Biochemical oxygen demand
420598
222528010
2008-06-29T20:47:29Z
68.175.56.211
/* Dilution method */
'''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 [[Sewage treatment|wastewater treatment]] plants.
==Typical BOD values==
Most pristine rivers will have a 5-day BOD below 1 [[mg/l]]. Moderately [[pollution|polluted]] rivers may have a BOD value in the range of 2 to 8 mg/l. Municipal [[sewage]] that is efficiently treated by a [[sewage treatment|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<sub>5</sub> test ==
BOD measures the rate of oxygen [[uptake]] by [[micro-organism]]s 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 BOD<sub>5</sub>. 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 compound]]s 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. <ref> {{cite journal | author = Katri Roppola et al. 2006 | title = Comparison Study of Manometric Respirometric Test and Common Chemical Methods in the Determination of BOD7 in a Pulp and Paper Mill’s Wastewaters | journal = Journal of Automated Methods and Management in Chemistry | volume = 2006 | issue = Article ID 90384 | pages = 1–5 | publisher = Hindawi Publishing Corporation | doi = 10.1155/JAMMC/2006/90384 | year = 2006}} Retrieved at [http://www.hindawi.com/GetArticle.aspx?doi=10.1155/JAMMC/2006/90384] </ref>
== 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 BOD<sub>5</sub> as the definitive test for organic [[Water pollution|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 BOD<sub>5</sub> 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==
*{{cite book | author=Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin | title=Chemistry for Environmental Engineering and Science | edition=5th edition | location=New York | year=2003 | publisher=McGraw-Hill | id=ISBN 0-07-248066-1}}
*{{cite book | author=Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton | title=Standard Methods for Examination of Water & Wastewater | edition=20th edition | publisher=American Public Health Association | location=Washington, DC | id=ISBN 0-87553-235-7}}Stevens Institute of Technology
==Notes==
<references/>
== See also ==
*[[Chemical oxygen demand]]
* [[Wastewater quality indicators]] discusses both [[BOD]] and [[Chemical oxygen demand|COD]] as indicators of wastewater quality.
==External links==
* [http://www.boddoctor.com/wiki/index.php?title=Main_Page BOD Doctor ]- a troubleshooting wiki for this problematic test
[[Category:Anaerobic digestion]]
[[Category:Environmental science]]
[[Category:Water quality indicators]]
[[de:Biochemischer Sauerstoffbedarf]]
[[es:Demanda biológica de oxígeno]]
[[fr:Demande biologique en oxygène]]
[[gl:DBO]]
[[it:Domanda biochimica di ossigeno]]
[[ja:生物化学的酸素要求量]]
[[pl:Biochemiczne Zapotrzebowanie Tlenu]]
[[sv:Biochemical Oxygen Demand]]
[[vi:Nhu cầu ôxy hóa sinh học]]
[[zh:生化需氧量]]