Microbial Fuel Cell with a Side of Betta Fish

Move over, potato batteries: DIY microbial fuel cells are here to stay! A microbial fuel cell (MFC) is a device that uses bacteria in an anaerobic (oxygen-poor) environment to convert chemical energy into electricity. [drdan152] posted steps on how to make a soil-based MFC  with a neat twist: it’s also a fishbowl for a betta fish.

[drdan152] used soil from the wetlands, referred to as “muck.” This nutrient-rich soil provided a hearty supply of bacteria, especially Geobacter species, known for their uncanny ability to transport electrons outside their cells using bacterial nanowires. The proton exchange membrane (PEM) was made up of salt, water, and agar. After some initial runs, [drdan152] determined that flat char cloth made the best anode, while red copper wire served as the cathode. Assembling the MFC was as simple as surrounding the anode with a thick layer of muck on all sides, adding the PEM on top, followed by water. The cathode was situated halfway out of the water.

After a couple of days, the voltage increased in proportion to the amount of bacteria growing on the anode. The betta fish can happily live in this habitat for a short period of time(it still has to be fed, of course), and the bacteria certainly won’t mind – the fish’s excrement provides an additional food supply. As a bonus, the water is kept clean. However, like any aquarium, the water will need to be changed periodically as carbon dioxide byproduct accumulates from the fish’s respiration and the MFC (high carbon dioxide levels = dead betta fish).

The MFC generates 725 mV. [drdan152] is not satisfied with that number, and is testing out charge pump circuits to generate as much as 3V. We are looking forward to seeing the results. We also wonder if a small aquatic plant could help make it a more self-sustaining environment for the fish. In the meantime, [drdan152] is encouraging others to try larger-scale versions of this MFC. Perhaps MFC-powered carnivorous robots doubling as mobile aquariums are in our near future.

35 thoughts on “Microbial Fuel Cell with a Side of Betta Fish

  1. That’s really neat. Any idea about the amount of current we can actually expect from there?

    CO2 exchange should be fine without water changes due to the surface area and exchange with the atmosphere. I suspect, however, that water quality will degrade quickly in this setup.

  2. The 300-600mV is, presumably, the open-circuit voltage; I wonder how much current one could actually draw from this contraption.

    I must say I’m a bit sceptical; with some ions in the water (from the soil, mostly), the mesh screen and copper electrode might just form a pair of galvanic electrodes. This could be avoided by using 2 identical electrodes, or by using non-metallic electrodes, such as graphite. Also, this hypothesis might be rejected by submerging the two electrodes in salt water without any soil contact.

    Also, I don’t quite understand why the electrons would pass through his circuit, instead of just recombining directly with the H+ ions, since both are on the same side of the proton exchange membrane. Exactly what is “persuading” the electrons to go through his circuit, and the protons to pass through the membrane instead? His explanation suggests that the electrons are only captured if the bacteria are actually touching the mesh; that would suggest an extremely low capture efficiency.

    Finally, he says the bacteria feed on the fish poop; how, exactly, is this fish poop going to end up in the bottom half of the container if the membrane is blocking the path?

    1. “I’m a bit sceptical”

      Hackaday really needs more of this type of comment, especially when it comes to “scientific” posts. Yeah, we all can agree that building whatever is great, but when someone sticks electrodes into dirty water and measures 0.something Volts, it’s not an microbial fuel cell — it’s a dirtwater battery at best, until proven otherwise.

      Occams Razor is a hacker’s best friend — when something works, look for the likeliest reason first, and if something fails, it’s usually not the weird neighbour granny’s curse, but the fact that you might have forgotten to plug it in.

      Not to mention editorial content that actually claims things “XY blows other XY-alike parts away”, without a single comparison to support that claim; I *love* hackaday, but it’s articles like that which give it an ugly tabloid/commercially involved kind of turn that I’m pretty sure the editors would like to avoid.

    2. 300-600 is in fact the open-circuit potential. Current density is around 40mAm^-2m of sediment-electrode contact( doi:10.1038/nature08790).
      While some of that potential is certainly simple galvanic interactions if you were to run the same cell w/o microbes you’d see a different self-potential profile. You would also get different ‘recharge’ responses since in the abiotic case the self-potential is limited by oxygen diffusion through the sediment.

      The microbes themselves are persuading the electrons to flow. It’s the same reason it takes a lithium battery 10 years to self-discharge but if you stick it in a highpower flashlight it’s ‘dead’ in 45 minutes.
      The microbes set up anoxic zones quite rapidly and in order to survive are getting energy from the pore fluid and possibly their neighbors.
      The electrons are mostly captured by the electrodes. The potential exists throughout the sediment but as you indicated it’s most efficient at the electrodes. As with any fuel cell or battery, more surface area gives higher current densities.

      I don’t know why he’s got an agar layer in there except maybe to keep the fish from disturbing the electrodes. The bacteria themselves are creating acidic conditions as they reduce/oxidise any metals in the sediment. Agar isn’t ion selective at all, it only serves to slow diffusion. It will eventually become saturated with water and break down. It’s just algae afterall.

  3. That’s not a “fish bowl” it’s a teeny tiny cage for a living creature. The whole “betta’s can live in a tiny cup” is a myth created by department stores that want to sell them to stupid people. 1 gallon (3.75 liters) is the absolute minimum, and 2-5 gallons will result in a much happier and healthier fish.

      1. We had a parrot that wanted to sleep in a covered cage. As in, he would climb across the bed and hop into the cage when it was bedtime.

        Sure, he didn’t spend the whole day in it, though…

    1. Bettas are a weird fish. My lady spent about a year and a half breeding them and raising the fry. There is a lot of highly conflicting info on how to best do this, so she just tried each technique in turn.

      My jaw hit the floor when she consistently achieved the best results (~97% survival from hatching to maturity and great health) in alkaline water so laden with ammonia from their own wastes that it should have been uniformly lethal. And when they were big enough to require separation, tiny cups. Though she did go at least a little bigger than the norm you see in pet stores.

      I can neither reconcile this with what I know about general fishkeeping, nor ignore what I witnessed. But I’m forced to concede that physiologically, they seem to be able to live in tiny cups; and even thrive there.

      Psychologically is a different story though. They are an intelligent fish. They can get bored, stressed, and depressed. Especially if male, and in constant view of rivals. They need mental stimulation. We added plants, algae, live foods, companion snails, even small toys. Interacted with them regularly. Rearranged things weekly, both inside and outside their containers. They got a kick out of having their containers surrounded by little plastic army men, solar dancing figurines, and other similarly silly things.

      And as they grew to adulthood, giving them a sufficiently stimulating environment eventually did impose a minimum size on their containers, even if water quality seemingly did not. Though she no longer breeds, she still has a lot of bettas. They’re kept in containers ranging from 0.5 to 5G, all with interesting environments, and all are doing well.

      While this range is close to your size recommendations, why we recommend it appears different. It’s not merely an issue of having a certain water volume, to dilute wastes and stabilize water parameters. A 1G that’s made interesting for a betta, can be superior to a barren and boring 5G.

  4. I’ve heard a lot of misconceptions when it comes to fishkeeping, but I’ve never heard the one about changing water to get rid of CO2 buildup.

    And even if you’ve never kept a fish, but have had a soda or beer sit out and go flat, you know this is false. CO2 outgasses from water readily, always trying to reach equilibrium concentration with the surrounding atmosphere. Since there’s a constant source of CO2, the concentration in the water will be a bit higher than equilibrium, but it won’t rise indefinitely. Unless you seal the container airtight, but in that case the fish will run out of oxygen before it suffers CO2 poisoning. Most fish, including bettas, can live comfortably in CO2 concentrations at least 60X higher than atmospheric equilibrium. I know this because I deliberately inject CO2 into many of my tanks to assist in plant growth.

    The vigorously growing plants in turn help remove some of the other pollutants which do build up, like ammonia, nitrite, nitrate, and so on; so the environment stays healthier between water changes.

      1. You’re still right that you need to change the water. The ‘cycle’ involved is fish waste to ammonia to nitrite to nitrate. Both ammonia and nitrite are toxic to fish. Nitrate not so much, but if you don’t change water out to keep this down you’ll be fighting algae constantly.

        Not to mention anything else that happens to be present or building up – regular partial changes can keep these from getting too high.

  5. This one deserves a thumbs down for many reasons.

    Did the author check if electrolysis can occur in case of a strong load, especially if multiple cells are connected in series? Besides the obvious suffering for the fish in a small container like that, and the high risk of jumping outside of it, I’m not sure it would like chlorine and hydrogen.

    Leave out animals from experiments, seriously!

  6. Betta fish can breath air from the surface, which is what allows stores that dont know any better or dont care to sell them in little cups. They do best in a tank like any other tropical fish.

  7. It is cool.
    The Betta will be just fine. They are quite robust. Some enjoy more room than others. Some are frightened by the horrible freedom of space and just stay in one place their entire 5 year existence, eating pellets and pooping. a lot.

    1. You could say the same about humans, but it’s still cruel to confine them in a tiny space. While I realize that fish aren’t humans, some effort should be taken to provide a living creature with some approximation of their home environment, which I would argue is not a small glass cup.

  8. I’m pretty sure to try this out shortly on a 100 liters barrel of nettle and other nitrated plants manure for maybe heating my small greenhouse (or at least make a small warm bed for raising salads)

    1. Dunno if that’s the best use, tho it certainly can’t hurt. You might be better off using the natural heat of decomposition, compost heaps are often warm to the touch, and rubbish dumps can get really hot. Or better, get a couple of barrels and start a methane digester.

  9. For anyone interested in the biochemistry, you can’t keep fish in a small unfiltered bowl because their waste produces ammonia, which is toxic to fish (and most other things). A mature tank with proper filtration will quickly build up a population of Nitrosomonas bacteria, which will convert poisonous ammonia into (still poisonous) Nitrite, and a population of Nitrobacter bacteria which will convert Nitrite into (fairly harmless and plant-fertiilising) Nitrate.

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