Science Buddies: "Ask an Expert"

I am making a Microbial Fuel Cell and testing the effects of various waste waters on the output of the battery. In the procedure, it states "Collect some of the stream water in the cleaned 1-gallon jug" and "Make a conductive salt … the salt solution." What is the importance of the water being from a stream? Can I use tap water to make the conductive liquid that is put in the cathode? Also, how long do you think I should run the experiment and how often should I measure the output. I have to do the expeirement three times so I cannot do a month as stated in the procedure. Thank You

Hello,

This is an interesting question. Could you explain the design of the battery a little more? Given that this is a "microbial fuel cell" the purpose of using stream water is so that there are microorganisms in the water. Your tap water should be fairly close to sterile. Tap water would actually be a good control sample as you would expect minimal microbial activity. Waste-water and stream water will have various kinds of microorganisms with different electrochemical activities and thus different effects on battery output. Could you post the procedure you have in full so I better help you plan your experiment, including number of conditions to test, duration, and replicates?

Cheers,
Colin

I am doing this experiment: https://www.sciencebuddies.org/science- ... ml#summary. I am making it my own by testing various waste waters such as chicken manure water, compost water, pond water and as a control purified water. In the experiment it states that the waste water is put into the anode container and a conductive salt water mixture is put into the cathode. The experiment says that the salt water mixture should be made with water collected from the same stream that the benthic mud sample used as the waste water is from. As I am not using a benthic mud sample in my experiment I did not collect any stream water. I went ahead with the experiment and used tap water to make the conductive mixture and it seemed to work as my cells are producing a little voltage. However a new question has come up, my control (the purified water) is producing more voltage than any of the other cells (0.2 volts vs. 0.1 and 0 volts). Also, on the other cells, when I try to read the resistance the Multi-meter reads OL (open line). How could it be that even though the circuit is not complete it still produces energy? When I attempt to test the resistance of the other cells the number does not settle down instead it continues to increase at a constant rate for minutes on end. What are some possible explanations for this? Thank you so much for your help and a timely response.

I think the reason the instructions said to use water from the same source as the benthic mud was so the water in both samples had similar trace mineral and ion contents other than the salt you add to the cathode and whatever the microbes produce in the anode. I, too find the results a little confusing, though the purified water you used as control anode solution will not be the same as the tap water you put in the cathode, so there is the possibility of creating a current. Did your electrodes test OK when you made them? Let me think about this for a few hours. You might try cross-posting this question in the physical science forum as this is really an electrical question at this point.

-Colin

Any progress? Your electrical results have me a little stumped. Any help from the physical science forums?

Cheers,
Colin