Effects of Substrate Diffusion and Anode Potential on Kinetic Parameters for Anode-Respiring Bacteria

The substrate-utilization rate of anode-respiring bacteria (ARB) directly correlates to the current density, one of the main factors in a microbial electrolysis/fuel cell. This study first evaluates the effects of donor-substrate diffusion and anode potential on the estimation of the half-maximum-rate concentration (K_s) and the maximum specific substrate-utilization rate (q_max) of a mixed culture biofilm in a microbial electrolysis cell oxidizing acetate. The intrinsic K_s value is 119 g COD/m³, substrate diffusion has a significant impact on K_s estimation, and the effect of the anode potential on K_s is small. The intrinsic q_max value is 22.3 g COD/g VS-d for an assumed biomass density of 50,000 g VS/m³ (q_maxX_f = 1120 kg COD/m³-d). The maximum specific growth rate (μ_max) is 3.2/d which is substantially faster than for acetate-utilizing methanogens and homoacetogens. Although the anode potential affects q_max, substrate diffusion has a negligible effect. The measured half-saturation anode potential (E_KA) is very negative, −0.448 V (vs Ag/AgCl), and this low value minimizes anode-potential limitation on the current density and the substrate-utilization rate. Thus, the ARB selected in our biofilm anode were relatively fast growers able to take advantage of their low E_KA value (−0.448 V).