80.

What do you mean by BOD and COD? How are these determined?

BOD: BOD stands for biochemical oxygen demand. It is a measure of the dissolved oxygen that would be needed by the microorganism to oxidise some of the inorganic and organic compounds present in the polluted water. BOD, therefore, is a measure of the contamination caused by the totality of these compounds which can be oxidised in the presence of micro-organisms. The BOD is taken as the measure of water quality. Clean water has BOD value of less than 5 ppm whereas highly polluted water has BOD value of 17ppm or more.

Determination of BOD: The water sample is first saturated with oxygen. It is then kept at constant temperature (usually 20°C) for five days. In this period of time, the micro-organisms in the water sample oxidise pollutants. The remaining amount of dissolved oxygen is determined and BOD is obtained by subtraction.

COD: COD is another parameter called the chemical oxygen demand. It is expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants.

Determination of COD: The water sample is treated with a known quantity of an oxidising agent such as potassium dichromate in acidic medium. This oxidises most of the polluting substances, including those which are resultant to microbial oxidation. The remaining K₂Cr₂O₇ is determined by back titration using standard solution of Mohr’s salt (reducing agent). From the concentration of K₂Cr₂O₇ used up, the amount of oxygen used in the oxidation may be calculated from the following chemical equation:



The results are expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants. This is called COD.