Households, industries, hospitals, offices, etc, dispose of wastewater that is considered to be sewage. During a storm or heavy rain, the sewage also involves rainwater that runs down the street. The washed-off water from roads and rooftops contains harmful substances in it. Sewage is considered to be a liquid waste. Usually, a large portion of sewage contains water, which has liquified and suspended impurities. These impurities are known as contaminants.