|Fig. 1: A plant microbial fuel cell. (Source: Wikimedia Commons)|
The problems of increasing waste and climate change are some of the biggest of our generation. Scientists are currently working on a potential solution to address both of these problems at the same time. They are working to generate electricity from organic waste. The way this works is using certain types of bacteria to break down organic waste converting the energy of the chemical bonds into electricity. 
There are two types of microbial cells: mediated and mediator free.  Mediated microbial cells use an electrochemically inactive microbial cell along with a mediator. The electrochemically inactive cells are more common than electrochemically active ones. The problem is that mediators can be too expensive or toxic.  There are also mediator-free microbial cells. Mediator-free microbial fuel cells do not need a mediator to transfer electrons to the electrode.  These mediator-free microbial cells can be run on organic waste and convert energy directly from plants. A diagram showing how a plant microbial fuel cell works is shown in Fig. 1. One interesting characteristic of these microbial fuel cells is that they can be modified to produce more hydrogen instead of electricity.  If oxygen is removed and a small amount of voltage is added to the circuit, hydrogen gas can be produced. 
One of the most widely accepted use of these fuel cells is for treating wastewater. Annually the United States spends $25 billion and the electricity cost constitutes 1.5% of used electricity to process this wastewater.  According to professors at Washington University in St. Louis, most of this cost could be saved using microbial fuel cells to treat the wastewater. Microbial fuel cells can produce energy to offset the other energy costs of wastewater treatment. The microbial fuel cell can produce about 0.27 kWh/m3 of wastewater.  Different kinds of wastewater also contain varying amounts of organic matter. This means that the fuel cell for U.S. household wastewater will need to be configured differently than say livestock production wastewater.  These fuel cells would be managed locally and could potentially provide large benefits for less developed nations.
© Wesley Olmsted. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 D. C. Holzman, "Microbe Power!" Environ. Health Persp. 113, A754 (2005).
 D. Lal, "Microbes to Generate Electricity," Indian J. Microbiol. 53, 120 (2013).
 B. E. Logan and J. M. Regan. "Electricity-Producing Bacterial Communities in Microbial Fuel Cells," Trends Microbiol. 14, 512 (2006).
 Y. Dong et al., "A Combined System of Microbial Fuel Cell and Intermittently Aerated Biological Filter for Energy Self-Sufficient Wastewater Treatment," Sci. Rep. 5, 18070 (2015).