Biochemical oxygen demand is an impoant test done on sewage. It is defined as the amount of oxygen adsorbed by a sample of sewage during a specified period, generally 5 days, at a specified temperature for the aerobic destruction or use of organic matter by living organisms. If BOD more than 300mg/L - strong sewage Less than 100mg/L - weak sewage Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 823