Effect of NOM and biofilm on the removal of Cryptosporidium parvum oocysts in rapid filters

Laboratory experiments were performed to evaluate the effects of biofilm and natural organic matter (NOM) on removal of Cryptosporidium parvum oocysts from water by filtration. The bench-scale rapid filters consisted of 2.54cm ID×30.5cm polycarbonate plastic columns packed with 0.55mm spherical glass beads to a depth of 25cm and a porosity of 40%. Calcium chloride (0.01M) served as the coagulant in most of the experiments. The oocyst removal efficiency decreased from 51±6% for a clean bed to 23±3% for the biofilm-coated bed and to 14±1% in the presence of 5ppm of NOM. The oocyst removal for an experiment with a combination of biofilm-coated filter media and NOM was similar to that for the experiment with NOM alone (15±1%). The zeta potential values for the oocysts pre-equilibrated with NOM were significantly more negative than those obtained for untreated oocysts. This suggests that NOM enhanced the electrostatic repulsion between the oocysts and the negatively charged glass beads. Fortunately, use of alum as coagulant at a dosage sufficient to neutralize the surface charge of the NOM-coated oocysts resulted in a high removal efficiency (73±6%). Pre-equilibration of the oocysts with NOM also increased the hydrophobicity of the oocysts, but this was deemed to have a negligible effect on deposition onto the glass beads. The results of these experiments suggest that water treatment facilities treating source waters with moderate organic matter concentrations and/or employing biologically active filters have a greater potential for oocyst breakthrough and proper coagulation is critical for effective removal of oocysts in the filters.