Ozone mass transfer in water and wastewater treatment: experimental observations using a 2D laser particle dynamics analyzer

Abstract

Ozone mass transfer in water and wastewater was studied by using a 2D laser particle dynamics analyzer (PDA) to simultaneously measure bubble size distribution and mixing intensity. Three types of water samples, including deionized water, tap water and pulp mill effluents were tested to demonstrate the enhancement of ozone absorption with the presence of chemical reactions. Bubble size distribution and apparent mass transfer rates were used to estimate individual mass transfer parameters and examine the effects of operating conditions and selected gas spargers. It was shown that gas bubbles in ozonation systems varied over a broad range of sizes and their distributions usually deviated from a simple normal distribution. Among the factors examined, the gas sparger and water types were identified as the most important factors affecting mean bubble size, whereas the impacts of ozone dose and gas flow rate on bubble size distribution were minor. Due to the higher gas holdup, however, the specific surface area was strongly dependent on the gas flow rate as well. Consequently, the mass transfer coefficient increased as the gas flow rate increased. A further enhancement of mass transfer was observed for pulp mill wastewaters as compared to tap and deionized water samples, because the occurrence of chemical reactions depleted the dissolved ozone within the gas–liquid film.