Aquatic Pollution and Coastal Management

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.2.Composition of Sewage

Sewage effluents contain a wide variety of dissolved and suspended impurities from municipal/domestic waste, rainwater and anything washed down. Eventhough, the quantity of sewage disposed varies from place to place, the chemical characteristics remain similar. In general, sewage contains dissolved solids, suspended solids, nutrients (N, P), sulphate, chloride and heavy metals (Fe, Cu, Co, Zn, Pb, Ni), bacteria and viruses. Sewage consists of 99% water and 1% solids. Of the solids present in sewage, 70% are organic and 30% are inorganic in nature. The organic fraction contains proteins (60%), carbohydrates (20%) and fats (10%). The inorganic fraction contains grit, salts and metals.

Components	Concentration (ppm)
Total Suspended Solids	200-300
Biological Oxygen Demand (BOD)	200-250
Chemical Oxygen Demand (COD)	350-450
Nitrogen	25-60
Phosphorus	5-10
Oil and Grease	80-120