Osmotic membranes for municipal wastewater reclamation: Insights into applications, transmembrane diffusion mechanisms and prospects

With the global advancement of the circular economy, integrating reverse osmosis (RO) or forward osmosis (FO) with anaerobic membrane bioreactor (AnMBR) offers a promising approach to simultaneously generate high-grade reclaimed water, produce energy, and preserve valuable nutrients from municipal wastewater. However, the selectivity of these osmotic membranes towards ammonia nitrogen, a major component in municipal wastewater and anaerobic effluent, remains unsatisfactory due to its similar polarity and hydraulic radius to water molecules. Therefore, enhancing the ammonia nitrogen rejection of osmotic membranes is imperative to maximize the quality of reclaimed water and minimize the loss of ammonia nitrogen resources. Unfortunately, the current understanding of the mapping relationship between ammonia nitrogen transmembrane diffusion and the micro/nano-structure of osmotic membranes is not systematic, making precise optimization of the membranes challenging. Hence, this review comprehensively analyzed the diffusion behavior of ammonia nitrogen through osmotic membranes to lay the foundation for targeted regulation of membrane fine structure. Initially, the desire for ammonia/ammonium-rejecting membranes was highlighted by introducing current and promising osmotic membrane-based applications in municipal wastewater reclamation processes. Subsequently, the connection between the micro/nano-structure of osmotic membranes and the transmembrane diffusion behavior of ammonia nitrogen was explored by analyzing the effects of membrane characteristics on ammonia nitrogen transport using the DSPM-DE model. Finally, precise methods for modifying membranes to enhance ammonia nitrogen rejection were proposed. This review aims to offer theoretical insights guiding the development of RO and FO membranes with superior ammonia nitrogen rejection for efficient reclamation of municipal wastewater.