Stanford scientists say they've figured out how, and now the challenge will be to make it cheap enough to do on a large scale.
Once upon a time, it was a triumph of technology when a gas-powered machine replaced an animal. The tractor rendered the ox-drawn plow obsolete. The automobile outperformed the horse-and-buggy.
But sometimes, Mother Nature knows best. Consider exoelectrogenic microbes, an unusual variety of bacteria that emit electrons as they consume organic material. Stanford researchers have developed a way to harvest electricity from these odd organisms as they feast on sewage.
"You clean the wastewater and you can also recover energy," explains Xing Xie, an interdisciplinary fellow at Stanford and one of the co-authors of a new paper published this week in the Proceedings of the National Academy of Sciences.
Thousands of these tiny "microbial batteries" cling to carbon filaments and transfer their output to a silver oxide electrode as they work through the organic material in wastewater. All told, the device can extract nearly a third of the potential energy in wastewater.
This could be a win/win for the U.S. Only 4 percent of North American wastewater is put to beneficial reuse right now, but the United Nations has forecast we'll need to rapidly increase that number in the coming decades. Wastewater is treated to feed crops, create snow for ski mountains, and even, occasionally, for drinking.
Exoelectrogenic microbes have previously attracted the attention of scientists who sought to harness their remarkable abilities. In a paper last year in Science, Bruce Logan and Korneel Rabaey wrote that exoelectrogenic microbes could be used to generate "biofuels, hydrogen gas, methane" and various industrial chemicals.
But until now, few efforts to apply these micro-organisms to wastewater treatment had been successful, because of the difficulty in efficiently harvesting electricity. In this case, Xie and his co-authors, professors Yi Cui and Craig Criddle, solved that problem with silver oxide to attract the electrons.
Unfortunately, silver oxide brings its own issues to the table. "The concept is scalable," says Xie, "but the problem is the cost of the material." If "microbial batteries" are to be widely deployed in urban water systems or polluted lakes, in other words, the next step is to figure out how to bring the cost down.