What is BOD (Biochemical Oxygen Demand)

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Biochemical Oxygen Demand or Biological Oxygen Demand (BOD) is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. 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.

BOD can be used as a gauge of the effectiveness of wastewater treatment plants.

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 or less. 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 BOD 5 Test
BOD measures the rate of oxygen uptake by micro-organisms in a sample of water at a temperature of 20°C and over an elapsed period of five days in the dark.

There are two recognized methods for the measurement of BOD.

Dilution method
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 BOD test is carried out by diluting the sample with de-ionized water with added nutrients, 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. For carbonaceous BOD (cBOD), a nitrification inhibitor is added after the dilution water has been added to the sample. The inhibitor hinders the oxidation of nitrogen. This inhibition allows for measurement of carbonaceous oxygen demand (cBOD).

BOD can be calculated by:

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

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.

Manometric method
This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxidation is inhibited.

The sample is kept in a sealed container fitted with a pressure sensor. A substance absorbing carbon dioxide (typically LiOH) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed and, as ammonia oxidation is inhibited, carbon dioxide is released. The total amount of gas, thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the electronics computes and displays the consumed quantity of oxygen.

The main advantages of this method compared to the dilution method are:

* its simplicity: no dilution of sample required, no seeding, no blank sample
* direct reading of BOD value
* continuous display of BOD value at the current incubation time
Furthermore, as the BOD measurement can be monitored continuously, a graph of its evolution can be plotted. Interpolation of several graphs on a similar water may build an experience of its usual evolution, and allow an estimation of the five days BOD after as early as the first two days of incubation.

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.

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