Water pollution is the contamiation by micoorganisms, chemicals, industrial and other wast, or sewage. These harmful materials deterioate the quality of water.
Nottable effects of water pollution is when it involes human health. Nitrates in drinking water can cuase disease in infants that sometimes results in death. Cadmium in sludge-derived fertilizers can be abosrbed by crops; if ingested insufficient amounts, the metal can cause an acute diarrheal disorder and liver and kidney damage. The hazardous nature of ingnorance 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. Runoff of chemical fertilizer from cultivated fields may trigger this. The process of eutrophication can produce aesthetic problems such as bad tastes and odors 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 the northeastern U.S. and Canada totally devoid of life.
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 treating municipal wastewater 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 using 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 percent of the capital and operational costs of a treatment plant.
The characteristics of industrial wastewater 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.