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What are the differences between Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD)?

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Courtesy of Camlab Limited

Ask the expert: What are the differences between Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD)?

Answer: Both COD and BOD test methods aim to give an indication of the amount of pollution in a water sample. COD is the amount of oxygen required to chemically breakdown the pollutants whereas BOD is the amount of oxygen required to do this biologically through micro-organisms. There is a strong correlation between COD and BOD, however COD analysis is a much faster and more accurate method.

BOD analysis is performed to determine what effect dirty water, containing bacteria and organic materials, will have on animal and plant life when released into a stream or lake. Bacteria will need to take in oxygen in order to break down the organic materials (pollution) in the water. The test measures the potential of the incoming water to deplete the oxygen of the receiving waters due to the bacterial activity.

If there are high levels of organic waste and bacteria in the water it will have a detrimental effect on the surrounding ecosystem. Low levels of organic waste and fewer bacteria present mean the BOD will be lower and the dissolved oxygen levels higher. The BOD test involves taking an initial dissolved oxygen (DO) reading and a second reading after five days of incubation at 20°C.

COD analysis on the other hand is a measurement of the oxygen-depletion capacity of a water sample contaminated with organic waste matter. Specifically, it measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water thus removing the pollution. COD is an integral part of all water quality management programmes.

For decades there has been a traditional COD analysis technique using potassium dichromate and mercury which have now been deemed unsafe under the Europeans Union’s REACH regulations. This has lead to new innovations such as the PeCOD® , an automated COD method using safe, rapid and green technology. For more information on these alternative methods for COD analysis see our detailed blog post on the subject here.

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