Efficient Methane Production from Beer Wastewater in a Membraneless Microbial Electrolysis Cell with a Stacked Cathode: The Effect of the Cathode/Anode Ratio on Bioenergy Recovery

A methane-producing microbial electrolysis cell (MEC) is a promising energy-recovery technology, yet its performance is generally inhibited by the insufficient cathode/anode ratio. In this study, a novel stacked stainless-steel-mesh cathode was designed to investigate the effect of the cathode/anode ratio on methane production in semi-continuous MECs. Overall, energy recovery was significantly enhanced by increasing the cathode/anode ratio. The methane production rate in R3 (cathode/anode ratio of 4 cm2/cm3) reached 0.14 m3m–3day–1with an applied voltage of 0.9 V, which increased by 56–180% compared to the methane production rates in R2 (2.5 cm2/cm3) and R1 (1 cm2/cm3). The overall energy efficiency in R3 was 66–94% higher than the overall energy efficiencies in R2 and R1. The cathode area was sufficient for obtaining and maintaining a maximum current when the cathode/anode ratio was higher than 2.5 cm2/cm3. According to electron balance analysis, when the cathode/anode ratio was less than 2.5 cm2/cm3, the methane production enhancement was mainly attributed to the promotion of bioelectrochemical performance, while the sole biomass contribution was enhanced and led to further improvement in overall methane production when the ratio was above 2.5 cm2/cm3. In general, increasing the cathode/anode ratio of the staked-style cathode would be an effective strategy to improve the methane production in the membraneless MECs.