite, Cryptosporidium, in water supplied in part of the Littlehempston water supply zone, which resulted in boil water notices being issued to consumers.

The Inspectorate instigated prosecution proceedings against South West Water Limited under section 70 of the WIA 1991, for supplying water unfit for human consumption.

The company pleaded guilty to the offence of supplying water unfit for human consumption at Exeter Magistrates’ Court on 4 March 2026. At the sentencing hearing on 2 June 2026, at Exeter Magistrates’ Court, the Judge imposed a fine of £1,853,000. Judge Smith concluded at the sentencing hearing that:

“the case concerned a serious failure by South West Water Limited, the statutory water undertaker for this region, which resulted in the supply of water unfit for human consumption to thousands of people within the Littlehempston water supply zone during the spring of 2024. For many households, basic activities, drinking, cooking, brushing teeth, preparing infant formula, washing salads, bathing children, became sources of anxiety. The boil water notice remained in place for up to eight weeks for some households.’“

Genetic analysis identified a unique “Brixham strain” linking water samples, soil taken in the vicinity of a damaged air valve, and stool samples from affected consumers. Taken together, this evidence established a clear causal connection between contamination of the supply system and the outbreak of illness.

The Inspectorate’s investigation identified serious deficiencies in the company’s management of known risks. In particular, the evidence indicated that contamination most likely entered the supply through a compromised air valve on the main between Alston and Hillhead service reservoirs. A private supply cross-connection at a farm was also identified as a material contamination risk.

The Inspectorate found that South West Water had failed to ensure effective inspection and maintenance of critical assets, had not implemented an adequate risk-based approach to controlling ingress risks, and had not acted effectively on previous regulatory recommendations in relation to air valves.

This event had a substantial and prolonged impact on consumers. There were confirmed cases of cryptosporidiosis associated with the outbreak, together with wider reports of gastrointestinal illness, disruption to households, businesses and schools, and a prolonged loss of normal use of the water supply for many consumers. The effect on public confidence was considerable and for some consumers, enduring.

There are important lessons arising from this event for South West Water and for the wider water industry. Water companies must maintain robust, risk-based arrangements for the inspection and maintenance of assets that may permit ingress, ensure effective oversight of high-risk premises and potential cross-connections, and provide clear, accurate and timely communications to consumers when incidents occur. These are essential requirements for the protection of public health and for maintaining confidence in public water supplies.

The full report on the Brixham event has been published on our website – https://dwi-production-files.s3.eu-west-2.amazonaws.com/wp-content/uploads/2026/06/22172046/2024-9654-Brixham-Report-final.pdf

South East Water – Pembury water treatment works loss of supply and boil water notice

On 29 November 2025, a major water supply and water quality event occurred at Pembury treatment works in the Tunbridge Wells area, affecting up to 60,170 consumers. The event resulted in a prolonged loss of supply and the issue of a boil water notice, and attracted significant media, stakeholder and political interest.

The event arose from escalating operational failures at the works, including unstable coagulation, increasing treated water turbidity, incomplete filter backwashing and rising headloss across granular activated carbon units. These issues led to repeated shutdowns. As downstream service reservoir storage was depleted, widespread interruption to supply followed. There was no evidence of deterioration in raw water quality, and the Inspectorate concluded that the event was foreseeable and preventable.

The underlying causes were longstanding deficiencies in operational resilience, process control and asset management. These included inadequate real-time monitoring of critical parameters, insufficient proactive jar testing to optimise coagulation, ineffective filter maintenance and backwashing practices, limited visibility of process performance, and missed routine maintenance activities. Collectively, these weaknesses reduced the site’s ability to operate reliably and ultimately led to a major disruption to essential services.

Significant weaknesses were also identified in the company’s emergency preparedness and response, including multiple failures under the Security and Emergency Measures Direction. The company did not notify the Inspectorate in a timely manner, despite clear notification triggers being met, including mobilisation of tankering, escalation of internal incident levels and requests for mutual aid. Statutory three-day and twenty-day reports were incomplete and required further investigation to establish key facts and timelines.

Inadequate contingency planning and preparedness compounded the impact of the event. The company did not have sufficient outage or alternative supply arrangements in place, and learning from previous incidents, including the 2018 freeze-thaw event and multiple Tunbridge Wells incidents in 2022 and 2023, had not been effectively embedded. Blackhurst service reservoir, a key supplying asset, had also been operating at low levels since August 2025 without appropriate escalation or mitigation.

Provision of alternative water supplies fell below statutory expectations. Based on the available evidence, bottled water provision did not consistently meet the minimum requirement of 10 litres per person per day during the first five days of the event. Non-potable water was incorrectly included within potable supply figures, and tanker deployment data was inconsistent and not sufficiently verifiable to provide assurance that customer needs were met.

Accessibility of alternative supplies was also inadequate. A limited number of bottled water stations, three at peak, served a population of more than 60,000 consumers, and locations were poorly distributed, disadvantaging consumers without access to private transport. Information on site availability and operating hours was often delayed, unclear or contradictory.

Support for vulnerable customers and critical sites was insufficient. Deliveries to Priority Services Register customers, healthcare facilities, schools and care homes were delayed or inconsistent. The Priority Services Register itself was incomplete and reactive, resulting in some critical facilities experiencing service disruption, including the temporary closure of a kidney treatment centre.

Communications throughout the incident were ineffective. The company lacked a coherent and documented communications strategy, and messaging through its website, direct messaging systems and media channels was frequently late, inconsistent or unclear. As seen in previous incidents, optimism bias affected decision-making and further eroded public confidence.

Command and control arrangements were also inadequate. Although criteria for escalation to the highest level of incident management had been met, the response remained at a lower level for an extended period, negatively affecting coordination, oversight and decision-making. The Inspectorate also identified ongoing failures to test and embed learning from previous incidents and exercises, particularly in relation to alternative water provision and planning for reasonable worst-case scenarios.

The Inspectorate concluded that the failings were systemic, repeated and evident across operational management, asset performance and emergency preparedness. Significant breaches of the Security and Emergency Measures Direction were identified, and further enforcement action is being pursued under section 18 of the Water Industry Act in relation to both the operation of Pembury works and the company’s response to the incident.

The Inspectorate has published its full findings on its website. The scale and seriousness of the event required extensive investigation, including multiple site visits, collection of witness statements and analysis of more than one million data points. Due to the level of political and public interest, the Chief Inspector was also called to give evidence before the Parliamentary Environment, Food and Rural Affairs Committee.

This event demonstrates the consequences of failing to maintain resilient operations, robust process control and effective emergency preparedness. It reinforces the expectation that companies must adopt a proactive, risk-based approach, embed learning from previous incidents, and maintain the capability to protect consumers during both operational and emergency conditions.

Water safety planning and risk assessment

The Inspectorate’s Risk Assessment team is responsible for understanding risks to water quality and sufficiency, adapting to new mitigation requirements, and developing approaches to Drinking Water Safety Planning (DWSP).

During 2025, the team assessed substantial volumes of data and documentation submitted by companies. Through systematic review of risk lines uploaded through regulation 28 reporting, the team strengthened its understanding of how effectively risks to water quality and sufficiency are being managed across the industry. This work enables the Inspectorate to identify where action is required and to support companies in complying with regulations 27 and 28 through DWSP audits, individual line assessments of regulation 28 data, and legal instrument closure audits. Collectively, these activities help maintain and improve risk management practice across the water industry.

The Risk Assessment team continues to engage with industry through the quarterly DWSP Forum, providing updates on current work and addressing issues raised by water companies and NAVs. To further support consistent implementation of DWSP requirements, the team has also introduced liaison meetings with companies and NAVs to provide additional support and guidance.

DWSP Guidance

In July 2025, the Risk Assessment team published the Guidance on risk assessment and risk management specific to Drinking Water Safety Plans (DWSPs) following collaboration with water companies through the Drinking Water Safety Planning Forum and a small in-person working group convened to resolve outstanding issues. The guidance provides a comprehensive framework for the delivery of a DWSP and introduced a new category, category J, for recording carried-forward risk. The introduction of category J is to enable consistent reporting of categories and methodology for carrying risk scenarios forward to downstream stages across the industries water supply systems and will be a requirement from February 2027. This increases the fairness of scoring within the Risk Assessment Risk Index (RARI) used to indicate companies’ current and trending risks.

Alongside this change to reporting categories, the use of hazardous events is being further endorsed by the Inspectorate through the DWSP guidance so that there is better clarity regarding root cause analysis and control measure/mitigation application for risks. Using hazardous events contributes to understanding risk scenarios at an asset, stage and cumulative supply system level to give a more accurate representation of potential risk to water quality and quantity at the consumer tap.

Sufficiency

In September 2024, the National Infrastructure Commission (now NISTA) published its report, Developing Resilience Standards in UK Infrastructure. In response, the Inspectorate sought to assess the extent to which companies understood their own sufficiency and resilience arrangements and wrote to them in December 2024 requesting further information.

The Risk Assessment team initially reviewed companies’ regulation 28 reports to determine which sufficiency-related risks had been reported to the Inspectorate and what mitigation measures were in place. However, this review proved inconclusive due to variations in the way companies reported hazardous events, with some companies not reporting sufficiency risks at all.

Consequently, the team drew on the three key recommendations set out in the NISTA report and requested this information from all water companies and new appointments and variations (NAVs). The responses provided an evidence base to inform the development of guidance on the delivery of a draft water sufficiency guidance in late 2025. The team has since revised the Sufficiency Guidance so that it aligns with the principles of DWSPs and this will be published in 2026.

Per and polyfluoroalkyl substances (PFAS)

Following the Inspectorate’s PFAS guidance issued in August 2024, companies were required to monitor 6:2 FTAB (6:2 fluorotelomer sulphonamide alkyl betaine), which was added to the list of PFAS parameters, and to report the ‘Sum of PFAS’ from January 2025.

The above monitoring requirement was extended to New Appointments and Variations (NAVs) also. The NAVs are also expected to exchange information and data and to maintain a forward-looking PFAS strategy. Annex C of the Information Direction lists all PFAS chemicals of interest. Companies are also required to notify the Inspectorate if additional PFAS chemicals not listed are identified at concentrations above tier 1.

The PFAS guidance was updated in March 2025, Information Letter 03/2025, to provide further detail on PFAS tiers and associated actions, alongside requirements for monitoring and reporting, regulation 27 risk assessments, regulation 28 reporting, and company PFAS strategies. The updated guidance also set out expectations for bulk supplies, including those involving NAVs.

Audits

Through DWSP audits, the Risk Assessment team has continued to identify recurring issues in the way companies record risks, contributing to inconsistency across the industry and an ongoing reliance on reactive rather than proactive measures to reduce risks.

Companies should take a proactive approach to identifying and addressing risks, rather than waiting for issues to emerge before considering investment needed to protect public health. A reactive approach weakens the effectiveness of risk management and makes it harder to compare risk profiles across companies.

Companies must strengthen their DWSP arrangements to support the early identification and management of risks, thereby protecting public health and securing compliance with regulatory parameters, rather than relying on retrospective action in response to water quality or sufficiency issues. Companies must also ensure full compliance with regulations 27 and 28. To date, the majority of DWSP audits have resulted in ‘minded to enforce’ decisions and/or the issue of legal instruments requiring companies to review regulation 27 risk assessments and regulation 28 reporting.

In July 2025, the team conducted a technical and drinking water safety plan (DWSP) audit at United Utilities at Watchgate Water Treatment Works(WTW) and Bird Hill Service Reservoir (SR). The audit revealed significant deficiencies around asset maintenance and risk management, highlighting the need for improved inspections, risk assessments, and control measures to ensure water safety and regulatory compliance as well as to develop holistic raw water contingency plans. At Bird Hill SR there was also a requirement for the company to address fencing and security deficiencies.

The team identified deficiencies in the Drinking Water Safety Plans including incomplete risk identification, misaligned PFAS risk assessments, and discrepancies in risk scoring and reporting. There were issues identified with control measures and misappropriated association with hazards and hazardous events. This means that risk assessments may not being carried out effectively to understand what control measures can be used for particular hazards/hazardous events, and therefore residual risk outputs from reg 27 assessments may not be accurate or understood correctly.

The DWSP did not sufficiently reflect significant risks, despite the company’s regulation 27 assessments and regulation 28 submissions showing that nearly all risks were scored 10 (amber) and categorised as A – ‘Target risk mitigation achieved, verified, and maintained’. The consistently applied risk scores limited differentiation between risks and reduced the ability to prioritise the most significant issues for improvement and investment.

Audit findings identified weaknesses in United Utilities’ DWSP risk assessment arrangements, requiring the company to revise its methodology, improve risk scoring and control measure evaluation, and complete reassessments across treatment works and connected raw water assets. Due to the deficiencies identified in the DWSP, the company was served with a companywide regulation 28 notice to address these.

Due to reduced team capacity, substantial input into legal instruments arising from audits undertaken in 2024, and wider internal Inspectorate commitments, this was the only onsite audit carried out in 2025. Other programmed audits were rescheduled to 2026.

RAR Reporting and Company Breakdown of DWI Categories

The annual submission of companies’ regulation 28 data took place in October. Companies used the Inspectorates’ online portal for this year’s submission including uploading of their DWSP methodologies and declaration with Director sign off. Any issues experienced with this process were easily resolved with good communication from companies and input from the Inspectorate’s Data Management Team.

Companies’ risk assessment record (RAR) data is submitted using the DWI categories shown with definitions in the table below in table RA1. Companies must apply DWI categories appropriately to allow data to be meaningful and to give clarity in risk management.

DWI category	Description
A	Target risk mitigation achieved, verified, and maintained
B	Additional or enhanced control measures which will reduce risk are being validated
C	Additional or enhanced control measures which will reduce risk are being delivered
D	Additional or enhanced control measures are required to materially reduce risk
E	Risk under investigation
F	Partial mitigation
G	No mitigation in place: control point downstream
H	No mitigation in place and none required
I	Future risk not requiring mitigation at present
J	No mitigation in place, carried forward risk: control point upstream
X	Line no longer relevant.

Table RA1 – DWI categories

Risk Assessment Risk Index (RARI)

The risk assessment risk index (RARI) is dependent on DWI category application to risks perceived by companies. Companies are required to report risk assessments and show where risks are carried forward through supply systems from catchment/source to the consumer tap. Since the introduction of DWI categories, companies have used different methods to show carried forward risks, either by indicating this by using the DWI categories applied at assets with active risks and applying the same categories at downstream assets or using residual risk scoring to demonstrate risks moving forward through a supply system.

As such, RARI values for companies have not been comparable across the industry and can only show changes over time when viewed on an individual company risk profile. RARI has been reported for companies within the DWI Annual reports since 2019. The index is a useful tool in showing active risks within companies and is being redeveloped as companies implement changes to how DWI categories are applied in line with the DWSP Guidance, gaining a more consistent approach over the coming years. This includes the introduction of a new category ‘J’ for carried forward risks although this category will not be included in the RARI calculations at this time, it will allow for a more consistent approach within the industry to category application across company supply systems.

This graph excludes NAVs and shows changes in company risk scores between 2024 and 2025. These changes reflect several factors, with some companies showing increased RARI scores due to an increase in assets and as the industry has increased hazard identification and recording of risks. It is expected that this is likely to be the case over the AMP 8 period through improvements made to this hazard identification processes companies use in line with DWSP Guidance.

NAVs

The graph shows a significant decrease in RARi scoring for LEEP. This reduced score is related to a removal of duplicate lines and an increase in assets. This leads to an increase in lines included in the calculation and a reduced RARI. This instance highlights why this metric is used for intra company comparison

Top 10 current identified risks in England (as a % sum of industry RARI)

Category I is used to indicate perceived future risks that may need to be investigated and mitigated. Such risks include hazards that may be associated with climate change, emerging contaminants, changes in raw water quality and sufficiency, and asset condition and longevity. These risks are not necessarily manifesting now, but companies are keeping a watching brief over them. However, increasing temperatures and periods of prolonged dry weather were seen across England in 2025 with some companies continuing to have a drought order into the winter months.

The top ten hazards where category I has been used in company submitted regulation 28 data is shown below.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS)

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) remain a significant focus for the Inspectorate because of their persistence in the environment and their potential to affect drinking water sources. The Inspectorate updated its industry guidance in August 2024, with a further update issued in 2025, setting out expectations for sample reporting, summary risk assessment information and prioritised mitigation. Companies have continued to use targeted sampling strategies to focus on sites most likely to be at risk from PFAS contamination. In 2025, companies across England and Wales undertook more than 666,000 analyses for individual PFAS. Since 2012, more than 2.3 million analyses have been completed, providing a substantial dataset to support understanding and management of PFAS risks in drinking water.

The Inspectorate’s PFAS guidance requires companies to apply the tier framework to all PFAS of interest listed in Annex C of the Information Direction where they are detected in raw or final water. The 2025 guidance consolidated the actions expected at each tier level and reinforced the need for companies to assess results in the context of source risk, treatment capability, blending arrangements and consumer exposure.

Sites are classified as:

Tier 1 where PFAS concentrations are <0.01 µg/L;
Tier 2 where PFAS concentrations are <0.1 µg/L; and
Tier 3 where PFAS concentrations are ≥0.1 µg/L

Monitoring by water companies has also highlighted 6:2 fluorotelomer sulfonamide alkylbetaine (6:2 FTAB) as a PFAS of potential concern. Companies are now expected to consider both individual PFAS results and the combined concentration of PFAS of interest. The requirement to implement a prioritised mitigation methodology has therefore been extended to include the “sum of PFAS” based on the 48 named substances in the parameter list. Where a site falls into Tier 2 or Tier 3 on a sum-of-PFAS basis, it should be incorporated into the company’s existing site prioritisation approach, with a proportionate risk reduction strategy designed to progressively reduce PFAS concentrations in drinking water.

PFAS monitoring is focused on source water because this provides information on the hazard and informs the most appropriate mitigation, which may include source substitution, blending, treatment or removal of an abstraction from supply. In 2025, two treated water samples supplied to consumers were reported in Tier 3. In raw water, 353 samples from abstractions were reported in Tier 3. These findings indicate that PFAS remains a source-water challenge, but also demonstrate the importance of the current industry strategy in identifying, prioritising and mitigating risks before they reach consumers.

Tier	Raw water tests	Treated water tests	Total
Tier 1	373,812	234,233	608,045
Tier 2	11,810	4,385	16,185
Tier 3	353	2	355
Total	238,650	385,975	624,595

Table 12 – Number of PFAS tests submitted in 2025

Affected sites

Where PFAS is identified, companies are expected to act in accordance with the Inspectorate’s risk prioritisation strategy. This includes confirming the source and extent of contamination, assessing treatment and blending capability, identifying potential consumer exposure and implementing proportionate mitigation to reduce concentrations over time.

In 2025, two Tier 3 PFOS detections in treated water were reported from the same treatment works operated by Southern Water. Previous detections had also been reported from this source, with two in 2022 and three in 2023. The works is relatively small, supplying approximately 1.7 ML/day, and is located close to a disused airfield. Water from the works is blended within an extended distribution network before reaching consumers’ taps.

Sum of PFAS

Sampling undertaken in early February 2025 identified a sum of PFAS concentration of 0.103 µg/L in treated water from Wessex Water’s Charlton Minety works. A subsequent sample collected in early March recorded 0.098 µg/L. The works supplies the Minety distribution system and Rodbourne works, where further blending occurs before supply to approximately 6,700 consumers. At the time of the event, the works did not include treatment capable of removing PFAS.

The company notified the Inspectorate promptly and confirmed that it relied on blending at Rodbourne to reduce PFAS concentrations before supply. However, this control was not supported by sufficiently robust verification or operational safeguards at the time. The company did not issue consumer advice, maintaining that subsequent marginally lower results did not warrant intervention. Longer-term options were already under consideration, including granular activated carbon (GAC) treatment and a new treatment works at an alternative location, but these were not then programmed for completion until 2035.

The Inspectorate concluded that water supplied during the event had PFAS concentrations above the published guidance and was therefore potentially unwholesome. Historical monitoring showed that this was not an isolated occurrence, with multiple results above the Tier 3 sum-of-PFAS threshold since mid-2024, including a peak of 0.205 µg/L. Although the March 2025 guidance formally extended the PFAS approach to the sum of 48 PFAS, the company had been calculating this value since June 2024. The risk was therefore evident before the event and should have informed earlier mitigation planning.

The Inspectorate required enhanced monitoring at Charlton Minety and Rodbourne works to provide assurance that blending controls were effective. Daily fast-track sampling was sought, but laboratory capacity and turnaround constraints meant that weekly sampling was implemented. The Inspectorate also required blending arrangements to be verifiable and protected by safeguards to prevent supply above Tier 3 levels. The company subsequently introduced a blending protocol supported by software flow interlocks and was required to review all assets where blending is used to control PFAS concentrations.

The company’s regulation 28(4) notice was revised to accelerate mitigation. The deadline for the permanent solution was brought forward to 2031, and temporary granular activated carbon treatment was expedited. The first GAC pod was installed in November 2025 to treat water supplied directly to the Minety area, and a second pod was commissioned in May 2026. Following installation of the first pod, results were below the limit of detection or within Tier 1, apart from one February 2026 result marginally above Tier 3 that was reported as an event and attributed by the company to a sampling error between adjacent pre- and post-GAC taps.

The key lessons from this event are that elevated sum-of-PFAS results should trigger prompt escalation and intervention, rather than reliance on future capital schemes. Where blending is used as a control measure, it must be demonstrably effective, supported by robust monitoring, operational safeguards and clear trigger levels. The event also highlighted practical constraints in increasing PFAS monitoring frequency, particularly laboratory capacity and analytical turnaround times, which companies must consider when designing assurance arrangements. PFAS monitoring is risk-based and targeted towards sources and assets most likely to be affected. Results should therefore be interpreted in that context, because a higher proportion of samples is collected from higher-risk locations. Of the 989 treatment works in supply across England in 2025, Tier 2 concentrations were detected in treated water at 82 locations, representing 8.29% of sites. Tier 2 results remain below the Inspectorate’s health-based guidance value of 0.1 µg/L for the sum of PFAS, but companies are expected to take a precautionary approach and develop strategies to reduce PFAS concentrations at these sites over time.

PFAS Tier	Raw water results	Treated water results	Total calculations
Tier 1	5,158	3,290	8,448
Tier 2	1,337	850	2,187
Tier 3	241	4	245
Total	6,736	4,144	10,880

Table 13 – Sum of PFAS calculations in 2025

Hanningfield sum of PFAS event

An event was reported at Hanningfield treatment works following detection of elevated PFAS in a sample collected on 6 October 2025. The sum-of-PFAS concentration was 0.106 µg/L, above the Tier 3 threshold of 0.1 µg/L. The result was largely driven by 6:2 fluorotelomer sulfonamide betaine (6:2 FTAB).

The elevated concentration was attributed to changes in raw water operation. Water from the River Blackwater was abstracted directly into the works rather than being blended with reservoir water from Hanningfield reservoir, because the proportion of reservoir water had been reduced in response to an algal bloom. Subsequent sampling showed PFAS concentrations reducing to Tier 2 levels, and catchment investigations indicated that the likely source of contamination was within the River Brain, a tributary of the River Blackwater.

The company undertook extensive sampling across the treatment works, supply zones and wider catchment, and engaged with relevant stakeholders. Although PFAS concentrations reduced after the event, evidence indicated that the existing treatment processes at the works were largely ineffective for PFAS removal. There was also evidence that ozonation may transform longer-chain PFAS compounds into shorter-chain species, which can be more difficult to remove using conventional treatment processes.

The company subsequently reverted abstraction towards Hanningfield reservoir water as raw water conditions improved. However, this operational change was driven by raw water quality considerations rather than by a defined PFAS mitigation strategy. The Inspectorate concluded that water supplied during the event was potentially unwholesome. Concerns were identified regarding the timeliness of sampling and analytical turnaround, the absence of a robust and effective PFAS treatment strategy, and the need for clearer evidence on treatment performance and laboratory capability. The Inspectorate is minded to take enforcement action requiring the company to strengthen PFAS risk management, improve sampling and reporting timeliness, develop effective mitigation measures and provide further evidence to prevent recurrence.

Raw water data – number of tests per tier greater than the limit of detection (LOD)

Parameter	Tier 1 results	Tier 2 results	Tier 3 results	Total results
PFOS	1449	636	125	2210
PHFHxS	1392	374	100	1866
PFHxA	1472	376	14	1862
PFOA	1480	165	13	1658
PFBS	1258	136	11	1405
PFPA	748	398	14	1160
PFBA	1085	63		1148
PFHpA	764	45	14	823
6:2 FTAB	120	166	9	295
6:2 FTSA; 6:2 FTS	86	188	13	287
PFecHS	125	131		256
PFPS	141	87	12	240
FBSA	94	83	14	191
FHxSA	40	102	14	156
PFDA	83			83
PFNA	72			72
HFPO-TA	62	2		64
PFUnDA	56			56
PFHpS	38	16		54
8:2 FTSA	31	11		42
9Cl-F3ONS	34			34
EtFOSAA	33			33
FOSA	16	14		30
MeFOSAA	29			29
PFNS	28			28
PFDS	16	2		18
PFDoDA	14	2		16
NFDHA	14	1		15
11Cl-PF3OUdS	10	1		11
PFTeA	11			11
HFPO-DA (Gen X)	7			7
PFDoS	6			6
DONA	5			5
MeFOSE	5			5
PFUnDS	3	1		4
5:3 FTCA	3			3
EtFOSE	3			3
MeFOSA	3			3
3:3 FTCA	1	1		2
EtFOSA	2			2
PFTrDA	2			2
7:3 FTCA	1			1
PFHxDA	1			1
PFMOBA	1			1
Total	10,844	3,001	353	14,198

Table 14 – Number of tests per tier greater than the limit of detection (LOD) in raw water

Raw water data – number of tests per tier greater than the limit of detection (LOD) 44 out of 48 had positive detections in 2025, meaning that there were no positive detections for 4 PFAS species.

Treated water data – number of tests per tier greater than the limit of detection (LOD)

Parameter	Tier 1 results	Tier 2 results	Tier 3 results	Total samples
PFHxA	1165	116		1281
PFOS	834	251	2	1087
PFBS	1056	21		1077
PHFHxS	929	146		1075
PFOA	873	58		931
PFBA	877	52		929
PFPA	642	150		792
PFHpA	611	6		617
PFecHS	197	16		213
PFPS	181	3		184
FBSA	161	4		165
6:2 FTSA; 6:2 FTS	83	74		157
6:2 FTAB	109	39		148
PFODA	76			76
FHxSA	27	29		56
PFDA	50	1		51
8:2 FTSA	48			48
PFHpS	43	1		44
PFUnDA	42	1		43
EtFOSAA	37			37
PFNA	35	1		36
PFNS	32			32
MeFOSAA	31			31
HFPO-TA	30			30
PFDS	29	1		30
PFTeA	24			24
3:3 FTCA	22			22
9Cl-F3ONS	22			22
PFDoS	20			20
PFDoDA	18	1		19
EtFOSE	17	1		18
PFUnDS	17	1		18
11Cl-PF3OUdS	17			17
PFTrDA	15	1		16
FOSA	15			15
MeFOSE	15			15
NFDHA	14			14
5:3 FTCA	13			13
EtFOSA	13			13
7:3 FTCA	12			12
MeFOSA	12			12
PFHxDA	12			12
HFPO-DA (Gen X)	11			11
4:2 FTSA	7			7
DONA	6			6
PFMOPrA	4			4
PFEESA	3			3
PFMOBA	3			3
Total	8,510	974	2	9,486

Table 15 – Number of tests per tier greater than the limit of detection (LOD) in treated water

PFAS at treatment works

The detailed PFAS tables show that detections are concentrated at a relatively small number of higher-risk sites. This reflects the targeted nature of company monitoring, which focuses sampling on sources and treatment works where PFAS contamination is most likely. The Inspectorate therefore interprets the data alongside source risk, treatment capability, blending arrangements and company mitigation programmes, rather than as a simple measure of average national exposure.

Company name	Total number of abstraction points	Actual number of affected abstraction points	Percentage affected	All company improvement programme issued?
Affinity Water Ltd	237	9	3.80%	Yes
Anglian Water Services Ltd	622	47	7.56%	Yes
Bristol Water Plc	30	2	6.67%	Yes
Isles of Scilly*	26	2	7.69%
Northumbrian Essex and Suffolk Water	120	4	3.33%	Yes
SES Water	93	1	1.08%	Yes
Severn Trent Water Ltd	347	1	0.29%	Yes
South Staffordshire Water Plc	87	6	6.90%	Yes
Southern Water Services Ltd	245	2	0.81%	Yes
Thames Water Utilities Ltd	238	4	1.68%	Yes
United Utilities Water Ltd	159	1	0.22%	Yes
Wessex Water Services Ltd	342	3	0.88%	Yes

Table 16 – Data on tier 2 PFAS detections and the number of affected sites

*The Isles of Scilly are incorporated into the SWB notice

Although only a small number of treated-water samples were reported in Tier 3, Tier 2 detections remain important because they identify sites where PFAS concentrations require ongoing control and reduction. Companies with Tier 2 detections are expected to maintain clear risk assessments, understand the contribution from each source or treatment works, and set out proportionate mitigation to reduce PFAS concentrations over time.

Anglian Water remains the most notable company in this dataset. The Inspectorate currently has five notices and one undertaking in place requiring PFAS mitigation measures at Anglian Water sites. The notices were put in place during 2023 and 2024 and are generally due for completion around 2031. The undertaking was entered into voluntarily by the company and commits it to a broad programme of sampling, risk assessment, catchment investigation, source identification and proactive risk reduction to progressively reduce PFAS concentrations in drinking water.

Audit programme completed by the Inspectorate

The Inspectorate undertakes an annual programme of technical audits to provide independent assurance that water companies are effectively managing risks to drinking water quality. These audits form a key part of regulatory oversight and enable the Inspectorate to examine how companies design, operate and maintain their systems to protect public health.

In 2025, the Inspectorate completed a programme of technical audits across England. This work included assessments of water treatment works supplied by surface water and groundwater sources, associated catchments and raw water assets, data and reporting systems, and arrangements for storing and supplying bottled water during interruptions to the normal supply.

Scope and approach of the audit programme

The 2025 programme included both routine and targeted audits. Some audits formed part of national or thematic programmes, while others were prompted by site-specific concerns, including water quality events, emerging risks, compliance failures or significant changes to treatment processes.

In total, the Inspectorate carried out 39 audits in 2025. The programme included on-site inspections, whole-company audits and remote vertical audits, enabling scrutiny of both individual assets and wider company systems.

Thematic audits are used to assess performance across the industry, identify good practice and recurring weaknesses, and inform wider regulatory expectations. Findings from audits may result in recommendations, formal requirements, enforcement action or wider communication to companies where common risks are identified.

Featured audits

Yorkshire Water – Headingley treatment works

The Inspectorate undertook a section 86 technical audit of Headingley treatment works following a confirmed total coliform detection in final water in February 2025. The audit assessed treatment processes, treated water storage arrangements and overall site resilience, and identified several issues requiring regulatory follow-up.

Treatment processes observed during the audit were generally operating satisfactorily. However, the internal inspection of the contact tank was overdue by more than ten years and was already subject to an existing enforcement notice. The company was required to complete the inspection and associated flood testing, and to report its findings to the Inspectorate, including its conclusions on the root cause of the coliform detection.

Further requirements related to treated water storage and associated assets. An apparent vent stack located between the contact tank and clean water tanks required investigation to determine whether it was connected to any live asset. The audit also identified deficiencies in the management of granular activated carbon assets, including the absence of a clearly defined risk-based regeneration programme and the need for firm timescales for refurbishment of a GAC unit that had been removed from service because of structural failure.

Poor practices were also observed in the handling and storage of removed GAC media, which was found to be unprotected and without clear status identification. The company subsequently provided evidence that the material had been quarantined, but the findings highlighted the need for robust controls and risk assessments to prevent inappropriate reuse.

Large white bulk bags containing vegetation and weeds are stored outdoors on a paved area beside a green metal security fence. One bag in the centre is filled with cut plant material, with shrubs and grass protruding above the top edge. Additional partially filled bags are visible to the left and right. Grass and weeds are growing around the base of the bags, with trees visible in the background. — Figure 40 – Unprotected GAC media and without clear status identification and there was clear vegetative growth coming from the media

This audit reinforces the importance of timely inspection and maintenance of treated water storage assets, clear understanding and control of site-specific risks, and effective management of GAC systems where they form a key barrier to water quality protection.

Bristol Water – Barrow treatment works audit

Barrow treatment works was audited as part of a themed programme examining the management of surface water treatment works and resilience to climate-related pressures. The audit resulted in a high number of recommendations, suggestions and formal requirements, reflecting the complexity of the site and the breadth of risks identified.

The Inspectorate identified significant concerns associated with ageing infrastructure and reliance on temporary mitigation measures. Of particular importance was the continued use of a temporary ozone generator following failure of the primary unit. The company was required to clarify its long-term strategy for ozone generation to ensure a resilient and compliant treatment process. The absence of online ozone residual monitoring was also highlighted, with the site relying instead on manual sampling. The Inspectorate advised the company to review and strengthen monitoring arrangements to support effective operational control.

Issues were also identified in the operation and maintenance of dissolved air flotation units and rapid gravity filters. Cleaning and inspection regimes had not consistently been completed within expected timescales, and there was limited evidence of a structured, routine assessment of filter condition and performance. The Inspectorate required the company to implement a more robust and systematic inspection and assessment programme to support sustained treatment effectiveness.

Further concerns related to treated water assets, including the condition of a rapid gravity filter backwash tank roof, which had partially collapsed and was being managed through temporary controls. The Inspectorate required the company to expedite permanent repairs and provide a clear, time-bound programme of works.

Overall, this audit highlights recurring industry challenges relating to ageing infrastructure, dependence on temporary mitigations and insufficiently robust inspection and maintenance regimes. The Inspectorate expects companies to identify such risks proactively and address them through sustainable, long-term solutions that ensure resilience, operational reliability and continued protection of water quality.

Northumbrian, Essex and Suffolk Water – Ormesby treatment works audit

The technical audit of Ormesby treatment works identified deficiencies across treatment processes, monitoring arrangements and procedural controls, resulting in several recommendations and formal requirements.

A key finding related to the operation of the granular activated carbon (GAC) system. The Inspectorate concluded that the process was not being managed appropriately, with multiple contactors operating beyond the company’s defined regeneration limits and without adequate interim mitigation measures. The company was required to review its regeneration programme and provide evidence of how associated risks would be effectively controlled while assets remained outside specification.

The audit also identified weaknesses in monitoring and verification. Handheld instruments were being used to verify critical online monitors without sufficient evidence of calibration or traceability, creating a risk of reduced confidence in monitoring data. The Inspectorate required the company to implement improved controls over portable instruments, including defined calibration standards, verification procedures and robust record keeping.

Procedural deficiencies were also identified. Site operating procedures were outdated or inadequately controlled, and there was no formal policy governing overrides on critical alarms and shutdown systems. The Inspectorate required the company to introduce clear governance for overrides so that their use is appropriately authorised, controlled and auditable.

This audit highlights the importance of maintaining effective control of treatment processes that act as key barriers to water quality risk, supported by accurate monitoring systems and well-governed operational procedures.

Affinity Water – Standon treatment works and Old Hall Green water tower

The Inspectorate undertook a technical audit of Standon treatment works and Old Hall Green water tower following a fuel contamination incident affecting raw water sources and a history of microbiological failures. The audit identified significant concerns relating to raw water protection, disinfection assurance and asset management.

At Standon works, detections of benzene and methyl tertiary-butyl ether in borehole sources were linked to contamination from a disused petrol station adjacent to the site. Although the company had implemented mitigation, including changes to abstraction regimes, the Inspectorate identified a lack of automated controls to prevent operation under elevated-risk groundwater conditions. The company was required to introduce preventative controls, such as alarm triggers or automatic shutdown mechanisms, to ensure unacceptable raw water conditions are effectively managed.

Serious deficiencies were also identified in ultraviolet disinfection assurance. The Inspectorate was unable to conclude that the company had an adequate approach for verifying UV performance. Calibration arrangements lacked sufficient evidence and traceability, and newly installed UV lamps had been brought into service without demonstrated calibration. The company was required to take immediate action to implement robust verification arrangements for UV disinfection at Standon works and to review similar systems across other sites.

At Old Hall Green water tower, the audit identified risks associated with low turnover, unprotected inlet valves and potential leakage from washout pipework. These conditions present credible pathways for contamination and require prompt investigation and remediation. The Inspectorate advised that further enforcement action may be considered if microbiological failures recur at the site.

This audit highlights the importance of protecting groundwater sources from contamination, maintaining robust and verifiable disinfection systems, and ensuring that treated water storage and distribution assets are operated and maintained in a way that safeguards water quality and protects public health.

Independent Water Networks Limited – Data systems audit

The Inspectorate undertook a technical audit of Independent Water Networks Limited focusing on data systems and regulatory reporting. The audit was notable for the scale and extent of data quality issues identified, particularly in sampling records, population estimates and regulatory submissions.

The Inspectorate identified widespread deficiencies in pesticide monitoring data, including inconsistencies in reporting and reuse of unique identifiers within regulation 28 risk assessments. These issues contributed to significant sampling shortfalls and inaccuracies in compliance calculations, undermining confidence in the company’s reported performance.

Although the company acknowledged these issues and had initiated system improvements, the audit highlighted the regulatory risks associated with inadequate data governance. The company was required to define clearly its approach to water supply zone delineation, population estimation and sampling strategies, and to ensure that future data submissions are complete, accurate and aligned with regulatory requirements.

The Inspectorate recognised the challenges faced by smaller water companies and welcomed the move towards improved, centralised data systems, including SharePoint-based platforms. Nevertheless, accurate, reliable and well-governed data remains fundamental to effective regulation and the protection of public health.

This audit reinforces the importance of robust data management across the industry. Weaknesses in data systems, even in the absence of operational failures, can compromise regulatory oversight, distort compliance assessments and erode confidence in reported water quality performance.

Bottled water storage and emergency arrangements

The Inspectorate undertook a thematic audit of bottled water storage and emergency arrangements to assess whether companies have sufficient, resilient and verifiable arrangements to protect consumers during interruptions to normal supply. The audit considered the availability, condition and rotation of bottled water stocks, arrangements for vulnerable consumers and critical sites, logistics, record keeping, and the ability of companies to demonstrate compliance with emergency planning requirements.

However, the audits also identified areas for improvement. These included inconsistent auditing of storage locations, unclear arrangements for checking storage conditions, and gaps in sampling and record keeping. In some cases, bottled water had been stored in conditions that did not fully meet national guidance, or information provided to the Inspectorate during incidents was found to be inaccurate.

Where deficiencies were identified, the Inspectorate required companies to improve their procedures, strengthen assurance arrangements and, where necessary, remove unsuitable stock from use. The Inspectorate expects companies to be able to demonstrate clearly that bottled water supplied to consumers is safe and wholesome at all times.

At Thames Water, the purpose built bottled water store in Didcot was well managed with good levels of traceability of when batches were processed through the store. The company were storing bottled water on lorries ready for deployment at short notice. However, the risks associated with temperature control to maintain water quality were not considered. It was also found that bottles were not stored appropriately at a satellite depot where the bottles were stored outside the designated container, open to contamination at a sewage works. Thames Water acted promptly to rectify the situation.

Curtain side of a lorry trailer opened to reveal pallets of bottled water stacked in rows and secured for transport. The blue curtain is partially pulled back, exposing the load inside a yard area next to a building. — Figure 41 – Bottles stored on a lorry, not in a temperature controlled environment

Blue shipping container positioned on a yard surface beside a stack of shrink-wrapped pallets of bottled water. The pallets are partially covered with protective wrapping, and additional blue containers are visible in the background along with trees and overcast sky conditions. — Figure 42 – Bottles stored outside store at sewage works

Several issues were identified at the Cambridge Water audit conducted at Fleam Dyke, with the company unable to provide evidence of up to date procedures, trained and competent staff, stock management, temperature monitoring, or bottled water supplier audits.

Indoor storage area with metal racking containing pallets of bottled water. Shrink-wrapped multi-packs are stacked on wooden pallets, some leaning slightly within the bays. The racking frames have yellow and black hazard markings, and a small sign is attached to one upright — Figure 43 – The small, bottled water store at Fleam Dyke – Cambridge Water

The storage conditions at Affinity Water’s Tyttenhanger site were found to be tidy and temperature controlled. However, the company had no evidence of routine sampling or checks on the bottled water in storage, nor was there evidence of bottled water storage checks and the annual audits had been missed.

Large indoor warehouse space with multiple pallets of bottled water arranged in rows on either side of a central aisle. The pallets are shrink-wrapped and stacked on wooden bases, with additional stacks visible at the far end of the room. Overhead lighting and wall-mounted fixtures illuminate the area, and an exit door is visible on the right-hand wall. — Figure 44 – Tidy store and temperature monitoring equipment at Tyttenhanger – Affinity Water

Digital thermometer probe placed on a flat surface next to a small bottle and wired sensor, displaying a temperature reading of 19.6 °C. — Figure 45 – Tidy store and temperature monitoring equipment at Tyttenhanger – Affinity Water

The designated storage area at Southern Water’s Falmer site was shared with other equipment such as pumps, pipework and spare filter cartridges. It is not clear that the risk of cross contamination had been appropriately considered. There was a large amount of sunlight entering the building. At Southern Water’s Timsbury site, the bottled water was stored in an old building at the back of the site, but it was generally clean and tidy. The company were unable to provide the necessary traceability on the movement of bottled from receipt through to deployment to consumers. The Inspectorate recommended the company formally assess the temperature risks.

Large warehouse or depot area with pallets of bottled water lined along a wall of windows. Several pallets are covered with a blue tarpaulin, secured over stacked supplies. Additional pallets and handling equipment are visible further along the open floor space. — Figure 46 – Bottles under tarpaulin at Falmer

Rural yard area with a metal-clad storage building featuring large green double doors. A white van is parked to the right, and a white container unit is positioned to the left. Pallets, a skip and stacked materials are visible around the site, with trees and vegetation surrounding the area in cloudy conditions. — Figure 47 – The bottled water store at Timsbury – Southern Water

Anglian Water had subcontracted one of their bottled water storage facilities to a third party. Storage and stock management were found to be good, but temperature monitoring was missing. However, at the company’s small, bottled water store at Isleham there was evidence of vermin access to the building and the risk of bottle contamination.

Close-up of a roof structure inside a building, showing metal beams and corrugated roofing sheets. There are visible gaps and damage at the junction between the beam and wall, with debris and staining present along the brickwork below — Figure 49 – Vermin access to bottled water store at Isleham works

Interior of a large warehouse showing an extensive, well-organised storage area for bottled water supplies. Palletised packs of bottled water are stacked on multi-level metal racking extending across the length of the wall, with goods shrink-wrapped and uniformly arranged on wooden pallets. The storage system makes use of vertical space, with multiple tiers allowing high-volume storage while maintaining clear separation of pallets. The floor area in the foreground is wide, clean, and unobstructed, enabling safe access for handling equipment and inspection activities. The warehouse structure includes a high insulated roof with exposed steel beams and overhead lighting, providing consistent illumination. The bottled water is stored off the floor, fully wrapped, and appears protected from contamination. The consistent stacking pattern and visible labelling suggest controlled stock management, likely supporting stock rotation and traceability. The overall layout indicates a dedicated and well-maintained environment suitable for the storage and distribution of emergency drinking water supplies, with clear access routes and minimal clutter reducing operational and hygiene risks. — Figure 50 – Bespoke contractor run bottled water facility – Anglian Water.

At Normanton, the Yorkshire Water bottled water was found to be stored appropriately, with restricted access, however, the cleanliness and maintenance of the site could be improved with the plastic wrapping dirty and weeds growing in the store. There was no temperature monitoring or water quality sampling at the time of the audit. The new small, bottled water store at Sand Hutton was found to have no temperature monitoring or control.

Interior of a warehouse aisle used for the storage of bottled water supplies, with palletised stock arranged on multi-level metal racking. Packs of bottled water are shrink-wrapped and stacked on wooden pallets, with several pallets marked in red spray paint with dates (for example “AUG 27”), indicating stock rotation or shelf-life management. The pallets are stored both at ground level and on upper racking tiers, maximising vertical storage space. The aisle is narrow but clear, allowing access for handling equipment. The storage environment includes overhead lighting, metal wall cladding, and exposed structural beams. Safety signage is fixed to the racking, including a sign stating “Safety helmets must be worn in this area”, and additional warning notices relating to pallet racking use. The bottled water appears to be kept elevated from the floor and fully wrapped, providing protection from contamination. The presence of visible date markings suggests an organised system for stock control and rotation, supporting the management of emergency drinking water supplies within a controlled warehouse setting. — Figure 51 – Normanton store, good stock control

Figure 52 – Dirty storage conditions at Normanton

Indoor storage area showing pallets of bottled water stacked on wooden bases, with shrink-wrapped multi-packs arranged in layers. To the right, a group of white and black cylindrical containers is lined up, and to the left a small desk and drawers are positioned against the wall. — Figure 53 – Small Sand Hutton store – Yorkshire Water

Several procedural improvements were suggested to South East Water related to storage and deployment. These included the need for improved security and restriction of access to the storage facility. Concerns were also raised relating to the proximity of the bottles to potential contamination risk from other activities including poor chemical storage at the Lenham depot.

Interior of a warehouse or workshop space with high ceilings and exposed metal roof beams. Shelving units on the left contain tools and equipment, while pallets covered with a blue tarpaulin are stacked towards the back. In the centre foreground, a person wearing a high-visibility orange vest and white hard hat stands facing away, near machinery and equipment including a trolley and a red container — Figure 54 – Bottled Water storage in close proximity to other items

Shelving unit in a storage area containing equipment and containers. A cardboard box on the upper shelf is labelled ‘CAUTION SHARP OBJECTS INSIDE’. Below, several plastic containers and tools are stored, with visible liquid staining and pooling on the floor in front of the shelving. — Figure 55 – Poor chemical storage – South East Water

All companies are advised to review their current practices against the new Guidance on Alternative Supply Operations published by the Inspectorate alongside Information Letter 01/2026, Guidance-on-Alternative-Supply-Operations-2026_v2.pdf. This guidance supersedes the good practice documented with TGN11 of the Principles of Water Supply Hygiene.

Operation and condition of works

Across the works audited in 2025, the Inspectorate saw examples of well‑run sites where treatment processes were operating effectively. Day‑to‑day operational controls were generally satisfactory, and companies were able to explain how treatment was adjusted in response to changes in raw water quality or demand. However, the audits also identified recurring issues relating to the condition and management of physical assets. In particular, concerns were raised at some sites about the inspection and maintenance of tanks, contact structures and other buried or enclosed assets that play a critical role in ensuring effective disinfection and storage of treated water.

In several cases, internal inspections of tanks had been delayed beyond expected timescales, sometimes for many years. While companies often had plans in place to address these delays, the lack of up‑to‑date information about the condition of these assets created uncertainty about the level of risk. In some instances. The Inspectorate required companies to conduct inspections or investigations and to provide evidence of the findings.

Temporary repairs and mitigation measures were also observed at a small number of sites. While these measures were often necessary to maintain supply in the short term, the audits highlighted the importance of ensuring that temporary arrangements do not become long‑term solutions without appropriate oversight. The Inspectorate expects companies to manage such risks actively and to progress permanent solutions in a timely manner.

Management of raw water sources and catchments

Nine audits focused on how companies manage risks associated with raw water sources and the catchments from which water is abstracted, including both surface water systems, such as reservoirs and rivers, and groundwater sources such as boreholes.

The Inspectorate observed examples of good practice at several sites. These included regular catchment walkovers, proactive engagement with landowners, and ongoing monitoring programmes designed to detect changes in raw water quality. In some cases, companies were undertaking detailed investigations into emerging contaminants. For example, the Inspectorate commended Yorkshire Water for its investigation into the source of PFAS contamination affecting Eccup reservoir, which contributed to Tier 2 PFAS detections at Headingley treatment works.

However, audits also identified instances where risks within catchments were not fully reflected in company assessments. This included cases where known activities, land uses, or potential contamination sources had not been adequately captured, and where changes over time had not been incorporated into risk assessments. Where such gaps were identified, the Inspectorate required companies to review and update their assessments and to demonstrate how identified risks would be effectively controlled.

The audits also highlighted the challenges associated with managing older or complex source systems, where infrastructure and operating practices have evolved over many decades. In such cases, maintaining a comprehensive and up-to-date understanding of system configuration and risk pathways is essential.

The Inspectorate expects companies to maintain accurate, current, and risk-based assessments of their raw water sources and catchments, supported by effective monitoring and engagement. This is fundamental to ensuring that risks are identified early and managed proactively, thereby protecting treatment performance and safeguarding the quality of water supplied to consumers.

Cross‑industry themes and learning

Across the 2025 audit programme, several common themes emerged. These included the need for timely inspection and maintenance of critical assets, the importance of ensuring that risk assessments remain aligned with actual site conditions, and the central role of accurate and reliable data in effective water quality management.

The audits also highlighted the value of proactive management and timely intervention. Where companies had identified issues early and taken decisive action to address them, outcomes were generally more positive and regulatory concern was reduced. Conversely, delays in recognising or responding to emerging risks often resulted in escalation and increased regulatory intervention.

A further recurring theme was the challenge of managing ageing infrastructure in the context of increasing environmental and operational pressures. While many companies are progressing investment programmes to address these risks, the Inspectorate expects such investment to be underpinned by robust evidence, clear prioritisation, and a comprehensive understanding of system risk.

Overall, the findings reinforce the expectation that companies adopt a proactive, risk-based approach, supported by effective asset management, accurate data, and timely decision-making, to ensure the continued protection of public health and the resilience of water supplies.

Air valve audits

Following the Cryptosporidium outbreak in Brixham, Devon in 2024, the Inspectorate undertook an industry‑wide audit desk-top programme focused on the management of air valves across water companies in England and Wales. Air valves perform an important role in the operation of water distribution network by protecting pipelines from collapse by allowing air into and out of water mains. This also therefore introduces a contamination risk to water supplies and all water companies are expected to have appropriate procedures, inspection and maintenance regimes to avoid contamination and protect public health.

The industry should be well aware of the risks related to air valves and the Inspectorate has shared findings and raised concerns with air valve maintenance regularly in Chief Inspector’s reports since 2017. The lessons from the Brixham Cryptosporidium incident should be learned by all companies.

Air valves present a contamination risk. Not just for Cryptosporidium but for other contaminants too. It is incumbent upon companies to ensure that their assets are inspected and maintained frequently based upon risk. Damage and other faults should be addressed promptly, taking steps to avoid air valves becoming submerged and, where possible to avoid locations with a source of contaminants Appropriate asset registers, maintenance records and GIS locations are important.

Reactive inspections linked to microbiological and other contamination risks should be standard practice.

Open underground chamber with a square access hatch and a hose fed into the opening from the surface. Adjacent image shows the interior of the chamber partially filled with cloudy water, with a submerged valve assembly connected to the hose. — Figure 56 – Pumping out an Air Valve Chamber

Rectangular underground chamber filled with standing water, debris, and leaf litter. A partially submerged access cover and fittings are visible within the chamber, with water reaching near the top of the structure. — Figure 57 – Submerged and barely visible air valve

Overview of industry findings

There is considerable variation in how companies manage and control air valves risks. Few companies demonstrated mature asset management arrangements and risk‑based inspection programmes. Several showed significant gaps in asset knowledge, inspection coverage and regulatory risk assessment. Enforcement was required for nine companies with inadequate or missing risk assessments found at:

South West and Bournemouth Water
Northumbrian Water
Dwr Cymru
United Utilities
SES Water
South East Water
Thames Water
Bristol Water
South Staffordshire Water and Cambridge Water
Affinity Water

Air valves are often poorly documented. In several audits, companies acknowledged that a significant proportion of air valves had not been inspected for many years, or that their precise locations were unverified. Site visits had been unable to locate air valves, in some cases, with companies uncertain whether they could not be found or had been removed, due to poor historical records.

The Inspectorate consistently emphasised that malfunctioning, poorly located or submerged air valves represent a credible risk of contamination and supply interruption, particularly where they are subject to flooding, agricultural runoff or negative pressure conditions. These conditions led to the ingress of Cryptosporidium in the Brixham outbreak.

Asset information, records and mapping

Portsmouth Water were one of a few companies that maintained a comprehensive asset register for air valves and regular inspection and maintenance regimes. Registers that were up to date and used to inform inspection planning, are a positive foundation for effective risk management.

Some companies were able to demonstrate that air valves were clearly mapped on GIS systems and that inspection outcomes linked back to the asset record, supporting traceability and auditability.

In contrast, several audits identified significant weaknesses in asset information. Uncertainty in the accuracy of mapped locations, with large numbers of air valves unverified because they had not been inspected in recent years. Other poor practices included the omission of certain air valves from asset registers altogether, particularly those located within treatment works or on raw water assets.

Recommendations focused on the need for companies to ensure that all air valves are recorded, accurately located on GIS systems, and subject to the same governance arrangements, inspection and maintenance controls.

Inspection and maintenance arrangements

Good practice was observed where companies had established risk‑based inspection programmes that treated air valves as standalone assets rather than relying on trunk main inspection cycles. In these cases, inspection frequencies were informed by factors such as flood risk, land use, proximity to watercourses and potential for negative pressure events.

Conversely, a common issue was the use of excessively long inspection intervals for low‑risk air valves, in some cases extending to 15 years without any form of routine inspection. The Inspectorate is critical of this approach that is not in keeping with good practice as land use and environmental conditions can change significantly over such periods.

Inspection records were often incomplete or lacking sufficient detail to demonstrate that meaningful checks had been undertaken. Some records merely recorded assets as “satisfactory” or “unsatisfactory” without documenting specific defects, maintenance actions or follow‑up requirements. In other cases, inspection durations were so short that it was unclear whether any maintenance activities, such as seal inspection or cleaning, had been carried out.

The Inspectorate also raised concerns where inspections were aborted due to access issues, such as landowner restrictions or safety hazards, without clear evidence that inspections were rescheduled or escalated based on risk.

Recommendations addressed inspection frequency, maintenance practices and follow‑up actions. While some companies had moved towards risk‑based inspection programmes, others rely on infrequent or generic inspection cycles that do not reflect risks such as flooding, agricultural activity or the potential for negative pressure events.

All companies should have implemented risk based inspection frequencies in line with air valve manufacturers guidance and good practice.

Remediation and follow‑up actions

Where defects were identified, good practice involved promptly raising of remedial jobs, clear prioritisation based on risk, and documented confirmation that repairs or replacements were complete. Some companies demonstrated structured approaches to remediation, supported by dedicated budgets and formal processes for tracking defective air valves to resolution.

However, for several companies poor practice was evident where significant numbers of defective or leaking air valves were identified but repairs were delayed or not documented and urgent defects were not rectified within appropriate timescales, raising concerns about the effectiveness of escalation and governance arrangements.

Companies are required to have robust processes for raising, tracking and closing out remedial work, ensuring that actions taken in response to inspection findings are recorded against the asset and subject to appropriate oversight

Risk assessment and regulatory compliance

A consistent theme across the audits was the inadequate integration of air valve risks into regulatory risk assessments. Several audits found that air valves were either absent from regulatory risk assessments or only referenced generically, without specific hazardous events or control measures. This approach does not meet the requirement for continual and comprehensive risk assessment and fails to demonstrate control of drinking water quality risks. Where upstream or bulk supply risks were relevant, reliance was sometimes placed on incumbent assessments without sufficient evidence of review or ownership.

Companies are required to explicitly include air valves within their regulation 27 and regulation 28 risk assessments, to identify relevant hazardous events (such as ingress via air valves), and to ensure that inspection and maintenance activities are reflected as control measures within drinking water safety planning.

Across the companies audited, the Inspectorate issued a total of 59 formal recommendations. (Table 17) These recommendations were directed at both water undertakers and inset appointees and reflect deficiencies identified through the review of asset information, inspection regimes, maintenance practices and regulatory risk assessments. Although the number of recommendations varied between companies, common themes emerged, indicating systemic weaknesses rather than isolated issues.

Company	Number of recommendations
Affinity Water	7
Bristol Water	4
Dwr Cymru Welsh Water	5
Anglian Water	4
SES Water	4
Wessex Water	4
Severn Trent / Hafren Dyfrdwy	3
South West and Bournemouth Water	3
Thames Water	3
Yorkshire Water	3
Albion Water	2
Icosa Water	2
Leep Networks Water	2
Matrix Water	2
MUA	2
Northumbrian, Essex and Suffolk Water	2
Southern Water	2
South East Water	1
Advanced Water Infrastructure Networks	1
Independent Water Networks	1
United Utilities	1
Veolia Water Projects	1
ESP Water	0
Total	59

Table 17 – number of recommendations raised against companies from the air valve audits

Additional information

Following on from the Inspectorate’s technical audit of United Utilities risk management of air valves, several deficiencies were identified, such as a lack of appropriate procedures for inspection and maintenance of air valves, a lack of proactive maintenance records for individual air valves, deficiencies in risk assessments for certain asset classes of air valves, and the lack of a clear strategy for remediation of air valves. A regulation 28(4) notice was served upon the company to improve the risk management of its air valves. The notice will be completed in January 2027 and covers all air valves located downstream of water treatment stages, thus benefitting the majority of the population served by the company.

Thames Water

The company demonstrated strong progress in 2025, most notably through the closure of the transformation notice relating to flooding risk. This represents a significant regulatory milestone and evidences sustained action to strengthen resilience in a key risk area. In addition, two further regulation 28(4) notices were satisfactorily completed, addressing Cryptosporidium and turbidity risks, as well as air valve risk assessment, reflecting continued improvement in risk management and compliance

Yorkshire Water – repeat coliform detections at High Bentham

High Bentham service reservoir is supplied from Embsay works via Cold Cotes service reservoir, all of which is a chloraminated supply. A routine compliance sample taken on 8 May 2025 identified elevated total coliforms, with follow up sampling confirming ongoing contamination primarily associated with compartment 2 of the reservoir with two out of four downstream sampled properties also demonstrating the presence of total coliforms. Compartment 2 was taken out of service on 11 May 2025, and a precautionary boil water notice was issued, supported by consumer communications, bottled water provision, and targeted flushing of affected DMAs.

Subsequent sampling identified one Clostridium perfringens detection at a consumer’s property but was otherwise satisfactory, while inspections and cleaning of the reservoir found no obvious defects. The boil water notice was lifted on 14 May 2025 following satisfactory sample results. The company identified there was a potential for stagnant water to back-syphon from a 3.5 metre section of riser pipework, connected to an air valve, upstream of the High Bentham reservoir inlet. The company could not be sure whether the air valve chamber had been flooded in the period immediately before the bacteriological detection. Compartment 1 of High Bentham reservoir had been refilled on 1 May 2025, following a clean and internal inspection on 30 April 2025, and if the inlet main had depressurised during filling, material or water could have been drawn in from the air valve chamber or the pipework.

The total coliform bacteria were identified as a mixture of Serratia species. Inadequate chlorine levels are regarded as a potential factor allowing Serratia and other coliform bacteria to survive and multiply within piping systems and in biofilms. The free chlorine residual was 0.07 mg/L and the total chlorine 0.13 mg/L on the initial compliance sample with the resample the next day demonstrating even lower residuals. The company has a policy that chlorine dosing systems should be operated to achieve a reference value residual total chlorine concentration of 0.4mg/L on the outlet of all service reservoirs. However, 10 out of 26 service reservoirs on this distribution system demonstrated residuals below this reference value.

The event demonstrated the importance of air valve inspection and maintenance especially if there are periods of depressurisation in the network. To prevent recurrence, the company removed the identified air valve and completed wider inspections of upstream air valves to provide assurance. This event also highlighted the potential risk to a low residual chlorine distribution system from local recontamination.

Enforcement, transformation and recommendations

Enforcement – England 2025

The Inspectorate publishes the drinking water quality legal instruments on its website under company improvement programmes. Security (SEMD and NIS) legal instruments are considered sensitive and therefore are not published in the public domain. The table below summarises the total legal instruments served in 2025.

Type of legal instrument	Number of legal instruments	Companies
Regulation 21(3) Notice for the protection of health in public buildings	1	Thames Water
Regulation 27(4) Notice for improvements to water safety plans	3	South East Water (1), South Staffordshire Water (2)
Regulation 28(4) Notice relating to risks identified in water safety plans	87	Anglian Water (21), Northumbrian, Essex and Suffolk Water (12), South East Water (33), Southern Water (8), South Staffordshire Water (1), South West Water (5), Thames Water (5), United Utilities (2)
Final enforcement order under section 18 of the Water Industry Act for drinking water quality risks.	1	Southern Water
Undertaking accepted under section 19 of the Water Industry Act for drinking water quality risks.	2	Severn Trent Water (1), Thames Water (1)
Regulation 17 Notice for improvements under the Network and Information Systems Regulations.	2	Northumbrian Water (1), Portsmouth Water (1)
Undertaking accepted under section 19 of the Water Industry Act for SEMD improvements.	3	County Water (1), South East Water (1), Southern Water (1)
Formally acknowledged improvement actions*	10	Anglian Water (2), Southern Water (1), Wessex Water (7)

Table 18 – Enforcement legal instruments issued by the Inspectorate in England, in 2025.

*Acknowledged improvement actions are not legal instruments and as such, have no legal status. They are a set of actions that the company set out are being delivered and which the regulator (the Inspectorate in this case) formally acknowledges. They are used for improvement schemes, that the Inspectorate has commended for support, but which do not meet the threshold for formal enforcement.

Details of the new enforcement served in the first half of 2025 was published in the interim report, therefore details of new enforcement from the second half of the year follow.

Enforcement served in England, in the second half of 2025

Notices served under regulation 21(3)

A notice under regulation 21(3) was served on Thames Water following persistent lead exceedances at a property. Initial sampling in March 2024 recorded concentrations above the parametric value, and although partial pipe replacement was undertaken, subsequent monitoring confirmed that a residual lead risk remained. Given that the risk had not been adequately mitigated, and in line with the enforcement policy, the Inspectorate required the company to exercise its powers under section 75 of the Water Industry Act to protect consumers.

Notices served under regulation 27(4)

Several notices were served where companies had failed to establish or maintain adequate risk assessments. South Staffordshire Water was required to complete risk assessments for a new non-potable supply at Eddington, ensuring full compliance with regulatory requirements where water is supplied for domestic use.

Deficiencies in air valve risk assessment were also identified across multiple companies. South East Water and South Staffordshire Water were both subject to regulation 27(4) notices requiring the development and implementation of risk assessments covering air valves across treatment works, distribution systems, service reservoirs and water supply zones. These notices reflect the importance of fully embedded Drinking Water Safety Plans that account for all critical assets and associated risks.

Notices served under regulation 28(4)

A significant number of regulation 28(4) notices were served to address risks to wholesomeness across treatment works, distribution systems, and catchments.

At Sawston Mill, a replacement notice was served on South Staffordshire Water following early breakthrough of short-chain PFAS through GAC media. A revised long-term scheme has been put in place to address contamination associated with the Duxford Airfield borehole, benefiting approximately 135,050 consumers.
Northumbrian Water received a notice requiring improvements to clarifier operation and supernatant return at Mosswood works, where audit findings identified unacceptable risks due to high turbidity return flows and the inability to safely isolate assets for maintenance.

South East Water was subject to multiple notices, including mains replacement following prolonged discolouration affecting consumers, and replacement notices for failure to meet inspection requirements at Blean No.2 and Crowborough No.3 reservoirs. Further urgent action was required at Pembury works, where a notice was served without consultation due to immediate risk, requiring installation of microfiltration. Failure to meet early reporting milestones led to escalation towards a Section 18 enforcement order.

Southern Water was served several notices following a chlorate exceedance at Brede works and delays in its groundwater hazard review programme. These included restrictions on operation where chlorate levels exceed 700 µg/L and multiple notices to complete outstanding hazard mitigation actions. Additionally, Itchingfield service reservoir was removed from an existing notice due to access constraints, with a long-term replacement solution now required.

South West Water was served a notice to address longstanding discolouration at Trevemper Road, where replacement of an unlined cast iron main has now delivered measurable improvements in water quality.
Thames Water was subject to further notices addressing multiple risks. Following Cryptosporidium detections at Taplow boreholes, a long-term filtration solution was required. In addition, a wider compliance issue was identified across 39 treatment works lacking adequate post-contact tank chlorine monitoring, prompting a large-scale programme of upgrades. Enforcement was also taken in relation to lead exceedances in the Bourton supply system, where evidence indicated broader systemic risk.

United Utilities was served a regulation 28(4) notice following a technical audit identifying significant deficiencies in air valve risk management, including gaps in inspection, maintenance, and strategic planning.

Section 18 Enforcement Orders

The Inspectorate made a Section 18 Enforcement Order against Southern Water following failure to comply with existing regulation 28 notices. The order, effective until November 2033, ensures the delivery of key actions required to protect water quality and public health.

Section 19 Undertakings for SEMD

A company-wide Section 19 undertaking relating to SEMD was accepted following identification of security risks during an audit. The company committed to reviewing and strengthening its security risk assessment framework and physical assets.

Actions formally acknowledged

The Inspectorate acknowledged several planned improvement schemes, including disinfection upgrades proposed by Wessex Water across multiple sites. While regulatory compliance risks were currently low, these schemes will significantly improve future resilience.

Enforcement Developments

The Inspectorate continued to develop its enforcement processes, including expansion of the milestone and closure reporting portal to support change applications and annual reporting. Further enhancements are planned to align with NIS and SEMD frameworks.

Transformation Programmes

The Inspectorate continues to oversee transformation programmes across several companies to address systemic risks.

Southern Water remains the longest-running programme, with notable progress in 2025 including early completion of enforcement orders and significant delivery against its transformation plan. However, delays persist at some sites, and the company remains subject to a large number of active legal instruments.

Thames Water continues to demonstrate steady progress, including closure of key transformation notices and improvements in risk management. Engagement with the Inspectorate remains strong, supported by structured governance and high-quality reporting.

South West Water has progressed its transformation programme, completing key notices and improving engagement and performance, although challenges remain, particularly in the Isles of Scilly.

Northumbrian Water continues its transformation, with progress in closing legal instruments but ongoing requirements to deliver hazard review actions and strengthen asset management.

South East Water entered transformation following concerns around risk identification, asset resilience, and operational capability. The programme will focus on improving risk assessment, maintenance delivery, legal instrument management, and overall system resilience.

Overall, enforcement activity during this period reflects the Inspectorate’s continued focus on addressing systemic risk, improving asset resilience, and ensuring that companies take timely, effective, and sustained action to protect water quality and public health.

Enforcement Performance Metric

During 2025, the Inspectorate developed and tested a new metric to measure water company performance in respect of their obligations contained within legal instruments served by the Inspectorate. The metric assists the Inspectorate in directing its limited enforcement resource to those companies that require the most attention, as well as publicly demonstrating water company performance.

Every legal instrument served is scored, using a five by five matrix, which has a set of rules attached, which the inspectors follow for consistency.

The first score, assigned when the legal instrument is created, is the seriousness score. This score reflects the origins of the legal instruments and ranges from voluntary programmes, through routine enforcement action, to escalated and transformation enforcement. The criteria are listed in Table X, below.

Seriousness	Seriousness score
Escalated enforcement – generally, enforcement orders but other forms too.	5
Further (additional) enforcement – “child” legal instruments, transformation legal instruments	4
Routine enforcement (realised breaches) – e.g. compliance assessments, events, regulation 26 breaches, data driven, consumer complaints, etc.	3
Routine enforcement (potential breaches) – e.g. likely to breach, risk of breach, audit findings, risk assessments, etc.	2
Proactive schemes – e.g. AMP schemes, voluntary undertakings, etc.	1

Table 19 – seriousness scores assigned to legal instruments

The second score is a Red, Amber, Green (RAG) (expanded to include Amber + and Red +) status of the progress of the legal instrument. This score is assigned when the legal instrument is first created, but is changed, as required, throughout its lifetime based on the progress of delivery. The criteria are listed in Table X below.

RAG scores assigned to legal instruments

Red+ (5)

The Inspectorate have rejected a change application for date extensions to the completion date of the legal instrument.
Any delays that affect the end date of the legal instrument.
Once a notice is in Red+ status, it can only ever recover to Amber +.

Red (4)

The most recently submitted milestone is delayed or was delivered late.
The most recently submitted milestone is insufficiently evidenced or incomplete (this is where a report is rejected entirely. Does not apply for routine follow-up questions).
The Inspectorate have been notified that a future intermediate milestone is going to be delayed and there is no way the company can meet it.
The Inspectorate have rejected a change application for date extensions for intermediate milestones.
Last-minute notification of not going to meet targets, when clearly company have known for some time.
Missed completion report deadline.
Red applies until the next milestone is received, when the score may reduce to Amber or Amber +.

Amber+ (3)

Escalated enforcement (see table above) always begins at Amber+
New child legal instrument from a delayed legal instrument begins at Amber + (for example an individual service reservoir (SR) being removed from a wider SR notice).
Change application has been submitted and accepted, with a new final delivery date.
Legal instrument was previously at Red+ status, but company have resolved issues and improved management of notice.
Cannot go lower than Amber+ once here.

Amber (2)

Intermediate delay which does not affect the final date of legal instrument.
There have been delayed/late milestones, but scheme is now on track.
Previously red, but company have resolved the issues and improved the management of the legal instrument.
Cannot go lower than Amber once here.

Green (1)

Default option for all new legal instruments.
All milestones are on target or completed on time.
Change of solution which does not affect the final delivery date (ie the company have found a cheaper or quicker way of delivering the scheme within the notice period.)
The Inspectorate direct a future change to a scheme (for example, the publishing of new PFAS guidance, which affects PFAS undertakings)
DWI have initiated a change.
Small administration changes (such as an annex update to a discolouration notice).
Cannot return to green once amber or red.

The metric is then calculated in a similar manner to the recommendations risk index. There is an acceptance that risk will always exist and the bigger the company (bigger catchment, more water treatment, more treated water storage, more kilometres of mains and more connections), the higher the residual risk. Total scores for each company are compared to those of the whole sector and normalised based on the proportion of overall population supplied to give an expected score for each company. The expected scores are then plotted against actual scores, for each company, using a linear regression. The upper 95% confidence value line is added, to highlight where there are statistical outliers, producing the graph shown below (Figure XX), which is the Enforcement Performance Metric for the end of 2025.

Figure 58 – Enforcement Performance Metric 2025

During the first half of 2025, we gave an amnesty to companies on NIS and SEMD legal instruments, as the reporting processes and requirements were all new to these areas. We sought to support, educate and advise, rather than reprimand where items were delivered late or incomplete. Whilst we are always available to advise companies, the period of grace has now ended and the NIS and SEMD legal instruments will be scored appropriately. At the present time, NIS and SEMD legal instruments are included with the drinking water quality enforcement into a single measure. This is due to there being only a small number of those legal instruments, compared to the overall number. This will be reviewed in the future, and they may be separated out if there is a benefit to doing so.

The metric shows that as of 31 December 2025, South East Water, Southern Water, South West Water and Anglian Water are the outlier companies. South East Water and Anglian Water both have a high number of notices served for individual service reservoirs and other treated water tanks after failing to complete the required inspections under their “all tanks” notices. These notices all score highly as they represent a failure to deliver on previous legal instruments. As these notices are delivered (tanks are inspected and repaired), they will be revoked and the scores will decrease accordingly. South East Water also has other legal instruments with high RAG scores due to the late reporting of milestones. The management of its legal instrument obligations has been raised with the company and will for a key part of its transformation programme.

Southern Water has historically failed to deliver its transformation notices on time, which has resulted in multiple final enforcement orders being made on the company which attract high scores. The company also have several “child” notices resulting from previous transformation notices, which will also score highly. The company do have a large number of closure reports due in 2026 and 2027, so we look forward to their score decreasing over the next two years, as we are able to close or revoke completed legal instruments.

South West Water has five highly scoring legal instruments for the Isles of Scilly, which are severely delayed, alongside some delays on other legal instruments. This is sufficient to push them above the 95% confidence interval line. With the length of delays on the island notices, this position is not likely to be fixed for some time.

Recommendations

In 2025, the Inspectorate issued 756 recommendations completing the assessment of 667 compared to 836 recommendations to water companies and new appointments and variations in 2024. This remained a substantial level of regulatory intervention and was higher than the 714 recommendations issued in 2023 and 564 issued in 2022. The company-level data shows that recommendations continued to be distributed across most companies, with the highest numbers issued to Thames Water Utilities Ltd (92), Dŵr Cymru Welsh Water (83), Severn Trent Water Ltd (69), Anglian Water Services Ltd (59), Northumbrian Essex and Suffolk Water (56), and United Utilities Water Ltd (51). These companies accounted for a significant proportion of the total recommendations, although recommendations were also made across a wide range of other water companies and NAVs. This indicates that the issues identified through the Inspectorate’s regulatory activity were sector-wide and not confined to a small number of companies.

Company	Number of Recommendations
Advanced Water Infrastructure Networks	3
Affinity Water Ltd	36
Albion Water Ltd	3
Anglian Water Services Ltd	59
Bristol Water Plc	13
Cambridge Water Company Plc	13
Dwr Cymru Welsh Water	83
ESP Water Limited	4
Hafren Dyfrdwy	6
Independent Water Networks Limited	31
Isles of Scilly	1
Last Mile Water Limited	15
Leep Networks Water Ltd	15
MUA Water Ltd	3
Matrix Water	2
Northumbrian Essex and Suffolk Water	56
Portsmouth Water Ltd	5
SES Water	6
Severn Trent Water Ltd	69
South East Water Ltd	40
South Staffordshire Water Plc	24
South West and Bournemouth Water	23
Southern Water Services Ltd	38
Thames Water Utilities Ltd	92
United Utilities Water Ltd	51
Veolia Water Projects Ltd	5
Wessex Water Services Ltd	15
Yorkshire Water Services Ltd	45

Table 18 – Number of recommendations given to each company in 2025

The distribution of recommendations reflects both the scale and complexity of company operations and the nature of the issues identified through audits, event investigations and other regulatory activity. Regulatory breaches remained the principal reason for making recommendations, particularly where a company had failed to comply with statutory requirements in circumstances that resulted, or could have resulted, in a risk to the wholesomeness or sufficiency of drinking water supplies.

Recommendation Type	Sum
Deficiencies with risk assessment	119
Deficiency relating to policy/procedure	103
Inadequate investigations	82
Deficiency relating to service reservoir or network management/operation/maintenance	61
Issues with sampling (appropriateness or accuracy)	34
Deficiencies with consumer communication	30
Deficiencies with other treatment operation / maintenance	29
Deficiencies with disinfection operation / maintenance	26
Inadequate follow up sampling	25
Issues with provision of inaccurate information	21

Table 19 – Type of recommendations given to each company in 2025 (Top 10)

Risk assessment failings were the most common area of non-compliance, with 119 recommendations These recommendations reflected shortcomings that the Inspectorate considered to have been within company control and which should have been identified, assessed and managed through effective drinking water safety planning. Failings associated with policy and procedures accounted for 103 recommendations, while investigation-related requirements accounted for 82. Deficiency relating to service reservoir or network management/ operation/ maintenance Disinfection and treatment failings resulted in 61 recommendations, underlining the continued importance of basic processes, effective operational monitoring and timely response to deterioration in source or treated water quality.

Figure 58 – Assessment and response of recommendations given to each company in 2025

This year an assessment of the average assessment and response score provides insight to those companies which are either creating serious recommendations or not responding adequately to recommendations driving up the outcome on the recommendations risk index seen in the graph below.

Figure 59 – Assessment of recommendations in 2025

Materials in contact with drinking water (regulation 31)

Regulation 31 prohibits a water company from applying any substance or product to, or introduce any substance or product into, water which may impact drinking water quality. This covers all chemicals, construction and treatment products used by water undertakers, from the source of the water, up to the point of delivery to the consumer’s building. This is a critical duty because materials can taint water by taste, odour or by leaching of harmful substances. The regulation sets out how approvals can be given to construction and treatment products and materials that do not prejudice water quality and ultimately consumer safety.

During 2025, the Inspectorate continued to receive and process applications for approval of products in contact with drinking water (under regulation 31). During this period a total of 545 enquiries were completed by the team. The volume of applications received was:

Figure 60 – Number of R31 applications

The Inspectorate has continued working with IT partners to further develop the regulation 31 portal for product approvals which has been well received overall. The new system replaced the previously used manual application forms with online, interactive application forms that will guide applicants in providing all the necessary information for an approval, product change or re-approval to be considered. The online process will have the benefit of meeting accessibility standards, allowing applicants to see what stage their applications have reached and making them available to more people with the next phase of the project delivering and end to end process.

In August 2025 we published a letter (DWI Ref: R31-06/2025) launching the new online interactive List of Approved Products. This has transformed from a monthly, published PDF document to an interactive, searchable website which is updated in real-time, something long requested by the industry and applicants. This has effectively become a live, online catalogue of approved products and can be found at: List of Approved Products for use in Public Water Supply in the United Kingdom – Drinking Water Inspectorate.

Identification of poor product stewardship continued throughout 2025 with a number of approved products being revoked from the Secretary of States List of Approved Products with immediate effect. All issues identified were as a result of unauthorised changes being made to a product. Where even the smallest unauthorised change has been made to a product formulation, material or component, the effect may lead to issues with unacceptable changes to water quality or a risk to public health. It is therefore imperative that all changes to formulation are approved prior to being implemented and supplied.

Where manufacturers have applied for changes to approved products, these will have been reviewed and where necessary testing undertaken prior to being accepted. This ensures that there are no issues with the products when in use.

The table below lists the products revoked and the associated letters issued during 2025 with some products completely revoked and requiring a full re-application and other products having conditions of use imposed.
Regarding Arlington 1000 Litre Bag-in-Box Combo Liner, conditions of use were applied to the product rather than a full revocation to allow water for domestic use to be available during the summer where many water companies were experiencing prolonged dry weather. This is an example of the pragmatic approach taken by the team during the year.

Date	Action	Product / Manufacturer	Reference number	Details / Conditions
Jan 25	Revocation of product approval	GF ABS Pressure Pipe / George Fischer	R31-01/2025
Jan 25	Revocation of product approval	8040-SBNF-DWI Membrane Elements / Mann+Hummel Water and Fluid Solutions	R31-02/2025
Apr 25	Revocation of product approval	PE100 High Performance Black Pipe / Radius Systems Ltd.	R31-03/2025
Aug 25	Update to requirements for approval	Arlington 1000 Litre Bag-in-Box Combo Liner	R31-04/2025	This product cannot be used for drinking or cooking purposes. Changes take effect from 06/08/2025.
Aug 25	Update to requirements for approval	Amazon SupaSpun II Cartridge Filters and Amazon SupaGard Cartridge Filters	R31-05/2025	Products supplied before 06/08/2025 shall be quarantined and cannot be installed. Stocks of already purchased product must be quarantined and clearly labelled as not to be used until further notice.
Oct 25	Removal of recognised grade status	Eurokote® 468 Thixo & Eurokote® 468 Thixo 500 Brun Rouge	R31-08/2025	Issue with naming of various products resolved.

Table 20 – products revoked and the associated letters issued during 2025

Issues with some of the revoked products have since been resolved and these products have returned to the List of Approved Products. It should be noted that the name of the product and reference number are likely to have changed. Others will be undergoing full review and additional testing to verify whether they can be approved.

The Inspectorate does not take the decision to revoke products or prohibit the use of products lightly however, where the stringent requirements to maintain the product approvals are not met, and constitutes a potential concern to public health then the decision to revoke the product can be taken. Use of an unapproved product is an offence and can be prosecuted under regulation 33.

In addition, several approval holders either made late applications or omitted to apply for product reapprovals which resulted in a delay in reapprovals being assessed. This resulted in an increase in expired products with manufacturers having to undertake the entire process of product approval from the beginning.

In December 2025, manufacturers were informed via letter, DWI Ref: R31-10/2025, that there will be a requirement for all recognised grades currently contained in Annex 3 of the List of approved Products, to go through the approval process with the requirement that they apply for this approval by end of June 2026. This will allow us to keep an updated list of recognised grade products and their current formulations.

There has been significant work undertaken by the Inspectorate to secure testing facilities for BS6920 and full regulation 31 testing throughout the year with five laboratories being added to the list of designated laboratories for BS6920 testing in February 2025. A laboratory was recently designated to undertake full regulation 31 testing capability, and we continue to work towards increasing this capacity throughout 2026. We are also collaborating with the water industry to resolve the issue, and this work continues. Laboratories are required to demonstrate and evidence compliance with the relevant standards and protocols to become designated for regulation 31 testing.

Following members of the regulation 31 team presenting on the challenges of regulation 31 testing and discussion at Northumbrian Water’s Innovation Festival in July 2025, the team has also collaborated with Ofwat’s Innovation Enablers team to publish an expression of interest for progressing further laboratory testing which is due for publication in Q1 of 2026. This work seeks to provide a pathway for laboratories to undertake full regulation 31 testing requirements.

Security and Emergency Measures Direction – SEMD

2025 was the fourth reporting year for companies since the Inspectorate has regulated the Security and Emergency Measures Direction (the Direction). Companies have demonstrated an overall improvement in self-identification of the areas that they need to improve and move towards compliance of the Direction, resulting in significant investment in AMP8 over a range of schemes consisting of security and emergency planning improvements. This recognition has led to an overall decline in the reported Red, Amber, Green (RAG) position since 2022.

The Inspectorate undertook a program of 22 on-site technical audits as well as event assessments and company interactions which resulted in a range of enforcement activity to secure improvement and compliance as demonstrated in Figure 61:

Figure 61 – SEMD Enforcement activity

Sufficiency

In September 2024, the National Infrastructure Commission (now National Infrastructure and Service Transformation Authority – NISTA) published a guide to developing resilience standards in UK Infrastructure, identifying gaps in water industry regulation and recommending resilience standards for public water supplies. Key recommendations included: setting a peak demand standard for short-term supply capability; defining limits on consumers reliant on a single asset; and introducing forward-looking asset health metrics.

In response to the publication, the Inspectorate undertook analysis and research. The Inspectorate then drafted a report with proposed guidance on maintaining a sufficient water supply in response to the key recommendations identified in the NISTA report and incorporating the findings of the independent research. The Inspectorate concluded that the measurement of sufficiency, serves as a practical indicator for both supply resilience and asset health.

Figure 62 – Sufficiency

There was also a wider acknowledgement that no single lens would deliver the results that are intended in the NISTA report, so whilst sufficiency was the focus of the Inspectorate’s response it touches on a number of areas as shown in Figure 62. This work then allows other standards to be developed and work in conjunction with the sufficiency work.

The Inspectorate’s proposed guidance supports the principles of, clear definitions, risk assessment planning, collection of empirical data, measurable outcomes, exceedance conditions, event conditions, consumer validation and regulatory reporting. This is overlaid in a systems approach to identify and target areas that need investment the most. The Inspectorate will continue to work further with water companies and other relevant stakeholders including regulators to improve the development of sufficiency risk assessments and reporting pathways.

RAG Summary (2025)

RAG Submission – The RAG assessment is a self-assessment tool, to report compliance with the direction over 33 outcomes covering the whole of SEMD.

Overview of RAG Movements – Since 2022, the companies red and amber self-classifications have increased, as expected in the initial years of assessment following regulator feedback from events and audits and peer to peer benchmarking.

Figure 63 – RAG outcomes

Notable RAG outcomes in 2025 include:

Outcome 3.2: Alternative water – Most companies reported an Amber position for alternative water in 2025. Common weaknesses included insufficiently adjusted stockpiles, challenges deploying static tanks, and over‑reliance on mutual aid, which is not always available in practice. Events during the year demonstrated that alternative water provision can be constrained by location and accessibility rather than scale alone, and that supply chains are vulnerable during concurrent weather-related incidents. The Inspectorate reiterates that companies remain responsible for all customers off supply and must ensure plans are deployable under reasonable worst‑case scenarios.

Outcome 3.3: Vulnerable customers – This remains a persistent area of companies reporting non‑compliance within the RAG. During incidents, companies frequently apply an overly narrow definition of vulnerability and do not consistently prioritise all customers who meet the emergency planning guidance (EPG) definition. While some good practice was observed, companies must ensure that all vulnerable customers, including those beyond tiered priority models, receive appropriate support within required timescales and that prioritisation arrangements are clearly defined and evidenced.

Outcome 3.9: Communication – Although not always reflected in RAG self‑assessments, another area that has been noted by the Inspectorate as requiring improvement is communication during live incidents. The Inspectorate continues to identify significant shortcomings in incident communications. Events reviewed in 2025 show that customers often receive infrequent or vague updates, contributing to confusion and frustration. The Inspectorate reviewed an event where in the absence of updates from a company, social media rumours took over and people were queueing at bottled water stations that had not been announced, opened or even stocked. Companies are expected to provide timely, accurate updates during incidents, including minimum likely restoration times and clear information on alternative water provision, in line with EPG requirements for 24/7 accessibility and responsiveness.

Outcome 4.4: External suppliers – The majority of companies assessed this outcome as Amber, reflecting limited visibility and assurance over extended supply chains, particularly subcontractors and overseas manufacturers. While contractual provisions are commonly in place, many companies report they lack effective mechanisms to test compliance in practice. Companies reporting stronger positions demonstrate clearer contractual expectations, routine reviews, and consistent application of security requirements across all suppliers and subcontractors.

Outcome 4.8: Personnel security – Requirements for role-based risk assessments have been in place since 1 March 2022, with guidance further reinforced following industry consultation in February 2024, and subsequently updated in protective security guidance (PSG). On 3 April 2024, the Inspectorate notified companies that audit evidence for personnel security procedures and measures would be required by 1 April 2025. Water companies have therefore received significant notice and time to advance their Personnel Security (PERSEC) measures. 

Analysis by the Inspectorate indicated a trend among companies to shift from a Green rating to Amber or Red on the outcomes associated with the audit topic. Based on the companies’ self-assessments of this audit topic, 10 companies reported a deterioration in their RAG (Red-Amber-Green) positions. This pattern suggests that most companies are overestimating their compliance with the RAG guidance. The Inspectorate therefore issued an industry-wide recommendation for companies to undertake a rigorous and candid evaluation of their risk profiles as part of the RAG self-assessment process. 

Analysis of audit evidence by the Inspectorate found that companies reporting a decline in outcome 4.8 have highlighted a range of vulnerabilities, including: 

A shifting risk environment and evolving threat landscape.

Weak key controls, with risks linked to lost and non-returned keys.

Absence of sufficient mover checks for internal transitions into sensitive roles.

Limited documentation and reliance on ad hoc local procedures.

No formal aftercare policy for personnel security.

Backlog, incomplete or no role-based risk assessments.

Inadequate personnel security checks for contractors.

Events and Incidents

2025 saw companies reporting a variety of security events as well as some large loss of supply events. Events reported included break-ins at works including attempted cable theft at one Critical National Infrastructure (CNI) site. Following the personnel security audit of companies as part of the RAG, this subsequently manifested in several events reported around how a water company manages its personnel security of staff as well as information handling of sensitive data.

One company in particular, suffered from a series of loss of supply emergency planning events, with the company attributing the cause to hot weather. The Inspectorate expects companies to have plans in place to continue to supply at all times as required under paragraph 4(a) of the Direction states that plans for water supply must be prepared on the basis that the company must (a) continue to carry out (i) all of its water supply functions.

Critical National Infrastructure fuel theft

This incident involved unauthorised access to a CNI site to steal fuel from alternative water supply tankers which in turn allowed access to parts of the treatment process, although no treatment interference took place.

The site’s perimeter and gates suffered no damage during the theft. In the days before the theft, the main gate’s combination padlock had been replaced due to wear and tear, but the same combination was retained. It was unclear from the Inspectorate’s assessment if the combination was known to the hostiles.

The company did not initially recognise this event as a serious incident because some operational assets were outside the inner boundary enclosure. Although internal escalation did occur, the company is updating its reporting processes.

Following the incident, the company undertook 24/7 guarding with strict access checks and regular patrols of both the perimeters and operational assets outside the inner fence. Longer‑term measures are now in place.

Serious organised crime

In January 2025, a water company reported a security incident at a CNI site involving attempted metal theft. Intruders accessed the site via adjacent farmland while the control room operator was undertaking site rounds. Although alarms were triggered, no access was gained.

Following the incident, the company implemented a range of mitigations. A perimeter fence upgrade was commissioned, and staff were briefed on intruder risks and escalation requirements. The Inspectorate determined that the seriousness of the event warranted a targeted audit of the site, which was undertaken on 19 March 2025.

Company	Water treatment works	Service reservoirs	Consumer taps	Number of tests per company
AFW	67,120 (91)	32,183 (160)	90,660 (92)	189,963
ALB	–	–	963 (3)	963
ANH	132,163 (129)	76,153 (311)	176,220 (293)	384,536
AWI	0 (0)	0 (0)	1,698 (19)	1,698
BRL	17,964 (13)	37,587 (151)	34,476 (40)	90,027
CAM	14,714 (18)	5,749 (30)	9,201 (27)	29,664
ESP	–	–	10,661 (37)	10,661
ICW	–	–	17,537 (127)	17,537
IWN	–	–	38,221 (214)	38,221
LNW	–	–	19,509 (99)	19,509
MTX	–	–	239 (1)	239
MUA	–	–	1,132 (12)	1,132
NES	59,130 (53)	70,611 (304)	136,461 (168)	266,202
PRT	20,204 (16)	8,136 (32)	20,530 (15)	48,870
SES	12,582 (7)	7,088 (34)	20,072 (29)	39,742
SEW	73,433 (83)	55,078 (226)	80,069 (91)	208,580
SRN	75,616 (84)	56,867 (232)	92,724 (159)	225,207
SST	21,117 (20)	6,290 (30)	38,485 (49)	65,892
SVT	110,341 (126)	88,801 (437)	253,161 (345)	452,303
SWB	52,886 (44)	73,333 (282)	88,747 (94)	214,966
TMS	93,392 (94)	77,875 (386)	264,939 (349)	436,206
UUT	98,937 (85)	86,254 (342)	200,482 (315)	385,673
VWP	811 (2)	1,252 (6)	597 (1)	2,660
WSX	49,857 (66)	81,410 (317)	47,734 (143)	179,001
YKS	86974 (55)	88,526 (349)	214,419 (185)	388,919
Total				3,608,411

Parameter	Standard	Total number of tests	Number of tests not meeting the standard	Number of tests not meeting the standard per company	Number due to consumer distribution systems	Percent compliance
Coliform bacteria (indicator)	0 number/100 ml	156,979	366	BRL(9) SST(5) TMS(63) LNW(5) NES(33) SWB(11) AFW(15) MUA(1) WSX(10) CAM(1) SRN(13) SEW(9) YKS(50) ICW(5) PRT(3) ANH(21) UUT(44) IWN(11) SVT(59)	260	99.767%
E. coli (confirmed)	0 number/100 ml	156,951	26	ANH(1) SRN(1) PRT(1) AFW(4) TMS(7) SEW(1) NES(2) IWN(1) YKS(1) SVT(3) UUT(4)	21	99.983%
Enterococci (confirmed)	0 number/100 ml	14,084	8	BRL(1) YKS(2) SWB(2) AFW(2) NES(1)	5	99.943%
Total		328,014	400		286	99.878%

Company	E. coli in water leaving treatment works	E. coli in water leaving service reservoirs	E. coli at consumers’ taps	Enterococci at consumers’ taps
AFW	0 – 13,272	0 – 8,047	4 – 9,863	2 – 697
ALB	0 – 0	0 – 0	0 – 29	0 – 15
ANH	0 – 21,271	0 – 15,213	1 – 13,243	0 – 1281
AWI	0 – 0	0 – 0	0 – 27	0 – 24
BRL	1 – 2,977	0 – 7,522	0 – 3,212	1 – 230
CAM	0 – 2869	0 – 1,435	0 – 960	0 – 61
ESP	0 – 0	0 – 0	0 – 336	0 – 119
ICW	0 – 0	0 – 0	0 – 732	0 – 271
IWN	0 – 0	0 – 0	1 – 2,355	0 – 797
LNW	0 – 0	0 – 0	0 – 986	0 – 342
MTX	0 – 0	0 – 0	0 – 12	0 – 4
MUA	0 – 0	0 – 0	0 – 37	0 – 12
NES	1 – 9,927	0 – 15,207	2 – 11,973	1 – 919
PRT	0 – 3,346	0 – 1,628	1 – 1,969	0 – 119
SES	0 – 2,097	0 – 1,772	0 – 1,920	0 – 176
SEW	0 – 12,145	1 – 11,019	1 – 5,746	0 – 568
SRN	0 – 12,443	0 – 11,296	1 – 6,917	0 – 564
SST	0 – 4,116	0 – 1,572	0 – 3,553	0 – 223
SVT	0 – 21,861	0 – 22,130	3 – 22,843	0 – 1727
SWB	0 – 8,586	0 – 14,641	0 – 5,992	2 – 546
TMS	1 – 17,802	0 – 19,468	7 – 26,211	0 – 2050
UUT	0 – 16,354	0 – 17,255	4 – 19,875	0 – 1816
VWP	0 – 159	0 – 313	0 – 48	0 – 8
WSX	0 – 8,199	0 – 16,276	0 – 3,746	0 – 515
YKS	0 – 14,127	3 – 17,705	1 – 14,364	2 – 1000

Company Code	Site Name	Parameter Name	Sample Date	Inspector Assessment	Company Corrected CRI
AFW	Iver	Coliform bacteria	2025/12/08	Recommendations made	2.929
TMS	Coppermills	Coliform bacteria	2025/09/15	Recommendations made	2.486
SRN	Otterbourne WSW	Coliform bacteria	2025/12/05	Covered by Legal Instrument	1.751
IWN	Rutland Grafham	E. coli	2025/08/11	Recommendations made	1.677
BRL	Purton Treated	Clostridium Perfringens	2025/04/16	Satisfactory investigation, did not identify cause	1.233
ANH	Wing WTW	Coliform bacteria	2025/07/28	Suggestions made	1.123
BRL	Clifton, Horfield, Redland And Kingsdown	Aluminium	2025/04/22	Covered by Legal Instrument	0.932
NES	Horsley WTW	Pesticides Propyzamide	2025/11/21	Covered by Legal Instrument	0.832
BRL	Whitchurch, Hengrove, Knowle And Knowle Park	Iron	2025/04/23	Covered by Legal Instrument	0.762
IWN	Rutland Grafham	Coliform bacteria (zone)	2025/01/17	Recommendations made	0.671
IWN	Rutland Grafham	Coliform bacteria (zone)	2025/08/11	Recommendations made	0.671
YKS	Langsett No.2 WTW	Coliform bacteria	2025/04/05	Covered by Legal Instrument	0.641
AWI	Audley End Wsz	Nickel	2025/09/26	Recommendations made	0.613
AWI	Audley End Wsz	Lead	2025/09/26	Recommendations made	0.613
SST	Cookley Pumping Station	Coliform bacteria	2025/02/16	Recommendations made	0.573
YKS	Headingley No.2 WTW Outlet 3	Coliform bacteria	2025/02/01	Covered by Legal Instrument	0.508
TMS	Swinford	Coliform bacteria	2025/07/07	Enforcement Considered	0.491
PRT	Northbrook Tw	Turbidity (WTW)	2025/05/18	Unlikely to Recur	0.450
SEW	Folkington Water Supply Zone	Iron	2025/09/12	Covered by Legal Instrument	0.445
SEW	Folkington Water Supply Zone	Iron	2025/12/08	Covered by Legal Instrument	0.445
ANH	Pitsford WTW (Pitsford Reservoir)	Turbidity (WTW)	2025/02/26	Recommendations made	0.341

Company	Event name	Cause of event	Assessment score	ERI
ANH	Heigham WTW	Other – Specify Below: Breach of regulation 31	2	12,225.087
SST	Hampton Loade – Elevated Geosmin	Treatment failure upstream of final disinfection	4	8,463.380
SST	Hampton Loade Main E – Enterococci	Microbiological Contamination – Treatment works	3	884.591
SRN	Tenants Hill WSR Potential Ingress	Structural failure – Service reservoir	4	600.692
TMS	Ashford WTW Bromate Issue	Treatment failure upstream of final disinfection	3	443.492
TMS	Cryptosporidium detection at Kempton park WTW	Cryptosporidium in Final Water	3	357.440
YKS	Chellow Heights WTW – bacti failure	Microbiological Contamination – Treatment works	4	167.468
SRN	Fairlight Old WSR Ingress	Structural failure – Service reservoir	4	144.261
SRN	Darland Chatham WSR West South Cell ingress	Structural failure – Service reservoir	4	74.582
ANH	Winterton Holmes WTW – Hardness	Other – Specify Below: Water hardness	3	73.694
NES	Wear Valley Network Discolouration	Plant failure – Specify Below: PLC fault causing a shutdown and a faulty valve in the network not closing	2	47.983
SEW	Kippings Cross 1&2 SR ROV Ingress	Structural failure – Service reservoir	2	43.758
YKS	Chellow Heights WTW Operational E. coli	Microbiological Contamination – Treatment works	4	41.435
TMS	Cryptosporidium detection at Dorney Taplow WTW	Cryptosporidium in Final Water	4	40.678
SEW	Polegate/Eastbourne Loss of Supply	Mains problem/ damage – Mains – Burst	3	27.446
AFW	Detection of Enterococci in a Sample from Clay Lane 36″ WTW	Other – Specify Below: Root cause not identified, however the company has not yet completed its investigation (internal inspection of backwash tanks required).	3	25.908
SRN	Yew Hill WSR Enterococci Detection	Structural failure – Service reservoir	4	23.193
SST	Multiple Coliform detections Penkridge Area	Microbiological Contamination – Distribution	3	21.819
TMS	Burst Main – 36” strategic main SE16	Mains problem/ damage – Mains – Burst	1	21.575
SEW	Sevenoaks Loss of Supplies / Media Interest	Plant failure – Failure – Power	3	21.384
SRN	Easton WSW Nitrate above PCV	Raw water deterioration	3	20.348
ANH	Welton WTW – post UV Enterococci detection	Microbiological Contamination – Treatment works	4	19.031
SWB	Dousland WTW – Loss of Supply	Plant failure – Specify Below: Raw water minas burst following comms failure.	4	17.494
TMS	Restriction of Use (Do Not Drink or Cook) issued to a property in London, EN7	Planned work – Company staff	3	17.260
NES	Saxmundham discolouration	Planned work – Company staff	3	16.937
WSX	Bourton WTC 2,4-D and MCPA	Ground Contamination / Spillage – Pesticide	3	16.690
AWI	Beaulieu Park E. coli	Other – Specify Below: Failing sample was collected from a hydrant by a non-accredited sampler. Sampling environment was a new housing development, and it is likely the sample was contaminated during collection.	3	15.528
TMS	Cryptosporidium detection at Dorney Taplow WTW	Cryptosporidium in Final Water	3	15.086
SEW	Trosley LOS	Plant failure – Failure – Disinfection System	2	11.356
UUT	Oswestry WTW Power Outage	Plant failure – Failure – Disinfection System	4	11.222
ANH	Stonewall Reservoirs – repeat coliforms		4	10.350
SWB	Colyton and Musbury Taste & Odour	Satisfactory investigation, did not identify root cause	3	10.293
SWB	Alderney Cryptosporidium Detection	Deliberate Sabotage	1	10.122
NES	S213 Mill Hill Outlet Discolouration	Emergency Work	3	9.338
SEW	Cottage Hill Discolouration	Planned work – Company staff	4	9.133
SEW	Hawkhurst Discolouration Event	Planned work – Company staff	3	8.760
UUT	Central Manchester discoloured water	Planned work – Company staff	3	8.679
YKS	Embsay WTW enterococcus	Other – Specify Below: No root cause found- investigation not satisfactory	3	8.024
ICW	Welsh Water Outage – Deeside	Mains problem/ damage – Mains – Burst	3	7.536
BRL	Chelvey Raw PFAS Tier 3 detection (Sum of 48)	Raw water deterioration	4	7.277
UUT	Winsford DW	Mains problem/ damage – Mains – Burst	4	7.199
SEW	Malders No.2 SR Ingress	Structural failure – Service reservoir	4	6.052
TMS	Burst 18 inch main in Crystal Palace, London	Mains problem/ damage – Mains – Burst	3	5.981
UUT	Ashworth Moor WTW – Chlorate	Structural failure – Treatment works	2	4.332
ICW	No Water Event	Mains problem/ damage – Mains – Damage by Third Party/Unknown	1	4.134
AWI	Failure to report sample result -ZAWIN037	Other – Specify Below: Failure to report a regulatory sample result due to miscommunication with analytical provider	3	3.662
SRN	Upper Enham WSR Enterococci Detection	Structural failure – Treatment works	1	3.569
SES	Godstone Air	Planned work – Company staff	2	3.486
YKS	Embsay WTW enterococcus	Other – Specify Below: No root cause found- Investigation not satisfactory	3	3.482
NES	Water Quality Zone T201 Discolouration	Plant failure – Specify Below: Electrical fault on coagulation dosing MCC	3	3.466

Parameter	Standard	Total number of tests	Number of tests not meeting the standard	Number of tests not meeting the standard per company	Percent compliance (3dp)
Clostridium Perfringens (sulphite-reducing clostridia) (confirmed)	0 number/100 ml	22,563	4	BRL(1), IWN(1), SRN(1), SST(0), SWB(1)	99.982%
Turbidity – indicator	1 NTU	171,559	11	ANH(2), PRT(1), SRN(2), SVT(1), SWB(1), TMS(4)	99.994%
E. coli (confirmed)	0 number/100 ml	171,595	3	BRL(1), NES(1), TMS(1)	99.998%
Total coliforms (confirmed)	0 number/100 ml	171,667	52	AFW(3), ANH(4), BRL(4), NES(3), SEW(5), SRN(6), SST(2), SVT(4), TMS(10), UUT(5), WSX(1), YKS(5)	99.970%
Total		537,384	70

Parameter	Standard	Total number of tests	Number of tests not meeting the standard	Number of tests not meeting the standard per company	Percent compliance
E. coli (confirmed)	0 number/100 ml	182,507	4	YKS(3) SEW(1)	99.998%
Total coliforms (confirmed)	0 number/100 ml	182,576	75	SEW(8) TMS(3) YKS(25) SVT(4) NES(4) CAM(1) UUT(8) AFW(1) SWB(4) BRL(1) WSX(5) SRN(5) ANH(6)	99.959%
Total		365,083	79

Parameter name	Standard	Total number of tests	Tests not meeting the standard	Tests not meeting the standard per company
1,2-Dichloroethane	3 µg/L	7,857	0
Benzene	1 µg/L	7,833	0
Pesticides (total by calculation)	0.5 µg/L	3,455	0
Trichloroethene & Tetrachloroethene – sum of 2 substances	10 µg/L	11,156	1	ESP (1)
Tetrachloromethane	3 µg/L	11,252	0
Benzo[a]Pyrene (total)	0.01 µg/L	13,644	1	SVT (1)
Polycyclic aromatic hydrocarbons (total by calculation)	0.1 µg/L	13,886	3
Trihalomethanes (total by calculation)	100 µg/L	14,175	0
Aluminium	200 µg/L Al	52,398	10	BRL (1), NES (4), UUT (5)
Arsenic	10 µg/L As	13,554	0
Bromate	10 µg/L BrO₃	10,026	1	ESP (1)
Cadmium	5 µg/L Cd	13,558	0
Cyanide	50 µg/L CN	3,654	0
Chromium	50 µg/L Cr	13,494	0
Iron	200 µg/L Fe	53,039	98	NES(9) SWB(7) AFW(1) SRN(3) SST(1) TMS(6) SEW(7) SVT(15) ANH(6) IWN(1) BRL(2) YKS(20) LNW(1) UUT(18) WSX(1)
Mercury	1 µg/L Hg	3,659	0
Manganese	50 µg/L Mn	52,918	18	NES(3) WSX(1) SVT(2) UUT(7) SWB(3) LNW(1) BRL(1)
Nickel	20 µg/L Ni	14,081	72	WSX(3) BRL(4) YKS(7) NES(2) TMS(20) AWI(2) IWN(3) SES(2) UUT(3) LNW(5) SST(1) ANH(3) ESP(3) SVT(6) MUA(1) AFW(3) SEW(1) SRN(3)
Lead	10 µg/L Pb	14,120	60	YKS(6) SST(3) VWP(1) SWB(1) AFW(6) NES(5) SRN(3) SEW(3) IWN(2) AWI(2) UUT(7) WSX(1) ANH(2) ICW(1) SVT(4) TMS(13)
Antimony	5 µg/L Sb	13,474	0
Selenium	10 µg/L Se	13,100	0
Electrical conductivity	2500 µS/cm at 20°C	48,731	0
Gross beta	1 Bq/L	1,254	0
Tritium	100 Bq/L	929	0
Gross alpha	0.1 Bq/L	1,257	15	ESP(4) LNW(1) IWN(10)
Odour	Acceptable to consumers and no abnormal change	57,660	48	UUT(4) TMS(1) ICW(1) ESP(1) NES(10) BRL(2) SRN(7) SWB(4) ANH(8) AFW(1) YKS(3) SST(2) SVT(5)
Taste (quantitative)	Acceptable to consumers and no abnormal change	57,502	72	SRN(6) UUT(5) SVT(6) SWB(8) YKS(5) PRT(1) ESP(1) IWN(6) ANH(17) TMS(6) NES(7) WSX(3) BRL(2)
Residual disinfectant – free	2 mg/L	112,910	0
Residual disinfectant – total	2 mg/L	134,171	0
Boron	1 mg/L B	7,114	0
Copper	2 mg/L Cu	13,785	2	ANH(1) WSX(1)
Fluoride	1.5 mg/L F	10,334	0
Sodium	200 mg/L Na	14,071	1	NES (1)
Ammonium (zone)	0.5 mg/L NH₄	39,132	0
Nitrite – consumers taps	0.5 mg/L NO₂	24,557	5	TMS(4) NES(1)
Nitrate	50 mg/L NO₃	24,518	5	ANH(1) IWN(1)
Colour	20 mg/L Pt/Co	57,066	0
Sulphate	250 mg/L SO₄	7,738	0
Total indicative dose (gross alpha and beta)	0.1 mSv/year	15	0
Hydrogen ion (pH) – zone	6.5 – 9.5 pH Value	57,141	0
Total		1,023,218	410

Pesticide name	Standard	Total samples	Total failures
Aldrin	0.1 µg/L	4237	0
Propyzamide	0.1 µg/L	5927	2
Dieldrin	0.1 µg/L	4790	1
Bentazone	0.1 µg/L	5877	0
Total			3

Dedication

Glossary

Figure 1 – Water industry compliance dashboard

Key facts

Key facts for private water supplies

Inset appointments

Sampling shortfalls

Compliance Risk Index by company

Figure 3 – 5 year CRI

Figure 4 – 5 year CRI

Figure 5 – Company CRI scores

Figure 6 – CRI scores by parameter

Microbiological failures

E. coli at treatment works

Bristol Water – Shipton Moyne

Northumbrian, Essex and Suffolk Water – Coldfair Green

Thames Water – Lullingstone treatment works

E. coli at reservoirs

South East Water – Broader lane

Yorkshire Water – Somerset Avenue

Yorkshire Water – Rudston

Yorkshire Water – Drighlington

Coliforms at reservoirs

Yorkshire Water – Scar House suction tank

South East Water – Cuckfields reservoir

Affinity Water Gunters Hill tower

Anglian Water – Pitsford treatment works

Anglian Water – West Pinchbeck treatment works

Nitrate

Lead

Iron, manganese and aluminium

Trichloroethene and tetrachloroethene

Bromate

Data integrity and reporting errors

Nickel

Nitrite

Copper

Wessex Water – Copper

Anglian Water – Copper

Sodium

Benzo(a)pyrene

Taste and Odour

Anglian Water

United Utilities

South West and Bournemouth Water

Portsmouth Water

Thames Water

ESP Utilities

Propyzamide

Dieldrin

Figure 7 – Reservoir inspection data

Figure 8 – assets not inspected within past 10 years

Events relevant to asset health in 2025

Anglian Water – Stonewall reservoir 2A

Discoloured water

Northumbrian Water

Figure 9 – Company discolouration trend

Severn Trent Water

Figure 10 – Company discolouration trend

Southern Water

Figure 11 – Company discolouration trend

South East Water

Figure 12 – Company discolouration trend

South West Water (including Bristol Water)

Figure 13 – Company discolouration trend

United Utilities

Figure 14 – Company discolouration trend

Wessex Water

Figure 15 – Company discolouration trend

Yorkshire Water

Figure 16 – Company discolouration trend

Company performance

Figure 17 – England vs industry BBO contact rate

Figure 18 – all companies BBO contact rate

Taste and odour

Figure 19 – Taste & Odour contact rate

Figure 30 – Water quality events in 2025

Figure 31 – Number of water quality events

Figure 32 – ERI since 2021, this is a sum of the raw score divided by the total population of each company

Event risk index by company