- Drinking Water 2024 – Summary of the Chief Inspector’s report for drinking water in England
- Foreword
- Water supplies and testing
- Compliance with water quality standards
- Water quality events
- Asset health and service reservoir integrity
- Consumer contacts
- Water safety planning and risk assessment
- Perfluoroalkyl and polyfluoroalkyl substances (PFAS)
- Audit programme completed by the Inspectorate
- Enforcement, transformation and recommendations
- Lead in water
- Materials in contact with drinking water (Regulation 31)
- Security and Emergencies (SEMD)
- Network Information systems (NIS)
- Research publications
- Raw water data
- Whistleblowers
- Working with stakeholders
- Annex A – Number of tests carried out by companies
- Annex B – Compliance with standards
- Annex C – Compliance failures and events
Asset health and service reservoir integrity
During 2024, there were 80 compliance failures for coliforms at water treatment works and 122 at reservoirs. There were three E. coli detections at treatment works and seven at reservoirs from a total of 3,631,092 tests. Ingress into contact tanks and service reservoirs is a recurrent problem and represents a residual risk as these results show.
In addition to these compliance breaches, there were 38 water quality events attributable directly to poor asset health and plant failure, including seven failures of the disinfection system and 12 structural failures of the treatment works, tanks or reservoirs. The expectation is that companies understand and maintain their assets through a programme of physical inspections, which may be supplemented with inspections by remotely operated vehicles (ROV). The Inspectorate considers physical inspections as being necessary because they provide clarity and better resolution than ROVs and allow for cleaning of walls and structures within the reservoir. Whilst 10 years is the maximum interval advised in the Principles of Water Supply Hygiene on treated water storage, the Inspectorate recommends more regular inspections are carried out on on a risk-based frequency.
Over the past five years the Inspectorate has increased scrutiny of service reservoirs and tanks through the service reservoir and tanks data return to ensure the quality of inspections and the robustness of remedial works. Analysis of the new data set highlighted gross failings around frequencies of inspections with 233 tanks not having been inspected for over 10 years. This represents an average of 3.1% compared to 2023’s 4.9% and 2022’s 6%, this is a significant improvement in the industry picture but reservoirs still represents a continuing risk requiring attention.
Figure 13 and table 19 – number of service reservoirs per company and the percentage overdue for inspection
Despite its low numbers of assets Portsmouth water has 17% of reservoirs which have not been inspected within the required frequency
Events relevant to asset health in 2024
Southern Water – Dunkirk Break Pressure tank ingress
In October Southern Water undertook a remote operated vehicle (ROV) survey at Dunkirk break pressure tank. The survey was a reinspection following a previous one in March 2024, to identify any new anomalies or potential points of ingress. Dunkirk break pressure tank is an asset which cannot be removed from supply for physical inspection and as such is covered by a regulation 28(4) notice which includes steps for the company to make the necessary enabling works and undertake ROV surveys as an interim measure.
The ROV identified suspected ingress in two places on the roof/wall joint. A sample taken on the same day from the tank reported a single coliform. The company applied overbanding externally to the areas of suspected ingress and have arranged for enhanced sampling and 3 monthly ROV inspections until the reservoir is able to be removed from service. A bypass needs to be installed to enable removal from service however, this has been delayed due to installation and commissioning of enabling works. The company expects to be able to complete this in May 2026, and will be installing a new roof membrane as a short term mitigation measure.
Thames Water – Animal Ingress in Farnborough Service Reservoir
A routine inspection completed by the company of Farnborough service reservoir on 14 March subsequently identified five rodent carcasses, the tank was isolated in advance of the inspection on 6 March. The company’s investigation concluded the most likely route of entry to be breaks in the ventilation system below the roof membrane level. The service reservoir supplies a maximum population of 209,468. A review of downstream consumer contacts the month prior to identification of the ingress confirmed no illness complaints. Sample results from routine monitoring programme in addition to samples from downstream locations did not detect the presence of E. coli or coliforms in the period preceding the removal of the tank from supply.
Southern Water – Baldslow service reservoir ingress
Following a consumer complaint of illness in April 2024 Southern Water undertook investigative sampling, including collecting samples from the supplying service reservoir. Although the sample at the consumers property did not contain any bacteria, however coliforms were reported in a sample taken from the upstream Baldslow service reservoir. The reservoir was removed from supply and ingress was identified. The points of ingress were repaired and a new roof membrane was installed in July before being returned to service.
Southern Water – Service reservoir
In May, a Southern water service reservoir in Sussex was removed from service for a routine flood test and inspection. The service reservoir had previously been inspected in January 2016 with no issues identified at the time. Two leaks were identified around the access covers, and a further inspection identified root ingress from trees in a neighbouring property. The company removed the cell from supply to enable the repairs to be completed and the cell was returned to supply in July following the completion of the works.
Southern Water – Wigmore 2 service reservoir ingress
In April, the company undertook a ROV survey and drop test of Wigmore 2 service reservoir ahead of a planned drain and clean. The ROV identified possible hairline cracking on the roof, and the drop test showed evidence of leakage. Due to a lack of flowmeters, the company could not confirm if the drop in level was due to a valve letting by or due to structural issues. The service reservoir was removed from supply and a flood test conducted, which confirmed multiple points of ingress. The Inspectorate reminded the company of the role of flowmeters in identifying uncontrolled or unmitigated flows and suggested the company identifies treated water storage assets where flow meters are not present and develops plans to install them.
Southern Water – Brede works Harland Tank potential ingress
in July 2023 an ROV survey was undertaken on the Harland tank which is pump suction tank at Brede works. The inspection identified potential points of ingress however the findings from the survey were not reported or escalated at the time. The findings from this survey were escalated in January 2024 after a coliform detection in an operational sample taken from Brede works. The Brede Harland tank requires enabling work to remove the tank from supply as it contains the backwash water pumps. The company has put in place short term measures including enhanced monitoring and the Inspectorate issued a regulation 28(4) notice following this event so that the company has defined short term measures and can plan and undertake the required remedial works to be able to remove the asset from supply. The Inspectorate also recommended to the company that the include defined escalation steps and procedures following any issues identified during ROV surveys.
Thames Water – Potential Ingress at Hampton works
A remotely operated vehicle (ROV) inspection of the contact tank outlet chamber was conducted. The inspection identified ingress of rainwater on roof joints. In response to the identified ingress Thames Water repaired the external membrane around upstands of kiosk buildings above the chamber. Sump pumps were also installed to displace accumulating rainwater. A scaffolding structure was constructed over the outlet chamber building to further protect against rainwater. Improved drainage channels were also installed.
South Staffs Water – Overpark service reservoir east
E. coli was detected in the east compartment of this two celled reservoir in July. The east compartment was subsequently removed from supply. Whilst the internal inspection identified no ingress, the company’s investigation identified an increased risk of aged water returning to reservoir from the network based upon demand. Under normal circumstances, booster chlorination is used to maintain sufficient residual disinfectant to this part of the water supply network, however, the dosing facilities tripped several times in the days before the E. coli detection, but alarms were not sent to the control room due to communication issues. These circumstances occurred before in June 2023 following a microbiological failure at Overpark service reservoir west compartment. Further work is required by the company to improve the turnover of water in this reservoir to prevent a repeated occurrence.
Northumbrian Water – Elford Reservoir
An operational large volume sample taken on 31 May 2024 from the SR contained 3 E. coli/1000 mL. Elford SR is included in NES-2020-00011 Tanks and Service Reservoir notice and the large volume sample was taken as required by this notice. The SR was removed from supply on 1 June 2024, four hours after the presumptive E. coli was reported to the company.
A dip sample was taken from the SR on 1 June 2024 and a sample from the sample tap prior to the SR being removed from supply. These samples contained E. coli and Enterococci. This sampling was repeated on 2 June 2024. All samples were analysed for standard microbiological parameters and Clostridium Perfringens and Enterococci and were satisfactory for all parameters, except from the isolated SR which again confirmed the presence of Enterococci.
An external audit was carried out on 3 June 2024. Air valves upstream of the SR were checked as part of the investigation and were found to be satisfactory. There was no evidence of poor drainage, however parts of the embankment had slipped, and the reservoir structure was visible.
An internal inspection and flood test of the isolated SR was carried out on 5 June 2024. Ingress was found on a joint between the roof and wall structure, in a location with external embankment slippage.
Issues with the overbanding were also found. Following the inspection, the embankment was backfilled and the overbanding was either repaired or replaced. The external waterproof membrane on both upstands for the access hatches was replaced following ingress detection during a second flood test and both access hatches have been lifted and resealed.
Anglian Water – Boughton Reservoir
An internal monitoring sample collected from Boughton Reservoir on 24 October 2024, was reported to contain four confirmed coliforms in a 100 mL sample. Further coliform detections were reported from Boughton Reservoir in both large volume 10 L samples and in routine 100 mL sample.
A remotely operated vehicle (ROV) inspection of Boughton Reservoir was completed in July 2024 (before the coliform detections) and repeated on 28 October 2024, with no changes or issues noted between the two inspections. Boughton Reservoir was removed from supply on 17 December 2024 and was internally inspected on 21 January 2025. The full internal inspection found six points of ingress at roof to wall joints, the western roof expansion joint and 17 wall joints; along with other integrity issues requiring remedial works.
ROV inspections alone may not be sufficient to identify all integrity issues—Boughton Reservoir, despite recent ROV assessments, had undetected ingress points that contributed to water quality failures. This underpins the importance of periodic full physical internal inspections.