What is the difference between COD and BOD in Sewage Treatment?

I have received a laboratory analysis report for effluent from my sewage treatment system and am wondering what the difference between BOD and COD is?  Do I need to get both BOD and COD tested?

Thank you for your question. COD stands for ‘Chemical Oxygen Demand’ where BOD stands for Biochemical Oxygen Demand.

It is a test of the amount of oxygen required to oxidise organic matter in a sewage sample by chemical oxidation with a powerful oxidising agent such Potassium Dichromate.  COD is closely related to BOD or Biochemical Oxygen Demand, the difference being that BOD is a test of the level of organic matter that can be biologically oxidised while COD is a test of the amount of organic matter that can be chemically oxidised.  (A full explanation of any of the parameters can be found by clicking on the blue links).

The higher the BOD/COD the more oxygen stripping capacity the discharged effluent has when discharged into receiving waters (oxygen is used biologically/chemically to break down the organic matter) and the more potential for damage to biological life in those waters.

COD is normally higher than BOD because more organic compounds can be chemically oxidised than biologically oxidised.  This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage as they will not be captured by BOD testing.

In regards to normal domestic effluent e.g. BOD & Suspended Solids (SS) 300 mg/l, which is usual for package sewage treatment systems, COD is not always stated as part of the discharge license.  If it is not part of your license you do not need to get it tested. However, COD does have a big advantage over BOD in that the test only takes approximately three hours, as opposed to the five days required for BOD testing and it is usually possible over a period of time to establish a ratio of BOD:COD, which allows extrapolation of the BOD.

More information can be found by clicking on the Chemical Oxygen Demand Link.

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