AWWA WQTC59010

A better understanding of the causes of and the techniques available for controlling the "filter ripening" phenomenon is essential to optimizing the filtration process and allowing fewer pathogens to enter into the finished drinking water supply. Cryptosporidium oocyst and polystyrene microsphere removals were evaluated during the filter ripening period following 24-hour continuously seeded filter runs of pilot scale conventional and biological filters. One of the main objectives of this study was determining whether or not a high concentration of oocysts and/or microspheres pass through the filter during the ripening period. Previous studies have not revealed significantly poorer removals during ripening, but most of those studies did not seed oocysts throughout the entire preceding filter run (allowing oocysts to be present in the backwash remnants, which compose a significant portion of the initial filter ripening turbidity spike). The concentration of oocysts and microspheres found in the filter effluent under the influence of backwash remnants ranged from no higher to 5-times higher than those in water passing through the filter after the ripening period depending on filter type under normal backwashing conditions (i.e., collapse-pulsing plus full-fluidization wash with low-turbidity filtered water). Biological filters exhibited marginally lower concentrations of oocysts and microspheres present in the effluent samples containing backwash remnants. Two other backwashing conditions were also investigated during this study to elucidate their impact on filter ripening pathogen passage via the microsphere surrogate. The two backwashing conditions were backwashing with finished water (containing chlorine and a blend of phosphates), and a new filter backwashing strategy called the extended terminal subfluidization wash (ETSW). Polyphosphates in the backwash water were shown to negatively impact filtered water quality immediately after backwashing, and the ETSW procedure was shown to markedly improve filtered water quality during the filter ripening period as measured by particle counts, turbidity, and microsphere passage. Discoveries were also made that may help explain why the delayed start procedure seems to work well for some treatment plants but not for others. Includes 31 references, tables, figures.