Comparative reductions of Cryptosporidium parvum oocysts, Bacillus subtilis spores, uncoated and glycoprotein-coated microspheres during water filtration through quartz sand

Contamination of drinking water by Cryptosporidium parvum oocysts poses a significant public health risk, as evidenced by numerous outbreaks of cryptosporidiosis worldwide. This study evaluated the effectiveness of Bacillus subtilis spores, unmodified yellow-green (YG) and yellow-orange (YO) microspheres, and glycoprotein-coated YO microspheres, in predicting C. parvum oocyst reduction during water filtration through quartz sand. Column filtration experiments were conducted at a flow rate relevant to slow sand filtration using Vienna tap water. Concentration breakthrough curves and data analysis using colloid filtration theory revealed the reduction order as B. subtilis spores ≪ YG microspheres ≪ glycoprotein-coated YO microspheres < C. parvum oocysts < YO microspheres. The normalized concentrations (C/C₀) were in the range of 10⁻² for spores, 10⁻³ for YG microspheres, 10⁻⁴ for both glycoprotein-coated YO microspheres and oocysts, and 10⁻⁵ for YO microspheres. Under the experimental conditions of this study, B. subtilis spores and YG microspheres were overly-conservative surrogates, while YO microspheres were under-conservative surrogates. Comparatively, glycoprotein-coated microspheres provided the closest predictions in oocyst reduction, though slightly conservative. The differences or similarities in physicochemical properties (size, shape, surface charge, hydrophobicity) and surface macromolecules between the oocysts and candidate surrogates were considered to be the determining factors influencing surrogate effectiveness. Glycoprotein-coated microspheres, exhibiting similar physicochemical properties to oocysts, emerged as the most effective surrogate, providing an accurate, albeit slightly conservative, prediction of oocyst reduction in sand media. The study highlights the importance of selecting appropriate surrogates for effective water treatment design and operation, balancing safety margins and cost efficiency.