Development of reaeration coefficient for Indian condition using mathematical approach and field studies – A research report by National Institute of Hydrology

The report deals with development of reaeration coefficient for Indian condition using mathematical approach and field studies. Of the various constituents, which determine the quality of water, Dissolved Oxygen is the most important parameter indicating the health of a stream. There is a continual replenishment (reaeration) or utilization (deoxygenated) of Dissolved Oxygen due to inflow of waste load at different points in the stream. When a waste load is discharged into a flowing stream, it is mainly subjected to advection, dispersion and reaction kinetics. Waste Load Allocation (WLA) studies provide information to assist in making effective decisions on levels of treatment required for a source or sources of pollutant loads.

To model and allocate waste loads in a stream, it is necessary to estimate the deoxygenation rate coefficient (K₁) and aeration coefficient (K₂). Rate coefficients, in contrast to loads, sources and sinks generally cannot be measured directly under natural conditions. Indirect measurements, supplemented by calculations provide one of the most reliable techniques for estimating rate coefficients. In addition, laboratory and field experiments can provide information on relative ranges of reaeration and deoxygenation rates.

Numerous equations employing depth, velocity and slope have been developed to estimate the stream reaeration and deoxygenation rate coefficient. This leads to uncertainties in modeling analysis because these equations are empirical in nature and may yield very different K₁ and K₂. Realizing the uncertainties involved in determining K₁ and K₂, many modelers have resorted to model calibrations as a way of adjusting values. In fact, in the pioneer work of stream analysis by Streeter and Phelps (1925), K₂ was evaluated indirectly by using DO budget and the K₁ was evaluated by carrying out laboratory and field experiments.

The following conclusions have been drawn from the results:

• The predictive reaeration equation developed in the present study showed promising results and can be used to determine reaeration in river Kali without using DOBT technique, which is exhaustive, costly and time consuming. For known velocity, depth and temperature reaeration coefficient can be obtained easily and can be used as input to water quality modeling.
• The developed predictive equation can be successfully used for rivers having similar and homogenous hydraulic and geographic conditions.
• Better results can be obtained if radioactive tracer technique is also used with the DOBT and PE approaches. However, in the tracer technique high precautions are required to avoid the effect of radioactive tracers on aquatic and human life.
• Longitudinal profile of the organic matter illustrates that at Mansurpur the water quality of the river Kali has been deteriorating exceedingly due to disposal of untreated municipal, industrial and sugar mill effluents of Muzaffarnagar town. 

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