Vacuum membrane distillation for seawater concentrate treatment coupled with microbubble aeration cleaning to alleviate membrane fouling

Abstract

In this study, a laboratory-scale vacuum membrane distillation (VMD) system was applied to treat seawater concentrate using a hydrophobic polytetrafluoroethylene (PTFE) membrane with an effective area of 120 cm2. At the beginning, the VMD performance was investigated under different temperature and vacuum pressure. Results indicated that running temperature and vacuum pressure were optimal at 65 °C, 0.09 MPa, respectively in order to obtain an appropriate permeate water flux. Furthermore, four types of synthetic seawater concentrate with different ionic compositions, namely, SW-1, SW-2, SW-3, SW-4, was used as feed solutions for studying the VMD performance at 65 °C, 0.09 MPa. Results confirmed that the existence or absence of scaling ions would observably affect the VMD performances. For instance, concentration factor in the SW-4 test without scaling ions can reach 3.99 (i.e., 20.09 wt%), compared to the SW-1 test. Besides, we briefly elucidate the whole fouling behavior in VMD, the reason is that scaling ions could easily generate sediment substances on the surface or in the pores of membranes. In view of appearance of membrane fouling, finally, unlike conventional membrane cleaning methods, microbubble aeration (MBA) cleaning as a green approach, without chemicals consumed during cleaning, was proposed to alleviate membrane fouling, meanwhile, the mechanism of performance enhancement by MBA cleaning was also illustrated, with the introduction of microbubbles (MBs), a gas-liquid two-phase flow could intensify the surface shear and adhesion effects near the membrane surface, and also generate an electrostatic repulsion effect between membrane surface and MBs. Together, these results show that MBA cleaning is an effective method to mitigate membrane fouling in VMD.