Microorganism control in open recirculating cooling water systems has been practiced for years. Without such control, cooling tower systems quickly will become septic. A number of problems will manifest in the cooling systems, including lost evaporative efficiency, reduced water flow, corrosion and, in some cases, sources of infection.

     Algae, aerobic slime forming bacteria, fungi, and, in some situations, protozoa and pathogenic bacteria, is among the microorganisms that can infest an untreated cooling system.

     Algae will grow in the cooling tower or basin area if it is exposed to either direct or indirect sunlight. Direct sunlight entering the interior cooling tower structure will promote the growth of green algae species. Dimly lighted areas create an environment suitable for the growth of blue green algae. Algae can be either unicellular or filamentous, forming algal slimes or mats. In all cases, the uncontrolled growth of algae on the cooling tower fill or air flow paths will inhibit the evaporative cooling process.

     Excessive algae growth in a cooling tower will reduce cooling efficiency by as much as 50 percent. Algae are capable of manufacturing their own food from light and the inorganic salts dissolved in water. Contemporary cooling water management practices based on nontoxic treatment additives, mildly alkaline pH, and high dissolved solids provide the ideal environment for the rapid growth of algae.

Microbiological Fouling

     Like algae problems, microbiological fouling caused by slime forming bacteria will occur in an untreated cooling system. Waterborne aerobic bacteria prefer a warm, dark environment. In addition, these types of bacteria require oxygen and organic nutrients to sustain life and promote growth.

     Bacteria can be either planktonic (free swimming), or sessile (Biofilms). The concentration of bacteria freely swimming in the recirculating water is an indication of the nutrient level in the cooling system, but normally isn't a direct problem. When the bacteria attach to the structural components of the cooling system, including heat exchanger tubes, baffle plates, distribution lines and cooling tower fill, biofouling problems develop.

     Bacterial Slimes commonly referred to as biofilms, are gelatinous mats that resist the flow of water in distribution lines and through heat exchangers/condenser tubes. The growth of bacterial slimes can significantly increase the energy requirements of the cooling water recirculation pumps.

     Another problem caused by bacterial slimes is corrosion. Bacteria produce organic acids, giving the biofilm a mildly acidic environment. There organic acids can attack the cooling system metals underneath the biofilms.

     Fungi can cause microbiological problems, ranging from lumber decay or wood rot in wooden cooling towers, to fungal biofilms in condensers. Fungi are filamentous organisms ranging from mold to mushrooms. Although mushrooms are not ordinarily found in cooling tower systems, mold hyphae is common and can cause plugging of heat exchangers/condenser tubes.

     Like bacteria, fungi prefer warm, dark locations within the cooling system and require oxygen and organic nutrients. They prefer cellulose or starch containing nutrients, such as leaves, grains or flour. Fungal fouling commonly occurs in food processing plants and bakeries, or in cooling towers located near trees or shrubbery. Since the outbreak of Legionnaire's disease in 1976, pathogens in cooling tower systems have been a concern and a problem. Cooling towers exposed to airborne dirt will accumulate mud, providing a breeding ground for the Legionella bacterium. Decaying vegetable matter and bird droppings provide a source of organisms that can cause intestinal disorders.

     The best way to prevent microbiological problems in open recirculating cooling tower systems is to maintain a clean system. A clean system is free of silt, sediment, decaying organic debris, waterborne deposits and microbiological slimes.

     Clean systems can be achieved through the continuous use of treatment chemicals containing scale and corrosion inhibitors and polymeric dispersants. The regular use of broad spectrum microbiocide is essential for controlling microbiological populations. Only through proper treatment can a clean and efficient cooling system be maintained.