The direct release of sewage into streams can have corrosiveeffects on stream water quality by spreading disease and creating a high oxygendemand on the receiving water. Microorganisms present in the stream andwastewater will slowly oxidize the introduced waste, and so doing, will consumethe available oxygen originally present in the stream. Under these conditions,the oxygen levels cannot be replaced by re-aeration at a faster rate for largerorganisms (such as fish, macro-invertebrates) to survive.
Microorganisms such asbacteria are responsible for decomposing organic waste/matter. When organicmatter which might be dead plants, leaves, grass clippings, manure, sewage, oreven food waste is present in a water supply or body, the bacteria will beginthe process of breaking down this waste. When this happens, much of the availabledissolved oxygen is used up by aerobic bacteria, robbing other aquaticorganisms of the oxygen they need to survive. Biological Oxygen Demand (BOD) isa measure of the oxygen used by microorganisms to decompose this waste. Ifthere is a large volume of organic waste in the water supply, there will alsobe a lot of bacteria present working to decompose or bioremediate this waste.In this case, the demand for oxygen will be high (due to all the bacteria) sothe BOD level will be high.
As the waste is consumed or bioremediated throughthe water, BOD levels will begin to reduce. The high BOD values are signs of thepresence of organic and inorganic pollutants, respectively. The dissolved oxygen (DO) andthe biochemical oxygen demand (BOD) are two useful parameters in tracingpollution profile and natural purification of rivers upon which engineeringcalculations of permissible pollution loadings arebased (Fair et al.
1971; Garg 2006). The BOD defines in a comprehensive manner the degradableload added to the receiving water body or remaining in it. It is both time andplace specific. BOD therefore measures the oxygen absorbing capacity of aneffluent. The DO defines the capacity of the body of water to assimilate theimposed load by itself or with the help of reaeration through oxygen absorbedmainly from the atmosphere and also through photosynthesis. The amount ofdissolved oxygen that can be held by the water depends mainly on the watertemperature (Garg 2006; Agunwamba et al.
2006). The determination of dissolved oxygen concentrationrelative to its saturation value and the rate of oxygen utilization measured asits BOD become a good measure for identifying the pollutional status of a waterbody. The knowledge of the progressive utilization of oxygen in a water bodyhas been widely used as a measure of the amount of decomposable or organicmatter contained in it at a given time. Also, it has been used to predictaerobic decomposition and the degree of self-purification accomplished in agiven interval of time.
It therefore follows that the oxygen economy of anyreceiving water is of paramount aesthetic and economic importance. Determinationof the self-purification capacity of water bodies has been the subject of researchesby scientists around the world (Villeneuve et al.1998; Rounds 2001; Radwan et al.
2003; Agunwamba et al.2006; Alam et al. 2007). Nitratesand phosphates in a body of water can contribute to high BOD levels. Nitrates(N)and phosphates(P) are rich nutrient for plant and can aid the quick growth ofplant and that of algae.
It adds to the organic waste/matter in the water,which is then decomposed by bacteria. This results in a high level of BOD. Whenthe levels of BOD are high, dissolved oxygen (DO) levels will decrease becausethe oxygen that is available in the water is being used up by the bacteria.Since less dissolved oxygen is available in the water, aquatic organisms maynot survive.
Standardized methods for the quantification of BOD in wastewatershave remained virtually unchanged for decades despite numerous deficiencies. Differenttechniques and estimation methods have been proposed. The kinetics of dissolvedoxygen consumption resulting from BOD released have been formulated intoseveral mathematical models for simulating surface water quality. The manyhistory of the BOD test and its application into many major water qualitymodels ensures that it will continue to be quantified for decades in thefuture.