Environmental Considerations
Water is a favorable environment for many life forms. Water cooling may alter natural water environments and create new environments. Flow characteristics of recirculating cooling water systems encourage colonization by sessile organisms to use the circulating supply of food, oxygen and nutrients. Volumes of water lost during evaporative cooling may decrease natural habitat for aquatic organisms. Water temperature increases modify aquatic habitat by increasing biochemical reaction rates and decreasing oxygen saturation capacity of the habitat. Temperature increases initially favor a population shift from those requiring the high-oxygen concentration of cold water to those enjoying advantages of increased metabolic rates in warm water. Temperatures may become high enough to support thermophilic populations.
Biofouling of heat exchange surfaces can reduce heat transfer rates of the cooling system; and biofouling of cooling towers can alter flow distribution to reduce evaporative cooling rates. Biofouling may also create differential oxygen concentrations increasing corrosion rates. OTC and open recirculating systems are most susceptible to biofouling. Biofouling may be inhibited by temporary habitat modifications. Temperature differences may discourage establishment of thermophilic populations in intermittently operated facilities; and intentional short term temperature spikes may periodically kill less tolerant populations. Biocides have been commonly used to control biofouling where sustained facility operation is required.
Large OTC flow rates may immobilize slow-swimming organisms including fish and shrimp on screens protecting the small bore tubes of the heat exchangers from blockage. High temperatures or pump turbulence and shear may kill or disable smaller organisms passing the screens entrained with the cooling water. More agile aquatic predators consume organisms impinged on the screens; and warm water predators and scavengers colonize the cooling water discharge to feed on entrained organisms.
Manufactured metals tend to revert to ores via electrochemical reactions of corrosion. Water can accelerate corrosion as both an electrical conductor and solvent for metal ions and oxygen. Corrosion reactions proceed more rapidly as temperature increases. Preservation of machinery in the presence of hot water has been improved by addition of chemicals including zinc, chromates and phosphates. The first two have toxicity concerns; and the last has been associated with eutrophication. Residual concentrations of biocides and corrosion inhibitors are of potential concern for OTC and blowdown from open recirculating systems. With the exception of machines with short design life, closed recirculating systems require periodic cooling water treatment or replacement raising similar concern about ultimate disposal of cooling water containing chemicals used with environmental safety assumptions of a closed system.
Industrial cooling water regulations
The U.S. Clean Water Act requires the Environmental Protection Agency (EPA) to issue regulations on industrial cooling water intake structures. EPA issued final regulations for new facilities in 2001 (amended 2003). Other EPA regulations for existing facilities were challenged in litigation and EPA issued new proposed regulations in March 2011.
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