Soil is naturally rich in microorganisms that harvest their own energy. To stay alive and function, the microorganisms consume, or "digest," the organic compounds in the ground — a process that results in the generation of electrons.
And when you make electrons, you can make electricity.
Some microorganisms have the ability to transfer electrons across their cell membranes to a conductive material, like an electrode.
Dr. Mirella Di Lorenzo and a team at the University of Bath are using this concept of "bioelectricity" to power an electrochemical reactor for water treatment.
To support the decontamination of water, the researchers created an array of soil microbial fuel cells (SMFCs). Each of the cells has an anode embedded in the soil and a cathode exposed to air.
The soil acts as a kind of electrode separator and as a source of both electroactive bacteria and organic matter. Each SMFC generates a power of 0.4 mW, which is increased up to 12.2 mW by electrically connecting 16 of the cells in parallel.
The cells were successfully tested in Icapuí, a fishing village in northeast Brazil where access to a reliable power network is scarce. The semi-arid location uses rain water for its main drinking source.
The team demonstrated that SMFCs can purify about three liters of water per day. The fuel cells produce energy to filter enough water for a person’s daily needs, with potential to increase scale.
“This project shows that SMFCs have true potential as a sustainable, low-energy source,” says project lead Dr. Mirella Di Lorenzo.
The effort in Icapuí is a collaboration with a team of geographers from Universidade Federal do Ceará and a team of chemists from Universidade Federal do Rio Grande do Norte.
What is a Soil-Microbial Fuel Cell?
The fuel cell, developed by researchers Bath’s Department of Chemical Engineering and Department of Electronic & Electrical Engineering, consists of two carbon-based electrodes positioned at 4 centimeters apart and connected to an external circuit. One electrode, the anode, is buried inside the soil, while the other, the cathode, is exposed to air on the soil surface.
The molecules that emit electrons, known as electrigens, consume the organic compounds to generate charged particles. These electrons are transferred to the anode and travel to the cathode via the external circuit, producing electricity.
By building a stack of several SMFCs, and by connecting the group to a battery, the energy can be harvested and stored to power an electrochemical reactor for water treatment.
The cost of a single SMFC unit could be further reduced with mass production and with the use of local resources for the electrode fabrication, says Dr. Di Lorenzo.
"This is a carbon-neutral technology, sustainable, clean, and extremely simple," Dr. Di Lorenzo told Tech Briefs in the Q&A below.
The project, which is funded by Research England under the Global Challenges Research Fund (GCRF) frame and by The Brazilian National Council for Scientific and Technological Development, has now been granted further funds to continue its work and improve the design and efficiency of the fuel cells.
In an edited interview below, Dr. Di Lorenzo reveals how the team expects the technology to be used in locations beyond Brazil.
Tech Briefs: Can you help us visualize the setup in the field test?
Dr. Mirella Di Lorenzo: The set-up is extremely simple and also cheap. The actual soil microbial fuel cell, or SMFC, device consists of two carbon-based electrodes — one buried inside the soil and one exposed to air and placed onto the soil surface. The two electrodes are connected to each other via an external conductive wire. To scale-up the power, we have electrically assembled together several SMFCs.
The energy generated by the SMFCs stack is harvested, stored, and released when required via a power management system, to which the reactor for water purification is connected.
Tech Briefs: How well did the system perform in that environment (in Icapuí)?
Dr. Mirella Di Lorenzo: The high temperature and humidity actually enhanced the performance of our system. So we can say that operations in a hot climate are preferable.
Tech Briefs: You mentioned in an earlier press release that using soil microbial fuel cell technology to treat a family's daily water was “trickier” in an outdoor setting vs. a lab setting. How so?
Dr. Mirella Di Lorenzo: It is always very hard to reproduce results obtained in the lab in the field. Things in the lab are always simpler, as everything like temperature, soil composition, soil moisture, and so on is under control.
A great result in our work was to demonstrate perfect reproducibility of the experimental results performed in the lab in Bath with the field test in Icapuí in northeast Brazil. This achievement shows the reproducibility and practicality of our system.
Tech Briefs: What are you working on next with the technology?
Dr. Mirella Di Lorenzo: We are now aiming to demonstrate its use in sample households.
Tech Briefs: What are the advantages of using SMFCs?
Dr. Mirella Di Lorenzo: This is a carbon-neutral technology: sustainable, clean, and extremely simple. We were working with pupils in the school who have helped us with setting up the device and with its maintenance.
Most importantly, the technology is affordable to everyone in the world.
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