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Biochemical oxygen demand
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== History == The ''Royal Commission on River Pollution'', 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 its sixth report the Royal Commission recommended that the standard set should be 15 parts by weight per million of water.<ref name="Royal">[https://archive.org/details/cu31924003641929 Final Report of the Commissioners Appointed to Inquire and Report What Methods of Treating and Disposing of Sewage. 1912 ]</ref> However, in the Ninth report the commission had revised the recommended standard: <blockquote>An effluent taking up 2β0 parts dissolved oxygen per 100,000 would be found by a simple calculation to require dilution with at least 8 volumes of river water taking up 0.2 part if the resulting mixture was not to take up more than 0.4 part. Our experience indicated that in a large majority of cases the volume of river water would exceed 8 times the volume of effluent, and that the figure of 2β0 parts dissolved oxygen per 100,000, which had been shown to be practicable, would be a safe figure to adopt for the purposes of a general standard, taken in conjunction with the condition that the effluent should not contain more than 3β0 parts per 100,000 of suspended solids.<ref name="Royal" /></blockquote> This was the cornerstone 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. The [[United States]] includes BOD effluent limitations in its [[sewage treatment|secondary treatment]] regulations. Secondary sewage treatment is generally expected to remove 85 percent of the BOD measured in sewage and produce effluent BOD concentrations with a 30-day average of less than 30 mg/L and a 7-day average of less than 45 mg/L. The regulations also describe "treatment equivalent to secondary treatment" as removing 65 percent of the BOD and producing effluent BOD concentrations with a 30-day average less than 45 mg/L and a 7-day average less than 65 mg/L.<ref>U.S. Environmental Protection Agency (EPA). Washington, DC. "Secondary Treatment Regulation." ''Code of Federal Regulations,'' {{uscfr|40|133}}</ref> === Typical values === Most pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L. Rivers may be considered severely polluted when BOD values exceed 8 mg/L.<ref>{{cite book |last=Connor |first=Richard |title=The United Nations World Water Development Report 2016: Water and Jobs, chapter 2: The Global Perspective on Water |publisher=UNESCO |date =2016 |location=Paris |page=26 |isbn=978-92-3-100155-0}}</ref> 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/inflow]]. The generally lower values in the U.S. derive from the much greater water use per capita than in other parts of the world.<ref name="Sawyer" />
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