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To learn about an alternative method for creating electricity, the microbial fuel cell. The goal is to build a microbial fuel cell using a benthic mud sample from a stream and determine if this device can harvest the electrons that the anaerobic bacteria (present in the mud sample) create.

"Gross! What isin 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—anaerobic bacteria—create...only, you'll be using mud, which is much safer to handle than wastewater. If you would like to learn how to reuse and recycle an unlikely substance, this is the science fair project for you!

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. There exist many different microbial fuel cell designs such as one-chambered or two-chambered MFCs. The traditional H-shaped two-chambered microbial fuel cell is made of several components: the two electrodes (the anode and the cathode), a proton-exchange membrane (PEM) and an external circuit. The anode chamber holds the bacteria and organic material in an anaerobic (without oxygen) environment. Here, the anaerobic bacteria consume the organic waste material while extracting electrons from their food source and oxidizing it to carbon dioxide. As part of their digestive process, the bacteria create protons (H+) and electrons (e-). This process is also known as oxidation. The electrons are pulled out of the solution onto the anode and are conducted through an external circuit into the cathode. The cathode chamber holds a conductive saltwater solution. The protons generated by the bacteria travel through the proton-exchange membrane (PEM) or a salt bridge to meet with the electrons at the cathode. The PEM or salt bridge separates the anode and cathode chambers, but at the same time allows protons to move from one electrode chamber into the other. At the cathode, the protons and electrons combine with oxygen to create water. All the processes happening in a microbial fuel cell are summarized in Figure 1. To read more about the basics of electricity, see the Science Buddies Electricity, Magnetism, & Electromagnetism Tutorial.

Diagram of a microbial fuel cell that uses a proton-exchange membrane to allow hydrogen ions to pass between the anode and cathode side of the cell. Incoming waste water brings bacteria which attach to an anode and produce electrons and hydrogen ions. The electrons travel up the anode to an external circuit and flow back down into the cathode. Hydrogen ions pass through a membrane that separates the anode from cathode and react with oxygen on the cathode side of the cell which forms water that exits from the cathode side of the 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, also called electrogenic bacteria carry the electrons they create through digestion of organic material to the electrode. There are several hypotheses how this electron transfer works including the generation of nanowires or direct electron transfer through special outer membrane proteins. However, the detailed mechanisms of how electrogenic bacteria transfer electrons are still not completely understood.

In this science fair project you will build a mediator-less microbial fuel cells. Since working with wastewater samples can be challenging, you will use a mud sample. The 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. Both, the wastewater sample and the benthic zone mud sample contain anaerobic bacteria that are electrochemically active. In fact, wastewater, benthic mud and even simple soil is packed with bacteria that generate electricity when placed in a microbial fuel cell (MFC). Because such bacteria-laden soil and mud is found almost everywhere on Earth, microbial fuel cells can make clean, renewable electricity nearly anyplace around the globe.

Do you think that electricity can be harvested from mud? In the case of this MFC, seeing is believing. Have fun experimenting, and remember that you are working in an area that is contributing to the well-being of our planet Earth.

Terms and Concepts

Microbial fuel cell

Bacteria

Anode

Cathode

Proton-exchange membrane

Anaerobic

Proton

Electron

Electrogenic bacteria

Nanowire

Lower order stream

Benthic

Power output

Resistor

Ohm's Law

Questions

How do bacteria respire organic waste material?

How do different microbial fuel cell designs look like and which one do you think works best?

What is the difference between a mediator and mediator-less microbial fuel cell?

Is the benthic zone an aerobic or anaerobic environment? What about topsoil?

How do you think the bacteria in benthic mud and topsoil compare to one another? Are they likely to contain the same or different bacteria?

Bibliography The following website has a lot of great information about the microbiology and electrochemistry of microbial fuel cells. Illumin. Mercer, Justin. (2010, May 4). Microbial Fuel Cells: Generating Power from Waste.. Retrieved June 28, 2016. 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. Logan, B. (2007, December 19). Microbial Fuel Cell Research. Retrieved August 20, 2008. NASA is interested in reusing human waste. Read this website to learn how! Miller, K. (2004, May 18). Waste Not. Retrieved August 15, 2008. Watch this video in which Prof. Bruce Logan explains the technology and applications of a microbail fuel cell: Youtube (video presented by The American Chemical Society) (2013). Electrifying Wastewater: Using Microbial Fuel Cells to Generate Electricity.. Retrieved June 28, 2016.