Evaluation of the effect of recycle of waste filter backwash water on plant removals of Cryptosporidium

Pilot-scale experiments investigated the removal of Cryptosporidium oocysts and any effects from the recycle (10% rate) of untreated waste filter backwash water (FBW) for two treatment plant types: dissolved air flotation (DAF) and plate sedimentation, with dual media filtration for both plants. Experiments assessed performance for design hydraulic loadings and detention times and for two seasons (summer and winter). DAF clarification achieved about 2 log oocyst removal for both seasons either with or without recycle of FBW. Plate settling in the summer achieved 1.8–1.9 log oocyst removal with no effects from recycle of FBW. Plate settling performance was poorer in the winter at about 1 log oocyst removal without recycle, and decreased slightly with recycle of FBW. No effects were found from recycle of FBW on DAF and filtration performance for turbidity and particle counts (2–15μm). There were no or minor effects for the same parameters on plate settling performance and no effects on filter performance. For both treatment plants, no oocysts were found in the dual media filter effluent either with or without recycle of FBW.