In ecology, biochemical (biological) oxygen demand (BOD) is an indicator for the concentration of biodegradable organic matter present in a sample of water. It can be used to infer the general quality of the water and its degree of pollution. BOD measures the rate of uptake of oxygen by micro-organisms in the sample of water at a fixed temperature and over a given period of time. 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-ionised water. All samples being tested in any one batch are innoculated with an equal volume of seed including the sampele control of de-ionised water saturated with oxygen. The BOD in the UK is carried out by diluting the sample with de-ionised water saturated with oxygen then sealing the sample (to prevent further oxygen dissolving in), and placing it in the dark (to prevent photosynthesis and thereby the addition of oxygen). This sample is kept at 20 deg C and tested for dissolved oxygen after five days. The apparent BOD for the control is subtracted from the test 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 UK allyl thiurea is also added at the start of the test to prevent oxidation or ammonia. Results from such tests are represented as BOT5(ATU)

However, the act itself without proper technical measures could not stop and prevent further pollution of the rivers. From this point of view, the formation of the Royal Commission on Sewage Disposal in 1898 can be understood as the milestone in the development of wastewater treatment technology. This commission co-ordinated the activities leading to better understanding of factors affecting the water quality in receiving waters and to evaluation of new treatment procedures. One of the best known outputs of the commission is the BOD₅ test recommended in 1908. The famous "30:20 + full nitrification" effluent standard was adopted in 1912 which was a great challenge for the development of wastewater treatment processes capable of meeting this standard.

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 approriate test period because this is supposedly the longest time that river water takes to travel from source to estuary in the UK. In 1912 the commission also set a standard of 30 ppm BOD₅ as the maximum concentration of 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 30:20 +full nitrification standard which was used as a yardstick in the UK up to the 1970s for sewage works efluent standard.

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 total organic levels rather than just levels of biologically active organic matter.

The rate of decomposition in lakes and rivers is seasonally variable, as the amount of dissolved oxygen varies inversely with the water temperature. It is also influenced by the availability of organic matter. If the decomposition rate is high, decay microorganisms can consume all of the available oxygen, denying it to other aquatic animals such as fish.

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

References

• Moran, Joseph M.; Morgan, Michael D., & Wiersma, James H. (1980). Introduction to Environmental Science (2nd ed.). W.H. Freeman and Company, New York, NY