Cooling Towers

Cooling tower water monitoring microbiological activity (also legionella)

Every cooling tower should be monitored by routine sampling and testing for the presence of bacteria, both general (aerobic) bacterial species and legionella bacteria.

Normally bacteria are monitored weekly by dip slides. They are dipped into the water and incubated for 48 hours.
Sampling should not be carried out if a biocide has recently been added.

A general caution - relatively clear water may be heavily contaminated with bacteria.

Legionella sampling should be undertaken at least quaterly by an accredited laboratory

Below action levels following for cooling towers are indicated.

Aerobic count (cfu/ml at 30°C (minimum 48 hours incubation	Legionella bacteria cfu/litre	Action required
10.000 or less		System under control
More than 10.000 and up to 100.000	More than 100 and up to 1000	Review programme operation – a review of the control measures and risk assessment should be carried out to identify any remedial actions and the count should be confirmed by immediate resampling.
More than 100.000	More than 1000	Implement corrective action – the system should immediately be resampled. It should then be ‘shot dosed’ with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions.

Above guidelines are derived from the approved code of practice and guidance manual of the Health and Safety Commission third edition 2000 UK.

More specific guidelines hereof can be obtained from Lenntech

For information on dip slides click here.