Biochemical Oxygen Demand Calculator

When organic waste enters a water body, bacteria decompose it by consuming dissolved oxygen. If the oxygen demand exceeds the water's natural reaeration rate, dissolved oxygen falls below the 5 mg/L threshold for fish survival — a dissolved oxygen sag that can make kilometers of river uninhabitable. BOD (Biochemical Oxygen Demand) measures this decomposition pressure by quantifying exactly how much oxygen a sample will consume over a standard 5-day incubation period at 20°C. The BOD calculator applies the standard 5-day dilution method to your dissolved oxygen measurements.

BOD₅ Calculation: The Dilution Method

Because raw sewage would consume all available oxygen before 5 days, samples are diluted with oxygen-saturated water before incubation. The standard BOD₅ formula:

BOD₅ (mg/L) = [(D₁ − D₂) − (B₁ − B₂) × f] / P

where D₁ = initial DO of diluted sample (mg/L), D₂ = final DO of diluted sample after 5 days at 20°C (mg/L), B₁ = initial DO of blank control (seeded dilution water only), B₂ = final DO of blank after 5 days, f = ratio of seed in diluted sample to seed in blank, P = decimal fraction of sample used (volume sample / total bottle volume). For D₁ = 8.2, D₂ = 3.1, blank B₁ = 8.2, B₂ = 7.9, no significant seeding (f × (B₁−B₂) negligible), P = 0.02 (2% dilution): BOD₅ = (8.2 − 3.1) / 0.02 = 255 mg/L. Use this online calculator for any dilution setup. The BOD kinetics calculator models the first-order BOD decay rate constant.

Reference BOD Values by Source

Typical BOD₅ values provide context for interpreting measurements:

• Clean stream water: 1–2 mg/L — healthy, oligotrophic conditions
• Slightly polluted stream: 3–5 mg/L — eutrophic but still supporting most aquatic life
• Polluted water: 5–10 mg/L — significant oxygen stress on sensitive species
• Raw domestic sewage: 150–350 mg/L — primary treatment target
• After primary treatment: 100–200 mg/L
• After secondary treatment (activated sludge): 20–30 mg/L
• After tertiary treatment: below 5 mg/L — river discharge quality
• Food processing wastewater: 1,000–10,000+ mg/L — high organic load requiring industrial treatment

BOD vs. COD: Why Both Are Measured

Chemical Oxygen Demand (COD) measures all oxidizable organic matter (biodegradable and non-biodegradable) using a strong chemical oxidant (potassium dichromate). BOD measures only the oxygen consumed by biological decomposition. The BOD/COD ratio characterizes wastewater treatability:

• BOD/COD above 0.5: readily biodegradable; biological treatment is efficient
• BOD/COD 0.3–0.5: moderately biodegradable; biological treatment feasible
• BOD/COD below 0.3: significant non-biodegradable fraction; chemical treatment likely needed

Municipal sewage typically has BOD/COD of 0.4–0.6; industrial wastewater with synthetic chemicals may fall below 0.1. The alkalinity calculator and water chemistry calculators provide complementary water quality analysis tools.

The Dissolved Oxygen Sag Curve

The Streeter-Phelps model (1925) predicts how dissolved oxygen in a river changes downstream from a wastewater discharge point, balancing oxygen depletion (from BOD decomposition) against oxygen re-aeration from the atmosphere. The critical DO deficit and its location downstream determine whether the river violates water quality standards. The model requires knowing the BOD of the discharge and the river's reaeration coefficient — with the BOD calculator providing the discharge characterization needed for this analysis. Modern water quality models (QUAL2K, WASP) are more sophisticated descendants of the original Streeter-Phelps framework.