Technical and Economic Evaluation of the Use of Variable Speed Pumps in the Backwashing of Rapid Sand Filters (A Case Study)

It is observed that physical changes in the filter as bed media size variation, nozzle breakage, or water temperature variations lead to changes in the backwash regime throughout the year. The remedy proposed in this article is to use variable speed pumps in order to maintain a constant and steady optimum regime. In this study, experiments were performed using a full-scale single-layer rapid sand filter to find the optimum bed expansions of 7-8%. In addition, previous research has shown that air and backwash water flows need to be slowly and gradually increased to the optimum design values in order to avoid sand boil. In a typical treatment plant, this is normally controlled by globe valves, which results in the waste of energy and variation in pump and blower pressures. Application of variable speed pumps has the advantage that a constant pressure can be maintained throughout to yield an optimum backwash regime. Variable speed pumps provide various backwash regimes which enable design engineers and water plant operators to determine the optimum expansion in sand bed filters. This range of optimum bed expansion was found to lead to such improvements as shorter filter ripening, reduced effluent turbidity to levels below 0.3 NTU, conservation of backwash water by up to 15‒20%, and savings in backwash pump power consumption by up to 30-50%.