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Sewage is municipal waste water which consists mainly of human excreta, wash waters and industrial and agricultural wastes . It contains a large amount of organic matter and microbes.
In general, sewage contains 95.5% water and 0.1 to 0.5% organic and inorganic matter. They are very harmful to us due to presence of variety of microorganisms in them, most of which are highly pathogenic. Sewage water has less oxygen concentration which results in death of water animals and it also emits foul smell due to incomplete oxidation of organic materials in the sewage.