Table of contents

• 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

Audit programme completed by the Inspectorate

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

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

Scope and approach of the audit programme

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

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

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

Featured audits

Yorkshire Water – Headingley treatment works 

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

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

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

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

Figure 40 – Unprotected GAC media and without clear status identification and there was clear vegetative growth coming from the media

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

Bristol Water – Barrow treatment works audit

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

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

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

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

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

Northumbrian, Essex and Suffolk Water – Ormesby treatment works audit

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

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

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

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

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

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

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

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

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

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

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

Independent Water Networks Limited – Data systems audit 

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

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

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

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

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

Bottled water storage and emergency arrangements 

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

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

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

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

Figure 41 – Bottles stored on a lorry, not in a temperature controlled environment

Figure 42 – Bottles stored outside store at sewage works

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

Figure 43 – The small, bottled water store at Fleam Dyke – Cambridge Water

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

Figure 44 – Tidy store and temperature monitoring equipment at Tyttenhanger – Affinity Water

Figure 45 – Tidy store and temperature monitoring equipment at Tyttenhanger – Affinity Water

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

Figure 46 – Bottles under tarpaulin at Falmer

Figure 47 – The bottled water store at Timsbury – Southern Water

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

Figure 49 – Vermin access to bottled water store at Isleham works

Figure 50 – Bespoke contractor run bottled water facility – Anglian Water.

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

Figure 51 – Normanton store, good stock control

Figure 52 – Dirty storage conditions at Normanton

Figure 53 – Small Sand Hutton store – Yorkshire Water

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

Figure 54 – Bottled Water storage in close proximity to other items

Figure 55 – Poor chemical storage – South East Water

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

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Operation and condition of works 

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

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

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

Management of raw water sources and catchments 

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

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

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

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

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

Cross‑industry themes and learning 

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

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

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

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