Oxygen is used by microorganisms in the oxidation of organic material. The greater the amount of organic material, the greater the demand on the oxygen present in the freshwater ecosystem. Although dissolved oxygen (DO) levels give a good indication of the quality of a freshwater ecosystem, biological oxygen demand (BOD) determination gives a very clear indication of the level of organic pollution. The greater the level of organic substances, the greater the likelihood of high levels of microorganisms. The higher the bacterial count, the higher the demand of oxygen by the biota and consequently the lower the levels of DO. The BOD measure is the amount of oxygen consumed by organic matter and associated microorganisms in water over a five-day period.
Biological Oxygen Demand (BOD) determination requires a standard 5-day incubation period for a collected water sample. The sample is incubated in the dark for 5 days at 20 degrees Celsius. Bring back the water sample to the original stream temperature and conduct a further dissolved oxygen (DO) analysis by titration (Follow the link to the DO page for more information on the process for DO analysis).
Measure and calculate the BOD by using the following formula:
(initial DO level - final DO level) / (initial DO level) = BOD (%)
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