Minimising particulate passage during ripening of direct filters

A significant consideration in forward planning for water treatment works design and operation concerns the effectiveness of filtration plant in providing a barrier to particulates in the low micron size range, including Cryptosporidium oocysts. The filter ripening period has long been identified as a cause for concern with respect to particulate passage into the filtrate, this work has shown that up to 54% of all particles that pass into supply during a 48 hour run, do so in the first hour of operation The aim of this study was to investigate methods of reducing particulate passage into the filtrate during ripening and identify optimums. Thus strengthening the filtration barrier to Cryptosporidium oocysts. The work was carried out on a large direct filtration pilot plant treating an upland lake source. Variables investigated included, filter media type and size, backwash regimes and durations, start-up strategies such as slow-start, and filtration rate. The two filter media types and configurations that gave best removal were in beds of 0.5-1.0mm silica sand and 2m beds of 2.0-3.35mm quartz sand. A combined air/water backwash, at rates to achieve collapse-pulsing, reduced the numbers of particles in the filtrate during ripening and increased run lengths when compared to water only wash and air followed by water wash. Optimum backwash durations identified for the 0.5-1.0mm sarid and 2.0-3.35mm sand were 4 and 3 minutes respectively. The effect of start-up strategies on the numbers of particles in the filtrate during ripening was found to be variable. Slow start (both 1/2 hour and 1 hour) performed well under normal conditions experienced at the plant but performance was adversly affected by deteriorating raw water conditions. Delayed start was not as effective as slow start under normal conditions but was much more effective during periods of poorer raw water quality. Higher filtration rates (10 and 20m/h) resulted in much greater numbers of particles in the filtrate during ripening, thus fast-start was not a viable option. Cost benefit analysis of the technical options showed backwash optimisation and delayed start to be attractive retro-fit options in terms of cost effective reduction of particulate passage during filter ripening.