When collecting a sample for Legionella testing from a cooling tower, when should the sample be taken?

Collecting a sample for Legionella testing from a cooling tower is most effectively performed after the system has been shut down and before it is drained. This timing allows for the assessment of potential Legionella contamination while still capturing the biofilm and water that may harbor the bacteria within the system.

When the cooling tower is shut down, the stagnant water provides an opportunity to collect samples from areas where Legionella might proliferate, such as within the basin, fill material, or dead legs in the piping system. Sampling before draining ensures that the water being tested reflects the current state of the system without being influenced by the removal of water that may contain bacteria.

Choosing to sample before shutdown or maintenance may not accurately represent the conditions in which Legionella could be present during normal operation. Sampling only after hyperchlorination or strictly during operational hours does not account for the risk of Legionella growth during those times when the system is not actively circulating, making it less effective for assessing potential risks.

Thus, sampling after shutting down the cooling tower and before draining is the optimal procedure for identifying the presence of Legionella and assessing potential risks associated with the system.