South West Water Limited – Alderney Water Treatment Works, Replacement Treatment Process

Notice under regulation 28(4) of the Water Supply (Water Quality) Regulations 2016 (as amended)

Reference: SWB-2021-00008

Version Number: 2

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:

  • Alderney WTW (T9100F7, T9100F8)
  • Alderney Supply System (Y9100)
  • Longham Lakes (A9100R2, A9100R3 and A9100R1)
  • Alderney North Supply Zone (ZBW3)
  • Alderney South Supply Zone (ZBW1)
  • Alderney East Supply Zone (ZBW2)
  • Alderney West Supply Zone (ZBW11)

Details of this Notice:

  1. The Drinking Water Inspectorate (‘the Inspectorate’) has received multiple regulation 28(1) reports (of the Water Supply (Water Quality) Regulations 2016 (as amended) (‘the Regulations’)) from the Company dated 13 October 2020, 20 October 2020 and 22 January 2020, which state that there is or has been a significant risk of supplying water from Alderney Water Treatment Works (and associated assets as applicable) that could constitute a potential danger to human health or 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 raw water risks and potential risks generated by the treatment process. Risks associated with the following parameters:

Hazard/ Hazardous Event/ Parameter

Hazard I Ds Multiple hazard lines including:









Total organic carbon (including dissolved organic carbon)


Disinfection by-products, including trihalomethanes

7870; 2925







Colony counts

7788; 7838

E. coli




Total coliforms

7869; 31757; 32064

Clostridium perfringens




pH (and resulting corrosivity effects)






Table 1: Table of the hazards, hazardous events, and/or parameters covered by this Notice, including the Hazard I D numbers.
  1. 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.


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

  1. Proactively monitor and suitably respond to the risk of algal blooms and other pollution incidents at Longham Lakes (Lake 1 and 2).
    Date: Following the completion of measure (b)4. onwards.
  2. Maintain contingency plans to protect human health in the event of a risk to human health associated with drinking water quality, which should include proactive liaison with local and regional public health officials.
    Date: Following the completion of measure (b)4. onwards.
  3. Monthly inspections and cleaning of the roof drains on Reservoirs 7 and 8.
    Date: Following the completion of measure (b)4. onwards.
  4. Monthly inspection of hatch integrity and remediation of any identified issues, for hatches over partially or fully treated water.
    Date: Following the completion of measure (b)4. onwards.
  5. Continue effective liaison with relevant stakeholders to allow appropriate response to upstream sewage events.
    Date: Following the completion of measure (b)4. onwards.
  6. Ongoing suitable maintenance inspections for treated or partially treated water storage tanks.
    Date: Following the completion of measure (b)4. onwards.
  7. Raw water continuous monitoring for ammonia, dissolved oxygen, conductivity, pH, turbidity, oil and colour, with suitable alarm triggers and response procedures in place.
    Date: Following the completion of measure (b)4. onwards.
  8. Undertake catchment management initiatives within the River Avon and Stour catchments, as outlined in the National Environment Programme.
    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:

  1. Carry out a review of all potential pathways through which Cryptosporidium could enter source waters, with particular regard to upstream sewage assets.
    Date: Complete
  2. Complete a review of the long-term suitability of the use of Reservoir 7 and 8 as treated water storage tanks as part of the replacing water treatment process, to evidence whether the risk associated with these assets can be mitigated.
    Date: Complete
  3. Complete sufficiently detailed design to enable the construction of an adequate treatment process (capable of mitigating the raw water challenge, with or without Lake 2 (the larger of the two lakes) of Longham Lakes as part of the treatment process, to replace the existing treatment process at Alderney Water Treatment Works) to proceed. The design must also specify and justify any of the existing assets that are to be incorporated into the replacing treatment process.
    Date: Complete
  4. Complete the planning, procurement, construction and installation of the chosen new/replacing treatment solution designed under (b)3. to enable wet process commissioning.
    Date: 31 August 2024
  5. Commission the new/replacing treatment solution (providing wholesome water) into supply through a controlled blending scheme.
    Date: Between 31 August 2024 and 31 December 2024
  6. Provide all treated water from Alderney Water Treatment Works from the new/replacing treatment solution (to enable use as designed), and accordingly:
    • Issue an updated Site-Specific Disinfection Policy and any associated site-specific procedures;
    • Issue an updated Site Operating Manual and remove out-of-date material from circulation; and
    • Complete training and sign-off of operators on the chosen treatment solution installed and the revised documentation listed above.
    • Provide confirmation of the programme of the long-term filter inspections and maintenance for the slow sand filters and roughing pressure filters following completion of step (b) 5 including scope of inspection and frequency of inspection
      Date: 31 December 2024
  7. Undertake any necessary consumer engagement mitigation measures as determined on the basis of risk, to ensure consumer acceptability is maintained during and after the switch to the replacing treatment process. This must include the monitoring and review of consumer contacts during the blending phase (undertaken under measure (b)5.), with appropriate resulting actions taken to ensure consumer acceptability is maintained.
    Date: Between 16 June 2022 and 31 December 2025

(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). The audit strategy must include:
    • Detailed specification of enhanced monitoring to be undertaken (parameters and frequencies);
    • Ownership of the audit strategy (roles and responsibilities);
    • Specified review of all monitoring data and resulting action; and
    • Consumer acceptability.
      Date: Complete
  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 Alderney 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.
    Date: by 31 January each year.
  2. Report summarising the review of Cryptosporidium pathways (measure (b)1.).
    Date: Complete
  3. Report outlining the chosen replacement or modified treatment process design. The report must also clearly outline and justify any existing assets that are to become part of the new treatment process (measure (b)3.), including the conclusion of the long-term suitability review for Reservoir 7 and 8 (measure (b)2.).
    Date: Complete
  4. Confirm the completion of the construction and installation of the chosen new/replacing treatment process (measure (b)4.) to allow for wet process commissioning to begin.
    Date: 30 September 2024
  5. Confirm the new/replacing treatment process is solely in supply (measure (b)6.), associated updated documentation has been issued and necessary training completed. Provide the programme of longterm filter inspection and maintenance as detailed in step (b)6.d
    Date: 31 January 2025
  6. Provide regular updates, through Company – Inspectorate liaison meetings, on any consumer engagement actions taken and monitoring of consumer contact levels. (measure (b)7.).
    Date: Ongoing for the duration of the Notice
  7. Submit the audit strategy for auditing the effectiveness of the measures, as specified in measure (c)1.
    Date: Complete
  8. Submit any revisions of the audit strategy produced under measure (c)2.
    Date: Within 1 month of a revision
  9. 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
  10. Submit a satisfactory completion report accompanied by a revised regulation 28(1) report with Board level Director sign-off.
    Date: 31 January 2026

Further Details of this Notice:

  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).


Nicholas Adjei
Deputy Chief Inspector, on behalf of the Secretary of State for Environment, Food and Rural Affairs
04 December 2023
Reference: SWB-2021-00008

Back to top