If your risk assessment has flagged it, or a water sample has come back positive, you may need water tank disinfection. It is a key part of keeping stored water safe — but it is often confused with cleaning, and many building managers are unsure when it is actually needed. This guide explains what water tank disinfection is, when it is required, and how the process works.

What is water tank disinfection?

Water tank disinfection is the process of chemically treating a cold water storage tank and its associated pipework to destroy bacteria — including Legionella — and make the stored water safe to use. It is usually carried out using controlled chlorination, and most often follows a physical clean of the tank.

Disinfection vs cleaning: what's the difference?

The two go hand in hand but are not the same. Tank cleaning physically removes sediment, debris and biofilm from the tank; disinfection then kills the bacteria left on the surfaces and in the water. A thorough service usually does both — there is little point disinfecting a tank that is still full of sediment, as debris shields bacteria from the disinfectant.

When do you need water tank disinfection?

Disinfection is typically required in the following situations:

After a tank has been cleaned, as the second stage of the service.
On a new tank or installation, before it is brought into use.
Following a positive water sample or evidence of contamination.
After remedial work, repairs, or anything that exposes the system to contamination.
When a system has been out of use or water has been left to stagnate.
Where your Legionella risk assessment recommends it.

How the disinfection process works

A competent contractor will usually follow a controlled process:

The tank is drained and physically cleaned to remove sediment and biofilm.
The system is refilled and dosed with chlorine to a controlled concentration.
The solution is held in contact with the tank and pipework for a specified period.
The system is thoroughly flushed until chlorine levels return to normal.
A water sample is taken and sent to a UKAS-accredited laboratory to confirm the water is safe.

Where the work is part of commissioning a new or modified system, you should receive documentation confirming it was carried out — see our guide to chlorination certificates for what that should include.

Keeping tanks safe between disinfections

Good tank hygiene reduces how often disinfection is needed. Keep tanks fitted with a close-fitting lid and insect screens, insulated to control temperature, and inspected regularly, with cleaning carried out when an inspection shows it is due. Your risk assessment sets the right schedule for your system.

Water tank disinfection in London

London Water Treatment provides tank cleaning and disinfection and chlorination services across London, with UKAS-accredited water sampling to confirm results. Get in touch for a quote.

Frequently asked questions

How often should a water tank be disinfected?

There is no fixed interval for every tank — it is driven by your risk assessment and the condition found at inspection. Cold water storage tanks are commonly inspected annually and cleaned and disinfected when the inspection shows it is needed.

Do you get a certificate after disinfection?

For new or modified systems, disinfection is usually confirmed with a chlorination certificate, backed by a satisfactory water sample from a UKAS-accredited laboratory.

Water Tank Disinfection: What It Is and When You Need It