WATER TREATMENT

The goal of water treatment is to reduce or remove all contaminants that are present in the water. No water, irrespective of the original source, should be assumed to be completely free of contaminants. The most common process used for treatment of surface water and ground water consists of sedimentation, coagulation, filtration, disinfection, conditioning, softening, fluoridation, removal of tastes and odors, corrosion control, algae control, and aeration.

Sedimentation allows any coarse particles to settle out. Coagulation consists of forming flocculent particles in a liquid by adding a chemical such as alum; these particles then settle to the bottom. Filtration, as the name implies, is the passing of the water through a porous media; the amount of removal is a function of the filtering media. Disinfection kills most harmful organisms and pathogenic bacteria—chlorine is the most commonly used disinfecting agent. Softening means removal of materials that cause "hardness," such as calcium and magnesium. Corrosion is an electrochemical reaction in which metal deteriorates when it comes in contact with air, water, or soil.

In a typical municipal water treatment process, water flows through pumps to a rapid mix basin, then to a flocculation basin, to a settling basin, through filters to a clear well, then after disinfection, to storage tanks, and finally to the end users.

In areas that derive their water from rivers, pumps must be used since rivers are usually in low areas. Water enters the treatment plant at what is called the rapid-mix basin, where aluminum sulfate, polyelectrolytes, polymers, or lime and furic chloride are added as coagulants. The water flows next to the flocculation basins, where the coagulant mixes with the suspended solids. The coagulant is used to form suspended solids into clumps, or floc, which then settle out of the water. Floc forms when the particles from small solids gather to form larger particles. The water then slowly flows through settling basins where the floc settles from the water. Activated carbon is then added to the water to remove color, radioactivity, taste, and odor. Filtration then removes bacteria and turbidity from the water as it removes any remaining suspended solids and the activated carbon.

The water then enters a clear well, where additional chlorine is added to kill any pathogens which may be present. A minimum free-chlorine residual of at least 0.2 ppm is recommended in plants requiring sanitary protection through the whole water distribution system. In water supplies that are fluoridated, 1 milligram per liter of fluoride is added.

At this stage in the process, the water is potable, palatable, and ready for consumption. The water is moved into elevated tanks for storage through pumps. The water flows down from these tanks into the community.

Raw water and post-treatment water are tested for bacterial, physical, and chemical standards, particularly pH, color, and turbidity. The Safe Drinking Water Act of 1974 established maximum contaminant levels, which are the national drinking water standards. These apply to any water distribution system that serves at least twenty-five units daily. Standards may vary from state to state, but they cannot be lower than those prescribed by the federal government.

Mark G. Robson

(see also: Ambient Water Quality; Clean Water Act; Dissolved Solids; Drinking Water; Groundwater; Sanitation; Wastewater Treatment; Water Quality )

Bibliography

Koren, H., and Bisesi, M. (1997). Handbook of Environmental Health and Safety, Vol. II. Boca Raton, FL: Lewis Publishers.

Morgan, M. (1993). Environmental Health. Madison, WI: Brown & Benchmark.