Microbial electrochemical technologies
Microbial electrochemical technologies use microorganisms as electrochemical catalyst, merging the microbial metabolism with electrochemical processes for the production of bioelectricity, biofuels, H2 and other valuable chemicals. Microbial fuel cells and microbial electrolysis cells are prominent examples of METs. While MFC is used to generate electricity from organic matter typically associated with wastewater treatment, MEC use electricity to drive chemical reactions such as the production of H2 or methane. Recently, microbial electrosynthesis cells have also emerged as a promising MET, where valuable chemicals can be produced in the cathode compartment. Other MET applications include microbial remediation cell, microbial desalination cell, microbial solar cell, microbial chemical cell, etc.,.
History
The use of microbial cells to produce electricity was perceived by M.C. Potter in 1911 with the finding that "The disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy". A noteworthy addition in MFC research was made by B. Cohen in 1931, when microbial half fuel cells stack connected in series was created, capable of producing over 35 V with a current of 0.2 mA. Two breakthroughs were made in the late 1980s when two of the first known bacteria capable of transporting electron from the cell interior to the extracellular metal oxides without artificial redox mediators: Shewanella ''oneidensis MR-1 and Geobacter sulfurreducens PCA were isolated. In late 90s, Kim et al. showed that the Fe-reducing bacterium, S. oneidensis MR-1 was electrochemically active and can generate electricity in a MFC without any added electron mediators. These findings set basis for the development of electromicrobiology, and the field of MFC started. However, due to low power generation, it was also doubtful whether the MFC can be practical application on wastewater organics reduction. This view was changed when it was established that domestic wastewater could be treated to practical limits while simultaneously producing power. Furthermore, power densities two orders of magnitude higher was demonstrated in an MFC using glucose, without the need for exogenous chemical mediators. Building upon these works, a race to develop practical applications of MFCs initiated to emerge at a very fast pace, with the major goals being development of a large scale technology for the treatment of domestic, industrial, and other types of wastewaters.In 2004, extracellular electron uptake from cathodes to microbes was established with attached biofilm, where fumarate was reduced to succinate. This reverse reaction for electron transport generated the research field of MES. In 2010, Nevin et al. discovered that the acetogenic microorganism Sporomusa ovata'' can convert CO2 to acetic acid in MES cells by uptaking electrons from the cathode electrode. In the next years, also due to the growing concerns on greenhouse gas emissions, the field of CO2 bioelectroconversion in MES cell flourished. Several autotrophic microorganisms showed ability of capturing electrons from the cathode, either directly or through mediators. Besides specific microbial species, it was shown that CO2 reducing communities can be enriched in MES cells from inoculum sources such as sewage sludge, digester sludge or marine/river sediments. In the following decade, technical improvements led to an increase of acetate production rate from few to hundreds g/m2cathode/d. MES cells demonstrated also a promising technology for converting CO2 into biomethane, with production rates up to 200 L CH4/m2cathode/d. Furthermore, the MES scope was expanded to target more valuable products, including ethanol and caproate.
Principles
Microbial extracellular electron transfer
There are various mechanisms for bacteria to electrons with an electrode. These include a "direct" process, where redox components located on the cell surface, that can be multiheme cytochromes or nanofilaments, contact directly with the solid surfaces, and an "indirect" process that is mediated by soluble redox mediators that cyclically shuttle electrons between cells and electrodes . Electron shuttles can be humic substances that are not produced by the cells, or secondary metabolites that are produced by the organisms including phenazines and flavins . In addition, some primary metabolites of bacteria, such as sulphur species and H2, can convey electrons towards extracellular electron acceptors. In addition to heme cofactors in multiheme cytochromes, flavin mononucleotide also were shown to enhance the rate of electron transfer in some outer membrane cytochrome as redox cofactors . Because electrons are transferred from the interior to the exterior of microbial cells across the cellular membrane during EET, ions with positive charge need to simultaneously move in the same direction as the electron flow to maintain charge neutrality.Bioelectrochemical systems (principles, components, configurations)
A bioelectrochemical system is the device used in METs. A classic BES such as the MFC is typically composed of two sections : An anodic and a cathodic section separated by a selectively permeable, proton/cation exchange membrane or a salt bridge. In a MFC, the anodic section contains microbes that work as biocatalysts under anaerobic conditions in the anolyte, where the cathodic section contains the electron acceptor. Electrons generated from the oxidation of organic compounds are conveyed to the anode. Electrons produced by the microbes are transferred to the anode directly via 'nanowires' or outer-membrane proteins, or indirectly using electron shuttling agents. These electrons reach the cathode across an external circuit and for every electron conducted, protons react at the cathode for completing the reaction and sustaining the electric current.There are numerous types of BES reactors but broadly they all share the same operating principles. Various designs and configurations have been established to optimize the assembly of the three basic elements in a functioning system. The performance of BESs is significantly changed with their design. Table 1. shows a summary of the major BES components and associated materials for their construction.
Table 1. Major components of MFC
Applications
Energy recovery and generation
Wastewater treatment with MFC
It is well-known that pumping, aeration, and solids handling are the major energy consuming process in wastewater treatments. Aeration alone can account for 50% of the operation costs at a typical wastewater treatment plant. Eliminating these costs can save a large amount of energy. MFCs in wastewater treatment, besides electricity generation, also help in energy savings linked to these mentioned processes which add a great advantage. The MFC process is an anaerobic process and sludge production for an anaerobic process is approximately 1/5 of that for an aerobic process. Thus, using MFCs could reduce solids production at a wastewater treatment plant, ultimately reducing significant operating costs for solids handling. Moreover, this technology has seen a nearly exponential increase in power production from the start of this century. This evolution echoes a mounting appreciation by engineers that this technology is ready to emerge as practical applications and associated technologies will be in limelight very soon.The treatment of wastewater by MFC technologies is a promising and yet unique methodology as the process of wastewater treatment can become an approach of producing energy in the form of electricity, rather than energy expenditure. MFCs were used for the determination of lactate in water by K.I.M. and coworkers, and later showed that electricity production in an MFC could be sustained by starch using an industrial wastewater. A great variety of substrates have been used in MFCs for electricity production varying from pure compounds to complex mixtures of organic matter present in wastewater. The application of MFC for biotreatment of wastewater has also recorded effective conversion of organic matter in wastewater into electricity with about 40-90% COD and BOD reduction. Obviously, the energy that could be captured from wastewater is not enough to power a city, but it could be large enough to run a treatment plant. With the continuous advances, bagging this power could lead to energy sustainability of the wastewater infrastructure.