<prompt> Biochemical oxygen demand is determined by -Oranic matter and bacteriaOxygen contentAlage content in waterAgriculture feiliser content in water  <response>: 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 mgL,sewage is said to be strong ,if it is 100mgL 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 