Micro-organisms are both the enemy and the servant of those involved in the control of surface water pollution as well as constituting the bottom of the food-chain existing in all water-courses on which, basically, depend all the higher aquatic life forms. Pathogenic—disease-producing—micro-organisms are the enemy in that the classical water-borne diseases of cholera, enteric fever and bacterial dysentery, together with many others, are not the least of the plagues of mankind and are of particular importance while so many people throughout the world depend on untreated surface waters for drinking, bathing and washing clothes. In contrast, a wide range of heterotrophic (and some autotrophic) saprophytes are employed for the stabilisation of waste organic matter which could otherwise create unacceptably high oxygen demands in receiving streams. The same micro-organisms are responsible for much of the natural processes of self-purification by which rivers are able to deal with limited additions of pollution.
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