Explain Biochemical Oxygen Demand.
Biochemical oxygen demand (BOD) is a measure of oxygen that would be needed by the microorganisms to dicompose the organic and inorganic pollutants in polluted water. The microorganisms transform the pollutants into non-hazardous products (CO2, and H2O). During this process dissolved oxygen is consumed.
The inorganic pollutants may include sulphites, sulphides, thiosulphates and ferrous iron. These pollutants are attacked by oxygen in presence of enzymes released by microorganisms. For determination of BOD, the water sample is first saturated with oxygen and then incubated at constant temperature (usually 20°C) for five days. During this time the microorganisms in polluted water oxidize the pollutants completely. After five days, the remaining amount of dissolved oxygen is determined and the BOD is obtained by subtraction.
The result is called the 5-day BOD, and is expressed in milligram of oxygen per litre of water (mg/litre) or in ppm. The 5-day BOD analyses is considered as an accepted standard test. The BOD of 75 mg/litre of polluted water indicates that the biodegradation of organic matter in one litre of a sample consumes 75 mg of oxygen. Drinking water should have a 5-day BOD of less than 1.5 mg/litre. The BOD of raw sewage ranges from 200 — 400 ppm, indicating a strong sewage. The major contributors to BOD are the chemical industries, pulp and paper industries and the food processing industry. The BOD water status is given in below figure.
The BOD is not a true indication of pollution, but gives an idea of the problems arising due to waste materials in the water supplies. BOD is directly proportional to the organic matter oxidized.