Key Points
- UK regulations require cold water storage tanks to be cleaned and disinfected at a minimum annually.
- All cleaning procedure must comply with BS8558:2015.
- Chlorination is the standard disinfection method; a verification sample must confirm results before the tank is returned to service.
- Written records of all cleaning activity must be retained under ACoP L8.
- Additional cleaning is required after contamination events, significant remedial works, or extended periods of non-use.
Why cold water storage tanks need regular cleaning
Cold water storage tanks act as settlement chambers. Even where incoming mains water is of acceptable quality, tanks accumulate sediment, scale, organic debris, and in some cases mould or algae growth over time. Where access covers are damaged or missing, tanks can receive airborne contamination, bird droppings, or other material. In more extreme cases, small animals or insects may enter and die within the tank.
These conditions create a substrate within which biofilm — the structured community of micro-organisms attached to the tank surface and pipework — can form and persist. Biofilm provides a protective environment for Legionella pneumophila and other waterborne pathogens, allowing them to survive and multiply even when water temperatures are maintained at levels that would otherwise inhibit growth in free-swimming (planktonic) form.
Additionally, cold water storage tanks can act as catchment areas for debris entering from the mains supply during pressure events or network works. This debris introduces nutrients that further support bacterial proliferation.
Regulatory basis
Cold water storage tank cleaning and disinfection is a specific control measure required under the written scheme of control mandated by ACoP L8. The procedure must be carried out in compliance with BS8558:2015. The minimum frequency is annual, as specified in HSG274 Part 2.
Regulatory requirements
ACoP L8 — Legionnaires' disease: The control of Legionella bacteria in water systems
ACoP L8 has Approved Code of Practice status under UK health and safety law. Failure to follow its guidance can be used in criminal proceedings as evidence that a duty holder has breached health and safety law. ACoP L8 requires that duty holders implement a written scheme of control for water systems that present a Legionella risk. For cold water storage systems, the scheme of control must include regular cleaning and disinfection as a specified control measure.
HSG274 Part 2 — Technical guidance on hot and cold water systems
HSG274 Part 2 provides the technical detail that supports ACoP L8 for hot and cold water systems. It specifies that cold water storage tanks must be cleaned and disinfected at least annually. It also notes that more frequent cleaning may be required where tank condition, sampling results, or site-specific risk factors indicate an elevated risk.
BS8558:2015 — Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings
BS8558:2015 provides the procedural standard for cleaning and disinfecting domestic water systems, including cold water storage tanks. All cleaning and chlorination work carried out by Hydraclean Ltd under the Tricel aftercare programme is compliant with BS8558:2015. The standard specifies acceptable disinfectants, required concentrations, contact times, and the verification procedures that must be followed before a tank is returned to service.
The tank top must also accommodate the access provisions required for confined space entry — manway clearance, tripod rescue system height, and platform space. For the access design requirements that feed back into tank and plant room specification, see the Confined Space Entry Guide.
What the cold water tank cleaning process involves
Cold water tank cleaning is a structured procedure requiring the tank to be taken offline. The process, as carried out by Hydraclean in compliance with BS8558:2015, involves the following stages:
Isolation
The cold water storage tank is isolated from the incoming mains supply and the distribution system it feeds, to prevent contaminated water from entering the building.
Drain down.
The tank is drained completely. The drain-down water is directed to a suitable waste disposal point.
Physical cleaning.
The interior surfaces are thoroughly cleaned to remove sediment, biofilm, scale, and any debris. All surfaces — base, walls, and lid — are cleaned using equipment appropriate to the purpose. This stage cannot be omitted; chlorination alone does not remove physical contamination.
Pre-inspection.
The interior of the tank is inspected before disinfection. The condition of lining, fittings, float valve, overflow arrangements, and access cover is checked. Any deficiencies are recorded for remedial action.
Disinfection by chlorination.
The tank is refilled and disinfected using a chlorine-based solution at the concentration and contact time specified in BS8558:2015. The chlorination solution is circulated through the associated distribution system where the scope of work includes this.
Drain and refill with potable water.
Following the required contact time, the tank is drained and refilled with fresh potable water.
Verification sampling.
A sample is taken from the tank after refilling to verify that residual chlorine and water quality meet the required standard before the tank is returned to service.
Documentation.
A written report of all work carried out, including findings, chlorination details, and verification results, is issued to the client and uploaded to Watercompliance.online.
A note on competency
ACoP L8 requires that persons carrying out cleaning and disinfection of water systems are competent to do so. Competency includes adequate training, practical technical knowledge, and understanding of the relevant standards including BS8558:2015. Work should not be carried out by untrained maintenance staff. Hydraclean’s water hygiene technicians hold the appropriate qualifications for this work.
Single tanks and twin-compartment arrangements
Many larger commercial and multi-residential buildings install twin-compartment or twin-tank arrangements specifically to allow one section to be taken offline for cleaning while the other continues to supply the building. Where such an arrangement exists, disruption to the building water supply during cleaning is minimised.
Single-tank installations require a planned approach to maintain supply during the cleaning period — typically involving temporary storage arrangements or scheduling the work to coincide with periods of low occupancy. Hydraclean assesses the best approach on a site-by-site basis before work commences.
Tricel Water UK manufactures sectional GRP cold water storage tanks that can be configured in twin-compartment arrangements to facilitate maintenance access and continuity of supply. Where a new tank installation is being specified, this is worth considering at the design stage.
When additional cleaning is required
The annual minimum is the baseline requirement under UK regulations. Additional cleaning is required — irrespective of the last clean date — in the following circumstances:
- A Legionella or full potable microbiological sample from the tank returns an unsatisfactory result that cannot be resolved by other means.
- A tank inspection reveals a level of contamination — sediment, biofilm, debris, or foreign matter — that presents an unacceptable risk.
- Remedial works have been carried out to the tank or to the supply or distribution pipework connected to it.
- The building has been unoccupied for an extended period (typically defined in the site-specific written scheme of control) and no flushing or monitoring programme was in place during the period of non-use.
- A contamination event has occurred — for example, flooding, a burst pipe affecting the tank, or evidence of animal access.
- The Legionella risk assessment has been reviewed and additional cleaning identified as a necessary control measure.
Documentation requirements under ACoP L8
ACoP L8 requires that duty holders keep records to demonstrate that the written scheme of control has been implemented. For cold water tank cleaning, this means retaining:
- The date of each cleaning exercise.
- The name of the person or organisation that carried out the work.
- A record of what was found (tank condition, any deficiencies noted).
- Details of the disinfection method, chemical used, concentration, and contact time.
- The results of any verification sampling carried out after cleaning.
- Any recommendations for remedial action and the actions taken.
Under the Tricel–Hydraclean aftercare packages, all of this documentation is produced and held on the Watercompliance.online portal. Clients can access the full record of cleaning history at any time through their portal account — without needing to contact Hydraclean to retrieve documents.
Frequently asked questions
How often must a cold water storage tank be cleaned under UK law?
HSG274 Part 2 and ACoP L8 require cold water storage tanks to be cleaned and disinfected at least annually. This is the minimum legal requirement for commercial and multi-residential buildings in the UK.
What standard governs cold water tank cleaning procedure?
BS8558:2015 — Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages — specifies the procedures for cleaning and disinfecting cold water storage tanks in UK buildings.
Does the building need to be taken out of service during cleaning?
Whether the building can remain in partial operation depends on the tank arrangement. Twin-compartment or twin-tank systems allow one section to be taken offline while the other remains in service. Single-tank installations require alternative water supply arrangements during the cleaning period. Hydraclean assesses this on a site-by-site basis before work begins.
When is additional cleaning required beyond the annual minimum?
Additional cleaning is required when a sample returns an unsatisfactory result, when inspection reveals unacceptable contamination, after remedial works to the tank or connected pipework, after extended building non-use without a flushing programme, or following a contamination event. The site-specific written scheme of control should define the triggers for additional cleaning.
Who can carry out cold water tank cleaning?
ACoP L8 requires that cleaning and disinfection is carried out by competent persons — those with adequate training, practical technical knowledge, and understanding of the relevant standards, including BS8558:2015. Hydraclean’s water hygiene technicians hold the appropriate qualifications.
What should cleaning records contain?
Records must include the date of work, the name of the person or organisation who carried it out, the condition of the tank on inspection, details of the disinfection method and chemicals used, contact time and concentration, the results of any verification sampling, and any remedial recommendations made and actions taken.
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Request annual cold water tank cleaning for your building
Contact Tricel Water UK to enquire about the annual cleaning and chlorination package, carried out in compliance with BS8558:2015 by Hydraclean Ltd.
