Bench-scale filtration experiments were performed to evaluate the impact of biofilm and natural organic matter (NOM) on removal of Cryptosporidium oocysts from water. Calcium chloride (0.01 M) served as the coagulant most of the experiments. The oocyst removal efficiency decreased significantly for the biofilm-coated bed and in the presence of 5 ppm of NOM. The oocyst removal efficiency decreased from 51% for a clean bed to 23% for the biofilm-coated bed and to 14% in the presence of 5 ppm 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%). 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. Preequilibration of the oocysts with NOM also increased the hydrophobicity of the oocysts, but this was deemed to have a negligible effect on deposition. The results of these experiments suggest that plants treating source waters with moderate organic matter concentrations and/or employing biologically active filters have a greater potential for oocyst breakthrough and may benefit from enhanced treatment strategies to ensure that Cryptosporidium oocysts are effectively removed in the filters such as enhanced coagulation to remove NOM and destabilize the oocysts, limiting biofilm buildup in the filters, or the use of a second stage of filters when biologically active filters are employed.
展开▼
