Biochemical Oxygen Demand (BOD) is a way to measure organic pollution in water by looking at the rate at which micro-organisms in the water use up dissolved oxygen when they metabolize the organic pollutants. For each sample, dissolved oxygen (DO) is tested at the beginning and end of a 5-day, in-the-dark incubation at 20ºC. The difference in DO is the BOD result. Results are reported in mg (O2)/L.
BOD is a commonly required test for wastewater treatment facilities. The following post-treatment effluent limits have been established by the EPA (see CFR link below):
|7-day average (in mg/L)||30-day average (in mg/L)||min. % reduction during treatment|
What is biochemical oxygen demand and why is it important?
Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD also measures the chemical oxidation of inorganic matter (i.e., the extraction of oxygen from water via chemical reaction). A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 C). The rate of oxygen consumption in a stream is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.
BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die.
Sources of BOD include leaves and woody debris; dead plants and animals; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.
- EPA, "Dissolved Oxygen..." (see below)
Typical Scale of BOD Results
|result in mg/L|
|1 - 2||very good|
|3 - 5||moderately clean|
|6 - 9||somewhat polluted|
|1000+||food processing waste|
Carbonaceous Biochemical Oxygen Demand (CBOD)
Carbonaceous BOD is a subset of BOD. BOD results are based on DO depletion from both carbonaceous and nitrogenous actors in a wastewater sample. CBOD measures DO depletion from only carbonaceous sources. Regulators may assign either BOD or CBOD, or both, to wastewater treatment facilities. Method SM 5310B is used for both BOD and CBOD.
Encyclopaedia Britannica: BOD
Good critique of BOD methodology: AZO Materials, "Monitoring Wastewater Treatment Plants using On-Line TOC Analysis"
University of Georgia Extension, "Understanding Laboratory Wastewater Tests: I. ORGANICS (BOD, COD, TOC, O&G)"
Wikipedia: "Biochemical Oxygen Demand"
Washington State Department of Ecology: "Supplemental Guidance for the Determination of BOD and CBOD in Water and Wastewater"
40 CFR (Code of Federal Regulations) 133.102 - Secondary treatment: https://www.law.cornell.edu/cfr/text/40/133.102
MEL Test Method for BOD and CBOD
Wastewater - SM 5210B, 18th & 22nd ed. (NPDES compliant)
Container: plastic, glass
Volume: 1 liter
Hold Time: 24 HOURS