Small continuous flow rate fluctuations in rapid gravity filtration

Rapid gravity filters used in the treatment of drinking water are subject to small continuously occurring flow rate fluctuations known as surges. Large, step changes in the rate of flow have been shown to have a detrimental effect on filtrate quality. However, less is known about the effects of surging flow on rapid filter performance. Measurements by previous researchers have found that surges from 2 to 10 % of the flow rate are common and can occur as many as one hundred times per minute. It has been suggested that surging may significantly influence rapid filter performance but the effect has yet to be confirmed under well-controlled conditions and the mechanisms critically examined. Measurements taken by this author at local water treatment plants confirmed the presence of surging flow in the rapid gravity filters of a similar nature to other researchers' findings. Evidence suggested the degree of surging present was related to the design of the filtrate piping and some design recommendations are made on this basis. Two rapid gravity filters were developed in the laboratory to investigate the influence of surging flow on filter performance. The filters were constructed from Perspex pipe and comprised 600 mm of 0.5 to 1.0 mm filter sand. The filters were operated at 30°C at an approach velocity of 8.0 metres per hour with a test suspension of PVC particles. Reproducible performance was established before applying surges to one filter only. A range of surging characteristics similar to those observed at full-scale plants was applied during the test programme. Measurements of head loss and turbidity were taken at a range of depths within the filter media periodically during each test. Samples were collected for particle size distribution analysis from selected tests. The surging flow was found to inhibit the performance of the laboratory filters. The fluctuations in flow rate were found to reduce the removal efficiency of turbidity' and retard the rate of head loss development. The surges were found to inhibit the removal of all particle sizes present in the test suspension. The magnitude of the effect on filter performance was found to be dependent on the magnitude and frequency of occurrence of the surges applied. The experimental results obtained suggest that surging does have an effect on full- scale rapid filter performance and has implications for drinking water quality.