What is Biological Oxygen Demand (BOD) ?

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Aerobic bacteria use oxygen dissolved in water when breaking down wastes. Scientists can find out how much organic matter the waste water contains by measuring how much oxygen the aerobic bacteria use in breaking it down. The quantity of oxygen utilized by microorganisms in aerobic degradation of organic matter in a water body is called its biological oxygen demand (BOD). Since BOD value is generally proportional to the amount of organic matter present in water, it can be used as a measure of waste strength and also as an indicator of the degree of pollution. The more the oxidisable organic matter present in water, the greater the amount of oxygen required to degrade it biologically and hence the higher the BOD value of water sample. Unpolluted waters, on the other hand, show comparatively low values of BOD.

BOD value is helpful in water pollution control management as a criterion of usefulness of the process design, loading calculations, as a measure of efficiency of operation of the treatment plants, and in evaluating self-purification capacity of natural waters. BOD is evaluated by determining oxygen concentration in a water sample, before and after incubation at 20° C in dark for 5 days. The difference is represented in per cent values. The various factors which influence BOD value are types of organisms, pH, presence of toxins, presence of organic matter, available nutrients and the rate of nitrification. Further details regarding the scope of this concept appear in the section on Eutrophication.

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