Biochemical Oxygen Demand Calculators

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Biochemical oxygen demand (BOD) is the amount of dissolved oxygen consumed by microorganisms while decomposing organic matter in water under aerobic conditions over a specified time period — typically 5 days at 20°C (BOD5). It is the most widely used indicator of organic pollution load in water and wastewater. High BOD indicates high organic contamination, which depletes dissolved oxygen in receiving waters, creating hypoxic conditions that kill fish and invertebrates. BOD measurement is central to wastewater treatment compliance, water quality monitoring, and environmental impact assessment.

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What Is BOD?

BOD measures the oxygen demand created by organic matter in water. When organic compounds enter a water body, heterotrophic bacteria decompose them aerobically — consuming dissolved oxygen in the process. If oxygen depletion exceeds replenishment from atmosphere and photosynthesis, the water body becomes anoxic, eliminating aerobic life.

BOD5 Measurement Procedure

  1. Prepare sample at appropriate dilution (standard method requires final DO > 2 mg/L at end of test)
  2. Measure initial DO (D₁) using Winkler titration or DO probe
  3. Incubate at 20°C in darkness for 5 days (prevents photosynthesis from adding O₂)
  4. Measure final DO (D₂)
  5. BOD₅ = (D₁ − D₂) × dilution factor (mg/L)

Typical BOD5 Values

  • Clean river water: 1–3 mg/L
  • Moderately polluted stream: 5–10 mg/L
  • Treated wastewater effluent: ≤20–30 mg/L (EPA secondary standard ≤30 mg/L)
  • Raw domestic sewage: 150–300 mg/L
  • Food processing wastewater: 500–2,000 mg/L
  • Brewery effluent: 1,000–5,000 mg/L

BOD vs. COD

  • BOD: Measures only biologically degradable organic matter. Slow (5-day test). Not affected by non-biodegradable compounds. Specific to biological treatment performance.
  • COD (Chemical Oxygen Demand): Measures total oxidizable organic matter using a chemical oxidant (K₂Cr₂O₇). Faster (2–3 hours). Includes non-biodegradable compounds. COD ≥ BOD always (COD:BOD ratio > 1).
  • For typical domestic sewage: COD:BOD ≈ 2–2.5

Nitrogenous BOD (NBOD)

In addition to carbonaceous BOD (CBOD), nitrification of ammonium exerts nitrogenous oxygen demand: NH₄⁺ + 2O₂ → NO₃⁻ + H₂O + 2H⁺ (2 mol O₂ per mol NH₄⁺). NBOD can be suppressed with allylthiourea (ATU) inhibitor to measure only CBOD.

Glossary

BOD5 (Biochemical Oxygen Demand)
Dissolved oxygen consumed by microorganisms decomposing organic matter over 5 days at 20°C. Measures organic pollution load. Clean water: <3 mg/L; EPA secondary effluent standard: ≤30 mg/L; raw sewage: 150–300 mg/L.
COD (Chemical Oxygen Demand)
Oxygen equivalent required to chemically oxidize all organic matter using K₂Cr₂O₇. Faster than BOD (2–3 hours). Includes non-biodegradable compounds. COD is always ≥ BOD. COD/BOD ratio indicates wastewater biodegradability.
Nitrogenous BOD (NBOD)
Oxygen demand from nitrification of ammonium: NH₄⁺ + 2O₂ → NO₃⁻ + H⁺ + H₂O. NBOD ≈ 4.57 g O₂/g NH₄⁺-N. Suppressed in BOD tests using nitrification inhibitors (allylthiourea) to measure only carbonaceous BOD.

Frequently Asked Questions

BOD5 is the amount of dissolved oxygen consumed by microorganisms decomposing organic matter over 5 days at 20°C in the dark. Measurement: dilute sample, measure initial dissolved oxygen (D₁), incubate 5 days at 20°C, measure final dissolved oxygen (D₂). BOD5 = (D₁ − D₂) × dilution factor (mg/L). The test requires initial DO >7 mg/L and final DO >2 mg/L; outside this range, repeat with different dilution.

High BOD indicates high organic pollution loading — abundant organic matter for bacteria to decompose aerobically. As bacteria consume oxygen, dissolved oxygen drops, stressing or killing fish (< 5 mg/L) and invertebrates. Severely polluted waters (BOD > 20 mg/L) may go anoxic, creating dead zones with only anaerobic organisms (producing H₂S, methane). Clean rivers have BOD < 3 mg/L; treated wastewater must meet ≤30 mg/L (EPA secondary standard) before discharge.

BOD (biochemical oxygen demand) measures only the oxygen demand from biologically degradable organic matter — requires live bacteria and 5 days. COD (chemical oxygen demand) uses a strong chemical oxidant (K₂Cr₂O₇) to oxidize all organic matter in 2–3 hours — including non-biodegradable compounds. COD is always ≥ BOD. COD/BOD ratio indicates biodegradability: ratio near 1 = easily biodegradable; ratio > 3 = significant non-biodegradable fraction that will not be removed by biological treatment.

In addition to carbon oxidation (CBOD), ammonium in wastewater is oxidized by nitrifying bacteria, consuming significant oxygen: NH₄⁺ + 2O₂ → NO₃⁻ + H₂O + 2H⁺ (NBOD ≈ 4.57 g O₂/g NH₄⁺-N). In a standard 5-day test, NBOD begins to appear after a lag of ~5–8 days, so it is usually minor in BOD5. In receiving waters, NBOD is significant when ammonium concentrations are high. Nitrification inhibitors (allylthiourea, TCMP) are added to suppress NBOD and measure only CBOD.