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Slow sand filters are considered as a great alternative for supplying drinking water in rural and/or isolated areas where raw water that is treatable with this technique is available. Some studies used backwashing as an alternative for cleaning the slow sand filter with the goal of applying the technology in small communities, since filters that supply water to a small number of people do not require much space. In this study the influence of the effective diameter on water quality in the filters and cleaning system was evaluated. A pilot system with six filters was built: three filters were conventionally cleaned by scraping and the other three were cleaned by backwashing, each with a different effective diameter of filter medium. Most filters had an average turbidity of less than 1.0 NTU, the turbidity required at the output of the filters by the Brazilian Ministry of Health Ordinance. In the study, the filters cleaned by scraping with smaller-diameter filter beds effectively filtered water better but had worse effective production. The opposite occurs in the case of backwashed filters.

A B S T R A C TThe need for water production is increasing in urban populations. In conventional water treatment plants, the filters are backwashed by treated water, which represents high longterm costs. In order to obtain a highly effective production of water with a subsequent reduction in production costs, it was proposed to feature a granular filter element with low density and its potential to provide a down-flow rapid filter, which can represent a reduction in the volume of water used for backwash. The polymer used for this purpose appears to be favorable; in addition, there are possibilities of recycling this material. A velocity of 6.08 m/h for a 50% expansion was demonstrated, which is very low compared to conventional filter elements (i.e. sand and anthracite). So, this element has shown great potential in reducing the volume of water during the backwash process, as observed using mathematical models. The final quality of the filtered water still needs to be analyzed. However, the permeability coefficient was determinate at 1.79 × 10 −5 ± 2.87 × 10 −6 m/s being satisfactory as filtration. The outlook is that it may become an option for element filtration in down-flow rapid filters for water treatment.

In conventional water treatment stations, the filter cleaning is performed with the use of filtered water. To save water and obtain higher production, the use of polystyrene (PS) beads has been proposed as a granular filter element because it is a granular element with a low specific mass. By being lightweight, this material requires a lower water velocity during backwash. The PS beads were applied in a descending rapid filter and compared to a conventional sand and anthracite filter, and its hydraulic performance was evaluated during the backwash with air and water interspersed. Although it presents a high fluidity, with lower rates (compared to conventional filters) of backwash, this fact does not necessarily represent an economy of backwash water, because it requires more time for cleaning. It was also observed that there is an optimal value for the removal of particles collected during the filtration without loss of material.