Biochemical oxygen demand is determined by -

Strength of sewage is expressed in terms of -biochemical oxygen demand -chemical oxygen demand Biochemical oxygen demand is defined as the amount of oxygen absorbed by a sample of sewage during a specified period ,generally 5 days,at a specified temparature,generally 20 deg.C for aerobic destruction or use of organic matter by living organisms .BOD values range from about 1 mg pee litre of natural waters to about 300 mg per litre for untreated domestic sewage.If BOD is 300 mg/L,sewage is said to be 'strong' ,if it is 100mg/L it is said to be 'weak'. Hence BOD is a measure of organic matter and bacteria. Parks textbook of preventive and social medicine.K Park. Edition 23. page no:763