Dual-Stage Membrane Distillation for One-Step Separation, Concentration of Volatile Fatty Acids, and Simultaneous Water Recovery from Wastewater

The anaerobic digestate generated from municipal sludge treatment contains abundant volatile fatty acids (VFAs), which hold great recovery value due to their wide utility in the food, pharmaceutical, and chemical industries. However, conventional recovery technologies are confronted with inferior purity and low concentration of the VFA product because of their poor VFA selectivity over competing species and low efficiency in separating VFAs from water. In this study, a novel dual-stage membrane distillation (MD) coupled with alkali adsorption was designed to achieve one-step VFA separation, concentration, and simultaneous water recovery. When treating real anaerobic digestion wastewater, the system achieved a VFA recovery of 86.2%, VFA concentration of 103.3 g L –1, concentration factor of 20.3, and water recovery of 62.8%, which were the best performances reported to date. A literature review elucidates that the concentration, concentration factor, and purity of the obtained VFA product are far superior to those of the state-of-the-art technologies. Moreover, the system also exhibited excellent stability during 192 h of continuous operation with real wastewater. This study demonstrates the great potential of the dual-stage MD for separating and concentrating VFAs from anaerobic digestion wastewater, providing a novel solution for volatile component recovery.