Thermodynamic approach to simulate current densities of energy-harvesting microbial electrochemical systems fed with human urine

Abstract

Different theoretical models have been addressed to simulate electricity generation of microbial electrochemical systems (MESs) fed with a variety of organics. In the case of urine-fed MESs (u-MESs), the lack of input parameters to feed current bio-physical models has somehow restrained their theoretical approach. This study introduces a model, based on microbial energetics, to simulate current generation of u-MESs without the need of resorting to parameters typically used as simulation entries in bio-physical approaches (e.g., maximum substrate-utilization rates (qmax), Monod half-saturation coefficients (KS)). The strategy consisted in linking the catabolic energy density available in the urine medium to the specific electro-active microbial growth-rate, the latter calculated from metabolic thermodynamics (Gibbs free-energy changes for the catabolic/anabolic reactions, and Gibbs energy dissipation). Calculated growth rates were used to compute specific substrate-utilization rates, the effective flux of electrons into the anode biofilm and the resulting current densities as a function of urine concentration.