Pre-filtration followed by slow double-layered filtration: Media clogging effects on hydraulic aspects and water quality.

This study evaluated clogging effects on head loss and water quality in sand prefiltration columns, followed by innovative double-layered slow-filter filtration technology using anthracite coal and sand. The study was divided into two phases. Phase 1 (filtration rate 11 m /m ·d) had a layout of a pre-filter (PF) system followed by a slow filter (SF). In Phase 2 (filtration rates of 5.5 and 11 m /m ·d), two PF systems (PF1 and PF2) followed by SFs (SF1 and SF2) were monitored, be differed only by filtration rate. Phase 1 presents a mean turbidity removal of 45.6% in PF and 23.9% in SF. PF presented a lower head loss in the superficial layers of the filter media, whereas SF presented a more important contribution in the deeper layers of the bed. In Phase 2, the mean results obtained for turbidity removal in PF1 and SF1 were 84.7% and 22.4%, in that order. PF2 and SF2 promoted mean reductions of 84.5% and 22.0%, respectively. Greater clogging is observed in the PF's surface layers, whereas the SFs dissipate greater clogging with depth. Filtration rates and water quality were similar in filter media of different compositions. PRACTITIONER POINTS: Double-layer filters show better use of the porous media in depth. Different materials produce different forms of clogging. Higher filtration rates increase the number of pre-filter and slow filter runs.