The pollution profile and natural purification of rivers upon

The direct release of sewage into streams can have corrosive
effects on stream water quality by spreading disease and creating a high oxygen
demand on the receiving water. Microorganisms present in the stream and
wastewater will slowly oxidize the introduced waste, and so doing, will consume
the available oxygen originally present in the stream. Under these conditions,
the oxygen levels cannot be replaced by re-aeration at a faster rate for larger
organisms (such as fish, macro-invertebrates) to survive.

Microorganisms such as
bacteria are responsible for decomposing organic waste/matter. When organic
matter which might be dead plants, leaves, grass clippings, manure, sewage, or
even food waste is present in a water supply or body, the bacteria will begin
the process of breaking down this waste. When this happens, much of the available
dissolved oxygen is used up by aerobic bacteria, robbing other aquatic
organisms of the oxygen they need to survive. Biological Oxygen Demand (BOD) is
a measure of the oxygen used by microorganisms to decompose this waste. If
there is a large volume of organic waste in the water supply, there will also
be 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) so
the BOD level will be high. As the waste is consumed or bioremediated through
the water, BOD levels will begin to reduce. The high BOD values are signs of the
presence of organic and inorganic pollutants, respectively. The dissolved oxygen (DO) and
the biochemical oxygen demand (BOD) are two useful parameters in tracing
pollution profile and natural purification of rivers upon which engineering
calculations of permissible pollution loadings are
based (Fair et al. 1971; Garg 2006). The BOD defines in a comprehensive manner the degradable
load added to the receiving water body or remaining in it. It is both time and
place specific. BOD therefore measures the oxygen absorbing capacity of an
effluent. The DO defines the capacity of the body of water to assimilate the
imposed load by itself or with the help of reaeration through oxygen absorbed
mainly from the atmosphere and also through photosynthesis. The amount of
dissolved oxygen that can be held by the water depends mainly on the water
temperature (Garg 2006; Agunwamba et al. 2006). The determination of dissolved oxygen concentration
relative to its saturation value and the rate of oxygen utilization measured as
its BOD become a good measure for identifying the pollutional status of a water
body. The knowledge of the progressive utilization of oxygen in a water body
has been widely used as a measure of the amount of decomposable or organic
matter contained in it at a given time. Also, it has been used to predict
aerobic decomposition and the degree of self-purification accomplished in a
given interval of time. It therefore follows that the oxygen economy of any
receiving water is of paramount aesthetic and economic importance. Determination
of the self-purification capacity of water bodies has been the subject of researches
by scientists around the world (Villeneuve et al.1998; Rounds 2001; Radwan et al. 2003; Agunwamba et al.2006; Alam et al. 2007). Nitrates
and 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 of
plant 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. When
the levels of BOD are high, dissolved oxygen (DO) levels will decrease because
the 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 may
not survive. Standardized methods for the quantification of BOD in wastewaters
have remained virtually unchanged for decades despite numerous deficiencies. Different
techniques and estimation methods have been proposed. The kinetics of dissolved
oxygen consumption resulting from BOD released have been formulated into
several mathematical models for simulating surface water quality. The many
history of the BOD test and its application into many major water quality
models ensures that it will continue to be quantified for decades in the
future.

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