Rapidly fluctuating flow rates, known as surging, have been shown to produce poorer water quality from rapid gravity filtration. This paper compares observations of surging made at treatment plants in the UK with those reported in the USA. Two rapid filters were previously developed in the laboratory to study the impact of surging on filter performance. Head loss and turbidity removal were measured. The surges applied caused a deterioration in filtered water quality but unlike the previous work, filter performance was suppressed throughout the filter cycle and not just during the early stages. The surges applied here slowed the rate of head loss development in the filter, generating a longer filter run but at the expense of poorer quality water. The results suggest that surging flow similar to that found in full scale filters in the UK and US could have a significant effect on filter performance and may have implications for the removal of Cryptosporidium and Giardia. Recommendations are made on how to minimise the occurrence of surging in present and future water treatment plant designs.

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