| Biochemical Oxygen Demand |
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Information AboutBiochemical Oxygen Demand |
| CATEGORIES ABOUT BIOCHEMICAL OXYGEN DEMAND | |
| anaerobic digestion | |
| environmental science | |
| water quality indicators | |
| SHOPPER'S DELIGHT | |
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TYPICAL BOD VALUES Most pristine rivers will have a 5 day BOD of less than 1 mg/l. Moderately polluted rivers may have BODs in the range 2 mg/l to 8 mg/l. Efficiently treated municipal Sewage treated by a Three Stage Process would have a BOD value of about 20 mg/l. Untreated sewage is very variable but might average out at around 600 mg/l with slurry from dairy farms at around 2000 mg/l and Silage liquor around 6000 mg/l THE BOD TEST 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. The test generally takes place over an elapsed period of 5 days but other BOD tests are also used. METHOD The BOD test is carried out by diluting the sample with de-ionised water saturated with oxygen , innoculating it with a fixed aliquot of seed, sealing the sample (to prevent further oxygen dissolving in), and then placing it in the dark (to prevent Photosynthesis and thereby the addition of oxygen). The sample is kept at 20 °C in the dark and tested for dissolved oxygen (DO) after five days. 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 UK 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 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 everything that can be chemically oxidised rather than just levels of biology active organic matter. BOD is used as a gauge of the effectiveness of Wastewater Treatment plants. Various commercial devices are available for its determination. SEE ALSO
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