Biochemical oxygen demand

What Is Biochemical Oxygen Demand?

Biochemical oxygen demand, abbreviated BOD, is the mass of dissolved oxygen that aerobic microorganisms consume while decomposing the organic matter in a given volume of water under defined conditions. It is reported in milligrams of oxygen per liter and is measured rather than calculated, because the quantity depends on which compounds are present and on how readily the resident microbial community can metabolize them. BOD is therefore an operationally defined parameter: it describes an oxygen demand under a specified incubation protocol, not a fixed chemical property of the sample.

The parameter matters because dissolved oxygen in a river or lake is finite and slowly replenished. When organic loading is high, bacterial respiration draws oxygen down faster than reaeration restores it, and concentrations can fall below the roughly 4 to 5 milligrams per liter that most fish require. A high BOD reading is thus a warning about the receiving water body rather than a description of a toxin. The water science summary of biochemical oxygen demand published by the US Geological Survey explains how sewage discharge, agricultural runoff, and urban stormwater raise the value and what that implies for aquatic habitat.

The Five-Day Test

The standard procedure, known as BOD5, fills an airtight 300 milliliter bottle with sample, measures dissolved oxygen immediately, seals the bottle in the dark at 20 degrees Celsius for five days, and measures dissolved oxygen again. The difference, corrected for any dilution, is the result. Dilution with aerated buffered water is normally required because the test can only resolve depletion within the initial oxygen content, and strong wastewater would exhaust the bottle long before day five. Samples low in bacteria are seeded, and nutrient salts of phosphate, magnesium, calcium, and iron are added so that microbial growth is limited by the organic carbon rather than by anything else. Incubation in darkness suppresses photosynthetic oxygen production that would otherwise offset respiration. Field and laboratory protocols, including holding times, quality control checks, and dilution selection, are set out in the five-day biochemical oxygen demand chapter of the USGS national field manual.

Two distinct populations consume oxygen in the bottle. Heterotrophic bacteria oxidize carbon compounds from the first day, while nitrifying bacteria oxidize ammonia to nitrate on a slower schedule that usually becomes significant after roughly five to eight days. Where the nitrogenous contribution would confound interpretation, an inhibitor such as allylthiourea is added and the result is reported as carbonaceous BOD, or cBOD5. Chemical oxygen demand and total organic carbon serve as faster proxies, returning results in hours rather than days, but they oxidize compounds that microorganisms cannot reach, so their ratio to BOD varies by effluent type and must be calibrated locally. The five-day window itself is a historical compromise, chosen because it approximates the travel time of British rivers to the sea, and it typically captures 60 to 70 percent of the ultimate demand.

Regulatory and Operational Use

BOD is a conventional pollutant under the US Clean Water Act, and effluent limits for publicly owned treatment works are written directly in terms of it. Federal secondary treatment regulations at 40 CFR 133.102 require a 30-day average of no more than 30 milligrams per liter of BOD5, or 25 milligrams per liter of cBOD5, together with a minimum 85 percent removal. Plant operators also use influent and effluent BOD to size aeration basins, set sludge age, and verify that biological treatment is performing as designed. The five-day delay is a persistent operational drawback, which has driven development of respirometric analyzers and microbial fuel cell biosensors that estimate the value in minutes to hours.

Applications

Biochemical oxygen demand has applications in a range of fields, including:

  • Municipal and industrial wastewater treatment plant design and control
  • Discharge permitting and regulatory compliance monitoring
  • River and lake water quality assessment and modeling
  • Evaluation of agricultural and stormwater runoff impacts
  • Aquaculture and fish hatchery water management
  • Food, brewing, and pulp and paper process effluent characterization