Comparison of Conventional and Biological Filter Performance for Cryptosporidium and microsphere removal

Previous studies involving filter runs seeded with oocysts have not shown significantly poorer removals during ripening. However, these studies typically did not seed oocysts throughout the preceding filter run, which is necessary for representative numbers of oocysts to be present in the backwash-remnant water. In this study, four trials consisting of three to five consecutive filter runs were continuously seeded with oocysts and/or microsphere surrogates so that they would appear in the backwash-remnant water of the successive run. Compared with concentrations in the postripening period, the concentrations of oocysts and microspheres found in the effluent under the influence of backwash-remnant particles ranged from no higher to more than an order of magnitude higher, depending on filter type. Significant carryover of different-colored microspheres was not observed in consecutive filter runs following the filter-ripening period. During this study, conventional and biological filters showed similar average postripening oocyst and microsphere removals (roughly 1.7 and 2.1 log, respectively), but the biological filters exhibited significantly lower concentrations of oocysts and microspheres in the initial effluent samples containing backwash-remnant particles left in the filter at the conclusion of the backwashing procedure.