- Drinking Water 2025 – Summary of the Chief Inspector’s report for drinking water in England
- Foreword
- Water supplies and testing
- Compliance with water quality standards
- Learning from microbiological failures
- Learning from turbidity failures
- Learning from chemical, taste and odour failures
- Learning from pesticide failures
- Asset health and service reservoir integrity
- Consumer contacts
- Drinking water quality events
- Water safety planning and risk assessment
- Perfluoroalkyl and polyfluoroalkyl substances (PFAS)
- Audit programme completed by the Inspectorate
- Air valve audits
- Enforcement, transformation and recommendations
- Materials in contact with drinking water (regulation 31)
- Security and Emergency Measures Direction - SEMD
- Network and Information Systems - NIS
- Research publications
- Whistleblowers
- Working with stakeholders
- Annex A – Number of tests carried out by companies
- Annex B – Compliance with standards
- Annex C – Compliance failures and events
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.
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 |
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.