Solar enhanced membrane distillation for ammonia recovery

Directly recovering ammonia from waste streams is a sustainable approach for ammonia management since it saves energy from both the Haber-Bosch process, the major industrial method for ammonia synthesis, and wastewater treatment. Membrane distillation (MD), an evaporation-based membrane separation process, has been employed to recover ammonia from ammonia-rich wastewater due to the high volatility of ammonia. In this study, the photothermal effect is incorporated into MD to enhance the ammonia recovery from ammonia-rich wastewater. Carbon black particles are coated on the membrane surface to increase its absorption of solar irradiation at the solution-membrane interface and facilitate the ammonia transport across the membrane. We demonstrate that the system can recover ammonia at a maximum ammonia flux of 4.52 g-N·m⁻²·h⁻¹ with a solar intensity of 1.7 kW·m⁻². The estimated mass transfer coefficient of carbon black coated membrane is 2.67 × 10⁻² m·h⁻¹ with solar irradiation, enhanced by 30.8% when compared to that in a pristine membrane. We also confirm that the improvement of ammonia flux by photothermal effect is equivalent to heating the feed solution by 20–30 °C. Our study demonstrates a promising pathway for utilizing solar energy by photothermal effects to enhance MD for ammonia recovery from ammonia-rich wastewater.