Treatment of MF residuals for contaminant removal prior to recylcle This project focused on the recycle challenges posed by the potential presence of Cryptosporidium in microfiltration (MF) residuals. Among the various methods for treating MF backwash streams, clarification was selected because it was successful for treating spent filter backwash water from conventional plants, is economical, and is familiar to water plant operators and U.S. regulators. Successful results from bench-scale treatability tests guided the pilot- and full-scale studies. Microbial performance indicators for the pilot test included inactivated Cryptosporidium oocysts and aerobic endospores. At a sedimentation overflow rate of 0.25 to 0.5 gpm/ft2 (0.6 to 1.2 m3/m2/h) and with the appropriate coagulant type and dosage, coagulation/clarification of MF backwash streams produced a recycle quality similar to the original raw water. Unlike spent filter backwash water from conventional plants, polymer alone was not sufficient to treat MF backwash unless the feedwater had been previously coagulated with a metal salt coagulant.