Biochemical oxygen demand
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BOD - biochemical (biological) oxygen demand is a test used to measure 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 and it is used in water quality management and assessment, Ecology and environmental science. BOD is not an accurate quantitative test and should be considered as providing an indicator of the quality of a water body.
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[edit] 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 variable but might average out at around 600 mg/l in Europe and as low as 200 mg/l in the USA or where there is severe groundwater or surface water infiltration. The generally lower values in the USA derive from the much greater water use per head than in other parts of the world. Slurry from dairy farms is around 2000 mg/l and silage liquor around 6000 mg/l.
| BOD Level (in ppm) | Water Quality |
|---|---|
| 1 - 2 | Very Good |
| 3 - 5 | Moderate |
| 6 - 9 | Fairly Polluted |
| 10+ | Very Polluted |
[edit] 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.
[edit] Method
The BOD test is carried out by diluting the sample with de-ionised water saturated with oxygen, inoculating 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.
BOD can be calculated by:
, where DOi and DOf are the initial and final dissolved oxygen respectively and P is the dilution ratio
[edit] 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 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 quality.
[edit] See also
- Chemical oxygen demand
- Wastewater quality indicators discusses both BOD and COD as indicators of wastewater quality.
