Biochemical Oxygen Demand Calculator
Enter the dissolved oxygen before and after incubation and the dilution factor to compute biochemical oxygen demand BOD = (DO_initial − DO_final) × dilution factor (mg/L), assessing organic pollution.
Input Data
Results
At a glance:Enter the dissolved oxygen before and after incubation and the dilution factor to compute biochemical oxygen demand BOD = (DO_initial − DO_final) × dilution factor (mg/L), assessing organic pollution.
Formula
BOD = (DO_initial − DO_final) × dilution factor P (mg/L).
$$\text{BOD} = (DO_i - DO_f) \times P$$How to Use
- Enter the initial DO (day 0) and final DO (after 5-day incubation).
- Enter the dilution factor P (1 if undiluted).
- The calculator returns the BOD.
FAQ
Why a 5-day incubation (BOD₅)?
Microbes need time to decompose organics; 5 days at 20°C decomposes most easily degradable organics and is the internationally standardized time, allowing different labs and samples to be compared. Variants like BOD₇ or estimated ultimate BOD exist, but BOD₅ is the most common. Incubation must be sealed, at a constant 20°C, and in the dark.
Why dilute the sample? How to compute the dilution factor?
Without dilution, heavily polluted samples exhaust the bottle's DO within 5 days and the measured consumption is capped, underestimating the true value. Dilution keeps enough DO at the endpoint to back-calculate the concentration. Dilution factor P = final diluted volume ÷ original sample volume; clean samples may skip dilution (P = 1).
How is BOD different from COD?
BOD counts only the oxygen consumed by microbes degrading organics within 5 days and needs a 5-day incubation; COD (chemical oxygen demand) uses a strong oxidant to oxidize almost all organics and can be measured in hours, with a value usually higher than BOD. The COD/BOD ratio roughly indicates biodegradability — closer to 1 means more readily biodegradable.
What interferences should I watch for when measuring BOD?
Temperature, pH, and seed-microbe activity all affect results; toxic substances (heavy metals, bactericides) inhibit microbes and make BOD falsely low. Without a nitrification inhibitor, nitrification near day 5 adds extra oxygen consumption and overestimates carbonaceous BOD. Dark incubation avoids oxygen production from algal photosynthesis interfering.
How much BOD counts as pollution?
Clean natural river water BOD₅ is usually below 2 mg/L, light pollution about 2–8 mg/L, moderate about 8–20 mg/L, and untreated sewage can reach 100–400 mg/L. Actual grading follows each region's water-quality standards; high BOD means heavy dissolved-oxygen depletion after discharge, threatening the aquatic ecosystem.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.