<prompt> Biochemical oxygen demand is determined byOranic matter and bacteriaOxygen contentAlage content in waterAgriculture fertiliser content in water.  <response>: Ans. is a . Organic matter Bacteria. The strength of sewage is expressed in terms ofBiochemical oxygen demandChemical oxygen demandsSuspended solids demandBiochemical Oxygen demand -It is the most important 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. G, for the aerobic destruction or use by organic matter and living organisms. BOD value ranges from about 1 mg per litre for natural waters to about 300 mg per litre for untreated domestic sewage. If the BOD is 300 mgI and above, sewage is said to be strong ; if it is 100 mgl, it is said to be weak. 