General monitoring
Guidance
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:
* If routine inspection shows it to be necessary;
* If the system or part of it has been substantially
altered or entered for maintenance purposes
in a manner which may lead to contamination;
or
* During or following an outbreak or suspected
outbreak of legioellosis.
* Disinfection of the water services may be
carried out in two ways:
* by the use of suitable chemical disinfectants,
e.g. by chlorination (see BS6700:1997) when
it is necessary to disinfect the whole system
including the storage tanks; or by thermal
disinfection, i.e. by raising water temperature
to a level at which legionella will not survive.
Chemical disinfection
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
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 60C. 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 60C 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.
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