Gas-diffusion-electrode based direct electro-stripping system for gaseous ammonia recovery from livestock wastewater

Livestock wastewater (LW) typically contains a substantial amount of NH₄⁺ that can potentially be recovered and used in fertilizers or chemicals. In an attempt to recover NH₄⁺ from LW, a novel electrochemical approach using a gas diffusion electrode (GDE) was developed and its efficacy was demonstrated in this study. The GDE-based electrochemical device, when operated at an air-flow rate of 20 mL/min, was free of back-diffusion flux, which is a fatal drawback of any membrane-based NH₄⁺ separation approach. Continuous operation resulted in a nitrogen flux of 890 g N/m²d with synthetic LW and 770 g N/m²d with real LW at a current density of 10 mA/cm². The electrochemical energy input was 7.42 kWh/kg N with synthetic LW and 9.44 kWh/kg N with real LW. Compared with the traditional stripping method, the GDE-based electrochemical system has a certain potential to be competitive, in terms of energy consumption. For instance, a rough-cost estimate based only on operating costs regarding chemical usage, air blowing, and water pumping revealed that the system consumed 13.44 kWh/kg N, whereas the conventional stripper required 27.6 kWh/kg N. This analysis showed that an electrochemical approach such as our GDE-based method can recover NH₃, (particularly in gaseous form) from LW. In addition, with the future development of a smart operation method, as proposed and demonstrated in this study, the cost-effective implementation of a GDE-based method is feasible.