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Because much rier Cryptosporidium against attention of the this limitations has pathogen. oocysts been of directed and The many the ripening chemical fear toward of or waterborne filtration disinfectants maturation as disease the period for primary inactivating outbreaks, of the barfilCryptosporidium oocysts and the fear of waterborne disease outbreaks, much attention has been directed toward filtration as the primary barrier against this pathogen. The ripening or maturation period of the filter run may be the Achilles heel of this barrier. Several research groups have already examined Cryptosporidium removal during the filter-ripening period and found removals that were typically only slightly less than those of the same filters following the ripening period (Huck et al, 2000; Health Stream , 1999; Emelko et al, 1998; Patania et al, 1995). Many studies only seeded oocysts for a brief period (if at all) of the preceding filter run and allowed few oocysts to be present in the backwash remnant water that is responsible for a significant portion of the turbidity peak that follows backwashing. Amburgey and co-workers (2001a, 2001b) did seed the entire preceding filter runs of both conventional and biological filters and performed extensive sampling during the ripening period to more accurately assess the risk of Cryptosporidium passage. These researchers found that biological filters usually had effluent oocyst concentrations approximately equal to those found later in the filter run, but the conventional filter had oocyst concentrations from 3 to 18 times higher than the biological filters at the peak of the ripening period following an efficient backwash procedure. Hall et al (2000) found similar results following continuously seeded conventional filter runs. Overall, there appears to be only a brief period of increased Cryptosporidium passage during the ripening period of optimized pilot-scale filter runs, but this may not be the case for full-scale filters operating at suboptimal coagulation conditions and/or employing less efficient backwash techniques. Several enhancements of the normal filter-operating process have been explored in recent years in response to the risks encountered during the filter |
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