Concentrating Ammonia from Wastewater with Electrodialysis

Electrodialysis (ED) is a promising technology for the recovery of ammonia from wastewater. However, separating ammonia directly from complex wastewater mixtures using ED is challenging due to membrane scaling, low selectivity, and high energy consumption. Here, we evaluate the potential of electrodialysis for ammonia recovery from simulated and real wastewater mixtures. The specific energy consumption (SEC) of electrodialysis exceeded 31 kWh/kg-N for simulated wastewater but decreased 4-fold to 7 kWh/kg-N after hardness removal. Concentration factors (CFs), the final concentration relative to the initial concentration, of NH₄ ⁺ for real wastewater after ultrafiltration and for synthetic wastewater without hardness were 7.5 and 10, comparable to the CF of 9 for single-salt solutions (nonmixtures). We find that the concentrated product after ED with real and simulated synthetic wastewater 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 fertilizer, the resulting product will be 30/30/30 for Na⁺, K⁺, and NH₄ ⁺. Finally, staged electrodialysis achieved a CF of ∼50 (2.42 N wt %) with SECs of 15.2-18.1 kWh/kg-N for synthetic wastewater without hardness, demonstrating promise for recovering ammonia from wastewater with a high concentration and low energy demand.