Concentrating Ammonia from Wastewater with Electrodialysis

Efficient recovery of ammonia from wastewater is essential for a circular nitrogen economy. However, separating ammonia directly from wastewater using membranes is challenging due to the complex nature of the feed stream. The most significant challenges with membrane-based processes for ammonia recovery include membrane scaling, low selectivity, and high energy consumption. Here, we evaluate the use of electrodialysis for the recovery of ammonia from simulated and real wastewater mixtures. The specific energy consumption (SEC) of electrodialysis exceeded 31 kWh / kg-N for simulated wastewater, but decreased fourfold to 7 kWh / kg-N after hardness removal. The concentration factors (CF) of NH4+ for real wastewater after ultrafiltration and pretreated synthetic wastewater were 7.5 and 10, comparable to the CF of 9 for single-salt solutions (non-mixtures). We find that when concentrating ammonium with electrodialysis with real and simulated synthetic wastewater, the concentrated product includes K+ and Na+, as cation exchange membranes exhibit K+/NH4+ and Na+/NH4+ selectivities near one. Thus, if the concentrated product is directly used as an aqueous liquid phase fertilizer, the resulting product will be 30/30/30 for Na, K, and N. Secondary gas phase separations can be introduced in series to recover only ammonia. Finally, staged electrodialysis achieved a CF of ~50 (2.42 N wt %) with SECs of 15.2-18.1 kWh / kg-N for pretreated synthetic wastewater, demonstrating promise for recovering ammonia from wastewater mixtures with high concentration and low energy demand.