Electrochemical Resource Recovery from Digestate to Prevent Ammonia Toxicity during Anaerobic Digestion

Ammonia inhibition during anaerobic digestion limits the substrate loading rate and endangers process stability. Furthermore, digestates are interesting feedstocks for nutrient recovery. In this lab-scale study, an electrochemical cell was used to investigate the NH4+ flux from anode to cathode. Under optimal conditions with synthetic wastewater, an NH4+ charge transfer efficiency of 96% and NH4+ flux of 120 g N m(-2) d(-1) could be obtained at a concomitant electricity input of 5 kWh kg(-1) N removed. A more selective NH4+ transfer could be established by maintaining a high concentration of other cations in the cathode compartment. Comparable NH4+ fluxes could be obtained with digestate at an electrical power input of 13 kWh kg(-1) N removed and 41% current efficiency. The ammonium level in the digestate could be lowered from 2.1 to 0.8 - 1.2 g N L-1. Interestingly, also potassium fluxes of up to 241 g K+ m(-2) d(-1) could be obtained at 23% current efficiency. As the cathode can be operated at high pH without the need for chemical addition, stripping and absorption of dissolved ammonia could reach 100% efficiency. By valorization of the generated side products, this technology shows economic potential for practical application.