The biological oxygen demand indicates ?

Ans. is 'a' i.e., Organic matter Biochemical Oxygen demand It is the most impoant test done on sewage. It is defined as the amount of oxygen absorbed by a sample of sewage during a specified period, generally 5 days, at a specified temperature, generally 20 deg. C Pr the aerobic destruction or use of organic matter by living organisms. BOD value ranges from about 1 mg per litre far natural waters to about 300 mg per litre for untreated domestic sewage. If the HOD is 300 mg/1 and above, sewage is said to be strong ; if it is 100 mg/1, it is said to be weak".