Biochemical oxygen demand (BOD) refers to the amount of dissolved oxygen needed by microbes to break down organic material present in a given water sample at certain temperature over a specific time period. BOD is measured in a water sample during 5 days of incubation at 20℃, known as BOD5. BOD represents the amount of organic matter which can be decomposed by aquatic organisms. It also indirectly represents the degree of organic pollution of the water.