Pilot-Scale Evaluation Of The Effects Of Mixing On Ozone Disinfection OfEscherichia ColiIn A Semi-Batch, Stirred Tank Reactor

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Abstract
A research program was undertaken to examine the effects of mixing intensity and post-ozonation conditions on the survival of Escherichia coli ATCC 11775 in a high quality secondary wastewater effluent with a total organic carbon content of 8 mg/L and a chemical oxygen demand of 26 mg/L. The study was conducted using a pilot-scale, semi-batch stirred tank reactor with a 6-blade Rushton turbine. Two power conditions were investigated: 255 W/m3 and 870 W/m3. Two post-ozonation conditions also were investigated. In the first, a headspaceless sample was withdrawn and stored in the dark, in a quiescent condition. For the second, the gas flow to the reactor was stopped, but mixing was continued. For equivalent contact times, it was found that two orders of magnitude more E. coli survived under the second post-ozonation condition when compared with the first condition. There was also a significant difference associated with the mixing intensity in the contactor, with the higher power input resulting in less efficient inactivation of E. coli. The results confirm the importance of designing an ozone contactor to promote the maintenance of aqueous ozone in the contactor. In addition, optimum ozone mass-transfer may require different contacting conditions than those required for optimum disinfection performance. It was concluded that the design of ozone contactors should consider the use of at least two-stages: one optimized for ozone mass-transfer and one optimized for disinfection contacting.