Forward osmosis (FO) is a new membrane technology with lower energy cost and less irreversible fouling than reverse osmosis (RO). Therefore, FO is being considered as an alternative or supplement to RO for wastewater recycling. However, the performance of FO in removing the disinfection byproducts (DBPs) formed during disinfection of the membrane feed has not been studied. We first evaluated DBP rejection using clean and fouled commercial FO membranes. The FO rejection of <i>N</i>-nitrosodimethylamine (NDMA) and haloacetonitriles (HANs), two high priority (groups of) DBPs, was 31%–76%, higher than that reported in RO. Organic fouling generated by model polysaccharide and protein lowered DBP rejection. Comparison showed that aquaporin membrane exhibited higher rejection than cellulose triacetate membrane for all DBPs in both clean and fouled situations. Subsequently, the effects of reverse salt flux, a unique phenomenon in FO, on DBP transport was evaluated. Reverse salt flux refers to the transport of draw solutes from the saline solution to the feed solution (i.e., wastewater). It was found to hinder DBP transport. Two mechanisms were identified experimentally: the steric hindrance in the active layer caused by the occupation of free volume by draw solutes and the retarded diffusion of DBPs in the support layer via a “salting-out” effect. To enable the prediction of DBP rejection in FO, a modified solution-diffusion model was developed to incorporate these two mechanisms, using a decreased membrane pore radius for the steric hindrance and the Setschenow constant for the “salting-out” effect. Compared with current models, the modified model showed improved prediction for the transport of halogenated DBPs in FO.

Modeling the transport of disinfection byproducts in forward osmosis membrane process

ACS Fall 2021

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