Degree of water impurity

From the study of aquatic life the degree and type of water pollution can be known e.g. Daphuia and most of the fishes are sensitive to poisons and metals. Water pollution by organic wastes is measured in terms of biochemical oxygen demand (BOD). It is the oxygen required in milligrams for 5 days to metabolise waster present in one litre of water at 200C.

A weak organic waste will have BOD below 1500 mg/litre, medium between 1500-1400 mg/litre, while a strong waste above it. Another slightly better mode is COD or chemical oxygen demand. It measures all oxygen consuming pollutant material present in water.