A STUDY ON RIVERBANK FILTRATION

Abstract

Riverbank filtration (RBF) is a process during which surface water is subjected to subsurface flow prior to extraction from wells. During infiltration and soil passage, surface water is subjected to a combination of physical, chemical, and biological processes such as filtration, dilution, sorption, and biodegradation that can significantly improve the raw water quality. Increased applications of RBF are anticipated as drinking water utilities strive to meet increasingly stringent drinking water regulations, especially with regard to the provision of multiple barriers for protection against microbial pathogens, and with regard to tighter regulations for disinfection by-products (DBPs). In this thesis, the logistic equation has been applied to compute the time required for n log cycle reduction in bacteria concentration. Using hydrodynamic equation, the travel time of a parcel of water from the river to the well has been derived for a given pumping rate and specified location of the abstraction well. It is found that the travel time is directly proportional to (i) square of the distance of the well from the river; (ii) the thickness of the aquifer (iii) porosity of the aquifer medium and inversely proportional to the pumping rate. This travel time of the parcel of water corresponding to the pumping rate and location of abstraction point from the river needs to be more than the time required for 6 to 10 log cycle reduction in bacteria concentration. Considering dispersion equation, the variation of concentration with distance at different time has been studied for given pumping rate. The variation log cycle reduction in concentration of the pollutant with distance is linear at a dynamic equilibrium condition which helps in deciding the appropriate location of the abstraction well. An analytical method has been derived to find the safe distance of a well from a polluted river. The travel time has been iii derived both for steady and unsteady flow field. From the study it is found that, the travel time corresponding to unsteady flow field is slightly higher than that corresponding to steady flow field. This is because incase of unsteady state flow field, when pumping commences, the water is first taken from the aquifer storage and contribution of river commences at a later time. Steady flow equation can be conveniently assumed to fmd the safe distance of a well or safe withdrawal from an existing well. An analytical method has been described to find, the safe distance of a gallery from a river..