You’ve probably never thought of the toilet this way, but from a chemist’s perspective, your average flush is a goldmine of untapped chemical energy. Over the past few decades, scientists and engineers have tested various ways to reclaim raw sewage’s wasted energy and recycle it back into sewage treatment. The methods have ranged from the practical—extracting methane by fermentation—to the futuristic: creating bacteria-powered fuel cells.
In their test, documented in the study in Science Advances, the researchers grew a strain of bacteria (Shewanella oneidensis) on their new material. Placed in untreated sewage taken directly from a municipal wastewater treatment plant in Beijing, a large swath of the microbe-infested material produced enough energy to power a small, wristwatch sized timer.
“We’ve found the graphene is very bio-compatible for bacteria, and the small amount of platinum particles is a very good medium for [energy] transfer,” says Zhiyong Tang, a materials scientist with the team at the National Center for Nanoscience and Technology in Beijing. “It’s a very exciting step for future research,” he says, though noting that “our material still has many improvements that need to be made.”
Judging by the responses we got from independent engineers, that may be an understatement. Craig Criddle, an environmental engineer at Stanford University who was not involved in the research, says that while the research is interesting, it’s far from impressive—at least in terms of practical performance. The big challenge for microbial fuel cells like Zhao’s is to boost energy efficiency (how well they utilize sewage’s total chemical potential) while staying affordable. Criddle says this new material overcomes neither hurdle. It has less than 0.1 percent total efficiency and a requires expensive platinum metal.
That’s not all. “Any device like this is going to need to prove it can run for long periods of time. Not hours, nor days, but years. Because in a [treatment plant] waste water just keeps on coming, nonstop,” he says.
Bruce Logan, an expert on microbial wastewater treatment at Penn State who was likewise not involved in the new research, is in resounding agreement. “The system requires a precious metal for the anode (platinum),” and incredibly large surface areas to produce even a tiny amount of appreciable power, Logan tells PM via email.
By comparison, Criddle points to another method of energy reclamation that many sewage treatment plants across the country are already using: methane fermentation. By farming a slew of microbes that expel methane in sewage (imaging fermenting the worst beer imaginable), treatment plants can capture the gas, and burn it later to offset their energy costs. Any new system of energy generation, Criddle says, will have to prove itself substantively better than the existing methane fermentation approach.