Optimizing direct filtration in Brasilia

The waterworks serving Brasilia consisted of 2 pulsator clarifiers and 12 pairs of filters, and was heavily overloaded. Because of the high cost of extensions, studies were carried out on the feasibility of increasing plant capacity by changing to direct filtration. The water supply was obtained from 2 impounding reservoirs; it was very low in colour and turbidity, and had low hardness and alkalinity. Tabulated and graphical results of laboratory and pilot-scale experiments on direct filtration are presented. Ferric chloride gave longer filter runs and greater reduction in turbidity, compared with alum. There was no advantage in using cationic polyelectrolytes. Flocculation before direct filtration was unnecessary, and could be detrimental to the performance of the filters. The coarse filter medium used required high back-wash rates, but the total volume of back-wash water required was lower than normal.