Reliability of surrogates for determining Cryptosporidium removal

Testing of field-scale bag filtration systems yielded results indicating that 4-6-mu m polystyrene microspheres can be used as a reliable surrogate for determining Cryptosporidium oocyst removal in bag filtration processes. A nearly perfect linear correlation was observed between log removals of 4-6-mu m polystyrene microspheres and Cryptosporidium parvum oocysts. The correlations between Cryptosporidium removal and other potential surrogates (including 4-6-mu m particle counts, 1-25-mu m particle counts, and turbidity) produced slopes that were substantially >1 and may vary geographically because of variations in raw water quality. The similarities and dissimilarities in log removal between Cryptosporidium oocysts and the potential surrogates for the bag filtration system resulted from their differences in size distribution.