Impact of surface area and current generation of microbial electrolysis cell electrodes inserted into anaerobic digesters

Abstract

The integration of microbial electrolysis cells (MECs) and anaerobic digestion (AD) has recently been pursued to improve methane production through a combined system (AD-MEC). Analysis of 24 different experimental results published in 13 different studies showed that the only significant correlation between increased methane generation rate and other parameters was the surface area provided by the electrodes per volume of reactor, or the S/V ratio (R2 = 0.63, p-value < 0.01). Other possible relationships between hydrogen production in MECs, and methane enhancement in AD-MECs, were examined as a function of applied voltage and current density using data from previous studies. For the MECs, there was a high correlation between current density and hydrogen production rate. There was no significant relationship between methane production rate and current density in AD-MECs. A comparison of other factors between MECs and AD-MECs showed large differences in substrate concentration, substrate type (single substrates or wastewaters), and volumetric current densities. Adding electrodes increased methane production on average by 65%, but surface area was a more important contributing factor in AD-MECs than current generation. Based on our analysis of the results in these previous studies, increased area and conductive surfaces provided by the electrodes can have beneficial effects for methane production other than hydrogen gas production by increasing biomass retention and stimulating electrical syntrophy between microorganisms.