Using effluents from two-phase anaerobic digestion to feed a methane-producing microbial electrolysis

The integration of a methane-producing microbial electrolysis cell (MEC) into two-phase anaerobic digestion (TP-AD) was investigated, by using effluents from a pilot-scale TP-AD treating the organic fraction of municipal solid waste. The MEC was aimed at exploiting residual COD of TP-AD effluents at the MEC anode in order to support CO2 removal and methane generation at the MEC cathode (fed by a CO2-rich gas phase, simulating a biogas). Feeding by 2nd phase digestate caused a loss of MEC performance, due to poor biodegradability of digestate COD under chosen anodic operating conditions (+0.2 V vs SHE, HRT 13 h, organic load 2.3 g COD/L d). On the other hand, by using the 1st phase fermentate (rich in volatile fatty acids, VFA), good MEC performance was recovered with a current of 60 ± 4 mA and a methane production rate of 33 ± 3 meq/L d. However, periodic baskwashing was also necessary to recover fouling effects. Moreover, partial nitrogen removal (228 mg N/L d) from the fermentate was obtained because ammonium was transported across the separation membrane and then recovered through the cathodic concentrated spill (at 3177 ± 97 mg N/L). Cation transport also generated net alkalinity which strongly contributed to CO2 removal (besides methane generation).