Oxygen Demand (BOD/COD)

Oxygen demand measures how much dissolved oxygen the organic and oxidizable matter in water or effluent will consume, indexing organic pollution strength. Biochemical oxygen demand (BOD) is the oxygen used by aerobic microorganisms to break down organic matter, measured as BOD5 over five days at 20 degrees C. Chemical oxygen demand (COD) is the oxygen equivalent to oxidize organic and inorganic matter chemically, run with dichromate in a few hours and always at least as high as BOD. The COD-to-BOD ratio indexes biodegradability. Sewage works track both to size treatment and to set effluent limits, since high-demand discharge depletes oxygen in the receiving water and can drive hypoxia.