The effects of recycling waste filter backwash (FBW) on the performance of clarification and filtration were studied with pilot-scale experiments. Two different clarification methods, dissolved air flotation (DAF) and inclined plate sedimentation, were studied alone or in combination with subsequent dual-media (anthracite/sand) filtration. Untreated waste FBW from the on-site full-scale plant was used to examine the effects of four recycling operational variables on treatment performance: FBW recycle rate (10 to 40% of raw flow), FBW pH (baseline of 7, high of 8.5), FBW solids level (baseline and high), and coagulant dosing (constant and reduced). Mixed influent flow (raw plus FBW recycle) was maintained constant throughout the study.Recycling FBW caused slight to moderate increases in clarifier effluent turbidity, with impacts generally increasing as recycle percentage increased. However, the increases in clarifier effluent turbidity were not in proportion to the large increase in clarifier influent solids, i.e. clarifier percent removal performance increased as clarifier loadings increased. Recycling high pH FBW with high soluble NOM and more negatively charged particles resulted in the worse treatment performance. Recycling FBW with baseline or high solids level at pH 7 generally caused moderate impacts on clarifier turbidity and minimal impacts on NOM removal. No impacts were found on filter effluent quality from 20% recycle at high solids and at pH 7, however, recycling high pH FBW caused moderate increases in filter effluent turbidity and NOM during the recycling period.

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