Under current guidance, cold water storage tanks should be inspected annually for evidence of silt or biofilm, plus water temperatures should be recorded on a six-monthly basis.
It is a common misconception that all cold water storage tanks should be cleaned every year. This is in fact only the case if the tank supplies drinking water outlets (as required by the Drinking Water Inspectorate).
In this instance the tank should be drained, cleaned and chlorinated on an annual basis, it should meet Water Regulations standards and bacteriological samples should be taken every six months.
If, as is common on most sites, the stored water supplies toilet flushing, bathing and hand washing then the tank only needs cleaning when ‘routine inspection shows it to be necessary’ or where there has been a contamination incident. Contamination can be introduced into the water system during remedial plumbing works, vandalism or by ingress of insects, birds or other pests. Disinfection of the entire water system may be required where water sample results fail to meet the required standard.