"From Foul to Fuel" Feature Article, June 2004

FEATURE FOCUS: Advanced Energy Systems

from foul to fuel

A new approach to fuel cells turns wastewater into clean water and electricity.

by Gayle Ehrenman, Associate Editor

Operating and maintaining a wastewater treatment plant is a costly proposition. New fuel cell technology that generates power while it cleans wastewater may offer a way to make clean water more available for developing and industrialized nations.

Microbial fuel cells are nothing new. Teams from around the world have developed microbial fuel cells that produce electricity as bacteria metabolize glucose, lactate, or acetate.

Now researchers from Pennsylvania State University have come up with a microbial fuel cell that produces electricity from domestic wastewater, while it cleans that water.

"Nobody has ever tried this with domestic wastewater," said Bruce Logan, a professor of environmental engineering at Penn State and the director of the project. "We're using something thought to be completely useless."

Logan came up with the idea for the project about two years ago, but didn't start working with the current design until six months later. Funding for the project has come through an $87,000 grant from the National Science Foundation Small Grants for Exploratory Research program. Such "sugar" grants are meant to foster small-scale, innovative preliminary research on untested ideas, according to the NSF.

The concept behind Logan's microbial fuel cell is so simple, he says he's surprised that no one else thought of it first.

The single-chamber microbial fuel cell has eight anodes around a central cathode that's made up of a catalyst and a PEM fused to a plastic tube.

Organic matter in wastewater has energy value; wastewaters have high concentrations of organic matter and a particularly high energy value, according to Logan. Where a typical fuel cell runs on hydrogen, a microbial fuel cell relies on the anaerobic oxidation of organic matter in a material—in this case, the wastewater—to produce electricity.

According to Logan, the wastewater produced by 100,000 people has the potential to generate 2.3 megawatts of electricity, if you could recover all the energy. This would be enough to power 1,500 homes, he said.

Logan's microbial fuel cell is a single-chambered Plexiglas device, 6 inches long x 2.5 inches in diameter. Inside, eight graphite anodes surround a cathode that's made up of a carbon/platinum catalyst and proton exchange membrane layer fused to a plastic support tube. The graphite rods were abraded to make it easier for bacteria to attach to them. A copper wire connects the circuit, providing the path the electrodes will follow.

A steady flow of wastewater pumped into the chamber feeds the bacteria. Bacterial digestion of the wastewater's organic matter releases electrons into the electrical circuit and positively charged hydrogen ions into the wastewater solution. Those ions reduce the solution's oxygen demand, a goal for all wastewater treatment.

The hydrogen ions also pass through the proton-exchange membrane to reach the cathode, which is exposed to air. At the cathode, oxygen from the air, hydrogen ions coming through the membrane, and electrons traveling down the circuit come together to create clean water.

Where other microbial fuel cells have inoculated the system with some pure bacterial cultures, Logan's tests showed that inoculation was not necessary to produce electricity in the microbial fuel cell. All it took was the bacteria already present in the wastewater, according to Logan.

One Chamber or Two?

The single-chamber design of this microbial fuel cell is also significant, because it allows a continuous flow-through system that is consistent with existing wastewater treatment systems, Logan said.

Most microbial fuel cells use a two-chamber system. One chamber contains bacteria growing under anaerobic conditions on the anode. In the other chamber, where the electrons combine with oxygen and protons to form water, the cathode is maintained under aerobic conditions. A bridge containing the proton-exchange membrane connects the two chambers, and allows the protons—but not the solution or oxygen—to diffuse between the two chambers.

The problem with the two-chamber approach is that you need to force air into water to provide dissolved oxygen to the cathode. The single-chamber microbial fuel cell, which is easily run as a continuous-flow reactor, uses passive direct airflow, rather than forced airflow, which cuts down on the costly aeration step required in traditional wastewater treatment.

Logan's experiments used wastewater that was primary clarifier effluent from Penn State's wastewater treatment plant. Primary clarifier effluent is water with the settleable and floatable solids removed. The wastewater has a fairly complex mix of organic matter, but contains little pure acetate or glucose.

The wastewater had a pH ranging from 7.3 to 7.6 and a chemical oxygen demand of 210 or 220 milligrams per liter. Chemical oxygen demand, a measurement of pollutants in natural and wastewaters to assess the strength of discharged waste, is defined as the amount of specified oxidant that reacts with a sample under controlled conditions.

In this two-chambered microbial fuel cell, the chamber with the anode is gas sparged with nitrogen to maintain anaerobic conditions for bacteria growth, while the cathode is maintained under aerobic conditions.

While the variation in the wastewater stream didn't greatly affect the performance of the microbial fuel cell, higher organic content in the water could produce more electricity, Logan said. Wastewater from food processing would provide the ideal "fuel" for the microbial fuel cell. "The process benefits from food to eat, rather than food that's already been eaten," he said.

The microbial fuel cell has generated about 26 milliwatts of power per square meter of electrode surface. That means you would need 38 square meters of surface area just to light up a Christmas decoration-style light bulb, according to Logan. But the process still removes up to 78 percent of the organic matter from the water, as measured by biological oxygen demand, and anywhere from 50 to 70 percent of the chemical oxygen demand. (Biological oxygen demand refers to the amount of oxygen that would be consumed if all the organics in one liter of water were oxidized by bacteria and protozoa.)

After the system acclimates to the wastewater, it takes about 70 hours to achieve steady current, Logan said.

Increasing Power Production

The first generation of the design proved that it's possible to generate fuel and clean water using wastewater as a medium. Now Logan and his team are working on ways to boost the power production of the microbial fuel cell, lower the cost to produce it, and transition it from the lab to a mass-production device.

First up is reducing the cost of materials, Logan said. "We're looking at ways to substantially reduce the cost of the PEM, in particular, " he said. "We're also looking at the catalyst materials, configuration, and design of the device."

Increasing the power output is another major goal. While the first-generation device didn't provide much power, a more recent iteration of the microbial fuel cell generates enough electricity to power a small fan. Still, this is a far cry from the amount of power required to make a commercially valuable device.

"Our goal is to be able to generate a steady 500 kilowatts of electricity, which would be enough to power 300 homes," Logan said. He estimates that the upper limit to the electricity generating capacity of the microbial fuel cell is on the order of 1,000 milliwatts per square meter of anode surface area.

Such a device is not as far from reality as it would seem. Logan expects to be ready to have a pilot-scale device tested within one to three years, and treatment-scale systems ready within the decade.

According to the NSF, about 33 billion gallons of domestic wastewater are treated every year in the United States, at a cost of $25 billion. Much of that cost goes to pay for the energy needed to run the processing systems. If the microbial fuel cell can be made cheaper and more efficient, it could reduce the energy costs of wastewater treatment.

"Even in places fortunate enough to have wastewater treatment plants, there's little incentive to fix the plant when it breaks," Logan said. "It's just too expensive to run.

"But, if the treatment plant also generates electricity, then it's viewed as a moneymaker and there's a great incentive to keep it running," he said. "If we can increase the power generation and decrease the cost of the microbial fuel cell, we can make clean water more available for both developing and industrialized nations."