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Understanding water quality
Understanding water quality

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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.
PollutantNatureCommon sourcesEffects of pollutionControl
natural organic materialbiodegradable organic materials; normally decomposed by aerobic bacteria (which require water-dissolved oxygen)domestic sewage; food-processing industries; farmsexcessive depletion of oxygen in water damages aquatic life; complete removal of oxygen causes anaerobic bacterial action on pollutants, resulting in offensive smellssewage treatment works, by physical and biological processes; containment of sewage, cattle slurry and silage effluent
living organismsdisease-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 nutrientsprincipally nitrogen and phosphorus compoundsdomestic 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 toxicserious problem: not removed by ordinary sewage treatment methods; very expensive to reduce
organic chemicalsdetergents, herbicides, pesticides, industrial by-products, medicinesdomestic sewage and industrial waste; farmspoison — threat to fish and other wildlife; possible long-term hazards to human beingsvery often not removed by usual sewage or water purification treatments
inorganic chemicalssalt, acids, metallic salts, cyanides, etc.mining; industrial processes; natural deposits (e.g. salt); road salting in wintertoxic effects on humans and wildlife; interference with manufacturing processes; bad smells and tastes; corrosion of equipmentdifficult: non-standard processes necessary
sedimentsprimarily soils and minerals; also some industrial by-productsland erosion by storms; flood waters; some industrial, quarrying and mining processesobstruction or filling of rivers, lakes, reservoirs; increased cost of water purification; interference with manufacturing processes; equipment corrosion; reduced aquatic life and diversitycontrolled by use of soil conservation and flood control methods; also by improvement of industrial technology; reduced by settling ponds
heatheated water returned to rivers and lakeselectric power plants; steel mills; refineries and other industrial cooling unitsreduction of oxygen in the water, resulting in slower or incomplete pollutant decomposition and harm to aquatic lifeminimised by recirculation and reuse of industrial cooling waters