Insights into the coupling pre-ozonation with coagulation pre-treatment for mitigating biopolymer fouling of reverse osmosis membrane: Role of Ca2+

Abstract

Although extensive efforts have been devoted to mitigate reverse osmosis (RO) membrane fouling by ozonation + coagulation (O + C) pre-treatment, it still remained unclear that whether pre-ozonation benefited the coagulation to remove organics and reduce membrane fouling in the presence of calcium ion (Ca2+). In this study, coagulation and O + C pre-treatments were used to alleviate reverse osmosis (RO) membrane fouling by biopolymer with different concentrations of Ca2+. Bovine serum albumin (BSA) and sodium alginate (SA) were chosen to represent protein and polysaccharides in biopolymer, respectively. The physicochemical properties of BSA and SA (e.g., dissolved organic carbon (DOC) concentration, particle size and zeta potential), the residual Ca2+ and Fe3+ (coagulant) concentration, and membrane autopsy were analyzed to reveal the fouling mechanism. The results exhibited that the effect of ozonation on coagulation for RO membrane fouling mitigation strongly depended on the biopolymer species and Ca2+ concentration. Ozonation promoted coagulation to alleviate BSA fouling at low Ca2+ concentration of 0.5 mM, but it was contrary for SA. The elevated Ca2+ concentration within a certain range (i.e., from 0.5 to 2.0 mM) was beneficial for O + C pre-treatment to mitigate membrane fouling by BSA and SA, thus ozonation acted as a coagulation aid to lessen BSA and SA fouling at higher concentration of Ca2+. The positive impact of Ca2+ on fouling control by O+C pre-treatment was resulted from the synergistic impact of the increase in DOC removal, the decrease in hydrophobicity, the residual Ca2+ and Fe3+ (coagulant) concentrations according to the principal component analysis (PCA). However, when Ca2+ concentration further increased to 4.0 mM, BSA fouling mitigation by O + C pre-treatment deteriorated due to the negligible promoted BSA removal efficiency and sharply increased residual Ca2+ concentration. This study got new insights into the effect of O + C pre-treatment on biopolymer fouling alleviation in the presence of Ca2+, which might guide researchers to effectively control RO membrane fouling during wastewater reclamation.