BOD Calculators

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Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen consumed by biological organisms when decomposing organic matter in water at a specific temperature over a given time period — typically 5 days at 20°C (BOD₅). It is a key indicator of organic pollution in surface waters and wastewater. High BOD indicates high organic pollution → microbial decomposition consumes dissolved oxygen → hypoxia → fish kills. Clean water typically has BOD₅ < 2 mg/L; moderately polluted rivers: 5–10 mg/L; raw municipal wastewater: 200–400 mg/L. BOD₅ is a primary treatment performance parameter for wastewater treatment plants.

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BOD₅ Measurement

Standard test: incubate diluted water sample in sealed BOD bottle at 20°C for 5 days. Measure dissolved oxygen (DO) at day 0 (DO₀) and day 5 (DO₅). BOD₅ = (DO₀ − DO₅) × dilution factor. Dilution: samples must be diluted so DO drop is between 2–8 mg/L (prevents full depletion). Use nitrification inhibitor (allylthiourea) to prevent nitrification consuming additional O₂ (nitrogenous BOD vs. carbonaceous BOD separation).

Ultimate BOD and Kinetics

BOD kinetics: BOD_t = BOD_ultimate × (1 − e^(−k₁t)). k₁ = first-order BOD rate constant (typically 0.1–0.3 day⁻¹ at 20°C). BOD_ultimate = total oxygen demand for complete oxidation (all organic carbon → CO₂ + H₂O). BOD₅ ≈ 68% of BOD_ultimate for typical wastewater (k₁ ≈ 0.23/day).

BOD vs. COD

COD (Chemical Oxygen Demand): measures all oxidizable material (including non-biodegradable) using a strong chemical oxidant (K₂Cr₂O₇ reflux). COD > BOD₅ (all organic material counted in COD; only biodegradable counted in BOD). COD/BOD ratio indicates biodegradability: COD/BOD ≈ 1.5–2: easily biodegradable. COD/BOD > 4: significant non-biodegradable fraction → biological treatment may be insufficient.

Glossary

BOD₅ (Biochemical Oxygen Demand)
O₂ consumed during 5-day microbial decomposition of organic matter at 20°C (mg/L); clean water < 2 mg/L; raw sewage 200–400 mg/L; measures biodegradable organic pollution in water.
COD (Chemical Oxygen Demand)
O₂ equivalent of all oxidizable material (biodegradable + non-biodegradable) using K₂Cr₂O₇ reflux; always > BOD₅; COD/BOD₅ ratio indicates biodegradability; fast (3 hours) vs. BOD₅ (5 days).
Ultimate BOD
Total O₂ demand for complete oxidation of all organic matter; BOD_t = BOD_ult × (1 − e^(−k₁t)); BOD₅ ≈ 68% of BOD_ult for typical wastewater with k₁ ≈ 0.23/day.

Frequently Asked Questions

BOD (Biochemical Oxygen Demand) measures the amount of dissolved oxygen consumed by microorganisms when breaking down organic matter in water over 5 days at 20°C (BOD₅). It is an indirect measure of organic pollution — more organic pollution = more microbial activity = more O₂ consumed = higher BOD. BOD₅ values: Clean rivers: < 2 mg/L. Moderately polluted: 5–10 mg/L. Raw municipal sewage: 200–400 mg/L. Industrial wastewater: can exceed 1,000 mg/L. Why it matters: high BOD → microbial decomposition depletes dissolved oxygen in receiving waters → hypoxic or anoxic conditions → fish kills → ecosystem collapse.

Standard BOD₅ test (APHA method): (1) Dilute sample appropriately (target DO drop of 2–8 mg/L over 5 days). (2) Add seed microorganisms (from acclimated sewage or effluent) to ensure sufficient biodegradation. (3) Measure initial DO (DO₀) using dissolved oxygen meter or Winkler titration. (4) Seal BOD bottles (no headspace) to prevent O₂ exchange. (5) Incubate at 20°C ± 1°C for exactly 5 days in the dark. (6) Measure final DO (DO₅). (7) BOD₅ (mg/L) = (DO₀ − DO₅) × dilution factor. Correction: subtract blank BOD (oxygen demand of seed and dilution water without the sample).

BOD (Biochemical Oxygen Demand): measures only biodegradable organic matter (what microorganisms can break down). Time-consuming: 5-day incubation. BOD₅ ≈ 60–90% of BOD_ultimate. COD (Chemical Oxygen Demand): measures total oxidizable material using a chemical oxidant (K₂Cr₂O₇ + heat). Both biodegradable and non-biodegradable organics measured. Faster (3 hours). COD/BOD₅ ratio: COD/BOD₅ ≈ 1.5–2 → easily biodegradable effluent. COD/BOD₅ > 4 → significant non-biodegradable organics → may require advanced treatment beyond biological processes. For wastewater treatment design: BOD₅ is used to size biological treatment systems; COD is used for complete oxygen balance and assessing non-biodegradable fraction.

BOD removal at each treatment stage: Primary treatment (settling): 25–40% BOD removal. Removes settleable solids and associated BOD. Secondary treatment (activated sludge or trickling filter): 85–95% BOD removal from raw wastewater. Biological oxidation of dissolved and colloidal BOD. Effluent typically < 20–30 mg/L BOD. Tertiary/advanced treatment: > 95% overall removal. Polishing steps (coagulation, filtration, UV/chlorine disinfection). Effluent BOD: < 5 mg/L for sensitive receiving waters. Regulatory limits: US EPA secondary treatment standard: BOD₅ ≤ 30 mg/L (30-day average). More stringent limits for discharges to high-value or sensitive water bodies (rivers, estuaries).