From U-bend to fuel cell

On a two-year trip to Mars, a crew of six astronauts is likely to produce six tons of organic waste and most of it is quite unsavoury. Or is it? Karen Miller explains.

On a two-year trip to Mars, according to one estimate, a crew of six humans will generate more than six tons of solid organic waste–much of it faeces. So what do you do with all that?

Right now, astronaut waste gets shipped back to Earth. But for long-term exploration, you’d want to recycle it, because it holds resources that astronauts will need. It will provide pure drinking water. It will provide fertiliser. And, with the help of a recently discovered microbe, it will also provide electricity.

Like many bacteria, this one, a member of the Geobacteraceae family, feeds on, and can decompose, organic material. Geobacter microbes were first discovered in the Potomac River in 1987; they like to live in places where there’s no oxygen and plenty of iron. They also have the unexpected ability to move electrons into metal. That means that under the right conditions, Geobacter microbes can both process waste and generate electricity.

The right conditions might be found in a new type of fuel cell, a membrane microbial fuel cell. This device is currently being developed by a NASA-funded research team led by Dr. Bruce Rittmann, a professor at Northwestern University.

All fuel cells generate electricity by producing and controlling a flow of electrons. Conventional cells, including ones used onboard the space shuttle and in some prototype automobiles, obtain the electrons for their electron flow by pulling them off hydrogen atoms. In order to do that, these fuel cells must be given a constant supply of hydrogen.

Microbial fuel cells obtain their electrons, instead, from organic waste. The bacteria at the heart of the device feed on the waste, and, as part of their digestive process, they pull electrons from the waste material. Geobacter microbes, as well as a few other types, can be coaxed to deliver these electrons directly to a fuel cell electrode, which conducts them into a circuit, a wire, for example. As they flow through the circuit, they generate electricity.

Microbial fuel cells are already being experimented with on Earth. For example, one prototype is being used at Pennsylvania State University to generate electricity as it purifies domestic wastewater.

To make this idea practical for space travel, says Rittmann, you have to have “a very efficient, very compact configuration.” The fuel cell can’t take up much room. To meet this requirement, Rittmann is considering a fuel cell of tightly packed fibres, each one of which will be a fuel cell all by itself.

Each fibre would consist of three layers, one inside of another. Each layer corresponds to one of the layers of a fuel cell: the anode (outer), the electrolyte-membrane (middle), and the cathode (inner). A slurry of liquefied waste would be pumped past the outer layers where Geobacter microbes (or other similar bacteria) can grab electrons and move them to the anode, into the circuit, and then to the cathode.

Before any such designs can be put into practice, however, Rittmann and his team must first decipher the exact mechanism by which the bacterium transfers electrons to the electrode. In laboratory tests so far, the transfer rate is too slow. “We need to know how we can make that faster,” Rittmann says, “and so generate more power.”

He has a couple of ideas about what the hold-up might be. “The electron actually has to move from the outer surface of the microbe to the electrode, and it could be that it’s limited by physical contact.” Even though the bacteria lives attached on the surface of the anode, only a tiny bit of each microbe actually touches the metal, and that may be hindering electron movement.

Another factor is the voltage on the electrode. It has to be high enough to coax the microbes into giving up their electrons. “Microbes move electrons around in order to gain energy. In fact, they only move the electrons when they do gain energy,” he explains. What’s the best voltage? “That’s one of the questions we’re trying to answer.”

“Let’s say, for example, that the total voltage difference between the fuel and the anode is 2 volts. Then the microorganisms, as they give up their electrons, might take 0.5 volts to sustain themselves, leaving 1.5 volts for doing work in the circuit. These are just made-up numbers,” says Rittmann, “but they illustrate what we are trying to learn.”

The membrane microbial fuel cell is still in the early stages of its development. Yet, if the project succeeds, it could find its way into people’s homes.

“You have to treat the wastes anyway,” points out Rittmann. “So why not make the process an energy gainer, instead of an energy loser? By producing electricity, microbial fuel cells would make the process of purifying waste streams much more economical.”

Moreover, he says, “they change our focus. Microbial fuel cells transform something we think of as undesirable into a resource.”