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How Do Bacteria Produce Power in a Microbial Fuel Cell?

Difficulty
Time Required Very Long (1+ months)
Prerequisites Previous experience using a multimeter and familiarity with the physics of electricity is helpful, but not required. A basic knowledge of how to work with bacteria is also recommended to complete this science fair project. Consult the Microbiology Techniques and Troubleshooting guide for information on how to conduct microbiology experiments.
Material Availability The microbial fuel cell kit needs to be special ordered from the Science Buddies Store.
Cost Average ($50 - $100)
Safety Be sure to wear the gloves supplied with the kit when handling the microbial fuel cell's electrodes (its cathode and anode). The electrodes are made of a conductive material called graphite fiber and should not be placed near electronics or power plugs, or have their fibers dispersed in the air. Use sterile technique when growing the bacteria. Read the Microorganisms Safety Guide before starting any experiments. SRC approval may be necessary. Adult supervision is recommended.

Abstract

Bacteria are powerful little creatures. They can dispose of contaminants, make us sick and did you know they can even generate electricity? In devices, called microbial fuel cells (MFC) they can extract electrons from their food sources such as organic materials and feed them into an electrical circuit to generate power. This way they can even turn plain soil into a source of energy! Does it matter though what kind of food the bacteria "eat"? Find out how bacteria grow in a microbial fuel cell and how power generation in an MFC changes depending on the provided food source.

Objective

Monitor the power output of microbial fuel cells and compare power generation and bacteria quantities for two different substrates.

Credits

Svenja Lohner, PhD, Science Buddies
Teisha Rowland, PhD, Science Buddies

Cite This Page

MLA Style

Lohner, Svenja. "How Do Bacteria Produce Power in a Microbial Fuel Cell?" Science Buddies. Science Buddies, 23 Sep. 2016. Web. 25 Sep. 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p032/microbiology/bacteria-microbial-fuel-cell.shtml>

APA Style

Lohner, S. (2016, September 23). How Do Bacteria Produce Power in a Microbial Fuel Cell?. Retrieved September 25, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p032/microbiology/bacteria-microbial-fuel-cell.shtml

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Last edit date: 2016-09-23

Introduction

Soil is packed with bacteria that generate electricity when placed in a microbial fuel cell (MFC). Because such bacteria-laden soil is found almost everywhere on Earth, microbial fuel cells can make clean, renewable electricity nearly anyplace around the globe. As natural resources are being depleted, scientists' attention has shifted to pursuing alternative energy sources, such as MFCs, even more than before. A microbial fuel cell is a device that can use microbes to generate electricity. An MFC has two electrodes and an area that separates the electrodes (called a membrane). For an MFC to function, electricity in form of electrons must flow into one electrode and leave the other electrode. How is this accomplished? It has to do with the bacteria in the MFC. Some types of soil bacteria can help generate electricity. These bacteria, known as electrogenic bacteria, include the Shewanella species, which can be found in almost any soil on Earth and are shown in Figure 1, and the Geobacter species, which prefer living in soil deep underground or even under the ocean, where no oxygen is present. The soil bacteria eat what is in the soil, such as microscopic nutrients and sugars, and in turn, produce electrons that are released back into the soil. Electrons are subatomic particles that have a negative charge. These electrons can be harnessed and used to create electricity, which is a form of energy.


Figure 1. This is a high-magnification image of Shewanella bacteria, specifically the species S. oneidensis. The bacteria are the cylindrical-shaped rods scattered in this image. (The other parts of the image are ice pieces that the bacteria were submerged in to take this picture.) (Image credit: PLoS Biology)

In a soil MFC using these soil bacteria, one electrode (specifically the anode) is buried down in damp soil. Down there, the bacteria multiply and cover the electrode (creating a biofilm on it), supplying it with a lot of electrons from breaking down organic or inorganic substrates in the soil. At the same time, the other electrode (called the cathode) is placed on top of the soil, leaving one of its sides completely exposed to the air. Electrons from the anode travel up a wire to the cathode, and, once there, they react with oxygen (from the air) and protons from the anode (made by the bacteria as it digests nutrients in the soil) to create water. See Figure 2 for a visualization of this process. The more electron-producing, soil-munching bacteria are in the soil, the more electricity the MFC produces.

Diagram of the chemical reactions taking place inside a microbial fuel cell.
Figure 2. This diagram shows the reactions taking place in a microbial fuel cell (or MFC) that make it generate electricity. (Wikimedia Commons, 2010, MFCGuy2010)

To evaluate the overall performance of an MFC, usually its power output is determined. This is done by measuring the voltage across a fixed resistor that you attach to the MFC and from that, power is calculated using a derivation of Ohm's law, as shown in Equation 1.

Equation 1:

  • P is the power in watts (W),
  • V is the voltage (V), and
  • R is the resistance in ohms (Ω).

If you use several different resistors, you can generate a power-resistance curve, which allows you to determine the maximum power output of your microbial fuel, cell as explained in more detail in the Procedure. As the bacteria will start generating power from the food present in the soil, the power output will start increasing over time and eventually reach a steady state.

In this science project, you will monitor the power output in two microbial fuel cells over time and compare the power generated from two different substrates: just soil and table sugar (sucrose). At the same time, you will monitor and compare the quantity of bacteria over time in each MFC using the plate count method by plating the bacteria from the soil on nutrient agar plates. Each individual bacteria or group of cells present in your sample will form a bacterial colony that you can count as colony-forming units (c.f.u.). As 1 gram (g) of soil contains up to trillions of individual bacteria, you will have to make a serial dilution of your sample before you can plate it on the agar plates. This way you can make sure that you only get a small number of colonies on each plate, making them easier to count. What do you think will happen to the quantity of bacteria in the MFC as the power output increases? Will it increase, decrease, or stay relatively stable? Will you see more bacteria growing with or without the addition of sugar? Will you see different types of microbes growing? Do this science project and find out!

Terms and Concepts

  • Microbial fuel cell (MFC)
  • Renewable or alternative energy source
  • Electrode
  • Membrane
  • Electron
  • Electrogenic bacteria
  • Shewanella
  • Geobacter
  • Electricity
  • Anode
  • Biofilm
  • Cathode
  • Power output
  • Resistor
  • Ohm's law
  • Plate count method
  • Nutrient agar plate
  • Colony-forming unit (c.f.u.)
  • Serial dilution
  • Ohm (Ω)
  • Voltage
  • Watt

Questions

  • Where do you find electrogenic bacteria and what are their natural food sources?
  • What are the anode and cathode reactions in a microbial fuel cell?
  • How do you assess the performance of a microbial fuel cell and how can you measure it?
  • How can electrogenic bacteria transfer their electrons to the anode?
  • What are the current limitations of microbial fuel cells?
  • How can you improve the power output of a microbial fuel cell?

Bibliography

These resources will give you more information about soil microbiology, microbial fuel cells, and the bacteria involved:

For more information about electronics terms and using a voltmeter/multimeter, use this tutorial:

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Materials and Equipment Product Kit Available

These specialty items can be purchased from the Science Buddies Store:

  • Microbial Fuel Cell kit (1). Includes:
    • Microbial Fuel Cell vessels (2)
    • Anodes (2)
    • Cathodes (2)
    • Hacker boards (2)
    • Capacitors (2)
    • LEDs (2)
    • Digital clock/thermometer to be powered by the microbial fuel cell (1)
    • Set of 7 Resistors (1)
    • Alligator clips and jumper wires (2)
    • Digital multimeter (1)
    • Nitrile gloves (1 pair)

You will also need to gather these items (not included in the kit):

  • Optional: Old newspapers to protect your work area
  • Soil (at least 4 cups) or about 400 g of sifted soil
    • Soil from just about anywhere works — from a backyard, park, open space, or even a riverbed. Just make sure the soil has not been treated with pesticides and that you have permission to take some of it.
    • Soil can also be purchased from a plant nursery, a hardware store, or online. When you buy your soil, choose organic topsoil that has not been sterilized or treated with pesticides. Do not use soils with little white foam balls, vermiculite pieces, or perlite, which are used to aerate the soil; also, avoid soils with peat moss.
  • A sieve, plastic strainer, or colander to remove large particles from the soil
  • Large mixing bowl that can hold all the soil
  • Measuring cups or 100 mL graduated cylinder. A 100 mL graduated cylinder is available from Amazon.com
  • Distilled water (at least 1.5 L). This can be purchased at most grocery stores.
  • Paper towel or rag
  • Masking tape
  • Permanent marker
  • Mini cups with lids, 2 oz (at least 20)
  • Teaspoon
  • Table sugar (sucrose)
  • Transfer pipettes (at least 7), available from Amazon.com
  • Nutrient agar plates (10 or more), available from Amazon.com
  • Cell spreader, available from Amazon.com
  • Rubbing alcohol
  • Flame from either a candle or a gas stove
  • Stopwatch
  • Digital scale with 0.1 g increments; a digital scale that would be suitable is the Fast Weigh MS-500-BLK Digital Pocket Scale, available from Amazon.com
  • Lab notebook

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies does participate in affiliate programs with Amazon.comsciencebuddies, Carolina Biological, Jameco Electronics, and AquaPhoenix Education. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity. If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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Experimental Procedure

Microbial Fuel Cells in Science Fairs

Working with microbial fuel cells involves growing soil bacteria. Because of this, many science fairs, including those associated with the International Science and Engineering Fair (ISEF) have requirements which need to be met before you start your project. We recommend you:

  • Check with your teacher or science fair coordinator about any requirements
  • Read the Science Buddies Microorganisms Safety Guide to learn how to safely handle bacteria
  • Fill out a Risk Assessment Form (required by ISEF affiliated science fairs for all MFC projects)

Setting up the Microbial Fuel Cells and Bacteria Count

You will assemble two identical microbial fuel cells that contain the same soil material. Later in the procedure you will provide one of them with a sugar food source and compare its power output and bacterial quantities to the control microbial fuel cell with no treatment.

  1. First, click through the slide show or follow the step-by-step instructions to see how to assemble the microbial fuel cells.

Dry soil. Wet soil microbial fuel cell materials bend wires wire in anode prepared anode prepared cathode full 1cm with mud Anode in mud full 5cm with mud cathode in microbial fuel cell closed wires in hackerboard capacitor in hackerboard finished hackerboard resistor in place voltage sweep
slideshow mfc pee holder
Follow along with the pictures in this slideshow to assemble the microbial fuel cell.
  1. Prepare your soil mud.
    1. Place a plastic strainer or colander over a large mixing bowl.
    2. Measure a total of about 2 cups (about 500 milliliters [mL]) of your soil for each of the microbial fuel cells into the strainer (4 cups in total). Gently shake the strainer over the bowl so that the soil is strained and any small, hard particles (such as rocks, pebbles, twigs, etcetera) are removed from the soil. You will likely need to be patient; it may take several minutes to strain the soil. When you are done, you should have about 400 g of fine, sifted soil in the bowl.
      1. It is important to remove these particles from the soil because they can aerate the soil and inhibit the desired bacteria from growing (the bacteria do not want to be exposed to oxygen).
    3. Add distilled water and mix it in until your topsoil mud feels like cookie dough. Add more water if the mud is too crumbly, or add more topsoil if the mixture feels too wet. For 400 g of soil you will likely need about 1 cup (250 mL) of water.
    4. When you have prepared your topsoil mud, set it aside and wash your hands.
    5. Take a sample of your prepared soil for measurement of the initial bacteria quantity.
      1. Sterilize a metal spoon by dipping it in rubbing alcohol, shaking off the excess alcohol, and holding it into a flame (from a candle or gas stove). Wait until the flame on the spoon burns out and let the spoon cool down.
      2. Label a mini cup with the date and time, and use your sterilized spoon to scoop about 2–3 spoons full of your prepared soil (you will need 0.5 g) into the cup. Close the lid and set it aside for now.
  2. Carefully take the MFC pieces out of the box and lay them out. Identify the different components and use masking tape and a permanent marker to label one of the devices with "control" and the other one with "sugar treatment."
  3. Put on the gloves that came with the MFCs and start assembling the first microbial fuel cell.
  4. Take out the green and orange wires that came with the MFC electrodes. Bend each wire where the plastic part ends so that each wire is now at a 90° angle (shaped like a capital "L"), as shown in the slideshow.
  5. Remove the MFC anode from its bag. (The anode is the thinner, black, felt-like circle.)
    1. Safety Note: The MFC's cathode and anode (its electrodes) are made of a conductive material called graphite fiber. Do not put the cathode or anode near electronics or power plugs, and do not disperse the fibers in the air, as the fibers will cause electrical shortages when in contact with electronics.
  6. Straighten the metal part of the green wire and carefully insert it into the anode, as shown in Figure 3. Make sure the wire goes in straight and does not poke out on the top or bottom sides of the anode.
Inserting the green wire into the microbial fuel cell's anode.
Figure 3. Insert the metal part of the green wire into the anode circle.
  1. Repeat steps 6 and 7 using the cathode (the thicker, black, felt-like circle) and the orange wire (which is shorter than the green wire).
  2. Take the soil mud that you prepared in step 2 and use it to fill the first MFC vessel (that came with your kit) up to the line next to the "1" on the plastic vessel (marking 1 centimeter [cm]). Once filled, pat the mud so that its surface is smooth.
    1. Tip: You may want to cover the surface you are working on with old newspapers to prevent mud from getting on it.
    2. When you are finished, rinse the mud off your gloves and dry them (but do not take them off yet).
  3. Put the anode on top of the mud in the vessel, as shown in Figure 4.
    1. The green wire from the anode should be sticking up. The green wire should not be stuck down in the mud.
    2. Gently press the anode flat against the mud so that no air bubbles are under the anode. Note: Removing the air bubbles is important as the trapped oxygen can prevent the formation of an anaerobic bacterial biofilm and reduce the power output of your microbial fuel cell.
An anode added to a microbial fuel cell.
Figure 4. Place the anode on top of the 1 cm of mud.
  1. Use more soil mud to fill the vessel up to the line next to the "5" mark (marking 5 cm). Once filled, again pat the mud so that its surface is smooth.
    1. Run the green wire along the side of the vessel.
    2. Rinse the mud off your gloves and dry them.
  2. Gently place the cathode on top of the mud and press it as flat as you can, as shown in Figure 5.
    1. The orange wire from the cathode should be sticking out of the top side.
    2. Do not let any mud or liquid cover the top of the cathode.
    3. It is best to arrange the cathode so that its orange wire is about 1–2 cm to the left of the green wire.
    4. Let the mud rest in the vessel for a few minutes. Then carefully pour off any excess liquid.
Microbial fuel cell full of mud with cathode and anode added.
Figure 5. Add the cathode on top of the 5 cm of mud.
  1. Use a clean paper towel or rag to wipe any mud off the vessel's rim. Then take off your gloves.
  2. Take the white plastic lid and pass the wires through the small holes in the lid. Arrange the wires so that the orange wire is on the left and the green wire is on the right when the semicircular indentation on the lid is facing the front. Then carefully snap the lid onto the plastic vessel.
  3. Take out the hacker board (the small green circuit). Attach it into the lid's rectangular indentation.
  4. Locate the "+" and "-" ports (i.e., holes) on the hacker board. Plug the cathode's wire (orange) into the "+" port and the anode's wire (green) into the "-" port.
  5. Locate ports 1 and 2 on the hacker board. Plug the blue capacitor (the small, cylindrical item with two longer metal prongs) into these ports. The blue capacitor's longer prong should go into port 1 and the shorter prong into port 2. Note: The orientation of the capacitor is important. If you reverse the short and long prongs, it will not work and might even damage the capacitor. You may need to bend the capacitor's longer end slightly so that the capacitor's prongs fit into the ports well.
  6. Plug the red LED below the capacitor into ports 5 and 6. (Ports 3 and 4 will remain empty.) The LED's longer prong should go into port 5 and the shorter prong into port 6. Important: The LED will only work if inserted in this orientation. If you accidentally reverse the prongs, the LED will not light up. You may need to bend the LED's longer end slightly so that the LED's prongs fit into the ports well.
  7. Make sure that the wires, capacitor, and LED are all securely in place. The assembled hacker board on the top of the MFC should now look like Figure 6.
Close-up of the top of a completely assembled microbial fuel cell.
Figure 6. When you have finished assembling your MFC and its hacker board, the top should look like the one in this image.
  1. After assembling the first MFC, set up the second microbial fuel cell using the rest of the prepared soil from step 2.
  2. Once both microbial fuel cells are assembled, set the MFCs indoors, at normal room temperature (about 19–25° Celsius [C], or 66–77° Fahrenheit [F]), in a place where they will not be disturbed. The MFCs should remain in the same location the entire time after you set them up because if they are moved, this could disrupt the growth of the bacteria. It should take about 3–7 days before the red LEDs on the hacker board start blinking, but you will start taking measurements before that, as described in the next section. Note: Temperature variations can cause changes in the power output of the microbial fuel cells due to different bacteria activities.
  3. Now you can start doing your initial bacteria count using the plate count method. To do that, put your gloves on, take the cup with the set aside topsoil that you prepared in step 2.e. and continue as follows.
    1. Prepare three mini cups for the serial dilution and label them 10-2, 10-4, and 10-6. Note: 1 g of soil contains millions or even trillions of individual bacteria. Because these are impossible to count, you will do a dilution series of your soil in order to get a small number of colonies on each plate.
      1. Disinfect the area that you are working on with rubbing alcohol.
      2. Sterilize a metal spoon by dipping it in rubbing alcohol, shaking off the excess alcohol, and holding it into a flame (from a candle or gas stove). Wait until the flame on the spoon burns out and let the spoon cool down.
      3. Fill each cup with 49.5 mL of distilled water. You can use the digital scale to fill the cups with water until they weigh 49.5 grams.
      4. Weigh 0.5 g of your prepared soil and add it to the cup that says 10-2. Use a sterilized spoon to stir the mixture well.
      5. With a transfer pipette, take 0.5 mL of the soil water mixture and add it to the cup that says 10-4. Mix with a sterilized spoon.
      6. Using the same transfer pipette, take 0.5 mL from the 10-4 cup and add it to the 10-6 cup. Stir the solution with a sterilized spoon.
    2. Take two of the nutrient agar plates and label them with the date, the sample volume, and the dilution factor. You will plate 0.5 mL of the 10-4 dilution and 0.5 mL of the 10-6 dilution.
      1. Pipette 0.5 mL of the 10-4 dilution onto the surface of the nutrient agar plate.
      2. Disinfect the cell spreader by dipping it into rubbing alcohol, shaking off the excess alcohol, and igniting it in a flame (candle or gas stove). Let the alcohol on the spreader burn off and wait until the spreader cools down.
      3. Spread the liquid over the surface of the agar with the spreader until it has soaked into the agar and the surface is mostly dry. Then close the lid of the plate.
      4. Disinfect the spreader again and repeat the inoculation using the second plate with 0.5 mL of the 10-6 dilution.
      5. Keep all your dilution cups, as well as the rest of the soil, in the refridgerator with a closed lid until you do the plate count. Make sure all your cups are clearly labeled and kept away from food.
    3. Incubate the plates, inverted, for 24–48 hours in a warm place. Make sure to keep the same incubation time for all your plates throughout the experiment.
    4. After incubation, count the number of colonies on both plates and record the numbers in your lab notebook. The number of bacteria you will find on your plates will depend on the soil you chose. An example is shown in Figure 7. Note: Only count plates that have between 25 and 250 colonies on them. If there are less than 25 or more than 250 colonies on both of the plates, you need to re-plate from a different dilution (depending on your results, this could be 10-3,10-5, or 10-7). You can use the leftover soil or prepared dilutions (10-2,10-4, and 10-6) that you kept in the refridgerator to make other dilutions.
bacteria plate
Figure 7. Count the number of bacteria colonies on each of your nutrient agar plates.

Measuring Power Output and Adding Sugar

You will measure the power output for both of your microbial fuel cells every day. Once the power output seems to have stabilized, you will add a small amount of sugar to one of the microbial fuel cells and use the other one as a control. The sugar should change the power output of the MFC. How do you think the MFC's power output will be affected?

  1. One day after setting up your microbial fuel cells, check to see if the LEDs are blinking. Most likely, they will not be, but check to make sure. Watch the LEDs for 2 minutes to see if they are blinking.
    1. If one or both of the LEDs are blinking, time how many seconds apart the blinks for each one are.
      1. To do this, start a stopwatch as soon as you see the LED blink and stop the stopwatch when the LED blinks again.
      2. If the LED is blinking faster than once every 5 seconds, do not time the seconds between blinks, but instead time the blinks per second. Time a 10-second interval and count how many times the LED blinks in this period and then divide this by 10 to get blinks per second.
      3. Repeat step 1.a.i. or 1.a.ii. two more times so you have three total counts.
      4. Record your results for each MFC in your lab notebook in a data table like Table 1. (If you counted blinks per second, as in step 1.a.ii., change the heading from "Seconds Between Blinks" to "Blinks per Second.") Calculate the average for your three counts and record that, too.

  Seconds Between Blinks
Day Count #1 Count #2 Count #3 Average
1     
2     
3     
Table 1. Each day, check both MFCs to see if the LEDs are blinking. If they are, record how many seconds elapse between the blinks (or how many blinks there are per second) for each, making three separate counts. Record the results for each microbial fuel cell in a data table, like this one, in your lab notebook.

  1. Next, measure the power output for both of the MFCs using the multimeter that comes with the Microbial Fuel Cell kit. If you need help using a multimeter, consult the Science Buddies reference How to Use a Multimeter, as well as the instructions that came with your multimeter.
    1. To measure the MFC's power output, remove the capacitor and LED from the hacker board. Then remove the orange wire from the "+" port and plug it into port 3. This means that the orange wire should be in port 3, the green wire should still be in the "-" port, and all other ports should be empty.
    2. Place a resistor between ports 5 and 6, as shown in the slideshow. For the resistors, the orientation does not matter.
      1. Several resistors come in the MFC kit. Start with the largest-capacity resistor, which will be a 4.7 kΩ resistor. (Ω, the capital Greek letter Omega, is the symbol for ohms, the unit used to measure resistance. 1 kilo-ohm, or 1 kΩ, is 1000 ohms.)
      2. Resistors' values are labeled using color-coded bands. Use Figure 8 to determine the resistance for each resistor.
        1. Tip: If you want, you can confirm the resistance of any resistor using your multimeter by setting it to measure resistance (usually a "Ω" symbol for ohms) and connecting the multimeter's leads on the wire ends of the resistor.
        Resistor chart
        Figure 8. Use this resistor color chart to determine the resistance for each resistor.

      3. Leave the resistor plugged in for 5 minutes.
    3. After the resistor has been plugged in for 5 minutes, use your multimeter to measure the voltage across the resistor.
      1. Make sure the multimeter's black wire is plugged into the "COM" port and its red wire is plugged into the "VΩMA" port on the multimeter.
      2. Set the multimeter to measure DC voltage. This is marked as "V" with a straight line next to it. Specifically turn the dial to "2000 m."
      3. Clip the multimeter's red lead to the resistor's metal wire that is plugged into port 5. Then clip the multimeter's black lead to the resistor's metal wire that is plugged into port 6, as shown in the slide show. Read the multimeter's screen to see what the voltage is (in millivolts [mV]).
      4. If the voltage seems to be changing a little—such as decreasing slightly over the period of a few seconds—watch the readings on the multimeter for a few seconds more until they stabilize (and stay the same for a few seconds). Use the stabilized value.
        1. If the readings are still changing after several seconds, or if your readings are 0 mV, make sure all of the wires are correctly and securely plugged into the circuit (both the cathode and anode wires, and the resistor's wires), disconnect the multimeter's leads from the resistor, and come back in another 5 minutes. Then repeat step 2.c.
      5. Record your results for both of the microbial fuel cells in your lab notebook in a data table like Table 2.

      Day and Time:
      Resistance (ohms) Voltage (mV) Power (µW)
      4700   
      2200   
      1000   
      470   
      220   
      100   
      47   
      Table 2. In your lab notebook each day, create a data table like this one for each microbial fuel cell to record your voltage measurements. Do not forget to write down the date and the time you started taking measurements on the top line.

    4. Disconnect the multimeter's clips from the resistor. Remove the resistor.
    5. Repeat steps 2.b.–2.d. until you have tested the MFC with all of the resistors in the kit. Start with the resistor with the largest resistance value and end with the resistor with the smallest resistance value.
  2. Once you have finished taking your voltage measurements for the first MFC, plug the wires, the capacitor, and LED back into the hacker board, as described in steps 16–18 in the Setting Up the Microbial Fuel Cells and Bacteria Count section and repeat the voltage sweep (step 2 and 3) for the second microbial fuel cell.
  3. Calculate the power output (in microwatts, or µW) for each resistor and both microbial fuel cells. You can calculate this by using a derivation of Ohm's law, as described in the Introduction, Equation 1.
    1. Note: It is important to convert the voltage measurements into power output measurements. The power output depends on the resistors you use, so you cannot determine how well the MFC is performing by just looking at the voltage measurements alone; they need to be converted into power for them to be meaningful.
    2. To use Equation 1, you will need to convert your voltage readings from millivolts (mV) to volts (V). To do this, divide the millivolt values by 1000 to give you volts. For example, if you had a voltage reading of 45 mV, this would equal 0.045 V.
    3. Using Equation 1, your answer will be in watts (W). Convert watts to microwatts by multiplying your answer by 1,000,000.
    4. Once you have calculated it, record the power for each resistor in the data table (such as Table 2) in your lab notebook.
  4. Determine what the peak power of your MFCs is.
    1. In your data tables for both microbial fuel cells, look at the power produced using each resistor. The peak power is the highest power produced by any of the resistors.
    2. If you want to visualize this, you can plot your data for the day on a graph, putting the resistance of the resistors on the x-axis (horizontal axis) and the power on the y-axis (vertical axis) for both MFCs. A sample graph is shown in Figure 9.
      1. You should see a curve, with the peak power at the top of the curve, as shown in the sample graph.
    3. Make a note in your lab notebook of what the peak power for both MFCs is each day, by circling or highlighting this value in your data table.
    4. Although it will not be explored in this science project, you might like to know that the peak power tells you what the internal resistance of your MFC is. The resistor that gives the peak power is closest to the internal resistance of the MFC. This may change a little over time.
Sample graph showing resistance vs. power using the microbial fuel cell.
Figure 9. This sample graph shows possible power output data using the resistors with the MFC. In this sample, the peak power was found using a 1 kΩ resistor, and the peak power is about 45.5 μW.
  1. Repeat steps 1–5 each day until it looks like the power output (the peak power) for both of your microbial fuel cells is stabilizing.
    1. Take these measurements around the same time every day. This will limit variables affecting your results (such as changes in temperature).
    2. For step 1, it should take 3–7 days for the LEDs to start blinking. However, even if the LEDs never blink, you may still be able to do this science project; be sure to continue to take the power output measurements every day.
      1. Tip: See the Frequently Asked Questions section for what to do if the LEDs do not blink, or if they were blinking and unexpectedly stopped blinking.
    3. For step 2, you should see the power output slowly increase.
      1. For each day, make a data table like Table 2 for each MFC in your lab notebook in which to record your results, and use them to determine the peak power.
    4. After about 7–14 days, the power output should stabilize.
      1. It may stabilize anywhere between 10 μW and 200 μW or more. A lot depends on the topsoil you are using, and other factors. Wherever it stabilizes, it should make enough power to blink the LED at least once every 30 seconds.
        1. Tip: If the power output seems low, see the Frequently Asked Questions section for suggestions on what to check and try.
      2. When it stabilizes, the peak power should not change by more than about 10% for at least three days in a row.
        1. Do not worry if your peak power changes by a little more than this. If it has been at least 14 days and when you graph the peak power (as described in the step 6.d.iii) it looks like it is stabilizing (meaning, it is not steadily increasing or steadily decreasing from day to day), then it has probably stabilized enough.
        2. Keeping this in mind; if it still does not look like your peak power is stabilizing, see the Frequently Asked Questions section for suggestions on what to check and try.
      3. Tip: Making graphs of your data as you collect it for both MFCs may help you see if the power output is stabilizing. If you do this, put the date on the x-axis and the power output (peak power) for each day on the y-axis. Does the peak power appear to be stabilizing?
      4. The time between LED blinks should also stabilize.
  2. Once it appears that the power output has stabilized in both MFCs, carefully open up both microbial fuel cells and add sugar solution to the one you labeled "sugar treatment," and distilled water to the one you labeled "control."
    1. Take your measurements for the day, as usual, before starting the treatments.
    2. On the digital scale, weigh 0.5 g of sucrose (table sugar) into a mini cup and add 10 mL of distilled water. Mix with a sterilized spoon to dissolve the sugar completely.
    3. Prepare another mini cup and only add 10 mL of distilled water.
    4. Unplug the anode and cathode from the hacker board and carefully remove the lid of the first microbial fuel cell, which you labeled "sugar treatment."
    5. Put your gloves on and gently lift up the cathode, being careful not to get any mud on top of the cathode.
      1. Safety Note: Remember, the MFC's electrodes are made of a conductive material called graphite fiber. Do not put the cathode near electronics or power plugs, and do not disperse the fibers in the air, as the fibers will cause electrical shortages when they come in contact with electronics.
    6. Take a sterilized spoon and use it to dig into the top 1–2 cm of the soil. Be careful not to dig all the way down to the anode, as this disturbs the anode biofilm. Scoop about 1 teaspoon of soil into a fresh mini cup, labeled with time, date, and microbial fuel cell type (sugar treatment or control) for the bacteria plate count.
    7. After you have taken the soil sample, use a clean transfer pipette to suck up some sugar solution from the cup and spread it on top of the mud, mix it in, and then dig around inside of the mud, adding all of the sugar solution and mixing it in as you go. You want to mix the sugar as evenly into the mud as possible, but do not disturb the anode —leave it in place (as well as its wire and the mud below the anode).
      1. Note: Disrupting the anode could damage the developing biofilm and interfere with your results.
    8. Wait 5 minutes to let the sugar soak into the mud a little. (If you immediately put the cathode back on the mud, the cathode will likely soak up much of the sugar solution). Flatten the mud surface with a spoon again before you re-assemble the MFC.
    9. Then assemble the MFC exactly as you put it together before, following the instructions from the "Setting Up the Microbial Fuel Cells and Bacteria Count" section to make sure that the wires are twisted together properly and everything is reconnected to the hacker board correctly.
      1. Specifically, this will be following steps 16–18 from the previous section.
      2. Do not get any mud on the top of the cathode. If you do, carefully wipe it off, being careful not to grind it into the cathode.
    10. Repeat step 7 with the "control" microbial fuel cell, but this time add 10 mL of just distilled water without the sugar.
  3. Use the bacteria plate count method again to measure the numbers of bacteria in both microbial fuel cell soil samples, following step 22 of the Setting Up the Microbial Fuel Cells and Bacteria Count section above.
    1. Note: Remember that depending on your previous bacteria counts, you will have to adjust the dilutions that you plate on the nutrient agar plates. In this step you probably do not have to plate the 10-4 dilution but rather 0.5 mL of the 10-6 and 10-7 (or even up to 10-10) dilution.
    2. Again, keep the soil as well as the dilutions in the fridge until you have counted the bacteria in case you get too few or too many colonies and need to re-plate.
  4. Starting the day after you add the sugar, continue monitoring the power output of both microbial fuel cells by repeating steps 1–5 each day until it looks like the power output (the peak power) of both microbial fuel cells is stable again (as described in step 6). Shortly after it stabilizes, the power output may then clearly change again.
    1. Take these measurements at the same time every day.
    2. For each day, for each MFC, make a data table like Table 2 in your lab notebook in which to record your results, and use them to determine the peak power. Note: It is possible that the resistor you use to determine the peak power will change slightly. Make a note of this in your lab notebook if it happens.
    3. Depending on the exact conditions of your experiment—which can be affected by the soil you use—the power output could take anywhere from several days to over a week to stabilize after adding the sucrose.
      1. When the power output is stabilized, the peak power may not change by more than about 10% for at least three days in a row. How much the power varies when it is "stabilized" can depend on the amount of power being produced by the MFC.
      2. Overall, if it has been about two weeks after adding the sugar and the peak power has not been steadily decreasing or increasing each day for at least the last week, then it has probably stabilized enough.
      3. After it has stabilized, the power output may then clearly change again (steadily decreasing or increasing each day).
      4. Tip: Making a graph of your data as you collect it for each MFC may help you see if the power output is stabilizing. If you do this, put the days on the x-axis and the power output (peak power) on the y-axis. Does the peak power appear to be stabilizing?
      5. The time between LED blinks should also stabilize.
  5. Once the power output stabilized for both of the microbial fuel cells, take a final sample for the bacteria plate count following step 22 of the "Setting Up the Microbial Fuel Cells and Bacteria Count" section above.
    1. Note: Remember that depending on your previous bacteria counts, you will have to adjust the dilutions that you plate on the nutrient agar plates. In this step, you probably do not have to plate the 10-4 dilution, but rather 0.5 mL of the 10-6 and 10-7 (or even up to 10-10) dilution.
    2. Again, keep the soil, as well as the dilutions, in the refridgerator until you have counted the bacteria in case you get too few or too many colonies and need to re-plate.

Analyzing Your Results and Continuing Explorations

  1. Make two graphs of your data for each microbial fuel cell, one showing how the power output changed over time and one showing how the frequency of LED blinks changed over time.
    1. For the graph showing power output over time, put the number of days after setting up the MFC on the x-axis and the peak power output (in μW) on the y-axis.
    2. For the graph showing the frequency of LED blinks over time, put the number of days after setting up the MFC on the x-axis and the blinks per second on the y-axis.
      1. If you recorded the time between blinks in your data table, convert this to blinks per second by taking the data for the average seconds between blinks that you collected each day and calculate what 1 divided by this number is. For example, if your LED blinked an average of once every 15 seconds, 1 divided by 15 is 0.067, which is the number of blinks per second it made.
  2. Analyze your graphs.
    1. What happened to the power output and frequency of the LED blinks after adding the sugar? Was there an increase or decrease in power output? How large was the change? How did the power output compare to the control MFC?
    2. How quickly did the measurements stabilize after adding the sucrose? When they stabilized, were they higher or lower than they were originally, before adding the sugar? Were they higher or lower than the power output of the control MFC?
    3. When was the power output and blinking frequency the highest for each of the MFCs? What was their peak power at this time?
    4. Based on your results, do you think it would be feasible to use sugar as substrate to help generate power? Why or why not?
  3. Look at the numbers of bacterial colonies that you counted for each time point in your treated microbial fuel cell and control. Calculate the number of bacteria in 1 g of that soil using Equation 2.

    Equation 2:

    You can graph the number of bacteria in a bar graph, showing the time (initial, before and after treatment) on the x-axis, and the number of bacteria on the y-axis for both MFCs. Did the number of bacteria increase, stay stable, or decrease over time? Did the addition of sugar change the number of bacteria compared to the control?
  4. At this point in your experiment, there are several ways to continue your explorations. Check out the Variations section for some ideas.

Troubleshooting

For troubleshooting tips, please read our FAQ: How Do Bacteria Produce Power in a Microbial Fuel Cell?.

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Variations

Note: Carefully clean your microbial fuel cells before you start a new experiment. We recommend using tap water to rinse the electrodes while gently rubbing them (with gloves on) until the dirty water runs clear. Also completely rinse out the vessel with tap water. See the Frequently Asked Questions for details.
  • In this science project, you tested how the addition of table sugar (sucrose) affects the power output of the MFC. Do other food sources result in a similar outcome and what happens if you add even more sucrose? What about other sugars, such as glucose, or inorganic substrates, such as carbonates or nitrates? Can you change the power output by feeding the bacteria inorganic and organic substrates in the form of waste products? Think of examples such as compost, cow manure, or even pee (see the Science Buddies science project Powered by Pee: Using Urine in a Microbial Fuel Cell for more details. Hint: You may want to look into what the soil bacteria (discussed in the Introduction) like to eat and the environments they thrive in.
  • Is adding a food source the only way to improve the power output of a microbial fuel cell? How does changing other factors affect the MFC's power output? Investigate other parameters, such as temperature or soil conductivity and soil moisture. Develop a well-reasoned hypothesis for what you think will happen when this factor is changed, and create a way to test it. What factor has the biggest affect on power output? If your MFC is generating between 80 μW and 199 μW, it is doing really well, and if it is generating 200 µW or more, it is doing amazingly well! An example for investigating soil conductivity is given in the Science Buddies science project Spice Up the Power of a Microbial Fuel Cell with a Dash of Salt.
  • Do you think you counted all bacteria that were present in the soil or did you miss some of them? A common saying in science is: "you only get what you are looking for." Think about what type of bacteria you were looking for in this experiment and consider if all bacteria can grow under the conditions tested. What about the electrogenic bacteria? Shewanella bacteria can be aerobic [they need oxygen] or anaerobic [they grow without oxygen], and Geobacter bacteria are anaerobic, which means they only grow without oxygen present. Knowing this, do you think you were able to count all Shewanella and Geobacter in your experiment? If not, what could you do to improve the growth of electrogenic bacteria? Do you think that anaerobic incubation of your agar plates would make a difference? Do some research and find out a way to optimize culturing conditions for a better count of Shewanella and Geobacter numbers.
  • How well do different topsoils work in the MFC? Try collecting some different types of soils from different locations (such as from a yard or park compared to the bottom of a stream), or purchasing different soils. Think about what differentiates the different soils and how this could affect the MFC power output. Test two different soils at a time, over a set period of time. Which soil is best, resulting in the MFC generating the most power? Which soil is worst? Which soil contains the most bacteria, substrates, or nutrients? If multiple soils work well, what do they have in common? What factors do you think help make a soil "good" or "bad" for an MFC? Check out the Science Buddies science project Turn Mud into Energy With a Microbial Fuel Cell for more details. Note: If you use a soil with perlite, foam balls, or vermiculite pieces, strain these particles out before using the soil, as they may damage the MFC or severely inhibit bacterial growth.

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Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.
One easy way to figure out how to troubleshoot your experiment is reviewing the flow chart in Figure 10. You will find more detailed information in the FAQ section below.
flowchart setup microbial fuel cell
Figure 10. Flow chart for troubleshooting the microbial fuel cell experiments.
Q: The LED on the microbial fuel cell has never blinked. What should I do?
A: Even if the LED is not blinking, you should still take and analyze your peak power output measurements and you may be able to continue with the experiment, as described in the Measuring Power Output and Adding Sugar section in the Procedure. If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment. It is possible that the microbial fuel cell is making enough power for you to continue with the experiment even though the LED is not blinking. There are several reasons why the LED might not be blinking:
  • If the hacker board is not set up correctly, the LED will not blink. You should confirm that everything is set up as described in the Setting Up the Microbial Fuel Cells and Bacteria Count section of the Procedure.
  • The LED might not have enough power to blink. You will need a peak power output of at least 4 μW before the LED starts to blink, and the initial blinking may only be about once every 30 seconds, so you will need to closely watch the LED to see it blink.
  • Depending on the type of topsoil you are using, it may take about 3–7 days for the LED to start blinking, if everything is properly in place. Some types of topsoil might make the microbial fuel cell take longer to blink than other types of topsoil because it may take longer for enough power to be made to light up the LED. Make sure that you are not using any topsoil with little white foam balls, vermiculite pieces, or perlite, since these can aerate the soil and inhibit the desired bacteria from growing (they do not want to be exposed to oxygen). Also avoid types of topsoil with peat moss or topsoil that has become completely dry.
  • The temperature of the room that the microbial fuel cell is in can significantly affect how well the bacteria grow, which affects whether (and how quickly) the LED blinks. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° Celsius [C], or 66–77° Fahrenheit [F]), in the same location the entire time after you set it up. If the room gets too cold, the bacteria may not grow well. If the microbial fuel cell is moved to a different location, this could disrupt the growth of the bacteria.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions, and/or gently jiggling the wires around in the slots in the hacker board. Also make sure that none of the exposed parts of the wires are touching each other.
  • It is possible that the LED has become damaged. You can check to make sure the LED is working by hooking it up to a battery, as shown in Figure 5 of the science project idea: See the Light by Making a Cell Phone Spectrophotometer.
Q: The peak power output of the microbial fuel cell seems low. What might be the problem and what can I do?
A: If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment, as described in the Measuring Power Output and Adding Sugar section in the Procedure. It may take about 7–14 days for the peak power output to reach at least 4 μW, depending on the topsoil and other conditions used. If your peak power output does not reach at least 4 μW by 7–14 days after setting up the microbial fuel cell, there are several possible reasons for this:
  • If the hacker board is not set up correctly, it will not produce power. You should confirm that everything is set up as described in the Setting Up the Microbial Fuel Cells and Bacteria Count section of the Procedure.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions. Also make sure that none of the exposed parts of the wires are touching each other.
  • If it looks like the peak power output is still increasing each day, it may just take a few more days for the microbial fuel cell to reach 4 μW.
  • The soil you are using might not work well in the microbial fuel cell. First, be sure that you are using topsoil, and not potting soil. Make sure that you are not using any topsoil with little white foam balls, vermiculite pieces, or perlite, since these can aerate the soil and inhibit the desired bacteria from growing (they do not want to be exposed to oxygen). Also avoid types of topsoil with peat moss or topsoil that has become completely dry.
  • If there are air bubbles trapped in the damp topsoil in the microbial fuel cell, this can prevent the bacteria from growing well because they do not want to be exposed to oxygen. When packing the mud in the microbial fuel cell, pat down the mud and electrodes, as described in the Setting Up the Microbial Fuel Cells and Bacteria Count section of the Procedure, so that you do not have any trapped air bubbles in the mud. Air bubbles may also be caused by small, hard particles (such as rocks, pebbles, vermiculite, twigs, etcetera) being trapped in the mud, and so anything like this should be removed from the topsoil before adding it to the microbial fuel cell.
  • Check if your soil in the microbial fuel cell is still moist. The soil needs to be moist for good electron conductivity and for the bacteria to grow. If your soil is dry, carefully open the microbial fuel cell and mix distilled water into the soil until it is moist again. However, make sure not to disturb the anodic biofilm.
  • If the temperature of the room that the microbial fuel cell is in is very cold, the bacteria may not grow well, which would decrease the peak power output. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° C, or 66–77° F), in the same location the entire time after you set it up. Also, if the microbial fuel cell is moved to a different location (particularly if it is at a different temperature), this could disrupt the growth of the bacteria.

Even if the microbial fuel cell is not making more than 4 μW, this may still be an interesting result, especially if you are testing conditions different from the ones described in the main project idea. Keep in mind that a science project does not have to work as anticipated in order to be real, good science. It is important to communicate (on your Science Fair Project Display Boards, report, or however you are presenting your project) your question, your hypothesis, what you anticipated would happen, then what you actually saw (such as your peak power output measurements over time), your questions about what was happening, and your attempts to troubleshoot. You could also communicate any possibilities of what could be happening and how you would test that (if you do not have enough time to test it now).

Q: Is the voltage made by my microbial fuel cell too low?
A: When you take the voltage measurements with the different resistors each day, the voltage measurements need to be converted into power (in microwatts, or μW). This is described in the Procedure in the section Measuring Power Output and Adding Sugar. The power output depends on the resistors you use, so you cannot determine how well the microbial fuel cell is performing by just looking at the voltage measurements alone; they need to be converted into power for them to be meaningful. Once you have converted the voltage measurements into power outputs and determined your peak power output, you can determine whether the peak power output seems low or not. If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment, as described in the Measuring Power Output and Adding Sugar section in the Procedure. It may take about 7–14 days for the peak power output to reach at least 4 μW.
Q: The LED on the microbial fuel cell was blinking, but now it has stopped and this is unexpected. Why did this happen?
A: Even if the LED stops blinking, you should still continue to take and analyze your peak power output measurements and you may be able to still do the experiment, as described in the Measuring Power Output and Adding Sugar section in the Procedure. If the LED was blinking one day but then stopped blinking the next, there are several possible reasons for this:
  • If the hacker board is not set up correctly, the LED will not blink. You should confirm that everything is still set up correctly, as described in the Setting Up the Microbial Fuel Cells and Bacteria Count section of the Procedure.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions, and/or gently jiggling the wires around in the slots in the hacker board. Also make sure that none of the exposed parts of the wires are touching each other.
  • The LED may no longer have enough power to blink. You will need a peak power output of at least 4 μW, for the LED to blink, and the blinking may be as infrequent as about once every 30 seconds, so you will need to closely watch the LED to see it blink.
  • If the temperature of the room that the microbial fuel cell is in is very cold, the bacteria may not grow well, which would decrease the peak power output and affect whether the LED blinks. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° C, or 66–77° F), in the same location the entire time after you set it up. Also, if the microbial fuel cell is moved to a different location (particularly if it is at a different temperature), this could disrupt the growth of the bacteria.
  • It is possible that the LED has become damaged. You can check to make sure the LED is working by hooking it up to a battery, as shown in Figure 5 of this science project idea: See the Light by Making a Cell Phone Spectrophotometer.
  • Check if your soil in the microbial fuel cell is still moist. The soil needs to be moist for good electron conductivity and for the bacteria to grow. If your soil is dry, carefully open the microbial fuel cell and mix distilled water into the soil until it is moist again. However, make sure not to disturb the anodic biofilm.

Keep in mind that a science project does not have to work as anticipated in order to be real, good science. You should always record your observation.

Q: The peak power output does not seem to be stabilizing. Should I be concerned?
A: About 7–14 days after setting up the microbial fuel cell, the peak power output should stabilize to around at least 4 μW. For the peak power output to be considered "stabilized," it should not change by more than about 10% for at least three days in a row. But even if your peak power output changes by a little more than this, do not worry; if it has been at least 14 days and the peak power looks like it is stabilizing when you graph it each day (meaning, it is not steadily increasing or steadily decreasing from day to day), then it has probably stabilized enough. Keeping this in mind, if it still does not look like the peak power output is stabilizing, this could be because some conditions have changed (such as, the temperature has changed or the fuel cell has been moved to a different location). It is very important that conditions are kept as consistent as possible for the peak power output to stabilize and to do the rest of the experiment using the fuel cell, as minor changes in conditions can significantly affect the power output.

If the power output does not seem to be stabilizing after the addition of sugar, try waiting a little longer. It could take about two weeks for the power output to stabilize after adding sugar. If it has been about two weeks after adding sugar and the peak power is not steadily decreasing or increasing each day, then it has probably stabilized enough.

Q: After I set up the microbial fuel cell, the peak power output was increasing, and then decreased before I added any sugar. Should I be concerned?
A: Within the first 7–14 days of setting up the microbial fuel cell, it is normal to see some fluctuations in the peak power output. This may include seeing the peak power output increasing, and then decreasing a little. After about 7–14 days, the peak power output should appear to stabilize. If this does not appear to happen, refer to the other relevant questions in this FAQ.
Q: What is the purpose of using the different resistors?
A: You need to use multiple resistors so that you can determine what the peak power output of your microbial fuel cell is. The power output depends on which resistors you use, and you need to determine which resistor gives you your peak power output, as described in the Measuring Power Output and Adding Sugar section of the Procedure. The resistor that gives the peak power output is closest to the internal resistance of the microbial fuel cell, and this depends on the type of topsoil you are using and other factors.
Q: Why do I need to wait until the peak power output has stabilized before adding the sugar?
A: If you do not wait for the peak power output to stabilize before adding sugar then you will not know how the addition of sugar changed the peak power output because it would have still been fluctuating. You would not be able to know for certain whether any changes in peak power output you see, after adding the sugar, would then be due to normal fluctuations of the microbial fuel cell or actually to the addition of the sugar.
Q: Why is sugar added to the microbial fuel cell?
A: As discussed in the Background, sugar is a carbon source for many bacteria, and when the sugar is added to the soil, these carbohydrates should be transferred to the bacteria in the soil. The bacteria should consume the sugar and grow. Growing bacteria should produce more electricity in the MFC. This means that initially when sugar is added to the MFC, there should be an increase in the peak power output, but eventually if the sugar is used up, the peak power output will decrease again.
Q: During the voltage sweep the multimeter readings fluctuate a lot and do not seem to stabilize. Should I be concerned?
A: It is important to leave the resistor plugged in at least 5 minutes before you take the voltage measurements. It is normal to see the voltage readings decreasing slightly over the period of a few seconds. In this case, watch the readings on the multimeter for a few seconds more until they stabilize (and stay the same for a few seconds). Use the stabilized value. If the voltage readings fluctuate randomly over a large voltage range, check your wire connections. If your microbial fuel cells already run for a long time (longer than 4 weeks), there might also be the chance that the internal connections inside the hacker board got worn out and the hacker board is damaged. In this case, you need to replace the hacker board or end your experiment.
Q: Why are there no bacteria growing on my nutrient agar plates?
A: After preparing your soil dilutions, as described in the Setting Up the Microbial Fuel Cells and Bacteria Count section of the Procedure and plating your soil samples, you should see bacteria colonies coming up after incubating the nutrient agar plates for about 48 hours at room temperature. If you do not see any bacterial growth, there are several possible reasons for this:
  • The soil solution that you plated on your nutrient agar plate is too diluted so there are actually no bacteria present in the sample you plated. Make a lower dilution of your soil sample and plate it again.
  • The incubation conditions for your bacteria are not ideal. It is important to keep the plates in a warm place between 19–25° C, or 66–77° F. If it is too cold, some bacteria will not grow or it takes longer for them to form visible colonies.
  • The incubation time is too short. Most bacteria will need at least 24-48 hours at room temperature to form visible colonies. Make sure that you wait at least that long before you count your colonies. If you still do not see any colonies after 48 hours, incubate the nutrient agar plates for another 24 hours to give the bacteria more time to grow.
  • Though unlikely, the nutrient agar plates could be the reason for no bacterial growth. Make sure to keep them in the refridgerator once you receive them to keep them hydrated and do not use plates that show any signs of molding.
Q: I have big and tiny bacteria colonies on my nutrient agar plates. Do I count all of them?
A: For your bacteria plate count you do count all visible colonies that are present on your plate, even if these are tiny. If your nutrient agar plate has too many colonies on it (more than 250), use a lower concentrated dilution of your soil sample for your bacteria plate count. The ideal number of colonies on one plate is between 25–250.
Q: Is there a way to clean out the microbial fuel cell without damaging it so that I can use it in another experiment?
A: Yes, the microbial fuel cell can be cleaned out without damaging it. To do this, carefully rinse the vessel and electrodes with tap water. The electrodes should be gently rubbed while rinsing them until the water running off of them is not dirty anymore. Also, to make sure you are starting with a clean slate of bacteria for each test, the vessel and electrodes should be quickly rinsed with 70% isopropyl rubbing alcohol, which should kill the bacteria. To do this, you can put the electrodes in the vessel (using it as a cup) and fill it with just enough alcohol to submerge the electrodes. Get some of the rubbing alcohol on the inside sides of the vessel while you do this, too. Then pour off the rubbing alcohol and briefly rinse the electrodes and vessel with tap water (so the rubbing alcohol does not remain on these parts). Make sure to follow all proper safety precautions when using the rubbing alcohol.

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fuel cell engineer with fuel cell model

Fuel Cell Engineer

Most of the world's energy comes from fossil fuels. However, the amount of fossil fuels is finite, and many people are concerned about where our energy will come from in the future. We can turn to alternative, renewable sources of fuel, such as our sun (solar energy) and the winds (wind energy). But what happens when the sun doesn't shine or the winds don't blow? Would we be stuck? Well, that is where the fuel cell comes in. A fuel cell is an electrochemical device that generates electricity through a reaction between a fuel, like hydrogen, and an oxidant, like oxygen. This reaction produces few greenhouse gas emissions other than water or water vapor. The job of the fuel cell engineer is to design new fuel cell technology that improves the reliability, functionality, and efficiency of the fuel cell. Do you like the idea of using your math and science skills to work on mankind's future energy needs? Then start "fueling your future" and read more about this career. Read more
electrical engineer aligning laser

Electrical & Electronics Engineer

Just as a potter forms clay, or a steel worker molds molten steel, electrical and electronics engineers gather and shape electricity and use it to make products that transmit power or transmit information. Electrical and electronics engineers may specialize in one of the millions of products that make or use electricity, like cell phones, electric motors, microwaves, medical instruments, airline navigation system, or handheld games. Read more

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