Water Tank Cleaning and HSE Guidelines

Cleaning and disinfection of cold water storage tanks is an important aspect of the control programme for preventing the growth of Legionella bacteria. The bacteria may colonize water tanks if conditions are suitable; for example the temperature of the water is above 20°C and turnover is low leading to stagnation. Legionella bacteria can cause Legionnaire’s disease, a lethal form of pneumonia.

It is therefore advisable for owners and operators of non-domestic properties to clean and disinfect their water systems on a regular basis. The frequency of water tank cleaning should be determined by the findings of six monthly inspections. However, if the tank is used to supply drinking water, then it must be cleaned and disinfected annually and microbiological samples should be taken on a quarterly basis.

Legionella is definitely a concern, but there are other contaminants that may adversly affect stored water quality such as the presence of vermin or animal fouling. Debris, biofilm, corrosion products and silt could also taint the water.  The British Standard BS6700 details the approved methods for cleaning and disinfection of water systems.

The physical condition of a cold water storage tank and the way in which it has been installed can affect its susceptibility to contamination and such issues are readily identified during a Legionella Risk Assessment.

Some common issues with cold water tanks are:

1.       The tank lid is not adequately sealed allowing bacteria and other contaminants to pass in.

2.       Warning and overflow pipes are not screened to prevent ingress of vermin or insects.

3.       Insufficient cross flow inside the tank, which can lead to water stagnation.

4.       Overcapacity (water not turned over within 24 hours), which can also lead to stagnation.

5.       Tank lid or other part of the construction is made of natural substances such as wood providing a source of nutrients for bacteria.

6.       Insufficient insulation allowing the stored water temperature to increase above 20°C

7.       Deposits like rust, scale, silt and biofilm are present inside the tank.

Any of these factors can result in an environment that creates the ideal conditions for Legionella growth. It is therefore important to address the issues as part of a comprehensive control programme,

Alongside physical water quality testing, disinfection of the water system is equally important. Approved biocides such as chlorine, chlorine dioxide or silver peroxide can be introduced into the system to remove any bacteria released during the cleaning process

Water tank cleaning and water system disinfection should be carried out by accredited professionals who are trained in the safe use and handling of chemicals and the approved methods laid out in BS6700.