Enhanced treatment of reverse osmosis concentrates by ozone micro-nano bubbles coupled with catalytic ceramic membranes

Reverse osmosis is widely used for wastewater reuse, but efficient treatment of reverse osmosis concentrate (ROC) has become a huge challenge. In this study, catalytic ceramic membranes with active cobalt oxides loaded onto Al2O3 flat ceramic membranes are synthesized and combined with ozone micro-nano bubble (MNB) to treat a ROC from Iron & Steel industry. Compared with conventional aeration method, ozone MNBs show a much smaller bubble size (mainly concentrated at 80–200 nm) and doubled the dissolved ozone concentration. With the urea assisted impregnation method, Co3O4 particles were loaded on the outer surface and inner pore of ceramic membranes (Co-CMU), which showed the highest efficiency in COD removal for the ROC. The quenching experiments and electron paramagnetic resonance analysis confirmed that the main active species involved in this coupled process were 1O2 and ·OH. The ozone MNBs/Co-CMU not only enhanced the destruction of organic pollutants but also reduced the membrane fouling, resulted from a combined action of various physical and chemical processes. The Co-CMU catalysts showed high catalytic stability in five cycles of reuse with negligible cobalt leaching. Especially, the operation cost in this coupled process for per unit COD removal is decreased by 2.5 times, comparing with conventional ozonation process.