Understanding water quality

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.

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