TREATING Microfiltration Backwash

A recent project focused on the recycling challenges posed by the potential presence of the pathogen Cryptosporidium in microfiltration (MF) residuals. After reviewing the various methods available for treating MF backwash streams, clarification was selected because it (1) is successful in treating spent filter backwash water from conventional plants, (2) is economical, and (3) is familiar to water plant operators and US 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/sq ft (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 water treatment plants, polymer alone was not sufficient to treat MF backwash because the feedwater had not been previously coagulated with a metal salt coagulant.