The world population is estimated at 9.5 billion by 2050. Since most of our current energy is generated from fossil fuels, this poses significant challenges when it comes to providing sufficient sustainable electricity while mitigating climate change.
One idea that has grown in popularity in recent years is to use bacteria to generate electricity in devices called microbial fuel cells (MFCs). These fuel cells rely on the ability of certain naturally occurring microorganisms to “breathe” metals and exchange electrons to generate electricity. This process can be operated with substances called substrates, which contain organic materials found in wastewater.
At the moment, microbial fuel cells can generate electricity to power small devices like pocket calculators, small fans and LEDs. In our laboratory we operated the lights of a mini Christmas tree with “simulated sewage”. However, as the technology expands it shows promise.[Read: How Polestar is using blockchain to increase transparency]
How they work
MFCs use a system of anodes and cathode electrodes that direct current either in or out. Common MFC systems consist of an anode chamber and a cathode chamber, which are separated by a membrane. The bacteria grow on the anode and convert the substrates into carbon dioxide, protons and electrons.
The electrons generated are then transferred to the cathode chamber via an external circuit while the protons pass through the membrane. In the cathode chamber, a reaction between the protons and the electrons consumes oxygen and forms water. And as long as substrates are continuously converted, electrons flow – and that is exactly what electricity is.
MFC’s power generation has a number of advantages: systems can be set up anywhere; They generate less “sludge” than traditional wastewater treatment methods such as activated sludge systems. They can be small, but modular construction can be used to build larger systems. They have a high tolerance to salinity and can be operated at room temperature.
The availability of a wide range of renewable substrates that can be used to generate power in MFCs could revolutionize power generation in the future. Such substrates include urine, organic matter in sewage, substances excreted into the soil by living plants (root exudates), inorganic wastes such as sulfides, and even gaseous pollutants.
1. Pee power
Biodegradable substances in waste such as feces and urine can be converted into electricity. This was demonstrated in a microbial fuel cell latrine in Ghana, which indicated that toilets could be potential power plants in the future. The latrine, which operated for two years, could generate 268 nW / m² of electricity, enough to power an LED light in the latrine while removing nitrogen from the urine and composting the feces.
Scheme of an MFC latrine. Cynthia Castro, et al. Journal of Water, Sanitation and Hygiene for Development, 2014.
For off-grid sites or for refugee camps, using waste in latrines to generate electricity could be really revolutionary.
2. Plant MFCs
Another renewable and sustainable substrate that MFCs could use to generate electricity is plant root exudates, called plant MFCs. As plants grow, they produce carbohydrates like glucose, some of which are excreted 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 across a membrane and recombined with oxygen to complete the electron transfer circuit. By connecting a load to the circuit, the electricity generated can be used.
Plant MFCs could revolutionize power generation in isolated communities with no access to the grid. In cities streets could be lit with 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 waste water, and at the same time desalinate water. The water to be desalinated is placed in a chamber between the anode and cathode chambers of MFCs using membranes made up of negatively (anion) and positively (cation) charged ions.
When the bacteria in the anode chamber consume the wastewater, protons are released. These protons cannot pass the anion membrane, so negative ions migrate from the salt water into the anode chamber. Protons are consumed at the cathode, so that positively charged ions move from the salt water to the cathode chamber and desalinate the water in the middle chamber. Ions released in the anode and cathode chambers help improve the efficiency of power generation.
Conventional water desalination is currently very energy-intensive and therefore expensive. A process that achieves large-scale desalination while generating (not consuming) electricity would be revolutionary.
Desalination plant in Hamburg. Current desalination technology is very energy intensive. Andrea Izzotti / Shutterstock
4. Improvement of the natural gas yield
Anaerobic digestion – which uses microorganisms to break down biodegradable or waste materials without the need for oxygen – is used to recover energy from wastewater by producing biogas, which is primarily methane – the main component of natural gas. However, this process is usually inefficient.
Research suggests that the microbial groups used in these fermenters share electrons – what is known as electron transfer between species – which opens up the possibility that they could use positive energy to affect their metabolism.
By supplying anaerobic digesters with a low voltage – a process known as electromethanogenesis – the methane yield (and therefore the electricity that can be obtained from combined heat and power plants) can be significantly improved.
While microbial fuel cells are capable of generating electricity for small devices, researchers are exploring ways to enlarge the reactors to increase the amount of energy they can generate and to better understand how extracellular electron transfer works. Some start-ups like Robial and Plant-e are starting to commercialize microbial fuel cells. In the future, microbial fuel cells could even be used to generate electricity in regenerative life support systems during long-term human space missions. It’s early days, but the technology looks promising.
This article by Godfrey Kyazze, a Bioprocess Technology reader at the University of Westminster, has been republished by The Conversation under a Creative Commons license. Read the original article.
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