AWWA WQTC60701

Extended terminal subfluidization wash (ETSW) is an advanced backwash technique developed to reduce the passage of particles thru conventional filters during the ripening period after backwash. This technique was investigated on two source waters at the pilot scale under different coagulation conditions to represent a wide range of full scale operations based on turbidity and TOC: optimal coagulation (enhanced), sub-optimal coagulation, and sub-optimal coagulation with a low dose of chlorine at the rapid mix. All filters were monitored for ripening peak turbidity, recovery time (time to a less than 0.1 NTU), and total particle counts. In all coagulation scenarios, the effluent water quality during the ripening period improved with the filters that utilized the ETSW technique compared to the regular backwash alone. ETSW filters for the first scenario had ripening peaks less than 0.1 NTU, thus eliminating the recovery time. Suboptimal coagulation increased the ripening peak turbidities, primarily after the first filter volume of effluent had passed. In addition, a sub-optimal coagulation chlorination study with either a low pre-chlorination dose, or chlorination prior to the filter (with no other preoxidation) yielded an approximate one log reduction in steady state filter effluent particle counts compared to filters treating water with no oxidant addition. There was no observable effect on effluent turbidity with or without chlorination under sub-optimal conditions. Includes 11 references, tables, figures.