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BOD - What is Biological Oxygen Demand?


Biological Oxygen Demand (BOD) is a laboratory test that calculates the oxygen demand of a discrete sample incubated over five days, used to indicate water quality. Alternative recognised methodologies such as TOC (Total Organic Carbon) allow continuous measurement and the prediction of poor water quality. The Protoc TOC analyser may be configured to rapidly report TOC, TC (Total Carbon) or DOC (Dissolved Organic Carbon) and include MCERTS accreditation.


A grab sample is incubated at 20°C over a 5-day period. The sample BOD is the DO consumption expressed in mg/L.

Limitations Of Test

The method takes five days to perform and uses a discrete sample. The test has poor reproducibility and toxic chemicals can inhibit measurement. Short-term BOD determinations magnify errors and still require relatively long sample periods. The test is not suited for on-line, continuous measurement.

Alternative On-line Methodology

Total Organic Carbon - TOC can be used to report the dissolved organic compounds in less than 3 minutes. By determining the TOC or dissolved organic component of a sample stream, a correlation may be set up with BOD that is typically a 1:1 relationship, for samples containing low concentrations of suspended solids. Peak organic loads may therefore be determined in real-time.

More Information

TOC - Total Organic Carbon