The use of anti-scalants in gypsum scaling mitigation: Comparison with membrane surface modification and efficiency in combined reverse osmosis and membrane distillation

Abstract

Membrane scaling is detrimental to the efficiency of membrane desalination. While both membrane surface modification and the addition of anti-scalants have been investigated as possible methods to minimize scale formation on membrane surfaces, the efficacies of these two strategies have not been compared in the literature. In this work, we first modified a commercial polyamide reverse osmosis (RO) membrane with four different polymers to create various surface functionalities and properties. Then the anti-scaling efficiencies of the modified membranes were evaluated and compared with the use of poly(acrylic) acid (PAA) as anti-scalant in a bench-scale RO system fed with gypsum supersaturated solutions. The modified membrane with the best performance displayed limited effectiveness in delaying gypsum scaling, whereas gypsum scaling was greatly retarded in the presence of PAA. In contrary to the passive effect of membrane surface modification on gypsum scaling, anti-scalants actively disrupt gypsum crystallization. Further, we applied membrane distillation (MD) to desalinate the concentrated brines generated from RO desalination with and without anti-scalants. We demonstrated that the residual PAA in the RO brines was highly effective to alleviate gypsum scaling in MD, greatly enhancing the total water recovery of the combined RO-MD system. Our results provide comparative insights on the efficiencies of different scaling mitigation strategies in membrane desalination, and demonstrate the promising potential of applying anti-scalants to achieve high water recovery.