Biological Oxygen Demand or Biochemical Oxygen Demand (BOD):
Dead organic matter in streams decays. Bacteria carrying out this decay require oxygen. In case of enough bacterial activity, the oxygen in the water available to fish and other organisms get reduced to levels so low that they may die. A stream without oxygen is dead for fish and for many other organisms. A stream with an inadequate oxygen level is considered polluted for those organisms that require oxygen above the existing level. The amount of oxygen required for such biochemical decomposition is called the biological oxygen demand (BOD).
In other words, Biological Oxygen demand is defined as the amount of dissolved oxygen required by bacteria while oxidizing organic matter under aerobic conditions.
It is also defined as the amount of dissolved oxygen required to aerobically decompose biodegradable organic matter of a given volume of water over a period of 5 days at 20°C.
BOD measures the oxygen consumed by living organisms while assimilating organic matter present in the wastewater.
Chemical Oxygen Demand (COD):
Chemical oxygen demand is defined as the amount of oxygen used while oxidizing the organic matter of a sample with a strong chemical oxidant under acidic conditions.
Most types of organic matter are completely oxidized by boiling a mixture of chromic acid and sulphuric acid to produce CO2 and H2O.
Difference Between BOD and COD:
Biological Oxygen Demand measures the oxygen consumed by living organisms while assimilating organic matter present in the wastewater whereas Chemical oxygen demand is a measure of biologically oxidizable and biologically inert organic matter such as cellulose.
COD values are higher than BOD values. Also, COD determination is completed in 2-3 hours compared to 5 days required for the BOD determination.