The word "wastewater" once said it all -- water sent down the drain to undergo expensive treatment for reuse or its return to nature.
But a Penn State University research team views wastewater as an important energy resource, a raw material. By combining and refining energy technology, the research team has developed a two-pronged method of using wastewater to produce a more abundant output of electricity than either method could do individually.
The Penn State study, "Energy Capture from Thermolytic Solutions in Microbial Reverse-Electrodialysis Cells," written by Roland Cusick, Bruce E. Logan and Y. Kim, was published Thursday in Science.
The microbial fuel cell uses microbes to consume plant or human waste to produce electrons that the fuel cell converts into electrical current. But that system, Mr. Cusick said, is inefficient.
Adding a system that uses reverse electrodialysis, the research team pumped solutions of salt water and fresh water across specialized membranes that only allow positive and negative ions through, while also separating them. The separation of salt water and fresh water by the membrane generates an electrical voltage that directly increases fuel-cell power. As the ions migrate from the salt water to the fresh water, their movement also increases electrical current generated by the microbes.
Because the system uses expensive membranes, Mr. Cusick and his colleagues have combined the two approaches into a system known as an "microbial reverse-electrodialysis cell," or MRC, which produces substantially more energy than the microbial fuel cell alone and requires fewer of the expensive membranes when compared with a conventional reverse electrodialysis system, a Penn State news release explains.
Another key to the system's success is use of ammonium bicarbonate salt solutions -- a baking-soda-like compound -- rather than seawater as fuel for reverse electrodialysis. This salt solution continuously can be regenerated with low levels of heat, such as solar thermal or industrial waste heat in a closed loop system.
The MRC technology can produce nearly a kilowatt hour of electricity for every kilogram of organic matter in the wastewater. Compare that to the current use of activated sludge to treat wastewater that consumes 1.2 kilowatt-hours of electricity for every kilogram of organic matter in the wastewater.
The new MRC system, if used in the average-sized waste-treatment plant, could produce 1,100 kilowatt-hours of electricity per day, Mr. Cusick said. Nationally, the average daily electricity use per house is 24 kilowatt-hours, which means an average-sized treatment plant could generate enough electricity to power 46 homes.
While the new process is expensive, Mr. Cusick said greater demand and economies of scale will make the technology more affordable. The team is working to reduce the size of the membranes required for the reverse-electrodialysis system.
Eric M.V. Hoek, assistant professor of civil and environmental engineering at University of California, Los Angeles, whose research includes making membranes for water filtration, osmosis and desalination, said it will take time to see if Penn State's technology can be affordable but said the approach succeeds in combining "the best of both worlds."
"These latest research results suggest the potential to transform the necessary but normally energy-intensive process of purifying wastewater into an energy neutral or even energy positive process might be possible," he said. "Also consider that wastewater is probably the only raw material that increases in proportion to population and industrial development."
David Templeton: email@example.com or 412-263-1578.