Aggregation kinetics of Cryptosporidium parvum oocysts

Abstract

Cryptosporidium parvum (C. parvum), an organism of increasing importance in drinking water, spreads to the environment through faeces of infected animals and humans. C. parvum produces infective chlorine-resistant oocyst that are removed from contaminated raw waters primarily using physical methods such as entrapment in surface or depth bed filters and coagulation–flocculation. In this paper, the aggregation properties of gamma-irradiated C. parvum oocysts in dilute concentrations, as a function of stirring time in the presence of bivalent and trivalent cations were studied. The size and surface charge of C. parvum oocysts were characterized before they were used for aggregation experiments. The surface charge of the oocysts, measured using a zeta potential analyzer were in the range of −57±2mV, which increased to −31±2mV in presence of bivalent electrolytes. Aggregation of oocyst in dilute suspensions was monitored using confocal laser scanning microscope (CLSM) in presence of bivalent or trivalent electrolytes. Aggregation was not visible in the diluted C. parvum oocyst suspension in Milli Q water system in the time frame of the experiment. Addition of a bivalent or trivalent salt with stirring condition induced aggregation due to the suppression of the surface charge of oocysts, resulting in a decrease in the energy of interaction between oocysts, and a decrease in double layer thickness around the oocysts. Predictive models could be built to understand C. parvum oocyst aggregation in raw waters, which could significantly improve the removal of oocysts from untreated (raw) drinking water.