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State of California Department of Pesticide Regulation - page 69 / 104





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contaminants such as bacteria. These contaminants intensify the potential for corrosion. Higher water temperatures and the presence of scale deposits also increase the potential for corrosion. Corrosion weakens structural parts of the equipment, causes leaks in plumbing and holding basins, and reduces the efficiency of pumps.

Fouling. Suspended particles of organic and inorganic matter may cause fouling. These materials build up on the cooling tower fill surfaces and reduce the efficiency of heat transfer.

Wood Decay. Dissolved chemicals and certain microorganisms contribute to the decay of wooden components of a cooling tower system, which shortens the useful life of the equipment and results in the need for expensive repairs.

Microbial Growth

The major microbial pests found in water cooling systems include algae, fungi, and bacteria.


Algae produce their own food through a process known as photosynthesis. Algae grow in open areas of the water cooling tower where sunlight, air, and water are available. Growths of algae block screens and other openings that water passes through. Algae can form long strands or free-floating masses. Increased nitrate or phosphate concentrations in the water promote algal problems.


Under the proper conditions, fungi will grow on wooden surfaces of water cooling towers. They may appear as black, brown, or green spots above the water line. These growths contribute to surface and internal decay of the wood on which they are growing. Some fungi combine with bacteria to produce slimes or biofilms. You can find slimes and biofilms inside pipes, on heat exchanger surfaces, and on other unexposed surfaces as well as in the tower fill and sumps. Slime deposits range in color from off-white to dark brown.


Water cooling tower systems provide ideal environments for the growth of the bacteria Legionella pneumophila. This bacterium causes a pneumonia-like illness (Legionnaire’s disease), which is fatal to some people. The organism occurs naturally in most water sources. However, the bacterium appears only to infect people who happen to inhale water vapor or mist from contaminated cooling tower water. Proper water maintenance prevents this type of bacterial growth and infection.

Sulfate-reducing and iron oxidizing bacteria also occur in water cooling systems. Sulfate-reducing bacteria grow in oxygen-free areas under slimes, biofilms, and scale deposits, making them difficult to control. They produce foul smelling hydrogen sulfide gas, which is highly corrosive to steel and other metal parts of cooling systems. Iron oxidizing bacteria remove and oxidize soluble iron from the recirculating water. This material is then deposited as ferric hydroxide or iron oxide in the form of slime- like filaments. These deposits can restrict water flow in pipes and tubes.


Chemicals must be added to the water in cooling systems to control microbial activity and to reduce scale deposits, corrosion, and fouling. Proper maintenance of water quality will protect the public, preserve efficiency of the system, and extend



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