Water Pollution

Water Pollution, contamination of water by foreign matter such as micro-organisms, chemicals, industrial or other wastes, or sewage. Such matter deteriorates the quality of the water and renders it unfit for its intended uses.

MAJOR POLLUTANTS

The major pollutants of water are the following:

• Sewage and other oxygen-demanding wastes (largely carbonaceous organic material, the decomposition of which leads to oxygen depletion).
• Infectious agents.
• Plant nutrients that can stimulate the growth of aquatic plants, which then interfere with water uses and, when decaying, deplete the dissolved oxygen and produce disagreeable odours.
• Exotic organic chemicals, including pesticides, various industrial products, surface-active substances in detergents, and the decomposition products of other organic compounds.
• Petroleum, especially from oil spills.
• Inorganic minerals and chemical compounds.
• Sediments consisting of soil and mineral particles washed by storms and floodwater from croplands, unprotected soils, mine workings, roads, and bulldozed urban areas.
• Radioactive substances from the wastes of uranium and thorium mining and refining, from nuclear power plants, and from the industrial, medical, and scientific use of radioactive materials.

Heat may also be considered a pollutant when increased temperatures in bodies of water result from the discharge of cooling water by factories and power plants.

EFFECTS OF WATER POLLUTION

Notable effects of water pollution include those involved in human health. Nitrates (the salts of nitric acid) in drinking water can cause a disease in infants that sometimes results in death. Cadmium in sludge-derived fertilizer can be absorbed by crops; if ingested in sufficient amounts, the metal can cause an acute diarrhoeal disorder and liver and kidney damage. The hazardous nature of inorganic substances such as mercury, arsenic, and lead has long been known or strongly suspected.

Lakes are especially vulnerable to pollution. One problem, eutrophication, occurs when lake water becomes artificially enriched with nutrients, causing abnormal plant growth. Run-off of chemical fertilizer from cultivated fields may trigger this. The process of eutrophication can produce aesthetic problems such as bad tastes and odours and unsightly green scums of algae, as well as dense growth of rooted plants, oxygen depletion in the deeper waters and bottom sediments of lakes, and other chemical changes such as precipitation of calcium carbonate in hard waters. Another problem, of growing concern in recent years, is acid rain, which has left many lakes in northern and eastern Europe and north-eastern North America totally devoid of life.

SOURCES AND CONTROL

The major sources of water pollution can be classified as municipal, industrial, and agricultural.

Municipal water pollution consists of wastewater from homes and commercial establishments. For many years, the main goal of municipal sewage disposal was simply to reduce its content of suspended solids, oxygen-demanding materials, dissolved inorganic compounds (particularly compounds of phosphorus and nitrogen), and harmful bacteria. In recent years, however, more stress has been placed on improving the means of disposal of the solid residues from municipal treatment processes. The basic methods of treating municipal wastewater fall into three stages: primary treatment, including grit removal, screening, grinding, flocculation (aggregation of the solids), and sedimentation; secondary treatment, which entails oxidation of dissolved organic matter by means of biologically active sludge, which is then filtered off; and tertiary treatment, in which advanced biological methods of nitrogen removal and chemical and physical methods such as granular filtration and activated carbon adsorption are employed. The handling and disposal of solid residues can account for 25 to 50 per cent of the capital and operational costs of a treatment plant.

The characteristics of industrial wastewaters can differ markedly both within and among industries. The impact of industrial discharges depends not only on their collective characteristics, such as biochemical oxygen demand and the amount of suspended solids, but also on their content of specific inorganic and organic substances. Three options (which are not mutually exclusive) are available in controlling industrial wastewater. Control can take place at the point of generation within the plant; wastewater can be pretreated for discharge to municipal treatment systems; or wastewater can be treated completely at the plant and either reused or discharged directly into receiving waters.

Agriculture, including commercial livestock and poultry farming, is the source of many organic and inorganic pollutants in surface waters and groundwater. These contaminants include both sediment from the erosion of cropland and compounds of phosphorus and nitrogen that partly originate in animal wastes and commercial fertilizers. Animal wastes are high in oxygen-demanding material, nitrogen, and phosphorus, and they often harbour pathogenic organisms. Wastes from commercial feeders are contained and disposed of on land; their main threat to natural waters, therefore, is via run-off and leaching. Control may involve settling basins for liquids, limited biological treatment in aerobic or anaerobic lagoons, and a variety of other methods.