Understanding the interdependence of operating parameters in microbial electrosynthesis: a numerical investigation.

Siddharth Gadkari,J Annie Modestra,Jhuma Sadhukhan,Mobolaji Shemfe,S Venkata Mohan

doi:10.1039/c9cp01288e

Abstract

This study describes and evaluates a dynamic computational model for a two chamber microbial electrosynthesis (MES) system. The analysis is based on redox mediators and a two population model, describing bioelectrochemical kinetics at both anode and cathode. Mass transfer rates of the substrate and bacteria in the two chambers are combined with the kinetics and Ohm's law to derive an expression for the cell current density. The effect of operational parameters such as initial substrate concentration at the anode and cathode and the operation cycle time on MES performance is evaluated in terms of product formation rate, substrate consumption and coulombic efficiency (CE). For a fixed operation cycle time of 3 or 4 days, the anode and cathode initial substrate concentrations show linear relationship with product formation rate; however MES operation with a 2 day cycle time shows a more complex behaviour, with acetic acid production rates reaching a plateau and even a slight decrease at higher concentrations of the two substrates. It is also shown that there is a trade-off between product formation rate and substrate consumption and CE. MES performance for operation with cycle time being controlled by substrate consumption is also described. Results from the analysis demonstrate the interdependence of the system parameters and highlight the importance of multi-objective system optimization based on targeted end-use.

Highlights

This study describes and evaluates a dynamic computational model for a two chamber microbial electrosynthesis (MES) system
The bacteria in the anode are classified into two types, primary anodic bacteria (the ‘exoelectrogenic’ bacteria that consume the substrate (COD) and release electrons which can be transferred to the anode) and secondary anodic bacteria
This function is minimized using the simplex search method based on the Nelder–Mead optimization technique,[31] a method which is well known for handling unconstrained optimization problems where the gradient is not known or is difficult to find.[32]

Summary

Introduction

This study describes and evaluates a dynamic computational model for a two chamber microbial electrosynthesis (MES) system. The bacterial population at the anode is assumed to consist of fermentative, electricigenic, methanogenic acetoclastic, and methanogenic hydrogenophilic microorganisms, whose metabolic activity and growth rates are described using multiplicative Monod kinetics This model expressed in ordinary differential equations is based on the assumption that a single hydrolysis and fermentation step of complex organic matter conversion to acetate can be used to describe the anaerobic degradation of wastewater. An expression for overpotential is derived from electron balance and Ohm’s law, combining the electron flux in the conductive matrix with electron consumption (from CO2 reduction) and electron generation (due to self-oxidation of biomass) terms This model predicts the biofilm dynamics, substrate concentration and current density profiles; the analysis is limited to the cathode and does not account for the influence of electrode kinetics and mass transfer losses at the anode.[23]

Methods

Results

Conclusion