How to Clean a Cold Water Storage Tank

For many facilities managers, cold water storage tanks are out of sight and easy to overlook. That can be a problem. These tanks sit at the start of a building’s cold water system and play a direct role in water quality, compliance, and occupant safety. When inspection and cleaning regimes slip, sediment, biofilm and bacteria can build up, increasing the risk of water hygiene failures.

Cleaning a cold water storage tank is not just a maintenance task. It is part of a broader duty to manage risk under UK health and safety legislation. This guide explains how cold water storage tanks should be cleaned, when cleaning is necessary, and how the process fits into an effective water hygiene programme.

Understanding cold water storage tank hygiene

The role of cold water storage tanks

Cold water storage tanks are commonly used to supply water to larger buildings where mains pressure or flow is insufficient. They may serve potable outlets such as taps and showers, or non potable systems such as flushing or plant equipment. Tanks supplying drinking water demand closer control because any contamination can affect building users directly.

UK guidance such as ACoP L8 and HSG274 makes it clear that water systems must be managed to minimise the risk of Legionella and other harmful bacteria. Cold water storage tanks are specifically referenced due to their potential to accumulate sediment and support microbial growth if poorly maintained.

Inspection frequency and monitoring

Routine inspection is the first line of defence. As a general guide, cold water storage tanks supplying potable water should be visually inspected every six months. Tanks supplying non potable systems are typically inspected annually. These inspections should check for debris, corrosion, damage to lids or screens, and any visible signs of biofilm.

Inspection records should be kept as part of the site water hygiene logbook. Where inspection identifies issues, further action may be required. For a broader overview of inspection expectations, this water tank maintenance guide provides useful context on routine checks and responsibilities: Water Tank Maintenance Checklist - Expert Tips | Titan Water

When a cold water storage tank needs cleaning

Indicators that cleaning is required

Not every inspection leads to cleaning. However, certain findings should trigger action. These include visible sediment on the base of the tank, organic debris, surface slime, corrosion products, or evidence of pest ingress. Any deterioration in water quality, unusual odours, or unsatisfactory microbiological results should also prompt investigation.

Where a Legionella risk assessment identifies the tank as a potential risk point, cleaning and disinfection may be recommended as part of the control measures. Understanding this relationship between risk assessment and remedial action is central to effective water hygiene management. Further information on formal risk assessments can be found here: Legionella Risk Assessment Services | Titan Water

Situations where cleaning can be deferred

In well managed systems, tanks that are clean, structurally sound, and producing satisfactory water quality results may not require immediate cleaning. Over cleaning can introduce unnecessary disruption and risk. The key is evidence. Decisions should be based on inspection findings, risk assessment outcomes, and test results rather than arbitrary time periods.

Step by step process for cleaning a cold water storage tank

Preparing the system

Before work begins, a task specific risk assessment should be completed. This should consider confined space hazards, working at height, chemical handling, and isolation procedures. Where entry into the tank is required, appropriate training, supervision and rescue arrangements must be in place.

The tank should be isolated from the system by closing inlet and outlet valves. Booster pumps and associated controls should be safely shut down. Building occupants should be informed in advance if water supplies are likely to be affected.

Draining and internal inspection

The tank is drained to a suitable discharge point. Once empty, a detailed internal inspection can be carried out. Photographs taken at this stage can be useful for records and future comparison.

Sediment and debris should be removed from the base of the tank using appropriate equipment such as wet vacuums or manual tools. Physical removal is essential, as disinfection alone will not address built up deposits. This video from our Youtube channel shows the vacuuming process in action.

Cleaning internal surfaces

All internal surfaces, including walls, floor, supports and internal fittings, should be thoroughly cleaned. Particular attention should be paid to corners and joints where biofilm tends to accumulate. Removing biofilm is critical, as it can protect bacteria from disinfectants.

Disinfection and flushing

Following cleaning, the tank is disinfected using an appropriate method, commonly chlorination. For potable systems, a defined chlorine concentration is introduced and held for a specified contact time to ensure effective disinfection.

The disinfectant should be circulated through the downstream pipework to ensure outlets are treated. After the contact period, the system is flushed until chlorine levels return to normal supply conditions.

Once the tank is refilled and returned to service, records of the cleaning and disinfection should be updated in the site logbook. For a detailed overview of professional tank cleaning procedures, this service page provides further technical information:https://www.titanwater.co.uk/water-hygiene-services/water-tank-cleaning-services

Safety considerations during tank cleaning

Confined space and access risks

Some cold water storage tanks meet the definition of a confined space. Where this applies, entry must only be undertaken by trained personnel using appropriate controls. Even tanks that are not formally classed as confined spaces can present risks due to restricted access and poor ventilation.

Water quality testing after cleaning

Post cleaning water sampling is good practice, particularly where tanks supply potable outlets. Microbiological testing can confirm that cleaning and disinfection have restored acceptable water quality. Sampling also provides evidence that control measures are working as intended.

For sites where routine testing forms part of the management plan, additional guidance on Legionella testing is available here: Legionella Testing Services | Titan Water

Ongoing management beyond cleaning

Integrating cleaning into a wider hygiene programme

Cleaning should be part of a wider programme including risk assessment, inspection, and monitoring. Understanding your tank type and system arrangement helps you plan sensible inspection and maintenance routines. This overview is useful for that:https://www.titanwater.co.uk/water-tank-services/cold-water-storage-tanks

If your compliance plan includes wider system protections, you may also want to review backflow controls:https://www.titanwater.co.uk/water-hygiene-services/backflow-prevention-services

Using inspection and maintenance checklists

Structured inspection checklists help ensure consistency and prevent issues being missed. These typically include checks on lids, vents, overflows, insulation, structural condition and water clarity. Using the same checklist over time allows trends to be identified and addressed early.

Further guidance and next steps

Facilities managers responsible for cold water storage tanks should ensure that cleaning decisions are evidence based, properly documented, and aligned with the site Legionella risk assessment. Where in house expertise or resources are limited, specialist support can help ensure work is carried out safely and in line with current guidance.

For further practical information on managing water systems safely, you may find this overview of water hygiene services useful:https://www.titanwater.co.uk/water-hygiene-services

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