Sewage is basically wastewater that contains faeces, urine, household and laundry waste, sewage needs to be treated in water treatment plant before letting it in sea body. Being organic in nature, sewage is biodegradable pollutant which can be broken down into simple substance in nature in due course of time by the action of certain microorganisms like bacteria. This process is called biodegradation/decomposition. Plastic, asbestos and mercury are non-biodegradable pollutants.