Biochemical oxygen demand
Measures oxygen consumed by microbes degrading organic matter in water.
Biochemical oxygen demand (BOD) is an analytical parameter that measures the amount of dissolved oxygen consumed by aerobic bacteria growing on organic material in a water sample over a specific time and temperature. It is commonly expressed as milligrams of oxygen per liter over five days at 20 °C and is used as a surrogate for the degree of organic water pollution. BOD reduction is a key gauge of wastewater treatment effectiveness, and BOD of effluents indicates short-term impact on oxygen levels in receiving waters.
- Standard incubation
- 5 days at 20 °C
- Typical pristine river bod
- below 1 mg/L
- Moderately polluted river bod
- 2 to 8 mg/L
- Severely polluted river bod
- exceeds 8 mg/L
- Treated sewage effluent bod
- about 20 mg/L or less
- Untreated sewage average europe
- around 600 mg/L
- Untreated sewage average us
- as low as 200 mg/L
- Us secondary treatment 85 percent remova
- 85 percent BOD removal required
- Us 30 day average effluent limit
- less than 30 mg/L
- Us 7 day average effluent limit
- less than 45 mg/L
Lore & Background
The Royal Commission on River Pollution, established in 1865, and the formation of the Royal Commission on Sewage Disposal in 1898 led to the selection in 1908 of BOD5 as the definitive test for organic pollution of rivers. Five days was chosen because it was supposedly the longest time river water takes to travel from source to estuary in the U.K. The Royal Commission's eighth report in 1912 first proposed a standard temperature of 65 °F (18.3 °C), later standardized at 68 °F and then 20 °C. The 5-day test protocol with acceptably reproducible results emphasizing carbonaceous BOD was endorsed by the United States Environmental Protection Agency.
Reader's Guide
BOD analysis is similar to chemical oxygen demand (COD) analysis, but COD is less specific as it measures everything that can be chemically oxidized, not just biologically oxidized organic matter. The BOD test is used to measure waste loadings to treatment plants and to evaluate BOD-removal efficiency. The standard dilution method (EPA Method 5210B) uses 300 mL incubation bottles with buffered dilution water dosed with seed microorganisms, stored for 5 days in the dark at 20 °C to prevent photosynthesis. Dissolved oxygen is measured before and after incubation using the Winkler method, published in 1888, which remains one of only two analytical techniques used to calibrate oxygen electrode meters. The 5-day BOD result represents the amount of oxygen required for aquatic microorganisms to stabilize decomposable organic matter under aerobic conditions. The United States includes BOD effluent limitations in secondary treatment regulations, generally requiring 85 percent BOD removal and effluent concentrations with a 30-day average less than 30 mg/L and a 7-day average less than 45 mg/L.
Did You Know?
- Up to 99 percent of total BOD is exerted within 20 days, 90 percent within 10 days, and approximately 68 percent within 5 days.
- The Winkler method for dissolved oxygen analysis, published in 1888, is still used to calibrate oxygen electrode meters.
- Untreated sewage averages around 600 mg/L BOD in Europe and as low as 200 mg/L in the U.S. due to greater water use per capita.
Frequently Asked Questions
What is Biochemical Oxygen Demand in aquarium water chemistry?
BOD is a laboratory measurement that quantifies how much dissolved oxygen aerobic bacteria consume while metabolizing organic matter in a water sample. In plain terms, it tells you how much 'oxygen hunger' the organic load in your water creates.
How is BOD actually measured in the lab?
The standard protocol incubates a sealed water sample at 20 °C for five days and records the resulting drop in dissolved oxygen, reporting the figure in milligrams of O₂ per liter. That five-day, 20 °C window is the universal reference point used to compare samples across labs and regions.
What BOD numbers should I worry about in my tank or source water?
A pristine freshwater source typically reads below 1 mg/L, while values between 2 and 8 mg/L indicate moderate organic loading. Readings above 8 mg/L signal severe pollution and a serious oxygen-depletion risk for fish and invertebrates.
Why do hobbyists and treatment plants both track BOD?
Because it acts as a practical proxy for overall organic pollution, a falling BOD after treatment is the go-to indicator that a wastewater system is performing. For aquarium keepers, a creeping BOD warns that uneaten food, excess waste, or decaying matter is quietly stripping oxygen from the water column.
How does BOD compare between treated and untreated sewage?
Untreated sewage in Europe averages roughly 600 mg/L, whereas properly treated effluent should be brought down to about 20 mg/L or less. That dramatic reduction is the core goal of secondary treatment—preserving the oxygen budget of rivers and lakes that receive the discharge.
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