Treatment Options for Giardia, Cryptosporidium,and other Contaminants in Recycled Backwash Water OBJECTIVES: The overall objective of the project was to provide a thorough guide to the type of processes available to treat recycle streams-particularly spent filter backwash water (SFBW)-to a degree that will prevent finished water quality from being adversely impacted by the recycle streams. Specific goals included identifying effective, cost-efficient, and practical methods of treating SFBW; developing an appropriate set of treatment goals; and evaluating and costing a range of technologies that can achieve those goals. BACKGROUND: A recent survey of utilities in the Information Collection Rule (ICR) database showed that more than half of conventional plants in the United States routinely recycle, and a survey of AWWA utility members that recycle indicated that about 30 percent do not treat their SFBW prior to recycling it. Waste streams concentrate contaminants, including Giardia and Cryptosporidium. Those pathogens are reintroduced upon recycling, posing a threat to pass through filters into the finished water. HIGHLIGHTS: Giardia and Cryptosporidium were detected in SFBW at concentrations 16 to 21 times raw water levels and infectious Cryptosporidium were observed in some SFBW samples. It was also noted that chlorine dioxide and UV yielded Cryptosporidium inactivations of 2-logs or greater and 4-logs or greater, respectively, in the SFBW environment. Dissolved air flotation (DAF) emerged as a highly effective strategy for treatment of SFBW. Polymer was critical for effective removal of particles and turbidity by either sedimentation or DAF; the best polymer type varied. Pilot cyst spiking showed that Cryptosporidium removals through sedimentation of SFBW with and without polymer averaged 1.0-log and 0.6 log. The research also showed that scale-up of both sedimentation and DAF from bench-scale to pilot- or full-scale was excellent. APPROACH: The project approach was to conduct a two-phase, comprehensive study of the practice of recycling. Phase I was designed to identify treatment goals and promising treatment candidates for meeting these goals. Phase II was used to evaluate the technologies under actual operating conditions to determine actual treatment efficiencies. Phase I work included a review of current recycling practices, and extensive bench-scale testing for both disinfection and physical removal processes. Also, a field sampling program was conducted in which parasite enumeration and viability measurements were collected on full-scale raw and SFBW. Phase II assessments were performed to determine the efficiency of selected processes on a larger scale or continuous basis and to determine the impacts on overall treatment.