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Project Summary

Difficulty  10 
Time required Very Long (several weeks to months)
Prerequisites You will need to be familiar with the procedures for handling biologically hazardous material. Read the Microorganisms Safety Guide from the Science Buddies Project Guide to learn more about handling biologically hazardous materials. You also need to have access to a lower-order stream of water or a creek. See the Introduction for an explanation of this type of stream. Try to avoid streams in which the bed is rocky. The benthic mud sample should be from an area that has a thick, rich mud bed.
Material Availability Specialty items required
Cost High ($100 - $150)
Safety Adult supervision is recommended. Be careful when using a hot stove and a drill. Remember to always wear safety goggles when drilling. Exercise caution when working near a stream or creek. Water currents can be stronger than they look.

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Abstract

"Gross! What is that in the toilet?" But maybe it's not just gross. Did you know there are bacteria that digest organic waste and create electrons? What if there was a way to collect those electrons to power a circuit? In this science fair project, you will make a microbial fuel cell to collect the electrons that the bacteria in a benthic water sample create...only, you'll be using mud, which is much safer to handle. If you would like to learn how to reuse and recycle an unlikely substance, this is the science fair project for you!

Objective

To learn about an alternative method for creating electricity, the microbial fuel cell. The goal is to build a microbial fuel cell and test different samples of mud to see which creates more electricity.

Introduction

In order to reduce pollution, we need to develop alternative and renewable energy sources. When people think of alternative and renewable energy sources, they usually think of harvesting energy from the Sun (solar energy), Earth (geothermal energy), water (hydropower), or wind. But by using a microbial fuel cell (MFC), electricity can be extracted from wastewater! The microbial fuel cell converts organic material to electricity using bacteria, leaving behind clean drinking water in the process. This is an exciting prospect for people around the world who lack adequate sanitation and the means to afford it. In addition, water treatment plants require a lot of power to treat water. Each year, almost 25 billion dollars are spent in treating wastewater. A lot of money and resources could be saved if the wastewater could be used as a fuel!

The microbial fuel cell is a bio-electrochemical system in which bacteria are used to convert organic material into electricity. The fuel cell is made of four parts: the anode, the cathode, the proton-exchange membrane (PEM), and the external circuit. The anode holds the bacteria and organic material in an anaerobic (without oxygen) environment. The cathode holds a conductive saltwater solution. As part of the digestive process, the bacteria create protons (H+) and electrons(H-). This is also known as oxidation. The electrons are pulled out of the solution onto an electrode and are conducted through an external circuit. The electrons move through the circuit into the cathode (via the cathode's electrode). The protons travel through the proton-exchange membrane (PEM) or a salt bridge to meet with the electrons at the cathode. The PEM and salt bridge separate the anode and cathode, but protons are able to move through both to connect the anode and the cathode. The salt bridge is a mesh of proteins that allows the protons to move from the anode to the cathode, but keeps the solutions in the anode and cathode separate. At the cathode, the protons and electrons combine with oxygen to create water.

Cartoon of microbial fuel cell
Figure 1. This diagram shows how a microbial fuel cell (MFC) functions.

There are two kinds of microbial fuel cells: mediator and mediator-less. In a mediator microbial fuel cell, the bacteria are electrochemically inactive. The bacteria digest the organic material and create electrons. However, the bacteria have no mechanism to rid themselves of the electrons. This is where the mediator helps. The mediator is an inorganic substance, such as thionine, humic acid, or methylene blue, which crosses the membrane of the bacteria and frees the electrons. The mediator then carries the electrons away from the bacteria and deposits them on the electrode. One disadvantage to the mediator microbial fuel cell is that many of the mediators are toxic.

In a mediator-less microbial fuel cell, the bacteria are electrochemically active. The electrochemically active bacteria carry the electrons they create through digestion of organic material to the electrode. However, as mediator-less microbial fuel cells are a recent development, this process is not completely understood.

In this science fair project you will build two mediator-less microbial fuel cells. Since working with wastewater samples can be challenging, you will compare two mud samples. The first mud sample will be from a local lower order stream or creek. A lower order stream is one that is formed by the joining of other streams. So a first-order stream is one that does not have any other streams feeding into it. When two first-order streams merge, they create a second-order stream, etc. The mud sample will be taken from the floor of a second-order (or lower) stream. This area of the stream is called the benthic zone. The second sample will be one that you make from topsoil that you get from the garden store. Compare which sample produces more electricity. Have fun experimenting, and remember that you are working in an area that is contributing to the well-being of our planet Earth.

Terms, Concepts and Questions to Start Background Research

Questions

Bibliography

The following website has a lot of great information about the microbiology and electrochemistry of microbial fuel cells.

This website details the work being conducted at Pennsylvania State University. It also shows several pictures of homemade microbial fuel cells, including one built by Ian Bennet, as well as instructions on how to build one.

NASA is interested in reusing human waste. Read this website to learn how!

Materials and Equipment

Building the Anode and Cathode Containers

Making the Electrodes

Making the Salt Bridge

Making and Obtaining the Mud Samples

Assembling the Fuel Cell

Testing the Fuel Cell

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Experimental Procedure

The goal of this science fair project is to determine which of two mud samples creates more electricity. In most cases, the experimental procedure states to perform actions twice. Keep this in mind as you go through the procedure and also keep in mind that you will be asked to perform two additional trials. You will have three sets of two anode-cathode pairs. All good science fair projects are replicated at least three times.

The procedure is broken into six sections: Building the Anode and Cathode Containers, Making the Electrodes, Making the Salt Bridges, Making and Obtaining the Mud Samples, Assembling the Fuel Cell, and Testing the Fuel Cell.

Note: Once you collect the mud samples, you'll need to use them within 24 hours. Make sure you have everything you need to assemble the fuel cell and start your experiment before collecting the samples. Since you don't want to take too many trips to your local stream, get enough of a benthic sample to use for three trials.

Building the Anode and Cathode Containers

  1. Unscrew the two ends of the compression fitting and discard the rubber fitting. Using the sandpaper, roughen the endcaps of the compression fitting.

Roughening the endcap with sandpaper
Figure 2. Roughening one of the endcaps.

  1. Take the sandpaper and roughen two opposing sides of two of the plastic containers. Just roughen two 1-inch by 1-inch patches across from each other. Making a rough patch will help join all of the parts together. See Figure 2.
  2. Using the permanent marker, make a mark in the center of the roughened side of one of the plastic containers. Use the ruler to measure the location of the mark and then make a mark at exactly the same location on one of the roughened sides of a second plastic container. Make sure that the marks are exactly opposite of and facing each other.
  3. Using the ruler, measure the outer diameter of the aquarium air pump tubing. Record this number in your lab notebook.
  4. Put on the safety goggles. Carefully drill a hole 2 millimeters (mm) in diameter on top of two of the plastic container lids. Brush off any plastic debris from the lids. In one of those lids, drill another hole the same diameter as the plastic tubing of the aquarium air pump next to the first hole.
  5. Using the 3/4-inch spade drill bit, drill a hole on the permanent marker marks on the sides of both plastic containers. Note:Drill slowly or the acrylic might crack. Brush off any plastic debris. You should now have two plastic containers that each have a hole in one side.

Hole in side of one container
Figure 3. Plastic container with a hole drilled into the side.

  1. Read the safety recommendations on the acrylic cement. Squeeze acrylic cement around one of the 3/4-inch spade drill bit holes. The acrylic cement is watery so be careful not to get it in the hole or on your fingers. Now squeeze acrylic cement around the flat part of one compression fitting endcap. Center the endcap over the hole on one of the plastic containers that you just squeezed cement around. Fit the two pieces together. Hold the two together for 30 seconds. Squeeze some additional cement around the outside of the endcap where it joins with the container. This is to ensure that you minimize liquid leaks. Let the assembly dry for 10 hours.
  2. After 10 hours, screw in the tube as tightly as possible. Hold the endcap firmly and just screw in the tube.
  3. Now screw the second roughened endcap into the tube. Make sure to tighten the endcap firmly.
  4. Lay the second container on its side with the 3/4-inch spade drill bit hole face up. Squeeze acrylic cement around the hole, making sure not to get any in the hole or on your fingers. Squeeze some acrylic cement on the second endcap on the assembly. Position the endcap and assembly over the 3/4-inch spade drill bit hole in the second container. Make sure that it is centered. Use the ruler to make sure that the containers are straight and level. Make adjustments as necessary. You need to make sure that the containers can sit flat on the table when all the parts are dry. Hold the entire assembly together for 30 seconds. Squeeze some acrylic cement around the outside of the endcap where it joins with the second container. Let the entire assembly (the two containers with connecting compression fitting), dry for 10 hours. See Figure 4 for a completed assembly.

Cementing the two containers
Figure 4. Aligning and cementing the second endcap and container.

  1. After 10 hours have elapsed, check to see if the two joints are watertight. Fill the containers with water past the holes/joints. Wait for 5 minutes. If there is no water leaking out, then proceed to the next section. If there is excess water coming out of a joint, empty the containers and dry them off completely with paper towels. Carefully squeeze acrylic cement around the endcap joint that leaked. Squeeze out enough cement that you make a seal, but not so much that it becomes messy or that you seal the tube. Wait for 10 hours and retest the watertightness. Try again if this doesn't work. If it still doesn't work, remake the assembly with fresh parts.
  2. Once the parts are watertight, carefully unscrew the tube from the endcaps. Set the tube aside.
  3. Repeat steps 1-12 five more times. You should end this section with six anode and cathode pairs, like the one shown in Figure 5.

Completed anode cathode pair
Figure 5. This is a watertight anode and cathode pair.

Making the Electrodes

  1. With the scissors, cut the carbon cloth into four equal squares. Each square should be 5 cm x 5 cm .
  2. Take each of the four pieces of copper wire and with the wire strippers, strip off 6 inches of the insulator on one end of each piece. Strip off 1 centimeter (cm) from the other end of each wire.
  3. Prepare the nickel epoxy according to its directions.
  4. Epoxy the 6 inches of bare copper wire to the carbon cloth along the edges of the square. Repeat with the other three carbon cloths. Let the epoxy harden for 10 hours.

Cartoon of epoxied electrode
Figure 6. This is a visual of how the electrode should look.

  1. Once the epoxy has hardened, test the connection between the carbon cloth square and the copper wire with the digital multimeter. Turn the digital multimeter to the resistor/resistance mode. Place one lead on the carbon cloth and the other lead on the free bare end of the copper wire. There should be no or very low resistance. If there is a large resistance, then remake the electrode.
  2. Repeat steps 1-5 three more times. You need to make enough electrodes to use in six anode-cathode pairs.

Making the Salt Bridges

  1. Place some plastic wrap along the bottom of a petri dish so that the ends of the plastic wrap are overlapping the edges of the petri dish. Set the covered dish aside.
  2. Cover one end of the tube section from the compression fitting securely with aluminum foil. Repeat with the tubes from all of the compression fittings. Place all tubes, open end up, vertically on the petri dish and then set it aside.
  3. Measure 300 milliliters (mL) of water and pour it into the pot.
  4. Using the scale, measure out 30 g of agar. Set the measured agar aside. Now measure out 6 g of salt.
  5. Place the pot of water on the stove and bring it to a boil. When the water is boiling, add the agar and stir it with the glass rod until it is dissolved.
  6. Once the agar is dissolved, take the pot off of the heat and add 6 g of salt. Stir with a spoon until the salt is dissolved.
  7. While the solution is still warm, carefully pour the solution into the tubes in the petri dish. If the tubes leak, tighten the foil and refill them. Once the tubes are filled and stable (i.e. haven't fallen over, leaked liquid, etc.) for 10 minutes, carefully move the petri dish to the refrigerator. Let the tubes sit in the refrigerator overnight. These tubes are the salt bridges.
  8. The next day, come back and place the salt bridges into a 1-qt. plastic baggie and seal it. This prevents the salt bridges from drying out. Take the bridges out when you are ready to use them.

Making and Obtaining the Mud Samples

  1. Go to the location of your stream where you have found the richest riverbed. The sample you get should not be full of rocks or twigs, just rich mud.
  2. Tie the nylon rope securely around the middle of the PVC tube.
  3. Throw the tube into the stream or creek. Make sure that you keep a good hold onto the end of the rope! Try to scoop as much of the stream bed as you can.
  4. When you think that you have gotten a large enough sample, drag the tube back to shore. Gently tap the pipe with the hammer and transfer the sample into the bucket. Cover the bucket with plastic wrap and set it aside. Make sure to get enough of the benthic sample to fill the anode chamber and remember to use the sample within 24 hours.
  5. Collect some of the stream water in the cleaned 1-gallon jug. Be careful when retrieving this sample. Always exercise caution when you are near a stream or a creek, as the water current can be stronger than it looks.
  6. To make the second mud sample, use your trowel to place a few scoops of topsoil into the second bucket. Mix some tap water into the topsoil until you get a sludge-like mud. Cover the bucket with plastic wrap and set it aside.

Assembling the Fuel Cell

  1. Retrieve the 12 containers built in section 1 and the salt bridges made in section 3. Remove the aluminum foil from the salt bridges. Connect a pair of containers with a salt bridge. Repeat six times. You now have the anode-cathode pair for six microbial fuel cells.
  2. Make a conductive salt solution. Measure out 12 cups of distilled water. Add 6 tbsp. of salt to the measuring cup and stir with a spoon until the salt has been dissolved. Fill one of the containers of three of the fuel cell pairs with the conductive salt solution. This container is the cathode for the topsoil mud sample (fuel cell type A). Fill the cathodes of the other three fuel cell pairs with the water sample from the 1-gallon jug (fuel cell type B).
  3. Take an electrode and thread it through the smaller hole of one of the lids with two holes. Place the lid with the two holes and the connected electrode back onto the cathode. Make sure the electrode is submerged. Repeat this step with another electrode and the other lids with the two holes. Seal each cathode with a lid.
  4. Connect the tubing to the outlet of the aquarium pump. Push the tubing into the cathode containers through the larger hole in the lid. Be sure to submerge the end of the tubing.
  5. Now, wearing gloves and safety goggles, fill half of the anode chamber of fuel cell A with the topsoil mud. Make sure that there are no bubbles in the mud. Press the mud sample down or gently tap to remove any bubbles. Take one of the electrodes and carefully bury it in the topsoil mud. Then place some more of the topsoil mud into the anode, covering the electrode. Push the free end of the electrode copper wire into the 2-mm holes in the container lids. Replace the lid onto the container to make sure that the electrode is hanging freely without hitting any of the walls or the bottom. You can use a little electrical tape on the outside top to hold the electrode in place. Repeat this with fuel cell B and the benthic mud sample. Repeat step 5 for the two additional trials. The fuel cells are complete and you should have three fuel cells of type A and three fuel cells of type B.

Testing the Fuel Cells

  1. Turn on the aquarium pump. This aerates the solution in the cathode and creates more oxygen.
  2. Now test the voltage between the anode and the cathode with the digital multimeter. Test fuel cell A, the topsoil fuel cell. Turn on the multimeter and put it in "voltage" mode. Clip one of the alligator clips to the anode electrode. Clip the other end of the alligator cable to one of the multimeter leads. Attach the second alligator cable. Clip one end to the cathode electrode and the other end to the multimeter. Is there a voltage reading? Note down your data in your lab notebook in a data table.
  3. Take voltage readings twice a day at the same times, every day for 30 days.
  4. Repeat steps 1 and 3 of this section for fuel cell b, the benthic fuel cell. Always record your data in your lab notebook.
  5. Repeat steps 1-4 for the other two trials. Always record your data in your lab notebook.

Analyzing Your Data

  1. Plot your data on a scatter plot. Make two plots; one for the fuel cell type A and another for the fuel cell type B.
  2. Which sample produced more electricity? Did the fuel cells start producing electricity right away? Did the electricity production ever peak? How did the electricity production vary over one day?

Variations

Credits

Michelle Maranowski, PhD, Science Buddies

This project idea is based on the following 2008 California State Science fair project, a winner of the Science Buddies Clever Scientist Award: Bennet, I. (2008). Generating Electricity from Wastewater Using a Microbial Fuel Cell. Retrieved August 23, 2008, from http://www.usc.edu/CSSF/History/2008/Projects/J0804.pdf

The design in this science fair project is based on Abbie Groff's design in her science fair project, titled "Identification of Benthic Microbes Utilizing Bioremediation and Microbial Fuel Cells."

The author would like to thank Sandra Slutz, PhD, Science Buddies, for very helpful discussions.


Last edit date: 2008-12-08 16:14:00

Career Focus

If you like this project, you might enjoy exploring careers in Energy & Power.

Power Distributors and Dispatcher
Think of all the things in your home or school that use electricity, like the lights, TV, refrigerator, washer, microwave, music players, computer, and electronic devices. Now think of how you feel when the power goes out, even for just a moment. Power plant distributors and dispatchers have an important job—they work to keep electricity flowing to homes and businesses by carefully watching and planning for problems like big storms that could damage transmission lines, heat waves that cause a big surge in demand for power, or normal construction work, which could take transmission lines out of service. 		  Power Plant Operator
No matter what time of the day or night, or what the weather is like, power plant operators work to ensure that homes and businesses have a reliable source of power. They switch the plant generators on and off, as needed, and monitor and maintain generators, turbines, and pumps to prevent failures.
Nuclear Power Reactor Operator
One in five United States homes and businesses is powered by nuclear power, and nuclear power reactor operators are the people who ensure that those reactors are operating safely and efficiently at all times. They monitor all equipment continuously, and implement procedures if malfunctions are observed. They also control and adjust the amount of power being generated, and the reactor coolant temperature as power demands change through the day and during weather events, like heat waves. 		 


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