Synergistic ammonia recovery from high-ammonia anaerobic digestate via coupled bipolar electrodialysis and membrane contactor system

This study evaluates ammonia gas recovery from high-strength anaerobic digestate using a bipolar membrane electrodialysis (BPED) and membrane contactor (MC). Ammonia is a promising carbon-neutral energy carrier, while digestates present both environmental challenges and opportunities for ammonia recovery. The BPED was tested at 2,000---10,000 mg-N/L under varying voltages and flow rates, achieving up to 87.6 % ammonium separation at 10,000 mg-N/L. Subsequently, the MC enabled direct gas-phase NH₃ recovery, using synthetic base solutions under different vacuum pressures and sweep gas flow rates. Recovery reached 80.9 % at low gas-to-liquid ratios (180-720), outperforming air stripping. Ammonia recovery followed first-order kinetics, with overall mass transfer (K·a) governed by vacuum pressure, sweep gas, and pH-dependent speciation. NH₃-N flux showed nonlinear dependence on the ionization fraction (α₁), underscoring synergistic effects of pH and gas-phase configuration. The BPED-MC offers a scalable, chemical-free, and energy-efficient approach for sustainable ammonia recovery from high-ammonia waste streams.