Novel air gap membrane absorption structure for ammonium salt crystallization

As a vital technology of renewable-nitrogen-fertilizer production, the conventional membrane distillation for ammonia nitrogen recovery from wastewater suffers from two challenges including high energy consumption and low product value, thus necessitating to develop a more efficient technology. Herein, a novel air gap membrane absorption (AGMA) structure was proposed to achieve superefficient ammonia nitrogen recovery and crystallization. To minimize heat loss and water transfer, AGMA was designed as an adiabatic air gap module (thickness=3 mm). The introduction of air gap structure enabled the feed side and the permeate side to maintain constant temperature, where the overall heat flux of AGMA process was 1019 W/m² lower than that of the process without air gap. Meanwhile, the water flux (from the permeate side to the feed side) reached to 1.27 kg/(m² h), and the overall mass transfer coefficient of ammonia (from the feed side to the permeate side) maintained at 2.38×10^-6 m/s, promoting the concentration and crystallization of permeate solution. Importantly, the AGMA process yielded an ultrahigh ammonia separation factor (>200), which exceeded these of typical membrane-based ammonia recovery technologies. Moreover, the AGMA system could stably achieve the recovery of pure ammonium salt crystals in the 60-hour cyclic experiment using liquid digestate as feed solution. Economic analysis confirmed that this system exhibited low energy consumption of approximately 64.35 kJ/mol-NH₃ and a low treatment cost of $2.01/m³-liquid digestate. These results underscore the great potential of the AGMA structure for high-efficiency and low-cost ammonia nitrogen recovery.