Ozone–water contacting by ceramic membranes

Abstract

Abstract A common process in water treatment is the wet oxidation for the removal of certain organic and inorganic pollutants. The strongest oxidant technically applied in this process is ozone, which is an unstable gas under normal conditions, and therefore is produced from oxygen on site, usually by electrical discharge. After that the ozone has to be transferred from that gas into the water to be treated. Conventionally ozone transfer is achieved by bringing the gas and water in direct contact by means of bubble columns, injectors or other similar devices. Under unfavorable conditions, however, these methods suffer from excessive formation of foam requiring an extra treatment and a high-energy demand for pumping gas or water. This project's approach was to improve the transfer by better control of gaseous and aqueous phase's conditions at the contact surface. This was achieved by means of a membrane both separating the two phases and allowing for an ozone transfer between them. Due to ozone's high oxidation potential, chemically inert ceramic membranes were chosen for that purpose. In experiments, it was found that the transfer of the unstable ozone molecules is not obstructed by ceramic membrane material. Transfer rates between gaseous ozone and model water were measured for conventional ceramic membranes, as well as specially designed ones. They are comparable to conventional methods or better on the base of mass transfer per reactor volume. In conventional oxide membranes, water enters the pores because of capillary effects in the hydrophilic material [Burggraaf, A.J. and Cot, L., 1996, Fundamentals of inorganic Membrane Science and Technology Elsevier Science, The Netherlands]. The water in the pores raises the diffusion resistance for the ozone thus decreasing the transfer itself. Consequently, the modification of the hydrophilic material features into a hydrophobic behavior was one promising approach for the optimization of the process. It was achieved through the application of a hydrophobic coating to the membrane surface, which greatly improved the transfer efficiency.