1. Definition of sewage: Sewage is the municipal waste, water containing large Quantities of human excreta.
  2. Composition: Sewage is formed of 99.9% water and only 0.1% of solid, impurities which are formed of suspended solids(e.g. sand, silt and clay), colloidal compounds(e.g. faces bacteria, cloth, paper fibers, etc.) and dissolved material(e.g. nitrates and phosphates of sodium, calcium, etc.

Treatment of sewage

Primary treatment: In this method mechanical screening and sedimentation of un-dissolved solids in raw sewage (e.g. floating polythene bags and other objects, large lumps of organic matter, sand and silty is done, so the sewage is passed through mesh screens of successively smaller pore sizes. After screening the sewage is passed through a grit chamber to separate sand and small pebbles by sedimentation. Finally the sewage is passed particles settle down to form the primary sludge. It fails to remove any dissolved substance in water. It does not remove the pathogens.

Secondary or Biological treatment: Sewage treated in primary treatment is brought in contact with oxygen and aerobic micro-organisms. They breakdown the organic matter into harmless materials as CO2 and H2O. Further, chlorination is done to kill the bacteria. It may be further treated to Tertiary level. Two measures are adopted for secondary treatment.

  1. Trickling filter method.
  2. Activated sludge method: The bacteria commonly used in sewage treatment are: Coliform, clostridium, pseudomonas, Micrococcus etc.

Advanced waste management or tertiary treatment: In this salts like nitrates and phosphates are removed by precipitation technique. Water is now pure enough to drink.

Biochemical Oxygen Demand (BOD) is a measure of the organic pollutant in The waste water. The greater is the BOD of waste water. More is the amount of Organic matter in the water so more is its pollution potential. The waste water having high amounts of organic wastes has high BOD because the microbes require more oxygen to decompose them which decreases the amount of dissolved oxygen in the water body. This shows that BOD has a direct relationship with the organic wastes while has an inverse relationship with dissolved oxygen. A sharp decline in DO causes increased fish mortality.


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