Dual function filtration and catalytic breakdown of organic pollutants in wastewater using ozonation with titania and alumina membranes

Abstract

Water recycling via treatment from industrial and/or municipal waste sources is one of the key strategies for resolving water shortages worldwide. Polymer membranes are effective at improving the water quality essential for recycling, but depend on regular cleaning and replacement. Pure ceramic membranes can reduce the cleaning need and last significantly longer in the same applications while possessing the possibility of operating in more aggressive environments not suitable for polymers. In the current work, filtration using a tubular ceramic membrane (α-Al 2O 3 or TiO 2) was combined with ozonation to remove organic compounds present in a secondary effluent to enhance key quality features of the water (colour and total organic carbon, TOC) for its potential reuse. ‘Bare’ commercial α-Al 2O 3 filters (pore size ∼0.58 μm) were tested as a microfiltration membrane and compared with the more advanced catalytically active TiO 2 layer that was formed by the sol–gel method. The presence of anatase with a 4 nm pore size at the membrane surface was confirmed by X-ray diffraction (XRD) and N 2 adsorption. Filtration of the effluent over a 2 h period led to a reduction in flux to 45% and 60% of the initial values for the α-alumina and TiO 2 membrane, respectively. However, a brief dose (2 min) of ozone at the start of the run resulted in reductions to only 70% of the initial flux for both membranes. It is likely that the oxide's functional property facilitated the formation of hydroxyl (OH ) or other radicals on the membrane surface from ozone decomposition which targeted the breakdown of organic foulants thus inhibiting their deposition. Interestingly, the porous structure therefore acted in a synergistic, dual function mode to physically separate the particulates while also catalytically breaking down organic matter. The system also greatly improved the efficiency of membrane filtration for the reduction of colour, A 254 (organics absorption at the wavelength of 254 nm) and TOC. The best performance came from combined ozonation (2 min ozonation time with an estimated applied ozone dose of 8 mg L −1) with the TiO 2 membrane, which was able to reduce colour by 88%, A 254 by 75% and TOC by 43%. It is clearly evident that a synergistic effect occurs with the process combination of ozonation and ceramic membrane filtration demonstrating the practical benefit of combining ceramic membrane filtration with conventional water ozonation.