Water treatment by multistage filtration utilising gravel prefilters and fabric enhanced slow sand filters.

Pilot plant studies were carried out to investigate the performance of a multistage filtration water treatment system. The system comprised three gravel upflow prefilters of a new design linked in series to parallel slow sand filters. The prefilters contained 0.5m depth of 40mm, 20mm and 10mm nominal aggregate respectively, supported on a raised floor. The slow sand filters of 0.5m and 0.3m depth were ''enhanced'' with 6 layers of polypropylene fabric placed on top of the sand beds. The performance of a fabric only filter, comprising 12 layers of polypropylene fabric, was also evaluated and compared with those of the enhanced slow sand filters. The physical performance of the multistage system was investigated by monitoring turbidity, suspended solids and particle size. A progressive sustained removal of particulates was achieved with product water generally <1 NTU, and <5 NTU for a sharp ''natural raw water peak'' of 400NTU. As well as dealing with the ''natural peak'' turbidity levels the prefilters were subjected to a short challenge with turbidity created by dosing the raw water with a kaolin suspension. A 99% removal of thermotolerant (presumed faecal) coliform was achieved in the multistage system, a feature of the system being progressive faecal coliform removal through the prefilters as well as within the slow sand filters. In addition, short challenges with suspensions of the bacteriophage of Serratia marcescens were carried out.