AWWA JAW54010

Pilot studies were conducted to investigate the effect of recycling spent filter backwash water (SFBW) on Cryptosporidium and particle removal during conventional sedimentation and dual-media filtration. When SFBW recycle configurations (3,000-19,000 oocysts/100 L) were used, Cryptosporidium concentrations in settled water were as low as or lower than when no recycle was used (6,000-22,000 oocysts/100 L). Filtered water oocyst concentrations were typically below the detection limit of 1 oocyst/120 gal (~0.25/100 L). Overall process removal was about 5 log without recycle and about 5.7 log for each recycle scenario based on calculations using raw Cryptosporidium concentrations and filtered water detection levels. As with Cryptosporidium removal, removal of turbidity and particles > 2 um for the entire process (sedimentation + filtration) was similar for operations with and without recycle. Typical median filtered water concentrations for particles > 2 um were < 0.5 particles/mL for all recycle scenarios, including no recycle. Similarly, settled and filtered water turbidity levels for all studies were typically < 2 and < 0.03 ntu, respectively. Includes 2 references, tables, figures.