Current improvement programmes

Notice of Regulation: Prewley Water Treatment Works


South West Water Limited: Green Economic Recovery Prewley Water Treatment Works

Reference: SWB-2021-00009

Water undertaker as appointed under section 6 of the Water Industry Act 1991 (as amended): South West Water Limited, with registered number 02366665 (hereinafter called ‘the Company’).

Assets affected:
Prewley WTW, T3832F;
Prewley Supply System, Y3832.

  1. 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 21 October 2020, 25 November 2020, and 20 May 2021, which states that there is or has been a significant risk of supplying water from Prewley water treatment works (and associated assets as applicable) that could be unwholesome.
  2. 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:

Hazard/Hazardous Event/Parameter

Hazard ID

Description of Risks







Consumer acceptability (discolouration)

  1. The Company are required to satisfy the following:


(a) to maintain the following measures for the period specified in each case:

  1. Enhanced operational monitoring for the iron, manganese and turbidity parameters for the water leaving Prewley works, and within the water supply zones covered by this notice.
    Date: Ongoing for the duration of the notice.
  2. Optimisation of current treatment processes within the operational limits that have an impact upon discolouration parameters.
    Date: Ongoing for the duration of the notice.
  3. Respond to any failures of the regulatory standard or consumer contact, relating to discolouration parameters, in line with company procedures.
    Date: Ongoing for the duration of the notice.
  4. Record, monitor and analyse consumer discolouration contacts received within the zones supplied by Prewley works and report as required for the annual return to the Inspectorate.
    Date: Ongoing for the duration of the notice.
  5. Define and carry out DOMS flushing in zones where necessary as dictated by an increase in consumer discolouration contacts within water supply zones supplied by Prewley works, or by a breach of the iron, manganese or turbidity parameter.
    Date: Ongoing for the duration of the notice.
  6. Continuously review the raw water risk assessment by undertaking enhanced monitoring for manganese to verify the raw water risk.
    Date: Ongoing for the duration of the notice.

(b) to review, revise or make operational the following measures, by the dates specified in each case:

  1. Install and commission an artificial mixing system and ultrasonic phytoplankton control at Meldon reservoir to provide interim mitigation for control of raw water manganese concentrations.
    Date: 31 March 2023
  2. Install a new raw water main connecting Meldon reservoir with Northcombe works.
    Date: 31 March 2025
  3. Install a new treated water main from Northcombe works to Prewley works.
    Date: 31 March 2025
  4. Undertake a strategic review of supply options for the current Prewley and Northcombe supply areas to identify the long-term solution for supply arrangements.
    Date: 31 December 2022

(c) to audit whether the measures have been effective by the following means:

  1. Define an audit strategy to monitor the effectiveness of the measures specified in sections (a) and (b).
    Date: 28 February 2022
  2. 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 Prewley water treatment works, or not to so supply unless the specified conditions below are satisfied:

  1. 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:

  1. Provide a progress report annually by 31 January and against the following milestones:
    Date: Annually by 31 January
    • Provide a report confirming the installation and commissioning of the artificial mixing system and ultrasonic phytoplankton control at Meldon reservoir.
      Date: 30 April 2023
    • Provide a report confirming completion of the raw and treated water mains, as required in measures (b)2 and (b)3.
      Date: 30 April 2025
    • Provide a summary report of the review of supply options for the Prewley and Northcombe supply areas and confirm the long-term supply arrangements.
      Date: 31 January 2023
  2. Submit the strategy for auditing the effectiveness of the measures, as specified in (c)1.
    Date: 31 March 2022
  3. Submit any revisions of the audit strategy produced under measure (c)2.
    Date: Within 1 month of a revision.
  4. Submit a report in the event that any completed measure of this Notice needs to be altered in any way for any reason.
    Date: Ongoing for the duration of this Notice.
  5. Submit a satisfactory completion report accompanied by a revised regulation 28(1) report with Board level Director sign-off.
    Date: 31 May 2026

  1. Any product or substance used as a result of the requirements in this Notice must comply with regulation 31.
  2. 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.
  3. 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).

Nick Adjei
Deputy Chief Inspector, on behalf of the Secretary of State for Environment, Food and Rural Affairs
15 December 2021
Reference: SWB-2021-00009
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