How to Sample Cooling Towers for Legionella Bacteria USA

The following presentation explains how to sample cooling towers for Legionella during Legionnaires’ disease outbreaks in the USA. The presentation looks at Legionella water sampling of cooling towers, how to take biofilm swabs, the measurement of other water quality parameters, and selection of typical sampling sites. The video featured here is one published by the US Centers for Disease Control and Prevention (CDC) gives an introduction to environmental health and engineering measures for Legionnaires’ disease outbreak investigations in the USA.

Sampling Cooling Towers for Legionella Bacteria

This Video is the fifth in a series addressing environmental aspects of legionellosis outbreak investigations. It will demonstrate the correct procedure for collecting environmental samples from a cooling tower for Legionella culture during a cluster or outbreak investigation, or when cases of disease may be associated with a facility.

Environmental Assessment & Sampling Plans

Conduct an Environmental assessment and make a sampling plan before collecting samples. This video should be used in conjunction with the written document CDC Sampling Procedure and Potential Sampling Sites.

Personal Safety & Protection during Sampling

Please be mindful of safety issues when sampling cooling towers, since they can be located in maintenance areas and on rooftops. When practical, cooling tower fans should be turned off to stop the spread of aerosols that may be contaminated with Legionella. When fans cannot be turned off, wearing a half-face air-purifying respirator equipped with an N95 filter may be appropriate.

Respirators must be used in accordance with a comprehensive respiratory protection program which includes fit testing, training and medical clearance ahead of their use. You may also want to wear gloves for protection from chemicals used to maintain the cooling tower.

Taking Water Samples from Cooling Towers

Cooling towers come in different shapes and sizes. Engage an engineer or maintenance technician familiar with the particular device to identify appropriate sampling sites. Collect a 1 Litre bulk water sample from the tower’s basin. If it is impossible to collect 1 Litre, collect as much as is available.

Add 0.5 millilitres of a 0.1 normal sodium thiosulfate solution to neutralize residual disinfectants, and tighten the top to prevent leakage, then label the sample with a unique identifier, and record it on the sample data sheet. Place the bottle into the cooler for storage and transportation.

Cooling Tower Water Quality Parameters

You will also need to measure the cooling towers water parameters. Collect some water from the basin in a separate plastic sampling bottle. If there is not enough water in the basin you may measure water quality parameters directly in the bulk water sample, but be sure to disinfect probes with an alcohol wipe to prevent cross contamination of samples.

Measure the water temperature; make sure there is enough time for the thermometer to stabilise. Measure pH by placing the paper strip inside the bottle. Compare the colours on the paper strip with the legend on the box; alternatively, you may use a digital pH meter.

If you are using a colorimetric wheel test kit to measure chlorine residual, add water from the same water source to the control tube. Open the pouch with the powder and add it to the second tube. There is a different type of powder for measuring free chlorine vs. total chlorine, so you’ll need to know which one to measure. Add water to the marked level.

Place both tubes into the box and compare them against a white background. Turn the wheel until the pink colour in the measuring tube matches the colour in the control tube – note the number on the wheel – if you are using a different chlorine measurement system, follow the manufacturer’s instructions. – Record data on the sample data sheet.

Taking Biofilm Samples

Next you will need to take several biofilm swabs from moist surfaces, such as the water line in the basin, the fill, or a drift eliminator. Rib the swab firmly along the surface. Collect 3 to 5 millilitres of water associated with the source into a 15ml sterile tube. The water is needed to keep the tip of the swab moist during transport. Place the swab into the tube, snap the stem about 1 inch from the top of the tube – add a drop of 0.10 normal sodium thiosulfate solution to neutralize residual disinfectants. Tighten the top to prevent leakage.

Label the tube with a unique identifier, and record it on the sample data sheet – place the tube into the cooler for storage and transportation. It is extremely important to make sure you collect, label, record and transport your samples accurately and carefully.

For more information, please reference the written document CDC Sampling Procedure and Potential Sampling Sites.

How to Sample Cooling Towers during Legionellosis Outbreak Investigations – Centers for Disease Control and Prevention (CDC)

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