Water pollution can be defined in many ways. Usually, it means one or more substances have built up in water to such an extent that they cause problems for animals or people. Oceans, lakes, rivers, and other inland waters can naturally clean up a certain amount of pollution by dispersing it harmlessly. If you poured a cup of black ink into a river, the ink would quickly disappear into the river's much larger volume of clean water. The ink would still be there in the river, but in such a low concentration that you would not be able to see it. At such low levels, the chemicals in the ink probably would not present any real problem.

Domestic sewage and industrial effluents:

• A mere 0.1 percent impurities make domestic sewage unfit for human use 
• Sewage contains dissolve salts like nitrates, phosphates, and other nutrients, and toxic metal ions and organic compounds. 
• The amount of organic matter in water is estimated by BOD
• Biochemical oxygen demand: the amount of Oxygen required oxidizing all organic matter present in one liter of water.

Changes take place on discharge of sewage into the river. 

• Micro-organism involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, hence there is sharp decline in dissolved oxygen downstream from the point of discharge.
• Due to low DO there is mortality of fish and other aquatic animals.