River Pollution

Some human activities are harmful to rivers: Large factories built alongside rivers use enormous amounts of water for cooling and other purposes. Then they return the water to the river at overheated temperatures. The unnaturally hot water disturbs the ecology of the river and kills fish. Industries dump harmful chemical waste into rivers. Cities near rivers contribute to the problem by releasing their wastes into the water.  Chemical fertilisers or pesticides (insect-killing substances) on surrounding land get into the groundwater and can enter a river. Pollution does not just affect the creatures that live in the water. When people eat fish taken from polluted streams, the pollution passes into their bodies and can cause cancer or other health problems. Water is usually cleaned before it is piped into people's homes. This involves: screening to remove any large objects, clarification where a chemical is added to cause particles to clump together, filtration through sand and gravel beds and disinfection often using chlorine.