The effect of surges on the performance of rapid gravity filtration

Abstract

Filters used in drinking water treatment are subject to small continuous flow rate fluctuations or surges. Large changes in rate are known to have a detrimental effect on filtrate quality. Less is known about the effects of surging. Past observations suggest that surging may significantly influence filter performance but the effect has yet to be confirmed under controlled conditions and the mechanisms critically examined. Two rapid filters were developed in the laboratory to investigate the influence of surging on performance. Reproducible performance was established before applying surges to one filter only. Measurements of head loss and turbidity were taken with depth and time. Surges were found to reduce filter performance. The fluctuations in flow were found to slow the rate of ripening of the filter, retard the rate of head loss development and reduce the removal efficiency. The experimental results obtained suggest that surging does have a significant effect on rapid filter performance.

Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

“… Ives and Pienvichitr [1965] noted that intermittent sampling and variation in flow rate disturbed deposited materials. Glasgow and Wheatley [1998] investigated the effect of surging by comparing two sand filters with equal average flow rates, observing that the surging filter clogged more slowly and produced higher effluent turbidity. Increases in flow rate have been shown to remobilize previously deposited Giardia cysts [ Logsdon et al , 1981], PVC particles [ Bai and Tien , 1997], polystyrene particles [ Bergendahl and Grasso , 2000] and illite and kaolinite clay [ Gao et al , 2004].…”