What is the relationship between BOD, COD and TOC?

Document ID

Document ID TE174

Published Date

Published Date 18/08/2020
What is the relationship between BOD, COD and TOC?
Comparing BOD, COD, and TOC results
Biochemical Oxygen Demand (BOD) can be estimated from Chemical Oxygen Demand (COD) measurements, if a correlation is established. 

This requires historical data of BOD and COD measurements taken over a period of time. Typically, the average BOD result is divided by the average COD result to find a "conversion factor" between the two parameters. One would then multiply their COD results by this factor to estimate BOD.

COD values are almost always higher than BOD values for the same sample. As a result, the multiplication factor will usually be less than one.

Once established, the correlation will only apply to the sample used to create it (one couldn't use the correlation with samples taken from other water sources). If the sample composition changes significantly (due to temperature or seasonal variations), a new correlation may need to be established.

Total Organic Carbon (TOC) measurements can also be used to estimate BOD using a correlation. The same methods and considerations would apply.

Ultra Violet (UV) absorbance measurements have also been used to estimate these parameters by correlation.

More information on the relationship between BOD, COD and TOC can be found in the following document (this document is fairly old, so many of the instruments that are referenced are obsolete):

The Science of Chemical Oxygen Demand


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