Ozonation is an increasingly attractive option for treating drinking water where it can effectively destroy microorganisms as well as oxidizing harmful micro-contaminants such as drugs and pesticides. It is also used to activate refractory pollutants present in industrial wastewaters in order to facilitate their removal by less expensive conventional means. It has also been proposed (in combination with coagulation) as means for controlling fouling in membrane separation units.

The use of ozonation in water/wastewater treatment has recently been growing as a result of increasing concerns about the quality of drinking water, and its adoption has been facilitated by the steady improvement in energy utilization efficiency achieved while generating ozone. Realization of full market potential is however impeded by the economic penalties associated with incomplete reactant utilization typically achieved in conventional gas/liquid contactors (such as bubble columns), and the consequent need to treat the off-gases before they are discharged into the environment. The capital costs associated with such large inefficient gas/liquid contacting units also provide additional constraints.

Overcoming these limitations has been facilitated by the recent observation that optimal performance of an integrated ozone contacting system can be achieved by using two segments, with the first one focusing on the dissolution of ozone while the reactive segment focuses on efficient microorganism reduction and/or pollutant oxidation [1, 2]. The hydrodynamic conditions in each section can thus be optimized to achieve optimal functional performance in each segment.

The use of co-current tubular gas/liquid contactors equipped with static mixers presents an attractive approach for dissolving ozone in a cost-effective and energy-efficient fashion. In this investigation, the impact of judiciously manipulating the turbulence structure in bubbly pipeline flow (by inserting a succession of static screen/grid elements across the flow) on: the inter-phase rate of mass transfer, k_La, the efficiency by which energy is utilized to achieve the desired mass transfer objectives, and the residence time needed to achieve 95% reactant utilization, was investigated. Several screen types and inter-screen spacing were tested using the rapidly-coalescent air/tapwater system. On the other hand, aqueous solutions containing trace quantities of SDS surfactant (1-50 ppm) were used to simulate the behavior of the slowly-coalescent systems encountered while ozonating wastewaters.

The ability of this static mixer element design to focus turbulent energy dissipation rate in a very thin layer adjacent to the screens resulted in the formation of microbubbles with very large interfacial area of contact; this, in turn, resulted in mass transfer coefficients in excess of 4 s^-1 being achieved. These k_La values are order of magnitude higher than those obtained in conventional and advanced ozone contactors [2 - 5] while maintaining a high energy utilization efficiency. Furthermore, the fact that both the gas and liquid phase processed through this contactor type exhibit essentially plug flow characteristics in the dissolution section resulted in better utilization of the reactants, with ≥ 95% being reached within contact times less than 1 s.  This is expected to facilitate the reuse of the oxygen-enriched air streams commonly used in energy-efficient ozone generating systems.

References

1. S.A. Craik; G. Finch; J. Leparc; M.S. Chandrakanth. The effect of ozone gas-liquid contacting conditions in a static mixer on microorganism reduction. Ozone Sci. Eng., 24, 91-103, 2002.

2. M.S. Baawain; M. Gamal El-Din; D.W. Smith; A. Mazzei, Hydrodynamic characterization and mass transfer analysis of an in-line multi-jets ozone contactor. Ozone Sci. Eng., 33, 449-462, 2011.

3. M.-T. Gao; M. Hirata; H. Takanashi; T. Hirokazu. Ozone mass transfer in a new gas-liquid contactor-Karman contactor. Sep. Purif. Techn., 42, 145-149, 2005.

4. O. Chedeville; M. Debacq; A.M. Ferrante; C. Porte, Use of an ejector for phenol containing water treatment by ozonation. Sep. Purif. Tech., 57, 201-208, 2007.

5. L.-B. Chu, X.-H. Xing, A.-F. Yu, Y.-N. Zhou, X.-L. Sun, B. Jurcik, Enhanced ozonation of simulated dyestuff wastewater by microbubbles, Chemosphere 68 1854–1860, 2007.