sewage treatment

the treatment of raw sewage to make it harmless for discharge into rivers, seas and oceans. Treatment systems may involve

(a) Primary treatment in which physical methods are used to remove solid matter (primary sludge) from the waste-water.
(b) Secondary treatment in which biological methods, involving a range of aerobic bacteria, are used to degrade the organic matter in the waste-water. The treated sewage may then be disinfected before discharge from the sewage plant. The sludge that accumulates during both primary and secondary treatments is acted on by anaerobic bacteria in an anaerobic sludge digester. The methane produced can be used as a fuel for heating the digester, whilst the treated sludge is dried and may be used as a fertilizer.
(c) Tertiary treatment in which chemical and physical methods are generally used to remove all the biochemical oxygen demand (BOD), nitrogen and phosphorus. After this treatment the water is disinfected and can be pumped direct into reservoirs. However, tertiary treatment is an expensive process, so water is often returned to the environment after secondary treatment.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

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