Ozone membrane contactor to intensify gas/liquid mass transfer and contaminants of emerging concern oxidation

A tubular porous borosilicate membrane contactor was investigated for ozone gas/water mass transfer and the removal of contaminants of emerging concern (CECs) in water. Ozone gas/water contact occurs on the membrane shell-side, which is coated with a photocatalyst (TiO₂-P25), as the ozone gas stream is fed from the lumen side and permeates through the pores generating micro-sized ozone bubbles uniformly delivered to the annular reaction zone where the contaminated water to be treated flows. Under continuous flow, water pH at 3.0 and temperature at 20 ºC, the volumetric mass transfer coefficient (K_La) ranged from 3.5 to 9.0 min^-1 and improved with the increase of gas flow rate (Q_G, 1.5-fold from 0.15 to 1.0 Ndm³ min^-1) and liquid flow rate (Q_L, 2.0-fold from 20 to 50 L h^-1), due to enhanced turbulence on the membrane shell-side and annular zone. The mass transfer efficiency was more pronounced as the Q_G decreased and the Q_L increased, which is advantageous for large-scale applications. The main resistances to ozone transfer were in the water phase boundary layer (53-76%) and in the membrane (24-47%; k_M = (1.14 ± 0.01) × 10^-4 m s^-1). For an ozone dose of 12 g m^-3 and residence time of 3.9 s, removals ≥ 80% were achieved for 13 of 19 CECs spiked in demineralized water (each 10 μg L^-1), demonstrating the applicability of this membrane contactor for ozonation treatment. Photocatalytic ozonation (O₃/UVC/TiO₂) did not significantly improve the treatment performance due to the low residence time inside the contactor.