AWWA WQTC52741

Approximately 3 to 5% of finished water produced at a typical conventional water treatment plant is used to backwash filters. Consequently, a treatment plant will need to process about 105% of distribution system demand in order to produce enough water to meet the demand, plus have enough for backwash.Recovery of part of this spent filter backwash water can decrease the raw water demand but there are potential negative consequences of this including hydraulic surges, changes in coagulation chemistry, and increased particle and cyst loading. This pilot study investigated the impacts of recycling spent filter backwash water on Cryptosporidium and particle removal during conventional sedimentation and dual media filtration. The study was conducted at the Erie County Water Authority in Tonawanda, New York. Recycle flow rates investigated included 0 (no recycle), 4.3, 10, and 20% of the influent flow rate. This paper presents the results of the study and describes the pilot plantoperations. Details of the Cryptosporidium spiking and measurement methods are also provided. Includes tables, figures.