Current State-of-the-Art of Non-Reverse Osmosis-Like Forward Osmosis Technology

Abstract

Forward osmosis (FO) has gained attention for its reduced fouling propensity. Traditional FO thin-film composite membranes, akin to reverse osmosis (RO)-like FO membranes, feature a dense and tight polyamide (PA) selective layer for high solute rejection but relatively low water permeability. Membrane technologists have therefore diversified their focus into non-RO-like FO technologies, particularly membranes resembling nanofiltration (NF)-like FO and ultrafiltration (UF)-like FO selective layers. Applying a looser PA layer in NF-like FO and a highly porous membrane in UF-like FO greatly mitigates the effects of internal concentration polarization (ICP). However, attention must be given to the serious reverse solute diffusion (RSD) and forward solute diffusion (FSD) in non-RO-like processes, especially considering the limited choice of draw solutes available. The choice of FO process is heavily reliant on the intended applications; non-RO-like FO membranes are currently less suited for water desalination but are favored for concentrating or recovering solutions containing large molecules. While RSD and FSD are hardly unavoidable, proper selection of draw solutes for the intended application should consider the properties and costs of the draw solutes. This review offers an overview of non-RO-like FO technology, covering the membrane synthesis, characteristics, performance and applications. The associated challenges, such as ICP, RSD, FSD and membrane fouling, are highlighted, and potential integrated FO-draw solute regeneration system are discussed.