The technology at play here, developed by Ioannis Ieropoulos and friends, is called a stack or cascade of microbial fuel cells. As the name suggests, a microbial fuel cell (MFC) contains microbes that feed on organic matter and produce electricity in the process. From the research paper, it sounds like Bristol’s MFCs were simply filled with the same activated sludge that’s used at sewage and water processing plants. The researchers introduced tryptone and yeast to encourage the growth of the energy-producing microbes (in this case, probably Geobacter sulfurreducens), and then poured the sludge into some MFCs.
In a microbial fuel cell there is an anode compartment, a cathode compartment, and in the middle there’s a membrane that only allows protons to pass through. The microbes break down the fuel (the organic compounds in your urine, such as proteins and hormones) in the anode compartment, producing CO2, electrons, and protons. The electrons are ferried to the cathode compartment via an electrical circuit, where they combine with the protons to form hydrogen — and then combine with oxygen to produce clean water and electricity.
The amount of electricity that can be produced this way is fairly low (on the order of microwatts), and so the Bristol researchers stack eight MFCs together, with your urine cascading down the line. The best result, which produced “practically useful power outputs,” was achieved by running four MFCs in parallel (for more power), in serial with another another block of four MFCs (for more voltage). In total, this setup produced 642 mV, 312 µA, and 206 µW — not a whole lot, but apparently enough to “charge a Samsung mobile phone.”
The MFCs themselves are small (and created with a 3D printer, incidentally), but the entire setup is currently around the size of a car battery. Ieropoulos says he is trying to secure funding to create a smart toilet with partners in the US and South Africa. There aren’t many details, but presumably this smart toilet will be equipped with a USB socket so that you can recharge while you discharge. The eventual goal, though, is to produce something that can be carried around easily and fully charge a smartphone. Whether the underlying science of microbial fuel cells will allow this, though, remains to be seen. The only real way of increasing an MFC’s power output is to increase the protein content of your urine — and generally, if you have a lot of protein in your pee, your kidneys are failing.
Research paper: “Energy production and sanitation improvement using microbial fuel cells” [Free PDF]
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