Make a Microbial Fuel Cell (MFC) - Part 1

Step 8: Assembling the Reactor - Part III Assembling the anode

Now we turn our attention back to the we obtained earlier. The collected water will be used as culture media and the mud in the pipe will be used to provide the culture.

We'll be diverging from Abbie's design a bit. Where Abbie used the actual mud we're going to try to extract a sample and culture the bacterium.

I have been advised by Dr. Logan at Penn State that they do not de-gas the influent since the bacteria will handle it due to the low solubility of oxygen. The material in italics may be bypassed but I left it in case anyone ever wants to de-gasify water for some reason.

"'To do that we're going to de-gas the water using nitrogen, argon or other inert gas. Strictly speaking you don't have to do this but it will give our bacteria a better chance to breed.

We're going to do it slowly. Since the options for obtaining gas range from inner tube to pressure tank I give you the following simple instruction. Connect a section of aquarium air tube to the end of a hose that is connected to the gas reserve. Hopefully you have some way to regulate the gas. If you're using an inner tube I suggest filling a balloon (insert it over the valve and push the valve in) and using that to dispense the gas by inserting the air hose into the end and letting the gas slowly out of the balloon. Yeah, I know, it ain't easy being that cheap. Get over it or spend some money for a tank with a valve.

Now having arranged by some mechanism to regulate the gas flow of the nitrogen into the air tube attach the bubbler stone to it. Fill the anode chamber (one bottle) with the culture media water. Agitate well. Insert the bubbler stone into it and slowly bubble nitrogen through the solution.

How much nitrogen to bubble and for how long? A very, very good question and as soon as I have an answer I'll update this. I've asked the folks at Penn State so its not impossible this will be updated. Try to stretch it out, say 15 minutes maybe stirring occassionally.

If you've got the nitrogen do both bottles since we'll be topping off the culture media.

Once you've de-gassified the water or not its time to introduce the culture (mud).

This is also the time to put in some algae sludge if you have some.

See I said it was an algae fuel cell....

Grab the sample pipe and place the mud end in the anode chamber preferably under water before removing the slip cap. You might need to push it off with a long screwdriver or kitchen fork. Pull the slip cap off of the other end and slowly begin pushing mud out with a dowel or broom handle. You want to push out the bottom 2-3 inches of mud then take out the pipe. This should give you a solid culture of primarily anaerobic bacteria. Top off the culture media if needed from the second jar and add 2 drops of vinegar for each quart of culture media in the jar to supplement the natural nutrients.

Now place the anode cap and electrode in the anode jar and seal the edge with hot glue. The anode should be air tight and is not designed to be recharged at this time. Once the food is exhausted the bacteria will die. This will be indicated by a drop off in the output voltages.

Because the MFC cannot be recharged it is properly called a battery. To rectify this (get, a little electrical joke...very little) use a container which gives a solid, air tight seal and add a feeder hole and tube to it. Remember to seal the feeder tube when not in use and preferably flush with nitrogen.

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reedmroue3 months ago

I'm a student who tried to build an mfc at home. I used waste water as the anode substance, and carbon electrodes, in both anode and cathode. but im not getting any voltage reading. is their something wrong?

egbertfitzwilly (author)  reedmroue3 months ago

There's not enough information for me to make any sort of an informed guess. What sort of waste water did you use and what did you use to create the individual cells? There are more detailed instructions in Part II.

nshankar2 years ago
I'm a student who tried to build an mfc at home. I used waste water as the anode substance, and graphite electrodes, in both anode and cathode. But my multimeter is showing a negative voltage reading. What have I done wrong?
egbertfitzwilly (author)  nshankar2 years ago
If you are showing a negative non-zero voltage you have connected your voltmeter backwards.
rwcrocker3 years ago
A couple of thoughts on de-gassing:

One quick way to degas a solution is to boil it for a few minutes. Gas solubility decreases a lot with temperature. Of course you won't be able to boil your culture, but sterilizing the media might help to keep stray yeasts out of it.
Other gases will work as well, such as helium, which is more readily available for party ballons in smaller disposable cylinders.

You can also remove the oxygen specifically by electrolyzing the solution since you have nice electrodes and a very fine salt bridge. During the electrolysis of salt water, at the cathode (-) side dissolved oxygen is reduced to hydroxide, OH-, and hydrogen gas is evolved. In the other compartment, the anode(+), oxygen gas is evolved, while acidifying the adjacent solution.
egbertfitzwilly (author)  rwcrocker3 years ago
Thank you for this excellent input. I will experiment with this for my other "How to make Sodium Hydroxide" instructable series...

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