By Kevin Wilcox
Researchers have developed a system that could generate power from the microbes in agricultural wastewater lagoons and use that energy to power aeration, which in turn improves the lagoons themselves.
October 28, 2014—A team of researchers at Washington State University (WSU) has successfully tested a microbial fuel cell that collects electrons from microbes during the biological processes that occur in agricultural wastewater lagoons and then converts that to small bursts of electricity that can power an aerator. In tests, this self-powered system improved some measures of lagoon efficiency by more than 50 percent.
Wastewater lagoons used by rural communities and large farms must be aerated to keep them healthy. Now a team of researchers has determined that the microbes present in such lagoons could generate enough energy to power small bursts of such aeration. Wikimedia Commons/Peter Facey
Lagoons are a common wastewater treatment option in rural and industrial farming. Lagoons without aerators release the greenhouse gasses methane and carbon dioxide, and eventually the ponds fill with sludge that must be periodically removed. Adding periodic aeration to these facultative lagoons can reduce both emissions and sludge accumulation. The tests at WSU seek to power that aeration using energy from the microbes in the wastewater.
The research team is led by Haluk Beyenal, Ph.D., a professor in WSU's School of Chemical Engineering and Bioengineering. The researchers recently published their findings, "Self-powered wastewater treatment for the enhanced operation of a facultative lagoon," in the Journal of Power Sources.
The researchers created a simulated lagoon in a laboratory and placed a carbon felt anode in the sediment and a cathode near the water's surface. When microbes consume organic waste they release electrons. The anode collects these electrons and stores them in a specially designed capacitor.
"There are not too many electrons, but once we collect them we can use them for applications. Theoretically, we know that we cannot catch all the electrons. They don't all go to the electrode. They go to other electron acceptors and sometimes even other organisms in the system," Beyenal says. Nevertheless, he says, "We have come up with an intelligent electron-collection system, and we have to collect them before they are utilized or accepted by other compounds in the environment."
These electrons are stored in the capacitor until a potential of approximately 300 mV is reached. "So what we do is discharge it by powering an aerator. While you are discharging, you can operate a DC-DC converter, which powers your aeration pump," Beyenal says. This can power an aerator for approximately 17 seconds at a time.
When the team tested the system in simulated wastewater, these short bursts of aeration improved chemical oxygen demand (COD) removal by 21 percent. When they later tested the system in a sample of agricultural wastewater, COD removal was improved by 54 percent.
"It was kind of a surprise to us," Beyenal says. "Our hypothesis is the buffering capacity [was higher] and the microbes in the real wastewater and organics are richer."
His team will conduct further investigations to determine the exact reasons for the difference between the lab and field tests, he says. But whatever the reasons, the results have bolstered optimism that the next step in the development of this technology—an eventual pilot-scale project in the field—could produce even better results.
"In the field we perhaps may have a better chance because it is open," Beyenal says. "If there is wind, you will transfer more oxygen, so you have a better reaction rate. [Also], if you keep this system running long term, performance increases. Starting up in the lab was difficult, because you have to get the sediments and put them in the lab. You aerate them, you mix them. And in doing this you damage the natural system. I believe in the field it will be easier for us to start up. I expect they will work better."
This optimism is counterbalanced by an understanding that field tests often reveal "things you never thought of," Beyenal says. For example, in earlier research when a microbial fuel cell was deployed in the ocean, it was attacked by organisms that burrowed into the anodes.
"This has to be tested multiple [times] in different locations before you can start commercializing this technology," Beyenal says. The system does not employ exotic materials or expensive components that could delay commercialization, however.
"We are using carbon fabric or felt. It is actually very affordable and a very durable material for electrodes. We've been using one system for five and a half years and still it is working," Beyenal says. "I'd like to develop a system so you build it, drop it, and then forget about it. [You] never come back to check it. That will be a big advantage for us."
Although the fuel cell generates small amounts of power, Beyenal says it could be supplemented by solar and wind energy to power longer aeration times. As the technology develops, he foresees possible applications beyond agriculture, perhaps in municipal wastewater systems in rural areas or in systems in the developing world.
He points out that although early work on microbial energy dates to 1911, the bulk of research in the field has been undertaken within the past 15 years. "We are at the very beginning of research in this field compared to solar cells," he adds. "I believe development will be faster because we have much better technology now to make advances."