Thames Water Utilities Limited – Dorney Taplow Works Cryptosporidium
Notice under regulation 28(4) of the Water Supply (Water Quality) Regulations 2016 (as amended)
Reference: TMS-2025-00007
Version Number: 1
Water undertaker as appointed under section 6 of the Water Industry Act 1991 (as amended): Thames Water Utilities Limited, with registered number 02366661 (hereinafter called ‘the company’).
Assets Affected:
- Dorney / Taplow Works T03U.318
- Aylesbury Supply system YTMS1170
- Chinnor Supply system YTMS1179
- Slough Supply system YTMS1221
- Tring Supply system YTMS1229
- Wycombe Supply system YTMS1235
Details of this Notice:
- The Drinking Water Inspectorate (‘the Inspectorate’) has received a regulation 28(1) report of the Water Supply (Water Quality) Regulations 2016 (as amended) (‘the Regulations’) from the company dated 19 May 2025, which states that there is or has been a significant risk of supplying water from Dorney/Taplow Works (and associated assets as applicable) that could constitute a potential danger to human health or could be unwholesome.
- Consequently, this Notice is served on the company by the Inspectorate on behalf of the Secretary of State, under regulation 28(4) for the assets listed above, for risks associated with:
Description of Risk | Hazard/ Hazardous Event/ Parameter | Hazard I D |
|---|---|---|
Inadequacy of the treatment process to prevent Cryptosporidium entering supply. | C111 –Cryptosporidium oocysts (confirmed) | TMS-4-T03U.318-MDCT007-T03U.318-C111A |
- The company are required to satisfy the following requirements.
The date given is that until or by which the requirements must be maintained or satisfied, as appropriate.
Requirements:
(a) To maintain the following measures for the period specified in each case:
- Continuous monitoring for Cryptosporidium at defined points at Dorney Taplow Works, when the associated raw water boreholes are running into supply.
Date: Ongoing for the duration of this Notice - Implement and keep under review triggers to remove the Taplow A borehole from supply, to include consideration for microbiological parameters.
Date: Ongoing for the duration of this Notice - Operation of all treatment processes at Dorney Taplow works, within defined operating ranges with appropriate triggers, responses, and contingency arrangements.
Date: Ongoing for the duration of this Notice - Water Quality response procedures in place for investigation of contraventions including process optimisation and reporting, consumer complaints and event response.
Date: Ongoing for the duration of this Notice - Maintain contingency plans for the protection of public health associated with drinking water quality, to include liaison with Local Authorities and the United Kingdom Health Security Agency (UKHSA).
Date: Ongoing for the duration of this Notice
(b) To review, revise or make operational the following measures, by the dates specified in each case:
- Produce the programme for delivery for the implementation of the preferred treatment option.
Date: 31 March 2026 - Complete the procurement strategy for the treatment option.
Date: 28 February 2026 - Complete the design for the treatment option.
Date: 31 July 2026 - Complete the construction, installation and commissioning of the chosen treatment process, designed under measure (b)3. Commissioning must include the following to account for the changes made:
- An appropriate maintenance regime for the installed process;
- Full integration of the new process into the site control and monitoring systems.
- Updating of any associated site specific procedures;
- Updating of the Site Operating Manual and removal of all out-of-date material from circulation;
- Provision of adequate training for all relevant personnel on the operation of the installed process and the revised procedures, policies and manual.
Date: 30 June 2027
(c) To audit whether the measures have been effective by the following means:
- Define an audit strategy to monitor the effectiveness of the measures specified in sections (a) and (b).
Date: 31 January 2026 - Implement, and keep under continuous review, the audit strategy defined in measure (c)1.
Date: Following completion of measure (c)1. onwards.
(d) Not to supply water for regulation 4(1) purposes from Dorney/Taplow Works, or not to so supply unless the specified conditions below are satisfied:
- Not applicable.
(e) To provide the following information in the time and manner specified below to enable monitoring of progress towards the mitigation of the risks:
- Provide a progress report annually.
Date: by 31 January each year. - Provide the delivery plan for the implementation of the preferred treatment process, as specified in measure (b)1.
Date: 30 April 2026 - Provide the procurement strategy, as specified in measure (b)2.
Date: 31 March 2026 - Provide the design for the treatment option, as specified in measure (b)3.
Date: 31 August 2026 - Provide evidenced confirmation that the new treatment process (designed under measure (b)3) has been installed and commissioned, as specified in measure (b)4.
Date: 31 July 2027 - Submit the audit strategy for auditing the effectiveness of the measures, as specified in measure (c)1.
Date: 28 February 2026 - Submit any revisions of the audit strategy produced under measure (c)2.
Date: Within 1 month of a revision - Submit a report in the event that any completed measures of this Notice need to be altered in any way, for any reason.
Date: Ongoing for the duration of this Notice - Submit a satisfactory completion report accompanied by a revised regulation 28(1) report with Board level Director sign-off.
Date: 31 July 2028
Further Details of this Notice:
- Any product or substance used as a result of the requirements in this Notice must comply with regulation 31.
- Under regulation 28(6) the Inspectorate, on behalf of the Secretary of State, may by notice served on the company, revoke or vary this Notice.
- Failure by the company to comply with this Notice may result in enforcement proceedings under section 18 of the Water Industry Act 1991 (as amended).
Nicholas Adjei
Deputy Chief Inspector, on behalf of the Secretary of State for Environment, Food and Rural Affairs
20 November 2025
Reference: TMS-2025-00007 v1