Cleaning and Disinfection
||The ACOP says the risk from exposure to legionella should be prevented and controlled; precautions include keeping the system and the water in it clean. The following section on cleaning and disinfection offers guidance on how to do this in hot and cold water systems.Hot water services and, exceptionally, cold water services, should be cleaned and disinfected in the following situations:
Before chemical disinfection is carried out it is essential that the system is clean and it is important to ensure that all parts of the system are disinfected, not just those that are readily accessible. Chemical disinfection is usually carried out by chlorinating the water in the cold water storage tank to 20-50 mg/ litre free residual chlorine. It is then allowed to flow to all parts of the system by successively opening the outlets in the system such as taps and showers (until there is a smell of chlorine), then closing them and leaving it to stand for an appropriate period. This depends on chlorine concentration (from at least one hour at 50mg/l to at least two hours at 20mg/l). The required concentration should be maintained in the header tank throughout the chlorination procedure and chlorine concentration needs to be monitored throughout disinfection to ensure that there is a sufficient residual chlorine level. The system should be thoroughly flushed following chlorination. Appropriate concentrations of chlorine dioxide, as recommended by the manufacturers, may also be used as a disinfectant.
This treatment should not be carried by untrained personnel and should be closely supervised. Building occupants should be warned that the water is heavily chlorinated. If tanks and calorifiers are heavily contaminated by organic materials, the system should be disinfected before cleaning to reduce risks to cleaning staff and also after cleaning. It may be necessary to add chemical dispersants to remove organic fouling from pipework, etc., and chemical descaling may also be necessary. Where possible, cleaning methods should not create an aerosol.
Thermal disinfection can be carried out by raising the temperature of the whole of the contents of the calorifier then circulating this water throughout the system for at least an hour. To be effective, the temperature at the calorifier should be high enough to ensure that the temperatures at the taps and appliances do not fall below 60ºC. Each tap and appliance should be run sequentially for at least five minutes at the full temperature, and this should be measured. For effective thermal disinfection, the water system needs to be well insulated.
Alternatively, the circulating pipework and deadlegs/ends may be thermally disinfected by means of trace-heating. As before, the system should be capable of raising the temperatures of the whole distribution system to 60ºC or more for at least an hour.
The risk of scalding should be considered and particular care taken to ensure that water services are not used other than by authorised personnel until water temperatures have dropped to their normal operating levels.