Bioelectrochemical protein production valorising NH3-rich pig manure-derived wastewater and CO2 from anaerobic digestion

Microbial electrosynthesis (MES) cell use is an innovative approach for single-cell proteins (SCP) production. Coupling MES with the valorization of CO2 from anaerobic digestion and nitrogen from livestock effluents has beneficial environmental effects, reducing greenhouse gas emissions and nitrogen overloading. In addition, the reducing power needed can come from surplus renewable energy. In this study, MES with a biocharfunctionalized cathode was tested at varying polarizations, i.e. non polarized, -0.6 V and -1.0 V vs Ag/AgCl, and biogas-derived CO2 and recovered ammonia from pig slurry was supplied. Negative polarization switched the microbial community from heterotrophic, typical of unpolarized MES, to a mix of both heterotrophic and autotrophic/electrotrophic communities at -0.6 V and to mainly autotrophic/ electrotrophic at -1.0 V. The more negative polarization allowed the highest CO2 and N capture, i.e. 39 +/- 2 % of the supplied CO2, and 6.7 +/- 0.8 % supplied N. Microbial biomass characterization indicated a protein content on dry matter basis of 33.1 +/- 1.3 % (unpolarized), 43.2 +/- 0.6 % (-0.6 V) and 69.1 +/- 1.0 % (-1.0 V). The amino acids profiles investigated showed a high nutritional value of the produced biomass, not far from those of conventional protein sources used for producing feed/food.