Biochemical oxygen demand
Typical BOD values
Most pristine rivers will have a 5-day BOD below 1 mg/l. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/l. Municipal sewage that is efficiently treated by a three stage process would have a value of about 20 mg/l. Untreated sewage varies, but averages around 600 mg/l in Europe and as low as 200 mg/l in the U.S. or where there is severe 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 micro-organisms in a sample of water at a fixed temperature (20°C) and over a given period of time in the dark. To ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionized water. The test generally takes place over an elapsed period of five days, but other BOD tests are also used.
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 oxidation of 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 chemical oxygen demand (COD), in that both measure the amount of 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 pollution of rivers. Five days was chosen as an appropriate test period because this is supposedly the longest time that river water takes to travel from source to estuary in the U.K. In 1912, the commission also set a standard of 20 ppm BOD5 as the maximum concentration permitted in sewage works discharging to rivers, provided that there was at least an 8:1 dilution available at dry weather flow. This was contained in the famous 20:30 (BOD:Suspended Solids) + full nitrification standard which was used as a yardstick in the U.K. up to the 1970s for sewage works effluent quality.
- 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