Evaluation of the performance of ultrasound-assisted membrane distillation crystallization process for water and sodium chloride recovery in hypersaline solution

A large amount of high saline solutions has been generated in various industries and management of hypersaline solutions is a critical issue. Membrane distillation (MD) integrated with crystallization (MDC) technology is a promising approach in the treatment of hypersaline solutions, while membrane scaling is still problematic. Herein, MDC assisted by ultrasound (US) was proposed to deal with 25.93 wt% NaCl solution, and the performance of four processes including MD, US + MD, MDC and US + MDC were systematically evaluated. Results showed that ultrasound and crystallization could effectively alleviate membrane fouling and the operation time was extended from 80 min to over 245 min. Compared with MD process, the weight of crystals on the membrane surface was significantly reduced in the presence of US or / and crystallization (by 61.24%–98.63%) while the water recovery rate increased by 122% with the help of ultrasound. Crystal characteristic analysis demonstrated that the average crystal size was narrow in crystallizer of US + MDC process, successfully verifying the stability and feasibility of the system. The superiority of US + MDC process was further confirmed by scanning electron microscopy — energy dispersive spectroscopy observation. Moreover, higher mechanical strength and more stable surface tension showed that US + MDC process had a potential application in treating hypersaline solution.