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.

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