Engineering Science

Filtering water and recovering electricity by wastewater treatment

A dual-function electrode in a microbial fuel cell combines two previously separate functions

June 7, 2021
The Scitech
 

Whether wastewater is full of “waste” is a matter of perspective. “Why is it waste?” asked Zhen (Jason) He, professor in the Department of Energy, Environmental & Chemical Engineering in the McKelvey School of Engineering at Washington University in St. Louis.

“It’s organic materials,” He said, and those can provide energy in a number of ways. Then there’s the other valuable resource in wastewater. He’s lab has developed one system that recovers both, filtering wastewater while creating electricity. Results from bench-scale trials were published in the journal Environmental Science: Water Research & Technology. The waste materials in wastewater are full of organic materials which, to bacteria, are food.

He’s lab took the two processes, filtration and energy production and combined them, integrating the filtration system into the anode electrode of a microbial electrochemical system. The system is set up like a typical microbial fuel cell, a bacterial battery that uses electrochemically active bacteria as a catalyst where a traditional fuel cell would use platinum. In this type of system, the bacteria are attached to the electrode. When wastewater is pumped into the anode, the bacteria “eat” the organic materials and release electrons, creating electricity. To filter that same water, however, requires a different system. He’s lab combined the systems, developing a permeable anode that acts as a filter. The anode is a dynamic membrane, made of conductive, carbon cloth. Together, the bacteria and membrane filter out 80% to 90% of organic materials — that leaves water clean enough to be released into nature or further treated for non-potable water uses. He used a mixed culture of bacteria, but they had to share one feature — the bacteria had to be able to survive in a zero-oxygen environment. “If there was oxygen, bacteria would just dump electrons to the oxygen not the electrode,” He said. “If you cannot respire with the electrode, you’ll perish.” To find the correct bacteria, He mostly defers to nature. “It’s not 100 percent natural, but we select those that can survive in this condition,” He said. “It’s more like ‘engineered selection,’” the bacteria that did survive and respire with the electrode were selected for the system. The amount of electricity created is not enough to, say, power a city, but it is in theory enough to help to offset the substantial amount of energy used in a typical U.S. water treatment plant.

Source: Washington University in St. Louis. news release