Potentiostatic vs galvanostatic operation of a microbial electrolysis cell for ammonium recovery and biogas upgrading

The experimental study reports the performance of a three-chamber Microbial Electrolysis Cell equipped with a two-side cathode, which combines the COD removal in the intermediate anodic chamber, the CO2 removal from a gas mixture in the two-side cathode and the recovery of ammonium as a concentrate solution. The MEC anode was fed by a synthetic dark fermentation effluent with a nitrogen load rate of 1.7 g N/Ld while the two-side cathode was operated with a gas mixture containing CO2. Indeed, the MEC configuration permitted the CO2 removal maximization from a N2/CO2 gaseous mixture simulating a biogas in terms of carbon dioxide composition, while the ammonium migration through the cation exchange membrane allowed for the recovery of a 5 times concentrated solution of ammonium. The +0.20 V vs SHE potentiostatic anodic condition and the two different galvanostatic conditions allowed the removal of 4.21 gCO2/Ld while 700 mg N/Ld were recovered as a concentrated ammonium solution. The current increase set by galvanostatic operation promoted the CO2 removal and ammonium recovery increase by the 113 % and 27 % in comparison with the potentiostatic condition. An increase of the energy consumption was promoted by the galvanostatic condition due to the loss of bioelectrochemical COD oxidation in favour of water oxidation which in turn was caused by the anodic overpotential decrease from 0.85 to 0.52 V.