Fouling process of membrane distillation for seawater desalination: An especial focus on the thermal-effect and concentrating-effect during biofouling

Membrane distillation (MD) is a promising seawater desalination technology for remote areas, although it is still limited by membrane fouling. In this study, a lab-scale direct contact membrane distillation (DCMD) system was designed for seawater desalination, operating in concentrating and non-concentrating modes to characterize the fouling process. Systematic analysis showed that scaling and biofouling are dominant fouling types in MD system, although the high temperature and salinity can inhibit the formation of biofouling layer. Biofouling can accelerate the formation of scaling, and the mixed foulants can block the membrane pores, leading to significant flux drop. With the application of high throughput sequencing, Proteobacteria, Bacteroidetes, Firmicutes and Planctomycetes were found to be the most abundant phyla. Microbial community succession was revealed during biofilm formation, in which Proteobacteria, Planctomycetes and Bacteroidetes played important roles. Additionally, a higher abundance of Firmicutes was observed in concentrating mode, representing the selective pressure from the heating and concentrating process. Finally, a three-phase model was suggested to describe the fouling process, which may reveal the vital fouling stage, and assist the development of anti-biofouling approaches in MD operation.