Maximizing the Enhanced Ozone Oxidation of Kraft Pulp Mill Effluents in an Impinging-Jet Bubble Column

Abstract

An impinging-jet bubble column that utilized two venturi injectors has been tested for the ozone mass transfer applications in water and wastewater treatment. The venturi injectors were utilized to create turbulent gas-liquid jets in the ambient fluid. The injectors were placed at an intersecting angle of 125° and the distance between the centers of the nozzles was equal to 60 mm. The intersecting of the gas-liquid jets caused an increase in the turbulence produced in the liquid phase and therefore, maximized the gas-liquid mass transfer. Pulp mill wastewater discharged from an aerated stabilization basin (ASB) of a Kraft pulp mill was ozonated in the impinging-jet bubble column. The ASB pulp mill effluent was ozonated in a continuous-flow mode with and without re-circulation of the liquid effluent leaving the bubble column in order to determine the overall mass transfer coefficient (kLa) and the enhancement factor (E). The gas absorption dynamics and the ozonation process treatment efficiencies were investigated. The ozonation process was more effective in removing the color and AOX-causing compounds than COD and TOC and it caused the biodegradability of the pulp mill effluent to increase. Ozone gas absorption dynamics were studied by continuously monitoring the ozone feed-gas and off-gas concentrations and ozone feed-gas flowrate. Rapid reactions occurred between ozone and the contaminants in the liquid phase, and as a result, the mass transfer process was enhanced by an enhancement factor (E). The two-film theory was applied and by assuming that the ozone reactions followed pseudo-first order irreversible kinetics, the overall mass transfer coefficient and the enhancement factor were determined. During the initial stage of the ozonation process, higher enhanced overall mass transfer coefficients (EkLa's) were observed. As the ozonation proceeded, EkLa values decreased significantly as a result of die enhancement factor dependency on the operating conditions and the wastewater characteristics. The process off-gas dynamics in terms of the ozone off-gas concentrations and their dependency on the process operating conditions and the wastewater characteristics were also examined in order to investigate the ozone off-gas emissions’ dynamics during the ozonation of pulp mill effluents. Accurate and reliable predictions of these emissions will help achieve optimized designs and reliable process-control of the ozone off-gas destruction facilities.