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How bacteria can produce electricity, treat wastewater

Chemical engineer César Torres leads the Torres Lab where students aid his research efforts to understand the diverse applications of microbial fuel cells. Photographer: Jessica Hochreiter/ASU

Chemical engineer César Torres leads the Torres Lab where students aid his research efforts to understand the diverse applications of microbial fuel cells. Photographer: Jessica Hochreiter/ASU


What if the bacteria found in wastewater could power the water’s own purification system?

Chemical engineering professor César Torres is exploring this possibility through research in microbial fuel cells (MFCs), supported in large part by a $1,900,000 grant from the Department of Defense.

An MFC is a bio-electrochemical device that converts the power of respiring microorganisms into electrical energy.

Specifically, MFCs contain anode-respiring bacteria (ARB) that can produce electricity when electrons from wastewater organics are transferred to an anode.

“In this system organic compounds can be removed from water, while electrical current is simultaneously produced,” said Torres, who earned his Ph.D. in environmental engineering from ASU in 2009.

The electrical current in MFCs is used to produce hydrogen peroxide — a powerful oxidant that can then be reused to treat and disinfect wastewater.

“This oxidant is rarely used in wastewater treatment because of its high cost, but MFCs allow on-site hydrogen peroxide production using energy from wastewater,” said Torres.

This reliable and energy efficient wastewater treatment option is of special interest to the U.S. armed forces in order to avoid the costs and risks normally associated with water transport in remote locations.

Torres is conducting this research in conjunction with Bruce Rittman, a Regents’ Professor and director of the Swette Center for Environmental Biotechnology in the Biodesign Institute and Konstantinos Tsakalis, an electrical engineering professor.

Powering sensors in the ocean’s depths

Two additional grants from the Department of Defense’s Office of Naval Research further the Torres Lab’s exploration in MFC research.

An effort to better understand the mechanisms that allow ARB to produce electrical currents is supported by a $450,000 grant. Rachel Yoho, a biological design doctoral student in the Torres Lab, is leading this portion of the research and focuses specifically on identifying key proteins used in ARB.

An additional $400,000 grant supports efforts to study the use of MFCs as power sources for sensors at the bottom of the ocean that collect oceanic data (like temperature and oceanic current flows) critical to efficient navigation.

These sensors require a self-sustainable power supply since a battery charge is not feasible — a problem the U.S. Navy has worked on for nearly a decade.

“MFCs produce low power densities, but enough to power sensors in remote locations where a continuous supply of fuel is possible due to the organic compounds found in nature,” said Torres.

The goal is to produce a one-watt MFC system that is able to power the sensors for several years.

Improving wastewater treatment technologies

Torres is also leading an effort to better understand microbial processes in municipal wastewater sludge treatment through the use of ARB. This sludge refers to the contaminant by-products that come from wastewater treatment processes.

This research involves using MFCs not as a treatment technology that generates power, but as a new tool for measuring microbial kinetics.

Microbial respiration can be measured in combination with electrical currents in MFCs, which provides insights into the microorganism’s behavior.

This allows the MFC to become an analytical tool for measuring the rate at which ARB respire, providing a more accurate and time-sensitive measurement of microbial processes related to hydrolysis than the current methods based on measuring methane gas production.

With these measurements Torres aims to develop comprehensive hydrolysis models that can better predict treatment in current and upcoming technologies.

The research is funded with a $333,000 grant from the National Science Foundation (NSF) and led by Torres and Steven Hart, an environmental engineering doctoral student in the Torres Lab and recipient of an NSF Graduate Research Fellowship.

Simple, natural solutions to complex problems

Torres’ efforts to understand the role of microbes in producing electrical currents have opened up a novel way to interface chemical energy and electrical energy.

His research in this area as a principal or co-principal investigator has been funded in excess of $4 million. He has published more than 30 peer-reviewed journal articles related to microbial cell research.

“There are many applications envisioned within the field of microbial electrochemistry, yet there are many aspects of these microorganisms that we do not yet understand,” said Torres.

“It is a field in which science and engineering make progress together and depend on each other — making it both challenging and exciting.”

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Ira A. Fulton Schools of Engineering

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