Chemical Oxygen Demand ( B.o )

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31. Biological Oxygen Demand ( B.O.D)
Introduction:
Biochemical Oxygen Demand (B.O.D.) is the most important parameter to determine the level of pollution in lakes and streams, their self-purification capacities, assess the biodegradable organic load of the wastewaters for designing wastewater management or treatment plants and thereafter to evaluate their efficiency or finding out the assimilative capacity of a water body. B.O.D. is an empirical standardized laboratory test defined as the amount of oxygen required for the aerobic decomposition of the organic matter by micro-organisms into stable inorganic forms at a given controlled condition of time and temperature in water. The quantity of oxygen required for above
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The greater the B.O.D., the more rapidly oxygen is depleted in the stream which means less oxygen is available to higher aquatic forms of life. As a consequence organisms that are more tolerant of lower dissolved oxygen levels may replace a diversity of more sensitive organisms.

A. Titrimetric method
Principle
The B.O.D. test is based on mainly bio-assay procedure, which measure the dissolved oxygen consumed by micro-organisms while assimilating and oxidizing the organic matter under aerobic condition. This test measures the oxygen utilized for the biochemical degradation of organic material (carbonaceous demand) and oxidation of inorganic material such as sulphides and ferrous ions during a specified incubation period. It also measures the oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand) unless their oxidation is prevented by an inhibitor. Temperature effects are held constant by performing a test at fixed temperature. The methodology of B.O.D. test is to compute a difference between initial and final D.O. of the samples incubation. Atleast 1.5 L of sample is required to perform the B.O.D. test. D.O. is estimated by iodometric or Winkler’s method titration. Since the test being a bio-assay procedure, it is necessary to provide standard conditions of temperature, nutrient supply, pH (6.5-7.5), adequate population of microorganisms and absence of microbial-growth-inhibiting substances. The low
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