How to Disinfect a Domestic Water System

Background Information

Tiosan will remove biofilm and therefore all Legionella from a water system providing it is applied at the correct concentration and is allowed to contact every part of the water system (no dead legs).

Houses built over the past 20 years do not present a Legionella risk because they do not have water storage tanks, or they have properly constructed tanks which have secure lids, have good front to back circulation and have screened overflows.

Method Statement

This method statement assumes that the system starts at the water storage tank which supplies all the domestic water to the premises - the hot water via a calorifier or some other water heating device, and the cold water to downservices.

The water storage tank should be given a preliminary disinfection. Calculate the size of the water tank and multiply its volume by 1.3.

From the Safewater Chart (see Table 1 at the end of this booklet) calculate the volume of Tiosan, which needs to be added to give 75 ppm hydrogen peroxide. Add this volume to the water storage tank and mix well into the water.

Drain the water through the taps in the premises, which are furthest away from the tank to ensure that the treated water contacts all the pipework. (please note that it is important that dead legs are found and, if possible, introduced into the main system flow or purged through with treated water).

Check the water leaving the furthest tap (hot and cold) until the presence of hydrogen peroxide is detected. When a level of 75 ppm hydrogen peroxide has been obtained on a test strip, close the taps and retain the treated water in the pipework. 75 ppm hydrogen peroxide is represented by a dark blue colour on the test strip. This ensures that the water in the system remains safe to drink should anyone inadvertently drink the system water while the disinfection is taking place.

The hot water system is particularly prone to harboring Legionella and care should be taken to ensure that there is Tiosan throughout the hot water system. Check drains from calorifiers and a selection of taps to ensure that there is sufficient Tiosan in the system.

If the tank can be cleaned, drain the tank, or pump out the remaining water using a submersible pump, taking care to ensure that there is sufficient local drainage to cope with the flow. Clean the tank. This will mean getting into the tank, and cleaning the walls and floor of the tank. Plastic tanks may only need a wipe down while metal tanks may have to be scraped and cleaned with a pressure washer. Wet Vacs are usually employed to remove debris and water from the bottom of the tanks.

When the tank is cleaned it should be filled with fresh water. The volume of Tiosan, which was added earlier should be added to the water in the tank. Check outlets over 10 minutes and if less than 75 ppm detected add more Tiosan until 75ppm is obtained consistently. The water in the outlets should continue to show a level of 75ppm hydrogen peroxide at the taps.

In many buildings it is not possible to clean and disinfect the water storage tanks as they cannot be taken out of service. The procedure outlined above can be carried out without the tank clean. Tiosan can be added to the tank drawn to the furthest outlets until a reading of 75ppm is obtained. This procedure will remove all biofilm from the system and remove persistent Legionella.