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.

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