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Biological oxygen demand (BOD)


The above graph has been plotted to depict the values obtained for various parameters of water quality analysis at different sites for river chelt. According to this graph, the water sample obtained at the source of origin of the river is virtually 100% oxygen rich, indicating a good balance between the production of oxygen by aquatic flora and consumption of oxygen by the resident microbes. However, fairly high concentration of nitrates and excessive presence of total culturable bacteria indicate an underlying fecal contamination. One could associate the presence of nitrates with the decomposition of organic waste originating from a point source. It might be for the same reason that the values of ammonia concentration are fairly low as the ammonia that is initially formed from the biodegradation is being rapidly converted into nitrate.

Biological oxygen demand (BOD) is defined as the quantity of dissolved oxygen that is required by the micro-organisms for the biodegradation of organic matter present in a water body. ( Organic materials such as dead plants & animals, sewage and other anthropogenic sources of waste promote the growth of aerobic bacteria primarily, that cause the breakdown of these complex molecules into simpler molecules such as nitrates, carbon dioxide and water. As this microbes utilize oxygen in this process, the level of dissolved oxygen decreases, therefore increasing the overall oxygen demand for the survival of native plants and animals. Dissolved oxygen levels below 5mg/l could have potentially profound implications on the sustenance of the water body itself. Drop in the oxygen concentration below 2mg/l for more than a few hours poses threat to the survival of fish and other oxygen sensitive aquatic species. Death of the native flora and fauna contributes to the carbonaceous and nitrogenous organic waste in the water body and their degradation by aerobic micro-organisms results in production of ammonia, nitrates and nitrites. ( These minerals further deteriorate the situation by promoting growth of algae and weeds in the water body, which die due to insufficient oxygen. ( ( Hence, continuation of the vicious cycle of decomposition, growth and oxygen are depletion. Adding further to the problem, oxygen concentrations below 1mg/l could even result in replacement of aerobic bacteria with anaerobic microbes. At this stage, anaerobic bacteria would start the decomposition of the dead aquatic life and cause the production of foul smelling hydrogen sulfide. Moreover, excessive discharge of industrial waste, overuse of fertilizers & non-biodegradable detergents and agricultural runoff aggravate the eutrophication by providing phosphates for the growth of algae. Although microbes form an innate part of any ecosystem, presence of the coliform bacteria is an indicator of sewage contamination in a water body. This group of bacteria is inhabitant of the human intestinal tract and although they do not cause any harm, their presence might be associated with the presence of other pathogenic micro-organisms. ( Presence of fecal coliforms like E.coli indicates fecal contamination of the water body by various species including man which further adds to their potential association with other disease-causing bacteria, viruses or protozoa. (

To summarize, the parameter values obtained down the river chelt are alarming and require prime attention by local environmental agencies. The data collected indicates a high rate of pollution and contamination, primarily due to human interventions such as sewage disposal, contamination by food waste (significant at the waitrose and sainsbury's sites) and fecal contamination.

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