The electrons that are produced are then transferred via an external circuit to the cathode chamber, while the protons pass through the membrane. In the cathode chamber, a reaction between the protons and the electrons uses up oxygen and forms water. And as long as substrates are continually converted, electrons will flow – which is what electricity is.
Generating electricity through MFCs has a number of advantages: systems can be set up anywhere; they create less “sludge” than conventional methods of wastewater treatment such as activated sludge systems; they can be small-scale yet a modular design can be used to build bigger systems; they have a high tolerance to salinity; and they can operate at room temperature.
Activated Sludge The activated sludge is a process with high concentration of microorganisms, basically bacteria, protozoa and fungi, which are present as loose clumped mass of fine particles that are kept in suspension by stirring, with the aim of removing organic matter from wastewater. From: Resource-Efficient Technologies, 2016
The availability of a wide range of renewable substrates that can be used to generate electricity in MFCs has the potential to revolutionise electricity production in the future. Such substrates include urine, organic matter in wastewater, substances secreted by living plants into the soil (root exudates), inorganic wastes like sulphides and even gaseous pollutants.
Mini-Review Published: Gaseous pollutant treatment and electricity generation in microbial fuel cells (MFCs) utilising redox mediators Evelyn, Yan Li, Aaron Marshall & Peter A. Gostomski Reviews in Environmental Science and Bio/Technology volume 13, pages35–51 (2014) Microbial fuel cell (MFC) is an emerging technology for sustainable energy generation and waste treatment. This paper reviews the potential of a gaseous substrate when it is combined with a mediator in an MFC to generate electricity and to treat toxic gaseous pollutants. Most MFCs for waste water treatment often cannot use mediator to enhance the electron transfer from the microbe to the anode because of the difficulty in recovering the expensive and potentially toxic compound. Combining gas feeds with mediators is possible since the soluble mediator would remain in the anode chamber as the gas passes through the reactor. In addition, this type of MFC is possible to be integrated into an anaerobic biofiltration system (BF-MFC), where the biofilter removes the gaseous contaminant and produces the reduced mediator and the MFC produces the electricity and recycles the reoxidised mediator. This paper also talks about the past research on gaseous feed MFCs, and reviews the mechanism and strategies of electron transfer in MFC using redox mediator. The advantages, process parameters and challenges of BF-MFC are discussed. This knowledge is very much required in the design and scale up of BF-MFC. This paper will be useful for those who work in the area of gaseous pollutant treatment and electricity generation.
1. Pee power
Biodegradable matter in waste materials such as faeces and urine can be converted into electricity. This was demonstrated in a microbial fuel cell latrine in Ghana, which suggested that toilets could in future be potential power stations. The latrine, which was operated for two years, was able to generate 268 nW/m² of electricity, enough to power an LED light inside the latrine, while removing nitrogen from urine and composting the faeces.
Deployment of the microbial fuel cell latrine in Ghana for
decentralized sanitation
Cynthia J. Castro, Joseph E. Goodwill, Brad Rogers, Mark Henderson
and Caitlyn S. Butler
For locations with no grid electricity or for refugee camps, the use of waste in latrines to produce electricity could truly be revolutionary.
2. Plant MFCs
Another renewable and sustainable substrate that MFCs could use to generate electricity is plant root exudates, in what are called plant MFCs. When plants grow they produce carbohydrates such as glucose, some of which are exuded into the root system. The microorganisms near the roots convert the carbohydrates into protons, electrons and carbon dioxide.
In a plant MFC, the protons are transferred through a membrane and recombine with oxygen to complete the circuit of electron transfer. By connecting a load into the circuitry, the electricity being generated can be harnessed.
Plant MFCs could revolutionise electricity production in isolated communities that have no access to the grid. In towns, streets could be lit using trees.
3. Microbial desalination cells
Another variation of microbial fuel cells are microbial desalination cells. These devices use bacteria to generate electricity, for example from wastewater, while simultaneously desalinating water. The water to be desalinated is put in a chamber sandwiched between the anode and cathode chambers of MFCs using membranes of negatively (anion) and positively (cation) charged ions.
MIDES aims to develop the World’s largest demonstrator of an innovative and low-energy technology for drinking water production, using MDC technology either as stand-alone or as pre-treatment step for RO. The project will focus on overcoming the current limitations of MDC technology such as low desalination rate, high manufacturing cost, biofouling and scaling problems on membranes, optimization of the microbial-electrochemical process, system scaling up and economic feasibility of the technology. This will be achieved via innovation in nanostructured electrodes, antifouling membranes (using nanoparticles with biocide activity), electrochemical reactor design and optimization, microbial electrochemistry and physiology expertise, and process engineering and control.
When the bacteria in the anode chamber consume the wastewater, protons are released. These protons cannot pass through the anion membrane, so negative ions move from the salty water into the anode chamber. At the cathode protons are consumed, so positively charged ions move from the salty water to the cathode chamber, desalinating the water in the middle chamber. Ions released in the anode and cathode chambers help to improve the efficiency of electricity generation.
Conventional water desalination is currently very energy intensive and hence costly. A process that achieves desalination on a large scale while producing (not consuming) electricity would be revolutionary.
4. Improving the yield of natural gas
Anaerobic digestion – where microorganisms are used to break down biodegradable or waste matter without needing oxygen – is used to recover energy from wastewater by producing biogas that is mostly methane – the main ingredient of natural gas. But this process is usually inefficient.
Research suggests that the microbial groups used within these digesters share electrons – what has been dubbed interspecies electron transfer – opening up the possibility that they could use positive energy to influence their metabolism.
By supplying a small voltage to anaerobic digesters – a process called electromethanogenesis – the methane yield (and hence the electricity that could be recovered from combined heat and power plants) can be significantly improved.
Spun out of the Massachusetts Institute of Technology (MIT) in 2006, Cambrian Innovation is commercializing a portfolio of environmental solutions based on newly discovered electrically active microbes. By harnessing the power of bio-electricity and advances in electrochemistry, Cambrian Innovation’s products help industrial, agricultural and government customers save money while recovering clean water and clean energy from wastewater streams. With support from the National Science Foundation (NSF), engineers and co-founders Matt Silver and Justin Buck are bringing their research from the lab to the market. One system, called EcoVolt, generates methane gas from the wastewater by leveraging what is called “electromethanogenesis.” It’s a newly discovered process for producing methane. “NSF funding of Cambrian Innovation’s research demonstrates our strong interest in supporting small business innovation that leads to novel and greener technological solutions to societal challenges,” says NSF program director Prakash Balan. The EcoVolt system sends wastewater through a bio-electrochemical reactor. As water filters through it, special bacteria in the reactor eat the organic waste in the water and release electrons as a byproduct. Those electrons travel through a circuit to generate methane, or CH4. A wireless signal allows the process to be monitored remotely. This very high quality methane is then piped out to an engine, where it’s burned with a small amount of natural gas. It then generates heat and energy. In addition, sensor systems built by Cambrian Innovation can also monitor pollutants, such as fertilizer run-off. The research in this episode was supported by NSF award #1230363, SBIR (Small Business Innovation Research program) Phase II: A low-cost real-time bio-electrochemical nitrate sensor for surface water monitoring; NSF award #1152409, SBIR Phase II: Exogen: Enhanced Anaerobic Digestion of Wastewater Using Bio-electrodes; and NSF award #1127435, SBIR Phase II: Energy Efficient COD Removal and De-nitrification for Re-circulating Aquaculture Facilities with a Combined Bio-electrochemical Process. Miles O’Brien, Science Nation Correspondent Ann Kellan, Science Nation Producer
While microbial fuel cells are able to generate electricity to power small devices, researchers are investigating ways to scale up the reactors to increase the amount of power they can generate, and to further understand how extracellular electron transfer works. A few start-up companies such as Robial and Plant-e are beginning to commercialise microbial fuel cells.
How Does Anaerobic Digestion Work?
Anaerobic digestion is a process through which bacteria break down organic matter—such as animal manure, wastewater biosolids, and food wastes—in the absence of oxygen. Anaerobic digestion for biogas production takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions (learn more about AD system design and technology). These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process) which is discharged from the digester.
Multiple organic materials can be combined in one digester, a practice called co-digestion. Co-digested materials include manure; food waste (i.e., processing, distribution and consumer generated materials); energy crops; crop residues; and fats, oils, and greases (FOG) from restaurant grease traps, and many other sources. Co-digestion can increase biogas production from low-yielding or difficult-to-digest organic waste.
The following figure illustrates the flow of feedstocks through the AD system to produce biogas and digestate.
Anaerobic Digester Outputs
Anaerobic digestion produces two valuable outputs: biogas and digestate.
Biogas is composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage (50 to 75 percent), carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems, among other uses. Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.
Digestate is the residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each have value that can be realized with varying degrees of post processing.
With appropriate treatment, both the solid and liquid portions of digestate can be used in many beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics), organic-rich compost (solids), and/or simply as soil amendment (solids), the latter of which may include the farm spreading the digestate on the field as fertilizer. Digestate products can be a source of revenue or cost savings, and are often pursued to increase the financial and net-environmental benefit of an AD/biogas project.
Environmental MicrobiologyVolume 8, Issue 3 p. 371-382Free Access
Exocellular electron transfer in anaerobic microbial communities
Alfons J. M. Stams,Frank A. M. De Bok,Caroline M. Plugge,Miriam H. A. Van Eekert,Jan Dolfing,Gosse SchraaFirst published: 14 February 2006https://doi.org/10.1111/j.1462-2920.2006.00989.xCitations: 224 Exocellular electron transfer plays an important role in anaerobic microbial communities that degrade organic matter. Interspecies hydrogen transfer between microorganisms is the driving force for complete biodegradation in methanogenic environments. Many organic compounds are degraded by obligatory syntrophic consortia of proton-reducing acetogenic bacteria and hydrogen-consuming methanogenic archaea. Anaerobic microorganisms that use insoluble electron acceptors for growth, such as iron- and manganese-oxide as well as inert graphite electrodes in microbial fuel cells, also transfer electrons exocellularly. Soluble compounds, like humic substances, quinones, phenazines and riboflavin, can function as exocellular electron mediators enhancing this type of anaerobic respiration. However, direct electron transfer by cell–cell contact is important as well. This review addresses the mechanisms of exocellular electron transfer in anaerobic microbial communities. There are fundamental differences but also similarities between electron transfer to another microorganism or to an insoluble electron acceptor. The physical separation of the electron donor and electron acceptor metabolism allows energy conservation in compounds as methane and hydrogen or as electricity. Furthermore, this separation is essential in the donation or acceptance of electrons in some environmental technological processes, e.g. soil remediation, wastewater purification and corrosion.
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