Enhancing selective ammonium transport in membrane electrochemical systems

Recovering ammonia nitrogen from wastewater is a sustainable strategy that simultaneously addresses both nitrogen removal and fertilizer production. Membrane electrochemical system (MES), which utilizes electrochemical redox reactions to transport ammonium ions through cation exchange membranes, has been considered as an effective technology for ammonia recovery from wastewater. In this study, we develop a mathematical model to systematically investigate the impact of co-existing ions on the transport of ammonium (NH₄⁺) ions in MES. Our analysis elucidates the importance of pH values on both the NH₄⁺ transport and inert ion (Na⁺) transport. We further comprehensively assess the system performance by varying the concentration of Na⁺ in the system. We find that while the inert cation in the initial anode compartment competes with NH₄⁺ transport, NH₄⁺ dominates the cation transport in most cases. The transport number of Na⁺ surpasses NH₄⁺ only if the fraction of Na⁺ to total cation is extremely high (>88.5%). Importantly, introducing Na⁺ ions into the cathode compartment significantly enhances the ammonia transport due to the Donnan dialysis. The analysis of selective ion transport provides valuable insights into optimizing both selectivity and efficiency in ammonia recovery from wastewater.