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(max)X(f) = 1120 kg COD/m3-d). The maximum specific growth rate (micro(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).