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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.