Ammonia recovery from wastewater: A critical review of technologies with emphasis on capacitive deionization

Ammonia-rich wastewater streams from municipal, agricultural, and industrial sources are increasingly recognized as valuable resources for nutrient recovery and, in some cases, clean energy applications. This review critically evaluates a broad range of current technologies for ammonium recovery, including physicochemical and electrochemical methods. Among these, Capacitive Deionization (CDI) systems have emerged as a promising low-energy and ion-selective approach. CDI systems electrostatically adsorb ions onto electrode surfaces, with zeolite-based electrodes enhancing ammonium selectivity. Recent studies report FCDI energy consumption as low as 0.45 kWh/kg-N, with removal efficiencies up to 99%, significantly outperforming other approaches. Integrating ion-exchange membranes further improves separation and operational stability. The review also explores opportunities for integrating CDI systems with renewable energy systems, applying modeling and machine learning for performance optimization, developing hybrid systems, and enhancing efficiency through magnetically assisted electrodes. A comparative analysis of capital and operating expenditure (CAPEX/OPEX), energy demand, recovery efficiency, system complexity, and technology readiness level (TRL)—using radar plots and heat maps—indicates CDI's low energy requirements and operational simplicity, although its current TRL remains lower than more mature alternatives. Future research should prioritize pilot-scale validation under real wastewater matrices with standardized datasets and reporting to enable machine learning (ML)-assisted monitoring and control, alongside the development of more selective ion-exchange membranes and electrode materials for ammonium recovery. This review stands out by providing a comparative analysis across physicochemical and electrochemical methods, covering efficiency, selectivity, scalability, cost, and energy, while positioning CDI as a transformative pathway for ammonium recovery. It also synthesizes advances in electrode materials and outlines forward-looking research directions. These advancements position CDI as a leading candidate for sustainable ammonium recovery in circular wastewater and resource management.