In Bruce Logan’s lab, scientists are using sewage to create electricity. They can use wastewater from households, companies, or farms—virtually any stream with organic material in it. Better yet, the process of harvesting energy from wastewater also cleans it. Logan has been working on this process for years, and the systems he’s developed are getting better at doing this work. One day soon, they could stand alone, with wastewater providing all the power needed for its own sanitation.
An environmental engineer, Logan began with the idea that bacteria can generate electricity. Wastewater has plenty of bacteria, which helps process the organic materials dirtying the water. But when Logan first examined the technology to capture bacteria-generated electricity, the amount of power he could produce from a given volume of wastewater was “very, very, very, very low,” he says.
The bacteria seemed the obvious culprit—perhaps they could be altered to produce more electrical power, he thought. But soon Logan and his colleagues realized that the bacteria were producing much more power than they given it credit for. Instead of fixing the bacteria, the engineers began tweaking the chemistry and physics of the system in which the bacteria were working.
These systems are called microbial fuel cells. To understand how they work, think about The Matrix, Logan suggests. “The premise of the movie was that humans were in these pods, and they were supplying electricity to the machines,” he says. “You and I eat food and generate energy,” and bacteria do the same thing. In aerobic conditions, the electrons they generate latch onto oxygen. But in a microbial fuel cell, the wastewater-dwelling bacteria are deprived of oxygen. Those electrons have to go somewhere, and in the fuel cell, they travel to an electrode. From there, they flow to the other side of the cell, creating an electrical current. On the other side, those electrons find oxygen and protons, with which they can combine. The end results: electricity and water.
Over time, Logan says, he and his colleagues improved the materials, their strategies for building the cells, and their understanding of the underlying microbiology and electrochemistry. Their results improved, but not enough. They kept trying out ideas to improve the cells. And every once in a while, Logan says, “You come up with a really good idea.”
The results of one such idea—to combine this technology with another in an effort to boost power—were published last week in Science. In this particular setup, Logan’s lab combined a microbial fuel cell with reverse electrodialysis, a technique to capture the energy between gradients of salty and fresh water. On their own, neither of these technologies could produce energy efficiently enough. Together, they worked much better, Logan found.
"One of the criticisms we received has been: 'You're taking two technologies that haven't made it on their own and putting them together. Isn't that a formula for disaster?'" he says. “But if you have a wooden stick and try to chop wood, nothing's going to happen. If someone gives you a piece of heavy metal and you try that, nothing’s going to happen. But if you put that stick and heavy metal together, you can split the wood. Sometimes things by themselves aren't as good as they are together.”
In this case, the energy the microbial fuel cell harnesses boosts the reverse electrodialysis system, which channels the power of ions as they move through a series of membranes from salty water to fresh water. The boost from the cell means that the system needs fewer membranes to be effective. That makes it cheaper. It’s also a closed system, so it doesn’t require new inputs. It just means that the amount of power generated by any particular volume of wastewater increases.
Logan’s goal is to eliminate the need to burn coal, oil or gas in order to process wastewater. He’s close: the amount of energy he can generate from a unit of wastewater almost matches the amount needed to process it. “We’re seeing this technology evolve very quickly,” he says. “If in 10 years, I’m telling you the same thing, it’s a problem. But we know a lot more than we did 10 years ago.” To find the energy needed to process wastewater in the United States — “that looked easily within our grasp,” he says. “This is a problem that we can solve.”
Photo courtesy of U.S. Geological Survey