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A better understandingof the causes of and the techniques available for controlling the "filter ripening"phenomenon is essential to optimizing the filtration process and allowing fewerpathogens to enter into the finished drinking water supply. Cryptosporidium oocyst andpolystyrene microsphere removals were evaluated during the filter ripening periodfollowing 24-hour continuously seeded filter runs of pilot scale conventional andbiological filters. One of the main objectives of this study was determining whether ornot a high concentration of oocysts and/or microspheres pass through the filter duringthe ripening period. Previous studies have not revealed significantly poorer removalsduring ripening, but most of those studies did not seed oocysts throughout the entirepreceding filter run (allowing oocysts to be present in the backwash remnants, whichcompose a significant portion of the initial filter ripening turbidity spike). Theconcentration of oocysts and microspheres found in the filter effluent under theinfluence of backwash remnants ranged from no higher to 5-times higher than those inwater passing through the filter after the ripening period depending on filter type undernormal backwashing conditions (i.e., collapse-pulsing plus full-fluidization wash withlow-turbidity filtered water). Biological filters exhibited marginally lowerconcentrations of oocysts and microspheres present in the effluent samples containingbackwash remnants. Two other backwashing conditions were also investigated duringthis study to elucidate their impact on filter ripening pathogen passage via themicrosphere surrogate. The two backwashing conditions were backwashing withfinished water (containing chlorine and a blend of phosphates), and a new filterbackwashing strategy called the extended terminal subfluidization wash (ETSW).Polyphosphates in the backwash water were shown to negatively impact filtered waterquality immediately after backwashing, and the ETSW procedure was shown tomarkedly improve filtered water quality during the filter ripening period as measured byparticle counts, turbidity, and microsphere passage. Discoveries were also made thatmay help explain why the delayed start procedure seems to work well for sometreatment plants but not for others. Includes 31 references, tables, figures.

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Edition: Vol. - No. Published: 11/02/2003 Number of Pages: 20File Size: 1 file , 570 KB