Removal of organic load from wastewater

Five-day biochemical oxygen demand, or BOD₅ , is one of the parameters of water quality. This BOD₅ measures the amount of biodegradable organic matter contained in a water. This biodegradable organic matter is evaluated through the oxygen consumed by the micro-organisms involved in the natural purification mechanisms. This parameter is expressed in milligrams of oxygen needed during five days to degrade the organic matter contained in one liter of water.

Chemical oxygen demand, or COD, is also one of the parameters of water quality. It represents the amount of oxygen needed to oxidize all the organic matter contained in a water.

This value is obtained by reacting water samples with a strong oxidant (potassium dichromate) and is expressed in milligrams of oxygen per liter of water.

COD consists of a particulate (or insoluble) fraction and a soluble fraction. The soluble fraction corresponds to the COD after filtration of the sample and the particulate fraction is obtained by the difference between the total COD and the COD after filtration of the sample (= soluble COD). It is essentially made up of Suspended Solids (SS).

The same applies to BOD₅: We can have the total BOD₅ of the wastewater analyzed, but also the soluble fraction of the BOD₅.

COD and BOD₅ are complementary parameters.

COD (Chemical Oxygen Demand) provides information on the total oxidizable material but does not provide information on its biodegradability.

BOD₅ (biochemical oxygen demand) indicates the amount of biodegradable material in an effluent.

The COD/BOD₅ ratio is used to qualify the biodegradability of an effluent:

• COD/BOD < 2: easily biodegradable effluent;
• 2 < COD/BOD < 4: moderately biodegradable effluent;
• COD/BOD > 4: effluent hardly biodegradable.

A physical-chemical treatment by coagulation and flocculation of wastewater can only abate the insoluble fraction of the water pollution (suspended matter, colloids, insoluble COD) but unfortunately this treatment step cannot reduce the dissolved load (soluble matter, soluble COD). The soluble fraction of the organic load can be reduced by a biological treatment.

It is therefore important when sampling the wastewater to ask for laboratory analysis of both total COD and soluble COD in order to have a clear picture of the type of wastewater to be treated and the best type of treatment to abate the organic load.