Biochemical oxygen demand
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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 sample 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 in the dark and tested for dissolved oxygen (DO) 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 of ammonia. Results from such tests are represented as BOT5(ATU) and referred to as Carbonaceous BOD (CBOD) in the US. Less frequently used is the Ultimate BOD (UBOD) test, in which DO is repeatedly measured by DO meter in the same specialized 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 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 wastewater 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
- Standard Methods for the Examination of Water and Wastewater - 20th Edition ISBN 0-87553-235-7. This is also available on CD-ROM and online (http://www.standardmethods.org/) by subscriptionde:Biochemischer Sauerstoffbedarf