Sedimentation occurs naturally in reservoirs and is accomplished in treatment plants by basins or settling tanks. Plain sedimentation will not remove extremely fine or colloidal material within a reasonable time, and the process is used principally as a preliminary to other treatment methods.
Suspended solids, colloidal material, bacteria, and other organisms are filtered out by passing the water through a bed of sand or pulverized coal, or through a matrix of fibrous material supported on a perforated core. Soluble materials such as salts and metals in ionic form are not removed by filtration. See also Filtration.
There are several methods of treatment of water to kill living organisms, particularly pathogenic bacteria; the application of chlorine or chlorine compounds is the most common. Less frequently used methods include the use of ultraviolet light, ozone, or silver ions. Boiling is the favored household emergency measure.
Municipal water softening is common where the natural water has a hardness in excess of 150 parts per million. Two methods are used: (1) The water is treated with lime and soda ash to precipitate the calcium and magnesium as carbonate and hydroxide, after which the water is filtered; (2) the water is passed through a porous cation exchanger which has the ability of substituting sodium ions in the exchange medium for calcium and magnesium in the water. For high-pressure steam boilers or some other industrial processes, almost complete deionization of water is needed, and treatment includes both cation and anion exchangers. See also Ion exchange.
Aeration is a process of exposing water to air by dividing the water into small drops, by forcing air through the water, or by a combination of both. Aeration is used to add oxygen to water and to remove carbon dioxide, hydrogen sulfide, and taste-producing gases or vapors. See also Water pollution; Water supply engineering.
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.