2 Pollutants
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.
| 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 |