We shall now look at the most common water pollutants, their sources, their effects, and how they can be controlled. A summary is provided in Table 1 and some points are discussed below in more detail.
Table 1 The nature, sources, effects and control of some major types of pollutants.
|Pollutant||Nature||Common sources||Effects of pollution||Control|
|natural organic material||biodegradable organic materials; normally decomposed by aerobic bacteria (which require water-dissolved oxygen)||domestic sewage; food-processing industries; farms||excessive depletion of oxygen in water damages aquatic life; complete removal of oxygen causes anaerobic bacterial action on pollutants, resulting in offensive smells||sewage treatment works, by physical and biological processes; containment of sewage, cattle slurry and silage effluent|
|living organisms||disease-causing organisms (bacteria, viruses)||human and animal wastes; certain industries (e.g. tanning, slaughtering)||curtailed recreational use of rivers, lakes, etc.||most commonly controlled with chlorine; seldom possible to remove all bacterial and viral contamination, but concentrations are greatly reduced|
|plant nutrients||principally nitrogen and phosphorus compounds||domestic sewage; industrial wastes; farms (especially from chemical fertilizers)||excessive growth of aquatic plant life leads to oxygen depletion, offensive smells, bad taste; excess nitrate in drinking water could be toxic||serious problem: not removed by ordinary sewage treatment methods; very expensive to reduce|
|organic chemicals||detergents, herbicides, pesticides, industrial by-products, medicines||domestic sewage and industrial waste; farms||poison — threat to fish and other wildlife; possible long-term hazards to human beings||very often not removed by usual sewage or water purification treatments|
|inorganic chemicals||salt, acids, metallic salts, cyanides, etc.||mining; industrial processes; natural deposits (e.g. salt); road salting in winter||toxic effects on humans and wildlife; interference with manufacturing processes; bad smells and tastes; corrosion of equipment||difficult: non-standard processes necessary|
|sediments||primarily soils and minerals; also some industrial by-products||land erosion by storms; flood waters; some industrial, quarrying and mining processes||obstruction or filling of rivers, lakes, reservoirs; increased cost of water purification; interference with manufacturing processes; equipment corrosion; reduced aquatic life and diversity||controlled by use of soil conservation and flood control methods; also by improvement of industrial technology; reduced by settling ponds|
|heat||heated water returned to rivers and lakes||electric power plants; steel mills; refineries and other industrial cooling units||reduction of oxygen in the water, resulting in slower or incomplete pollutant decomposition and harm to aquatic life||minimised by recirculation and reuse of industrial cooling waters|