Green electricity-driven simultaneous ammonia recovery and in-situ upcycling for microbial protein production

Currently, hydrogen-oxidizing bacteria (HOB) based power-to-protein is a promising approach to produce alternative microbial protein (MP), however, the nitrogen source used was either derived from commercial products or was firstly recovered from waste streams and then diluted for HOB growth. In the present study, simultaneous ammonium recovery from wastewater and in-situ utilization for the green MP (derived from Cupriavidus necator 335) production was successfully demonstrated using a microbial electrochemical recovery conversion cell (MERC). 0.41 ∼ 0.82 g/L of dried biomass (protein content 49 ∼ 63%) was yielded in 36 h with a power supply of 3, 4, and 5 V. C. necator 335 could grow in the MERC system receiving wastewater with a broad range of ammonium (0.05 ∼ 8 g N/L) and the highest biomass production of 0.9 g/L (protein content 54%) was achieved at 2 g N/L. 2.69 g/L of dried biomass containing 57% protein was obtained in 120 h with an initial supply of 1 L CO₂ and 2 g N-NH₄⁺. Applied voltages and ammonium concentrations showed a minor impact on the amino acid profile. Furthermore, the MERC system was tested with real waste streams (e. g., municipal wastewater, and digestate) and 0.45 ∼ 1.22 g biomass/L (protein content 52 ∼ 62%) were harvested. The characteristics of the wastewater streams (e. g., ammonium concentration and conductivity) could significantly affect the system performances. The harvested MP from real wastewater showed a high quality of amino acid profile and implied a potential in substituting traditional plant/animal-based protein.