Biochemical oxygen demand

Biochemical Oxygen Demand (BOD) is a Celsius. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it could be considered as an indication of the quality of a water source.

Typical BOD values

Most pristine rivers will have a 5-day BOD below 1 mg/l. Moderately groundwater or surface water infiltration. (The generally lower values in the U.S. derive from the much greater water use per capita than other parts of the world. |}

The BOD₅ test

BOD measures the rate of oxygen uptake by de-ionized water. The test generally takes place over an elapsed period of five days, but other BOD tests are also used.

Method

The BOD test is carried out by diluting the sample with de-ionized water saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample (to prevent further oxygen dissolving in). The sample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD The apparent BOD for the control is subtracted from the control result to provide the corrected value.

The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called the BOD₅. In the U.K., allylthiourea is also added at the start of the test to prevent ammonia. Results from such tests are represented as BOT5(ATU) and referred to as Carbonaceous BOD (CBOD) in the U.S. Less frequently used is the Ultimate BOD (UBOD) test, in which DO is repeatedly measured by DO meter in the same specialised bottles until it has reached equilibrium.

BOD is similar in function to organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidised, rather than just levels of biologically active organic matter.

BOD is used as a gauge of the effectiveness of wastewater treatment plants. Various commercial devices are available for its determination.

BOD can be calculated by:

Undiluted: Initial DO - Final DO = BOD
Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor

History of the use of BOD

The Royal Commission on River Pollution, which was 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 nitrification standard which was used as a yardstick in the U.K. up to the 1970s for sewage works effluent quality.

References

Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and Science, 5th edition, New York: McGraw-Hill. ISBN 0-07-248066-1.
Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton. Standard Methods for Examination of Water & Wastewater, 20th edition, Washington, DC: American Public Health Association. ISBN 0-87553-235-7. Stevens Institute of Technology