Cooling tower water pollution
|Pollutants in cooling tower water |
Water that is applied in cooling towers, even when this concerns tap water, often contains salts (such as chlorine, sulphates and carbonates), dissolved gases (such as oxygen and carbon dioxide) and metal ions (such as iron and manganese ions). The presence of these pollutants can cause a series of problems. The main problems that are caused, are fouling, limestone formation, corrosion and biological growth. The pollutants that are present depend on cooling tower building material, as well. Cooling towers are build of concrete, wood, plastic or metal.
Bacteria and other pathogenic mircroorganisms are present everywhere throughout the environment. They can often be found in cooling tower water. When cooling towers contain an open recirculation system, microorganisms can spread from air to water. Microorganisms can rapidly multiply, when a substrate is present and a number of conditions are ideal for microbial growth. Examples are pH, temperature, oxygen concentration and nutrients. The nutrient content in water increases, because of water evaporation. Process leaks and water use can also cause the nutrient content in the water to increase. This can cause problems.
When a significant microbial growth takes place, a slime layer is formed. This contains both organic and inorganic matter. Some microorganisms excrete polymers, which can form a gel-like network around cells after hydrolysis takes place. This is called a bio film. As a result of bio film formation, microorganisms can attach themselves to surface layers. This causes microorganisms to no longer be flushed away by cooling tower water. Bio films protect microorganisms from other microorganisms and from toxic disinfectants. This causes water disinfection to be much more difficult when a bio film is present.
Bio film partly consists of microbiological cells and components. Bio film, which is very sticky, also contains organic and inorganic matter that is present in the water and is absorbed by the film. This concerns chemical precipitation, organic flakes and dead cell mass. Bio film consists of 90% water.
Bio film causes a number of problems, knowing:
Within the protected slime layer microorganisms can cause a speedy corrosion, causing the walls of cooling towers and heat exchange systems to be corroded. The bio film prevents materials that cause corrosion protection from reaching the surface. Furthermore, microbiological reactions can accelerate corrosion reactions and microbial products can corrode materials.
Bio film creates an isolation layer on heat-exchange systems, causing them to no longer function properly. Microorganisms present in the bio film accelerate oxygen uptake. This can cause an oxygen deficiency in the system. Some microorganisms switch to fermentative metabolisms and produce a number of organic acids, which causes a decrease in pH. Anaerobic bacteria form sulphide byproducts, which are corrosive.
More information on water disinfection?:
What is water disinfection? Necessity of drinking water disinfection History of water disinfection Waterborne diseases Factors that influence disinfection Conditions of water disinfection Regulation drinking water disinfection EU USA