These parameters enable estimation of the quantity of organic matter present in water.
BOD ( biochemical oxygen demand ) expresses the quantity of oxygen necessary for the breakdown of biodegradable organic matter in the water by the development of microorganisms under certain given conditions.
The conditions most commonly assumed are 5 days , when partial degradation occurs , at 20°C, in the absence of light and air; this is known as BOD5.
This measurement is widely used for monitoring sewage-treatment plant effluent, because it provides an estimate of the loading of biodegradable organic matter.
It is expressed in mg/l ( mg of O2 consumed per litre ).
COD ( chemical oxygen demand ) shows the quantity of oxygen necessary to oxidise the organic matter ( biodegradable or not ) in the water, using an oxidant ( potassium dichromate ).
The result is expressed in mg/l.
Oxidability is a measurement similar to COD, and is used in the case of low concentrations of organic matter ( COD < 40 mg/l ).
The oxidant used is potassium permanganate.