Hydrodynamics And Bio-electrochemical Model For Microbial Fuel Cells

Microbial fuel cells (MFCs) are the devices that convert the chemical energy stored in organic/inorganic matter to electricity through bioelectrochemical reactions while using bacteria as catalysts, the MFC technique has become one of the research hotspots in recent years. So far the researches on MFCs mainly concentrated on seeking highly electroactive microbes, appropriate electron acceptors, excellent electrode materials and their modification methods and reasonable configurations. Although the hydrodynamics of MFCs would significantly affect the mass transfers and reaction kinetics, exerting a pronounced effect on reactor performances, however, the relevant studies are still limited. As many processes are involved in MFCs including mass transfer, biochemical and electrochemical ones, the modeling for MFC will be better to help us understand the operation mechanism of this complex system. But the relevant researches were seldom reported. In this article, the hydrodynamics of a novel MFC, i.e., electrochemical membrane bioreactor (EMBR), was investigated through various approaches. Moreover, a comprehensive mathematic model was established to simulate the mediator-less MFCs. The main contents and results are as follows:1. Lithium-ion tracer tests were adopted to generate residence time distribution curves at four different hydraulic residence times for the EMBR. Tank-in-series, axial dispersion and Martin models were developed to simulate the results of tracer studies in order to acquire the information such as dead zones and short-circuiting in EMBR. Compared to the tank-in-series and axial dispersion ones, Martin model could describe hydraulic performance of the EBMR better, and it reveals that the water flow primarily through the EMBR in three different channels. The detailed flow patterns of the EMBR were acquired from a computational fluid dynamics (CFD) simulation. The results demonstrate that fluid flow in the EMBR region had already reached a steady state by40s, and the dead zones of the EMBR were located primarily at the bottom and upper outer regions of the reactor. Besides, the possibility of existing preferential or circuitous flow in the reactor was proved by velocity vector field. A simple mathematical model was built to investigate the DO distribution in the cathode. A positive relationship was found between the current density of EMBR and multiplier of dead volumes and DO concentration, and the HRT determined that value of dead zones and DO concentration, thus, it is imperative to choose an optimized HRT for the operation of the EMBR.2. Based on the biofilm, a comprehensive one-dimensional model with multi-species was developed to describe the performance of the mediator-less two-chambered MFCs. Mainly aimed at the anode chamber, the model includes biochemical process, electrochemical process, mass transfer process, acid-base equilibrium, and liquid-gas process that taken place in MFCs. By using this model, the effect of various key parameters including substrate concentration, feed flow rate, biomass concentration and proportion of exoelectrogen on current density, coulombic efficiency and methane production of MFCs was investigated. The results demonstrate that methanogensis could be more competitive than exoelectrogen in MFCs when both substrate concentration and feed flow rate were low, resulting in generation of small amounts of electricity. When substrate concentration and feed flow rate were increased, both current density and coulombic efficiency in MFCs could have a significant improvement. Besides, the initial biomass concentration in anodic biofilm matrix would influence the mass transfer of substrate thus remarkably affecting the current density of MFCs.