Water Supply Vol 6 No 1 pp 121–128 © IWA Publishing 2006
Chemically enhanced gravel pre-filtration for slow sand filters: advantages and pitfalls
C.C. Dorea* and B.A. Clarke**
*Centre for Environmental Health Engineering (CEHE), School of Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom, (E-mail: c.dorea@surrey.ac.uk; caetanodorea@hotmail.com)
**Centre for Environmental Health Engineering (CEHE), School of Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom, (E-mail: c.dorea@surrey.ac.uk; caetanodorea@hotmail.com)
ABSTRACT
The chemical enhancement of gravel (or roughing) filtration with coagulants, i.e. direct (gravel) filtration, has been proposed as a pre-treatment alternative for slow sand filters. However, studies have frequently focused on the efficiencies of the pre-filters in terms of reduction percentages. The effectiveness of the pre-treatment on the subsequent slow sand filtration is not usually cited or even evaluated. By incorporating a pilot-scale slow sand filter in our trials, both aspects of the pre-treatment process were assessed: efficiency and effectiveness. In terms of turbidity reductions, our results demonstrated that chemically enhanced pre-filtration was substantially more efficient (93.2 to 99.5%) than conventional pre-filtration (50.6 to 79.3); this was also observed in terms of reductions in the level of other parameters (i.e. thermotolerant faecal coliforms and dissolved organics). Yet, the use of a coagulant can have a negative impact on the slow sand filtration run.
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