We’re back with Test4It Limited to discuss problems encountered with excess levels of bacteria in cooling waters and how to monitor and comply with legislation. As mentioned in our last article featuring Test4It, they have some great insight on the use of dipslides in various industries. We know dipslides are commonly used for regular monitoring of bacteria levels in cooling waters, so we asked Test4It to tell us more!
The Risks
Any water system with the right environment has the potential for legionella bacteria growth. The right conditions are rarely present in most water sources. However, if water is stored or re-circulated at a temperature between 20-45°C, contains deposits such as rust, sludge, scale and organic matter, and produces water droplets/aerosols, the risk of Legionellosis is increased.
Legionellosis is a shared term for disease caused by legionella bacteria. The disease you’re probably most familiar with is Legionnaire’s disease, which is a potentially fatal form of pneumonia. Legionnaire’s is contracted by breathing in small droplets of water (aerosols) containing the bacteria.
According to the Health and Safety Executive (HSE), the most common sources of legionella are in man-made water systems, such as cooling towers and evaporative condensers, hot and cold water systems, and spa pools. So what should you do to control these risks? Let’s take a look.
Monitoring Bacteria Levels
Figure 1: Colorized scanning electronic micrograph (SEM) under magnification of 8000X depicting Legionella pneumophila bacteria; Janice Haney Carr, CDC
The HSE lists several steps of risk assessment, as well as guidelines on how to prevent or control the risk of Legionellosis. We’re focusing on monitoring water quality using dipslides, and understanding what the results mean.
Identification of legionella bacteria requires sending a sample into a lab, which is expensive and time consuming. Instead, regular monitoring of aerobic bacteria levels is used to indicate the level of microbiological control. The most common test method is dipslides, and in particular, the Nutrient TTC / Malt dipslide.
It is important to note that legionella does not grow on these dipslides. They will, however, grow a variety of microorganisms, which form visible colonies. The dipslides can then be compared to a growth chart to determine the number of bacteria in the water, which is expressed as Colony Forming Units per millilitre (cfu/ml).
Weekly monitoring of bacteria levels in cooling water has been recommended by many legislators and professional authorities as a visual performance indicator to both system and treatment regime. It’s a useful method to gauge how effective a chemical or biocide product is, and a trend can be quickly established identifying changes. If an increase in the number of microorganisms is detected, a review of the system and control strategies is a good idea.
According to HSE guidelines, the dipslide should be used to sample the water downstream of the heat source. The water sample is usually taken from the return line to the tower. If a sample point is used, it is important to flush it to ensure a representative sample before the slide is dipped. The dipslide should be incubated for a minimum of 48 hours, typically at a temperature of 30°C.
The sampling point should be remote from the biocide dosing point and for biocides, which are applied in a shot dose, sampling should be taken when the residual biocide is at its lowest. Ideally, this test should be performed at the same time each week.
Below is a table from the HSE of dipslide appearances, corresponding aerobic counts and the action levels required. In general, counts should not exceed 103 cfu/ml. Counts above this are considered able to support legionella.
So remember, regular monitoring with dipslides is an easy way to evaluate your control system, and you don’t need a background in microbiology to do it. A special thanks to Robin Offord of Test4It Limited for providing the information above!
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