Use of Bacillus subtilis spores as model micro-organisms for ozonation of Cryptosporidium parvum in drinking water treatment

Spores of the aerobic bacterium Bacillus subtilis are potential models for evaluation of Cryptosporidium parvum inactivation by ozone processes in drinking water treatment. The kinetics of inactivation by ozone of prepared Bacillus subtilis ATCC 6633 spores were measured in a series of batch reactor experiments at temperatures of 3, 12, and 22°C and at a pH of 6 and 8. Spore inactivation curves were characterized by a pronounced lag phase at low ozone exposures followed by a logarithmic inactivation rate and, finally, by a tailing region at higher ozone exposures. A two-part kinetic model consisting of a multitarget term and a first-order term provided a good fit to observed inactivation at each temperature. Water pH had a statistically significant but limited effect on spore inactivation. When spore inactivation was compared to C. parvum oocyst inactivation predicted by a previously published kinetic model, substantial differences were observed. These differences may make it difficult to use this particular strain and preparation of spores to reliably predict C. parvum oocyst inactivation from the outcomes of B. subtilis seeding studies. Bacillus subtilis spores may otherwise serve as indicators of the hydrodynamic efficiency of ozone contactors. Key words: ozone, inactivation, Bacillus subtilis spores, Cryptosporidium parvum oocysts, drinking water, kinetics, modeling.