Improving the design of ozone contactors for microorganism reduction with a static mixer

Cryptosporidium parvum is an encysted parasite that has been linked to waterborne disease. In water treatment, the oocysts are resistant to chlorine but may be reduced or inactivated by aqueous ozone. A theoretical analysis based on established inactivation kinetics and chemical engineering principals showed that C. parvum inactivation by ozone will be limited by the mixing patterns that exist in many conventional ozone contactors. An alternative two-stage ozone contactor design concept, in which a static mixer is used to rapidly dissolve ozone, was proposed to improve the efficiency of microorganism inactivation. To test the fundamental assumptions of this concept, seeding experiments with Bacillus subtilis spores, C. parvum oocysts and Giardia muris cysts were carried out in a prototype contactor under rigorously controlled conditions. For C. parvum and G. muris , contactor efficiency was assessed by comparing measured inactivation to that predicted using kinetic models previously developed in batch reactor studies and available in the literature. For B. subtilis a batch reactor study was completed, and a new kinetic model, describing spore inactivation at temperatures ranging from 3 to 22°C and for pH 6 to 8, was developed.; In the prototype static mixer contactor, ozone gas-liquid mass transfer was determined mainly by the hydrodynamic conditions within the static mixer, which were given by the liquid superficial velocity (0.7 to 1.4 m/s) and the gas-liquid flowrate ratio (1.2 and 2.6%). For the dissolved ozone concentrations investigated, between 0.6 and 1.6 mg/L for B. subtilis spores and between 0.5 and 0.8 mg/L for C. parvum oocysts, inactivation was unaffected by static mixer hydrodynamic conditions and was determined mainly by contact with dissolved ozone in the reactive flow segment. Most importantly, spore and oocyst inactivation approached that predicted based on the batch kinetic models and the assumption of plug flow behaviour in the contactor. Results with Giardia muris cysts were similar, but less definitive. It was concluded that the primary role of the static mixer in the two-stage contactor was to dissolve gaseous ozone rapidly and efficiently, and to thereby facilitate efficient microorganism reduction by contact with the dissolved ozone in the reactive flow segment.