The actual BOD will be less than theoretical oxygen demand due to incorporation of some of thecarbon into newly synthesized bacterial cells. The test is performed under the conditions similarto those in actual natural water to measure indirectly the amount of biodegradable organic matterpresent. A water sample is inoculated with bacteria (1 to 2 mL of sewage per liter) that consumethe biodegradable organic matter to obtain energy for their life processes. The organisms alsoutilizes oxygen in the process of consuming the organic matter, the process is called as ‘aerobic’decomposition. This oxygen consumption is measured; more is the organic matter concentrationmore is the amount of oxygen utilized. Thus, the BOD test is the indirect measurement oforganic matter in terms of the oxygen requirement to convert them into stable end product.Although, not all organic matter is biodegradable within the stipulated incubation period, and theactual test procedure lack in precision, due to different inoculum seed and many fold dilutionrequired, it is still the most widely used method of quantifying organic matter because of thedirect conceptual relationship between BOD and oxygen depletion in receiving waters.
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