The Ingenieur Vol 57 January-March 2014 The Ingenieur Vol 57 January-March 2014 | Page 47

Guidance on Action to be taken if Legionella is found in the Water System
Table B: Action levels following Legionella sampling in hot and cold water systems.
Legionella bacteria
(cfu/litre)

Action Required

More than 100 but less than
1,000

Either:
1. If only one or two samples are positive, system should be
re-sampled. If a similar count is found again, a review or the
control measures and risk assessment should be carried
out to identify any remedial actions.
2. If the majority of samples are positive, the system may be
colonised, albeit at a low level, with Legionella. Disinfection
of the system should be considered but an immediate
review of control measures and risk assessment should be
carried out to identify any other remedial action required.

More than 1,000

The system should be re-sampled and an immediate review of the
control measures and risk assessment carried out to identify any
remedial actions, including possible disinfection of the system.

*cfu denotes Colony Forming Unit

Recommended Inspection Frequencies for Risk Systems
Table C: Checklist for Cooling Water Installations
System/Service

Task

Frequency

Cooling towers
and evaporative
condensers

Monitor water quality, water use and biocide/chemical use to
assess and ensure effectiveness of water treatment regime,
including key chemical and microbiological parameters, and
observations of internal condition of pond, pack and water

See Table A

Central control function, conductivity sensor calibration, blow
down function, uniformity of water distribution, conditions of
sprays/ troughs, eliminators, pack, pond, immersion heaters,
fans and sound attenuators

Monthly to
three monthly,
according to risk
(See Table A)

Clean and disinfect cooling towers/ evaporative condensers,
make-up tanks and associated systems, including all wetted
surfaces, de-scaling as necessary. Packs should be removed
and cleaned where practical

Six monthly

45