Biochemical Oxygen Demand MCQ - Practice Questions with Answers

Dissolved oxygen(DO) is essential for sustaining animal and plant life in any aquatic system. If the DO level in water decreases, the aquatic organisms may not survive. The wastes such as domestic, industrial and biodegradable organic compounds are oxygen demanding wastes. These are decomposed by the bacterial population which in turn decreases the oxygen from water.
BOD is a measure of the oxygen utilised by micro-organisms during oxidation of organic materials. It is the most widely known measure for assessing the water pollution of a given organic waste. BOD is directly proportional to the amount of organic waste which is to be broken down.
The determination of BOD of a sample of water requires 20-30 days for the complete decomposition of organic waste present in the sample. This is too long time for obtaining data. Therefore, the time has been fixed as five days, i.e, BOD is the amount of oxygen consumed in five days. The measurement is done at 20^oC. The sample of water is saturated with oxygen. It is then incubated for five days at 20^oC. Micro-organisms utilise oxygen to oxidise the waste during this period. The remaining oxygen is measured and subtracted from the amount of oxygen originally present to get the BOD value. It is expressed in ppm.
BOD is a real measure of water quality. Clean water has a BOD value of less than 5 ppm. The polluted river water could have a BOD value of 17ppm or more. The untreated municipal sewage has BOD value of 100-400 ppm.