Point Sources of Water Pollution
dissolved oxygen (DO) the oxygen freely available in water, vital to fish and other aquatic life and for the prevention of odors; DO levels are considered a most important indicator of a water body's ability to support desirable aquatic life; secondary and advanced waste treatment are generally designed to ensure adequate DO in waste-receiving waters priority pollutant a designated set of common water pollutants
Point sources of water pollution include municipal sewage treatment plant discharges and industrial plant discharges. Municipal sewage treatment plant point sources can contribute pollution in the form of oxygen-depleting nutrients and in the form of pathogens that cause serious health hazards in drinking water and swimming areas. Industrial point sources can contribute pollution in the form of toxic chemicals and heavy metals. Examples of nonpoint source water pollution include agricultural and urban runoff, and runoff from mining, and construction sites.
The Clean Water Act (CWA), passed by Congress in 1972, provides the basic structure for regulating the discharge of pollutants from point sources to waters of the United States. The CWA gives the EPA the authority to establish effluent limits. Effluent is the outflow from a municipal or industrial treatment plant. The CWA also requires the acquisition of a National Pollution Discharge Elimination System (NPDES) permit prior to the discharge of pollutants. States may be authorized to implement CWA programs, but the EPA retains oversight responsibilities.
The EPA manages effluent limits for point sources in two ways: through technology-based controls and through water quality-based controls. Industrywide effluent limits are established on a technology basis. These are minimum standards based on available treatment technology and pollution prevention measures. Effluent limits are also established on a water-quality basis. Water quality-based criteria are scientifically defensible standards that ensure protection of designated uses of a receiving water. Either standard may be superceded by the more stringent standard, as determined by the control authority.
Municipal point sources are the result of community sewage treatment systems. At the sewage treatment plant, wastewater is treated to remove solid and organic matter, disinfected to kill bacteria and viruses, and then often discharged to a surface water. Not all solids and organic matter are removed during treatment, resulting in degraded receiving water quality, due to a reduction in dissolved oxygen. Nutrients such as phosphorus that are not removed during treatment can cause overgrowth of algae and other organisms, also leading to lower dissolved oxygen. Many toxic substances can pass through conventional municipal treatment systems. Improperly treated sewage can be released as a result of upsets to the treatment process or as a result of operator error.
During heavy rain, discharges from sewage treatment systems can be a serious problem. In many municipalities, storm-water runoff is combined with municipal sewage in a common system. The increased water volume leads to reduced treatment. Combined sewer overflows occur when water flow exceeds treatment plant capacity, resulting in untreated sewage being discharged directly to rivers, lakes, or the ocean.
Industrial point sources are the result of industries using water in their production processes, and then treating the water prior to discharge. Some of the industries requiring process waters include pulp and paper mills, food processors, electronic equipment manufacturers, rare metal manufacturers, textile manufacturers, pharmaceutical manufacturers, forest product producers, leather tanners, and chemical manufacturers.
The National Pretreatment Program is charged with controlling the 126 priority pollutants from industries that discharge into sewer systems. These pollutants fall into two categories: metals and toxic organics. The metals include lead, mercury, chromium, and cadmium. The toxic organics include solvents, pesticides, dioxins, and polychlorinated biphenyls (PCBs).
Unlike municipal treatment methods, which are similar across the country, industrial treatment methods are industry-specific. For example, electroplating wastewater may require cyanide removal through oxidization. In general, physical processes may be used to remove solids and biological processes to remove organics. Chemical treatment, such as precipitation and neutralization, is also widely used.
The National Water Quality Inventory: 2000 Report is compiled based on the water quality reports required to be submitted to the EPA by states every two years. The report identifies "impaired" waters: water that cannot support its designated use, such as fishing or swimming, due to contamination. According to the report, municipal point sources contributed to 37 percent and industrial discharges contributed to 26 percent of reported water-quality problems in the impaired portion of estuaries. Municipal point sources were the leading cause of contamination in 21 percent of the impaired ocean shorelines, and industrial discharges were the leading cause in 17 percent. Municipal point sources were a leading source of contamination in 10 percent of the impaired river miles and 12 percent of the impaired lake acres. These figures are improved over the percentages recorded in the 1992 Report when municipal point sources were a leading contamination source in 15 percent of the impaired river miles and 21 percent of the impaired lake acres.
The NPDES permit program can be credited with achieving significant improvements to the water quality of the United States. Immediately following passage of the CWA, efforts focused mainly on regulating traditional point sources, such as municipal sewage plants and industrial facilities. In the late 1980s, efforts to address "wet weather point sources," such as urban storm sewer systems, began. Currently, there is a greater focus on nonpoint source pollution. The EPA is moving away from a source-by-source and pollutant-by-pollutant approach to a watershed-based approach. A watershed, or "place-based," approach is a process that emphasizes addressing all stressors within a hydrologically defined boundary or drainage basin. Equal emphasis is placed on protecting healthy waters and restoring impaired waters.
river mile one mile, as measured along a river's centerline lake acre an acre of lake surface
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