Thurston, Anne; (2003) The effects of flow rate changes on filter performance. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Filtration is an essential process within potable water treatment. Recent outbreaks of Cryptosporidiosis have brought filter performance into the spotlight, as it is widely acknowledged to be the most important stage for the removal of pathogens. There are many variables which can affect the efficiency of filtration, both within the filter itself, such as media type and size, and also stages preceding filtration such as coagulant dosing. This research focuses on the effects of flow changes on filter performance, because although researchers as early as the 1960s have suggested that increasing the flow will increase particulate breakthrough, no attempts have been made to actually quantify or explain these effects. Work has been carried out at laboratory and full-scale, and computer simulations have also been conducted. In the laboratory, temporary, permanent and gradual flow increases have been applied to a Im bed in a 2m high filter column. These increases were of varying magnitude and at different stages of the filter cycle, to determine which type of flow change is most harmful in terms of increased particle breakthrough. Various media configurations and coagulants have been investigated. The effluent quality was recorded using a particle counter, rather than a turbidimeter, so that the particle size distribution of the breakthrough could be assessed. Breakthrough in the specific Cryptosporidium size range (2-5um) could be distinguished. The results have shown that in general, flow increases do cause an increase in particle breakthrough, and that the larger and later the flow change is applied, the higher the level of breakthrough is observed. However, this pattern was not observed in all cases, and it was clear that external factors are just as important as the flow change itself in determining the amount of breakthrough from the filter. One of the most important variables is water temperature. Results have shown that when using alum as the coagulant, an identical flow change will cause more breakthrough at higher water temperatures than that observed during periods of colder influent water. Poly aluminium chloride provided more consistent breakthrough patterns through a range of water temperatures, but this coagulant did not perform as well as alum in cold influent conditions. The effects of mechanical vibrations on breakthrough was also investigated, and although there has been very little published research on this phenomena, this work found that vibrations have a great effect on the filter performance, both at laboratory and at full-scale. All of these results suggest a very complex pattern of filter breakthrough, whereby a number of factors must be considered when trying to predict the amount of particle shedding during a particular flow change.

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