BOD is biological oxygen demand, or the oxygen required by aerobic biological organisms to decompose inorganics in water. It’s measured in milligrams per liter just like COD which is chemical oxygen demand. It also is measuring the amount of oxygen required by chemical reaction to decompose organics and some organics in solution. As such, the COD will always be higher than the BOD. The simple difference between the two is the BOD is decomposition by bugs, essentially, and the COD is the composition by chemical reaction.