SR-38 : Dispersion of pollutants in streams

Abstract

A traditional concern is stream disposal of liquid wastes from municipal, industrial and sewage treatment plants. Beside these ,another common tendency of disposing blow down discharges from fossil or nuclear power stations to the nearest perennial open channel. For environmentalists the real concern of the problem is, how pollutants transport with the river water and when the river regenerates its assimilative capacity The most important aspect of river transport phenomenon is; how quickly pollutants mix with river water and how concentration distribution varies in the river water. From environmental considerations, the quality and quantity of effluents need to be so regulated that at every section between the point of release and till pollutants reached a safer limit, a reasonable wide zone of uncontaminated water to be made available available for movement of aquatic biota, if necessary after allowing the dilution of effluents.

An important parameter which characterize the ability of streams in disbursing the pollutants across the depth, width and towards the direction of flow is "longitudinal dispersion co-efficient" or simply "dispersion co-efficient". It seems that dispersion is relative unimportant for steady-state, one-dimensional flows, and usually considered to be important for unsteady flows and loads which are very common in rivers. Study of dispersion of pollutants in stream has many application starting from monitoring and ending with modelling of water quality parameters for both conservative and non-conservative materials. Determination of dispersion co-efficient is not a task but requires lot of data and due care of analysis. The algorithm which describes the dispersion process is analogous to the Fickian diffusion equation. However, there were numbers of opinion, some investigators have depicted that the Fickian diffusion equation can not be applied in the convective period ( convective period has been described as the time required for pollutants to reach from point of release to the point where it is completely mixed).while others opined that Fickian equation is not the correct description for mixing pattern mainly in natural streams where stream irregularities exist. Moreover, lot many opinions are also observed for mixing pattern and about the determination of dispersion co-efficient. It is clear from the studies that dispersion co-efficient is an empirical parameter derived from eddy-diffusivity assumption.

It is attempted in the report to describe the dispersion processes in stream in detail. Efforts have been made to elaborate the algorithm which describes the dispersion process, and how it can be used and had been used to determine the dispersion co-efficient has been highlighted. Studies related to dispersion of pollutants in stream have also critically been reviewed in this report.