We just finished getting all excited about a new Department of Energy project for teasing renewable hydrogen fuel out of municipal wastewater, when along comes Stanford University with a neat little wastewater trick of its own. The Stanford project involves harnessing a curiously evolved trait of exoelectrogenic microbes. These naturally occurring bacteria generate electricity as they react with oxide minerals in their environment, and if you get enough of them together you can organize them into a microbial battery.
We’ve covered similar microbial battery systems elsewhere, but getting microbes to follow orders has proven to be something of a challenge, kind of like herding cats but with microbes. The Stanford team seems to be on a new track so let’s see what they’ve got cooking.
A New Microbial Battery
The Stanford University team would be the first to admit that their microbial battery so far looks like a science fair project, but it could play a major role in the future energy landscape.
Wastewater treatment plants are huge, sprawling affairs that suck up tons of electricity for pumps and other equipment, accounting for about three percent of the electrical load in developed countries. Though wastewater is not very energy dense compared to other forms of renewable energy, on site microbial battery systems could provide enough power to take a significant chunk of that load off the grid.
Microbial battery systems could also be deployed to remediate “dead zones” in coastal and interior waterways that have been overloaded with organic waste from fertilizer runoff among other sources.
With that in mind, here’s how the Stanford microbial battery works.
Exoelectrogenic bacteria live in airless environments. Instead of breathing air, they react with oxide minerals to convert the nutrients in wastewater.
The battery basically consists of a jar of wastewater, including a colony of bacteria, with a positive and a negative electrode.
The negative electrode is engineered with carbon filaments, which serve as electrical conductors. Exoelectrogenic bacteria attach themselves to the filaments by putting out nanowires or “milky tendrils,” which the team observed using a scanning electron microscope.
The nanowires enable the microbes to shed the excess electrons that they produce while digesting food, and the electrons travel through the carbon filaments to the positive electrode.
At the positive electron is a silver oxide node, which gradually reduces to silver as it receives electrons. When the node is removed from the battery, it releases the electrons and converts back to silver oxide.
So far, the team has found that it takes about a day to “charge” the battery. There will be many next steps to a scaled-up prototype, but the simplicity of the system could help accelerate the development process.
Other Routes To Microbial Batteries
Over at the University of Massachusetts, researchers have been taking the genetic engineering route to microbial batteries by tweaking a microbe called Geobacter, which can produce electricity from mud as well as wastewater. When we last checked in, the team had created a strain eight times more efficient than others.
As for the renewable hydrogen from wastewater thing, the project we just covered is the Department of Energy’s teaming of Lawrence Livermore National Laboratory with a company called Chemergy.
That one involves producing hydrogen from wastewater through a chemical process, but you can do something similar with bacteria. One example is Arizona State University, which has figured out a way to make the process more efficient by neutralizing inefficient bacteria. Another is the University of Colorado, where researchers are working on an integrated system for treating wastewater and producing renewable hydrogen.
And let’s not forget the US Navy, which has been tinkering with microbial fuel cells that can scavenge fuel on the go in marine environments.