Biochemical Oxygen Demand
Oxygen consumed by microbes decomposing organic matter.
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How it works
Biochemical Oxygen Demand (BOD) measures how much dissolved oxygen microorganisms use while breaking down biodegradable organic matter in a water sample. The formula compares the dissolved oxygen before incubation (D1) and after incubation (D2), then divides by the fraction of the original sample present (P) to correct for dilution; this gives the oxygen demand of the undiluted water. A larger drop in dissolved oxygen means more microbial activity and therefore more organic pollution.
When to use it
Use this when assessing the organic pollution load of wastewater, river water, or effluent samples in environmental monitoring. It is especially useful in treatment plant testing and water-quality studies where you need a standardized measure of how much biodegradable material is present.
Worked example, step by step
A wastewater sample is diluted so that the original sample makes up 10% of the bottle (P = 0.10). Initial dissolved oxygen after setup: D1 = 8.6 mg/L Final dissolved oxygen after incubation: D2 = 2.1 mg/L BOD = (D1 - D2) / P BOD = (8.6 - 2.1) / 0.10 BOD = 6.5 / 0.10 BOD = 65 mg/L Final answer: the biochemical oxygen demand of the sample is 65 mg/L.
Common mistakes to avoid
- Forgetting to use the dilution fraction P, which underestimates BOD for diluted samples
- Reversing D1 and D2, which produces a negative value instead of oxygen consumption
- Using total dilution volume instead of the sample fraction actually present in the bottle
Did You Know?
Biochemical oxygen demand gauges water pollution by how much oxygen microbes need to digest the waste.
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