Bisulfite-activated permanganate oxidation plus coagulation as a pretreatment of SWRO desalination lines to enhance boron rejection

To address the concern of high level of residual boron in the filtrate of seawater reverse osmosis (SWRO), bisulfite-activated permanganate technology (PM/BS) combined with post-coagulation/sedimentation (C/S) was evaluated as a pre-treatment strategy. The results revealed that the combination (PM/BS + C/S) can remove 37.1 % of boron under optimal conditions. For PM/BS pretreatment, there was an optimal dosage of BS (450 μM) and PM (90 μM) and pre-oxidation time (5 min) to assist C/S in achieving boron removal. For post-C/S, the optimal flocculant dose (10 mg/L), GT value (G rapid T = 45948 and G slow T = 12480), and precipitation time (20 min) were also determined. BS/PM pretreatment weakened the dependency on strict GT control and shortened the precipitation time required. Moreover, compared to C/S alone, BS/PM + C/S showed a remarkable improvement in the elimination of dissolved organic matter (19.8 %) and nutrients (19.8 % for assimilable organic carbon, 30.8 % for total nitrogen, and 16.0 % for total phosphorus), which reduced membrane fouling. The synergistic mechanism for enhanced boron removal includes destroying the organic coating of colloid or suspended particles, changing the structure of dissolved organic matter, generating MnO 2 particles in situ as a boron absorbent and coagulation aid, and regulating the growth and development of flocs. A pilot-scale test integrating PM/BS into a typical SWRO process yielded an average boron rejection rate of 95.7 %. This study indicates that PM/BS + C/S may be a promising pretreatment technology for SWRO to control the amount of residual boron in the filtrate.