Photocatalytic sterilization of Escherichia coli (bacterium) or Saccharomyces serevisiae (yeast) was conducted with a rectangular bubble-column photoreactor (40 mm in width, 40 mm in breadth, and 250 mm in height) containing slurried TiO2 semiconductor particles. The profiles of cell deactivation with sterilization time could be expressed in fair agreement with experimental data, based on a series-event model and a second-order kinetics with respect to the concentrations of microbial cells and oxidative radicals generated by photoexcitation of TiO2 particles. Sterilization rate constants for the microbes were determined under various conditions of TiO2 concentrations (0−5 × 10-1 kg/m3) and average light intensities (0−223 W/m2) in the photoreactor. Linear relationships were obtained between the rate constants and average light intensity at TiO2 concentration of 1 × 10-2 kg/m3. When incident light intensity was kept constant (27 W/m2 for E. coli or 238 W/m2 for S. cerevisiae), the correlations between the rate constants and TiO2 concentration were interpreted considering a fraction of TiO2 particles adhered to the cells in slurry.

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The bubble column is commonly employed as a reactor in industrial processes such as chemical and fermentation processing as well as in the field of waste water treatment. It is often operated under conditions of liquid continuous flow. Correlations in predicting gas holdup in the bubble column with no liquid flow have been proposed by many workers1~5>7). However, the effect of liquid flow rate on gas holdup has not yet been elucidated quantitatively, and the findings obtained have been quite different amonginvestigators owing to the complicated flow situation of the two phases1~3)8).The objective of the present study is to ascertain the flow regimes of gas and liquid, and to obtain correlations useful in predicting gas holdup in both cocurrent and countercurrent flow systems. A criterion of flow regime distinction will be proposed and the effect of liquid flow rate on gas holdup will be examined.