Effects of Coagulant Type and Conditions on Cryptosporidium and Surrogate Removal by Filtration

摘要

Coagulation effects on Cryptosporidium oocyst and oocyst-sized polystyrene microsphere removals by filtration were investigated. Specifically, the impacts of alum and ferric chloride coagulation during stable (optimized) operating conditions, sub-optimal coagulation (50 % reduction in coagulant dose), and coagulation failure were studied. The pilot-scale results from this work indicated that: 1. Alum and ferric chloride coagulation generally resulted in similar removals of Cryptosporidium oocysts and oocyst-sized microspheres during optimized operating conditions when filter effluent turbidities were consistently below 0.1 NTU. The median oocyst removals were 4.5-log and 4.1 -log during filtration with alum and ferric chloride respectively; median microsphere removals were 3.7-log and 3.1-log respectively. 2. Sub-optimal coagulation conditions with both alum and ferric chloride coagulation resulted in deteriorated Cryptosporidium and microsphere removal by filtration (relative to stable operation). Median C. parvum removals were 2.1 -log and 1.3-log by filtration preceded by alum and ferric chloride respectively; median microsphere removals were 1.2-log and 0.4-log respectively. 4. The observed differences in Cryptosporidium and microsphere removal during sub-optimal coagulation conditions (and possibly during stable operation) may be associated with the different mechanisms of alum and FeCl_3 interaction with oocysts during filtration. Further analysis is necessary to determine if these differences are statistically significant. 5. Oocyst-sized polystyrene microspheres appeared to be reasonable indicators of Cryptosporidium removal by filtration, regardless of ferric chloride or alum coagulation.