Two-stage filtration is a potentially promising technology for treating water. It has been estimated that most of the water treatment carried out today can be achieved efficiently with this technology, although some design and operational aspects relating to high turbidity of the influent water have yet to be fully studied.

In the study reported here, a pilot plant was built comprising 2 two-stage filtration systems: system 1 involved an upflow coarse sand filter and system 2 consisted of an upflow gravel filter as the first stage of filtration. Both were followed by a downflow fine sand filter as the second stage. The experimental investigation was carried out using influent I with a turbidity of approximately 100 NTU and influent II with a turbidity of approximately 300 NTU. Coagulation was achieved using aluminium sulphate to obtain a final zeta potential of approximately zero. The main conclusions were that both systems can clarify influent I and influent II and that the downflow filter plays an essential role in the production of filtered water with a turbidity of less than 0.5 NTU. Intermediate downflushes on the upflow filters are more advantageous in increasing filter run times and in preventing the formation of any sludge blanket at the top of the upflow filters and the occurrence of breakthough in the rapid downflow sand filter. Another conclusion drawn from this research is that the effective water production is related to influent water turbidity, filtration rates, intermediate downflushes and the hydraulic head necessary for the retention of solids in the two filters of each two-stage filtration system.

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