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By Dave Christophersen, Vice President/Technical Manager

Because evaporative cooling tower systems are open to the atmosphere and sunlight, they are prone to microbiological (MB) activity and the resulting adverse affects. Pathogenic microbes such as Legionella pneumophila can create health concerns. Slime created by the commonly occurring gram-negative type of organisms can lead to heat exchanger fouling and loss of cooling efficiency. The biofilm can also lead to microbiologically influenced corrosion (MIC) and severe localized metal loss. Algae and fungal mats can impede water flow, cause an unsightly appearance, and damage wood and tower structures.

Good quality makeup water such as municipally supplied drinking water or well water should have negligible microbes and nutrients to support biological growth. Cooling towers, however, take in air-borne dust and microbes. The dust contains nutrients to support the growth of microorganisms. When there is minimal contamination, the activity level can become self-limiting and manageable.

The location of the cooling tower and the processes near the cooling tower can greatly affect the propensity for MB activity. Food plants, for example, can contribute plenty of organic compounds for the bacteria to have accelerated growth. Oils that contaminate a cooling water contribute to high MB growth. Process contaminations or the use of secondary wastewaters for makeup to the cooling towers improves the environment for MB growth. Phosphates in the water can increase algae growth and then algae can feed bacteria. The higher the biochemical oxygen demand (BOD) or total organic carbon (TOC) concentration of the cooling water, the greater the risk for increased biological fouling.