Making Bacteria Clean Water and Generate Electricity

Microbial fuel cells could turn wastewater treatment from a big electricity consumer to a generator

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[sound of flowing water]

Valerie Watson: This treatment plant collects the wastewater from the whole Penn State campus.

Glenn Zorpette: Ph.D. student Valerie Watson stands on a bridge surveying a brown waterfall.

Valerie Watson: Basically it's just sewage, it's what coming out of your toilets, it's what going down your drains or your garbage disposals.

Glenn Zorpette: Watson uses a stick scooper to fill liter plastic jugs with the yellowish water. But, crucially, there's more than just waste in wastewater—there are bacteria, feasting on it.

Valerie Watson: We're here to collect the bacteria that would normally be doing this process while exposed to oxygen. But we're gonna take them out and we're gonna put them in our own reactor.

Glenn Zorpette: Our “reactor” is a microbial fuel cell—a promising new energy technology, using bacteria, digesting wastewater, to produce electricity. And we'll come back to these jars of sewage.

[flowing water sounds fade]

Glenn Zorpette: See, wastewater of almost every variety contains organic matter. And that organic matter has energy, which bacteria are able to release in the form of electrons. Bruce Logan, who heads a Penn State environmental engineering lab, explains.

Bruce Logan: They rip electrons out of the organic matter and they have to go somewhere, and in our body, we send those electrons to oxygen, but we don't give them oxygen in a microbial fuel cell. So the only way the electrons can react with oxygen is to flow through the circuit.

Glenn Zorpette: Electrons flowing through a circuit is electricity. But the irony is, we currently spend electricity to remove organic matter from water—to the tune of 1.5 percent of our nationwide use. It goes to aerators, pumps, and lights for the buildings. Logan says that, someday, microbial fuel cells installed at wastewater treatment plants could generate that much electricity while cleaning the water.

Bruce Logan: We could look at not only not using that electrical energy but actually being a net electricity producer. That would be a really good thing for society.

Glenn Zorpette: For instance, that Penn State water treatment plant? Logan calculates its 2.6 million gallons of sewage daily could someday provide enough power to run 84 nearby houses.

[lab sounds]

Glenn Zorpette: So, back to that sewage. In the lab, Valerie starts assembling a microbial fuel cell. These test versions are clear plastic cubes small enough to hold in your hand. On one end: the anode, which looks like a bottle-cleaning brush. This is what the bacteria will grow on. On the other end, a circle of carbon cloth with platinum painted on. That's the cathode. Valerie fills the cell with the sewage sample we collected at the plant. A wire connects anode to cathode, with a resistor put in between. And voilà: a tiny bacteria battery.

Valerie Watson: And so now, here, freestanding on its own, we can have electron flow through the microbial fuel cell.

Glenn Zorpette: The bacteria continue doing what they were doing before, eating the waste. But the key difference is that now they're deprived of oxygen. In the absence of oxygen, some bacteria can pass electrons to a solid surface—the anode, in this case. In our test cell, electricity will begin to flow through the wires about two days later, as the electrons settle on the anode and begin their work.

[lab sounds fade; water sounds]

Glenn Zorpette: If you imagine this on a large scale, water at a wastewater treatment plant would flow through these chambers on its way through the processing. Microbes would digest the sewage, clean the water and produce electricity. It would look almost identical to how water treatment looks now—just, instead of electricity going in, it'd be coming out.

[water sounds fade]

Glenn Zorpette: On a smaller scale, the lab at Penn State still has lots to learn about the tiny critters that power their microbial fuel cells. Ph.D. student Rachel Wagner is studying which microbes thrive in a fuel cell, and why. She looks at how they've adapted to better pass their electrons to the anode.

Rachel Wagner: If we can figure out what genes are turned on when a microbe is in a microbial fuel cell and using the anode as a terminal electron acceptor, then we can manipulate those genes.

Glenn Zorpette: But the secret to success probably lies on the other side of the cell—the cathode—says Bruce Logan.

Bruce Logan: I think right now the central challenge is designing these systems so that they're not really big and really expensive. And, mostly, that boils down to designing an efficient cathode.

Glenn Zorpette: Today, the best cathodes are made of platinum, which is expensive. Other, less-efficient cathodes have to be really big to produce substantial power.

Bruce Logan: And so, we're really putting a lot of energy and effort into addressing that problem in a way that doesn't use precious metals and can be done with, you know, a reasonable amount of volume of reactor.

Glenn Zorpette: If these engineering hurdles get worked out, the potential energy savings and versatility of microbial fuel cells could transform water treatment. They could provide energy to treat water off the grid, in developing countries or remote areas. They could use almost any kind of wastewater as fuel. In State College, Penn., they're thinking: clean water at no net energy cost.

Bruce Logan: We're just trying to answer the question, can we guarantee society water? That's a pretty basic thing. You can argue about cars and buildings and heat and cooling or whatever, but you got to have water.

Glenn Zorpette: It's a big guarantee, carried on the backs of the tiniest of workhorses. I'm Glenn Zorpette.

To Probe Further

Check out the rest of the special report: Water vs Energy.

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