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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.
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