Editing Biochemical oxygen demand (section)

==Background==
Most natural waters contain small quantities of organic compounds. Aquatic [[microorganisms]] have evolved to use some of these compounds as [[food]]. Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively degrade the organic compounds, releasing [[energy]] which is used for [[Bacterial growth|growth]] and [[reproduction]]. Populations of these microorganisms tend to increase in proportion to the amount of food available.  This microbial [[metabolism]] creates an oxygen demand proportional to the amount of organic compounds useful as food.  Under some circumstances, microbial metabolism can consume dissolved oxygen faster than atmospheric oxygen can dissolve into the water or the autotrophic community (algae, cyanobacteria and macrophytes) can produce. Fish and aquatic insects may die when oxygen is depleted by microbial metabolism.<ref>{{cite book |last1=Goldman |first1=Charles R. |last2=Horne |first2=Alexander J. |title=Limnology |url=https://archive.org/details/limnology00gold/page/88 |date=1983 |publisher=McGraw-Hill |isbn=0-07-023651-8 |pages=[https://archive.org/details/limnology00gold/page/88 88, 267] |url-access=registration }}</ref>

Biochemical oxygen demand is the amount of oxygen required for microbial metabolism of organic compounds in water.  This demand occurs over some variable period of time depending on temperature, [[nutrient]] concentrations, and the [[enzyme]]s available to indigenous microbial populations.  The amount of oxygen required to completely oxidize the organic compounds to carbon dioxide and water through generations of microbial growth, death, decay, and cannibalism is '''total biochemical oxygen demand''' (total BOD). Total BOD is of more significance to [[food web]]s than to [[water quality]]. Dissolved oxygen depletion is most likely to become evident during the initial aquatic microbial population explosion in response to a large amount of organic material.  If the microbial population deoxygenates the water, however, that lack of oxygen imposes a limit on population growth of [[aerobic respiration|aerobic]] aquatic microbial organisms resulting in a longer term food surplus and oxygen deficit.<ref name="reid">{{cite book |last1=Reid |first1=George K. |title=Ecology of Inland Waters and Estuaries |url=https://archive.org/details/ecologyofinlandw00reid|url-access=registration |date=1961 |publisher=Van Nostrand Reinhold |pages=[https://archive.org/details/ecologyofinlandw00reid/page/317 317–320]}}</ref>

A standard temperature at which BOD testing should be carried out was first proposed by the [[Royal Commission on Sewage Disposal]] in its eighth report in 1912:
<blockquote>(c) An effluent in order to comply with the general standard must not contain as discharged more than 3 parts per 100,000 of suspended matter, and with its suspended matters included must not take up at 65&nbsp;°F (18.3&nbsp;°C) more than 2.0 parts per 100,000 of dissolved oxygen in 5 days. This general standard should be prescribed either by Statute or by order of the Central Authority, and should be subject to modifications by that Authority after an interval of not less than ten years.<ref>{{cite book |title=Report of the Commissioners appointed in 1898 to inquire and report what methods of treating and disposing of sewage (including any liquid from any factory or manufacturing process) may properly be adopted. Standards and Tests for Sewage and Sewage Effluents Discharging into Rivers and Streams |date=1912 |publisher=His Majesty's Stationery Office |page=17 |url=https://archive.org/details/b32179972_0008/page/n29}}</ref></blockquote>
This was later standardised at 68&nbsp;°F and then 20&nbsp;°C. This temperature may be significantly different from the temperature of the natural environment of the water being tested.

Although the Royal Commission on Sewage Disposal proposed 5 days as an adequate test period for rivers of the [[United Kingdom of Great Britain and Ireland]], longer periods were investigated for [[North America]]n rivers.  Incubation periods of 1, 2, 5, 10 and 20 days were being used into the mid-20th century.<ref>Norton, John F. ''Standard Methods for the Examination of Water and Sewage'' 9th Ed. (1946) American Public Health Association p.139</ref>  Keeping dissolved oxygen available at their chosen temperature, investigators found up to 99 percent of total BOD was exerted within 20 days, 90 percent within 10 days, and approximately 68 percent within 5 days.<ref>Urquhart, Leonard Church ''Civil Engineering Handbook'' 4th Ed. (1959) McGraw-Hill p.&nbsp;9–40</ref>  Variable microbial population shifts to [[nitrifying bacteria]] limit test [[reproducibility]] for periods greater than 5 days.  The 5-day test protocol with acceptably reproducible results emphasizing carbonaceous BOD has been endorsed by the [[United States Environmental Protection Agency]] (EPA). This 5-day BOD test result may be described as the amount of oxygen required for aquatic microorganisms to stabilize decomposable organic matter under aerobic conditions.<ref>Sawyer, Clair N. & McCarty, Perry L. ''Chemistry for Sanitary Engineers'' 2nd Ed. (1967) McGraw-Hill pp.&nbsp;394–399</ref>  Stabilization, in this context, may be perceived in general terms as the conversion of food to living aquatic [[fauna]].  Although these fauna will continue to exert biochemical oxygen demand as they die, that tends to occur within a more stable evolved [[ecosystem]] including higher [[trophic level]]s.<ref name="reid"/>
[[File:Influent raw wastewater (note grey colour in glass jar) (3232393634).jpg|thumb|Taking samples from the influent raw [[wastewater]] stream for BOD measurements at a [[wastewater treatment]] plant in Haran-Al-Awamied near Damascus in Syria]]