<prompt> O2 required to oxidize all pollutants in water isBiochemical oxygen demandChemical oxygen demandBiochemical indexoxygen index  <response>: Ans a Biochemical oxy... Ref park 20th ed p668The strength of sewage is expressed in terms of biochemical oxygen demand.BODBiological Oxygen Demand BOD is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high due to all the bacteria so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline.It is defined as the amount of oxygen absorbed by asample of sewage during a specified period, generally 5 days at a specified temperature, generally 20degc for the aerobic destruction or use of organic matter by living organism.BOD value ranges from 1 mg per litre for natural body waters to 300 mg litre for untreated domestic sewage.If BOD is 300m gl or above sewage is said to be strong and less tha lOOmgl it said to be weak. 