Why is water tank cleaning important and how do you do it?

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Cold Water Storage Cistern Clean and Disinfection (Tank Clean). The cleaning & disinfection of cold water storage cisterns is either carried out as part of a planned preventative programme, or when sample analysis demonstrates that there is evidence of sufficient bacterial contamination to justify the recommendation for cleaning & disinfection of the cistern. Wholesome water tanks should be cleaned and disinfected annually as a minimum in accordance with BS 8558 and BS EN 806.

 

Scope of works include:

• Confirm at job booking stage any disruption of water services, the time frame in which disruptions may occur and system availability which will be needed to complete the works.

• Ensure that all the building's health & safety documents have been seen & understood.

• Ensure site contact is aware of the procedure, possible disruption to water services (if applicable) and the time frame in which disruptions may occur.

• Confirmation of the foul drain for discharge and any safe disposal discharge permits are the responsibility of the client.

• Obtain permission to commence work, using the permit to work document.

• Where applicable, request schematic drawings of water systems.

• Undertake or review dynamic risk assessment.

• Identify the cold-water storage cistern(s) to be cleaned & disinfected

• Inspect cistern(s) & commence pre-condition report.

• Check for full and correct operation of the inlet and outlet valves.

• Identify the location and isolation points (if applicable) for any sensitive equipment (i.e Dosing Control Sensors, Water Softeners, Drinks Dispensers) and discuss the safe isolation of the equipment with Site Maintenance.

• Isolate cistern(s).

• Add the required volume of biocide solution to header tanks, to achieve the required concentration.

• Drain down cistern(s).

• Disinfect internal walls of the cistern(s).

• Thoroughly flush ball valve, to remove any sediment.

• Drain down residual water and ensure that cistern is cleaned thoroughly.

• Spray walls of cistern with biocide.

• Top up header cistern(s) with fresh water; and ensure that residual biocide levels do not exceed recommended concentrations.

• Inspect cistern(s) and complete the inspection report.

• Take TVC (total viable count), E-Coli & Coliform samples for analysis by an independent UKAS accredited laboratory.

• Obtain permit to work document completion signature.

• Supply a certificate of disinfection & sample analysis report, available in document & PDF electronic formats.

 

Why is water tank cleaning important?

Water tank cleaning is an important part of water hygiene and Legionella control. Dirt, dust, debris, mould and harmful bacteria, including Legionella, can build up in the water tank. Completing a legionella risk assessment will help assess the risks posed by any water tank and may recommend regular cleaning and disinfecting of the water tank.

What are the risks:

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• Stagnant, little use water sources can be a potential breeding ground for unwanted and harmful bacteria. Stagnation can be caused by many factors including under-utilising water, using an excessively large tank, and issues in your water system causing blockages or poor flow. These factors aside, there will always be some stagnation in water tanks. Therefore regular cleaning and disinfection is essential even if these risks are low.

• Cold water storage tanks are also a common source of a food source for biofilm and bacteria, including Legionella. A dirty tank can develop rust, sediment, and silt. If it is unchecked, tanks can become contaminated with organic matter such as leaves and animals.

• Another risk is debris coming in from the mains. The tank can act as a catchment area for debris, causing it to harbour nutrients and bacteria.

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How should water tank cleaning be carried out?
The following procedure complies with the requirements of BS8558:2015 ‘Guide to the design, installation, testing and maintenance services supplying water for domestic use within buildings and their curtilages’.
 
How often should you clean your domestic water tank?

When and how often you should clean will differ depending on whether the cold water storage tanks serve the building’s drinking water are potable (used for washing hands etc) or non-potable (toilet flushing, possibly from rain water recovery) systems. Drinking water storage tanks need to be cleaned more regularly because they feed drinking water outlets. In line with The Health and Safety Executive's HSG274 guidelines and code of practice (ACOP L8), you must adhere to the following:

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• The temperature of the tank from the inlet and outlet should be inspected at least annually. 

• An internal inspection for signs of possible contamination and debris should be carried out every 6 months for drinking water and every 12 months for potable/non-potable water.

• Microbiological samples should be taken every 6 months for drinking water, and only as required by a control scheme for potable and potentially non-potable water.

• Water tanks should be cleaned and disinfected annually (or more frequently if an inspection deems it necessary) for drinking water and only when inspection deems necessary for potable and non-potable water.

It is important to consider that although a water tank may contain non-potable water, aerosols may be created during usage and also maintenance procedures should be considered and risk assessed for activities such as testing fire tanks.