Biological and Chemical Oxygen Demands,Disadvantages of MPN Techniques

Biological and Chemical Oxygen Demands

	Biological and Chemical Oxygen Demands - Oxygen is sparingly soluble in water. Oxygen in the dissolved form in water is referred to as dissolved oxygen (DO). Because of the low solubility of oxygen in water, the DO content of natural waters never exceeds 8mg/l. Moreover, due to the heterotrophic microbial activity in water, this value is often very low. DO is used by aquatic microorganisms, plants and animals for respiration and other aerobic biochemical reactions.
Replenishment of DO in waterbodies occurs by reaeration from the atmosphere and/or by oxygen evolution during photosynthesis by plants and photosynthetic microorganisms. However, in the presence of excess organic substrates in the water, replenishment of DO is considerably lower than the utilization rate of oxygen by the aquatic forms. Thus, addition of sewage or other organic wastes to natural waters causes the exhaustion of their DO. Once the DO is exhausted, the self purification process ceases to function.
Ultimately, aerobic organisms including microorganisms and fishes tend to die due to oxygen depletion. Along with the fermentation produces of anaerobic reactions, these dead organisms cause obnoxious odors, tastes and color. Above all, decomposition of the dead organisms would consume further DO leading to a demand for oxygen.