How urine will get us to Mars

Every day, you flush a liter or two of urine down the toilet. Unless you live in one of the dry places considering toilet-to-tap systems, you probably never consider drinking it.

But if humans are ever going to get to Mars, we’re going to get there drinking our own pee. Now scientists have built a recycling system that can turn astronauts’ urine into both clean drinking water and energy. That combination could be an important step in making long-distance space travel viable.

The International Space Station would be a likely first candidate to get such a system.The station’s current water system, installed in 2008, uses a complex process to filter, distill and oxidize urine. “It makes yesterday’s coffee into today’s coffee,” astronaut Don Pettit said when it was installed. (Watch astronaut Chris Hadfield demonstrate the water recycling system on the International Space Station.)

Before the space station, space travelers really didn’t take advantage of pee. The Russian Mir craft had a recycling system that accepted urine, but it was notoriously glitchy and didn’t produce much drinkable water. The space shuttles jettisoned urine, creating lovely “shooting stars” of liquid waste visible from Earth (but they stored solid waste, which would have made for a really frightening form of space junk).

Astronauts report that the water made from recycled urine on the space station tastes great. But the system keeps breaking down, and it takes a lot of power to run it. The system “requires several consumables, and filtered components are discarded,” says analytical chemist Eduardo Nicolau of the University of Puerto Rico, a coauthor of the new study. The concept of the system he and his colleagues have come up with is to not only remove urea from wastewater, but also “to generate valuable components from human wastes instead of discarding them.”

The new system also generates electricity, scientists from NASA and the University of Puerto Rico report March 12 in Sustainable Chemistry & Engineering. It’s a clever setup, chemically speaking. To pull pure water out of urine, the system uses forward osmosis, which, as the name implies, works in the opposite direction of the reverse osmosis systems found at many kitchen sinks. Forward osmosis uses a concentrated salt or sugar solution to draw the water out of urine. Next, enzymes in a bioreactor convert the leftover urea into ammonia, which feeds into an electrochemical cell that uses the ammonia to generate electricity.

There’s no shortage of raw materials. You urinate about 50 percent more than you drink each day, says Sherwin Gormly  of Desert Toad Water Technology Research in Carson City, Nev., who helped design the urine recycling system for the International Space Station. That’s crazy, you’re thinking: How could you pee out more than you take in? Well, for one thing, your body turns some of your food into water. When you burn carbohydrates, your body makes energy with a side order of carbon dioxide and water.

All that pee ends up being one of the biggest obstacles to sending humans to Mars, or any other long-term space travel. Without urine recycling, water could make up 80 to 90 percent of the mass on a spaceship to get humans to Mars, Gormly says. And at a cost of up to $10,000 per pound launched into orbit, shooting tons and tons of water into space would quickly become ridiculously expensive.

Any recycling system that people will rely on for months or years of space travel has to be extremely efficient. The one aboard the ISS now can reclaim 93 percent of the water on board, a level that Gormly says is crucial. The new system is still just a prototype, but it also recovers more than 90 percent of the total water that goes into it.

But it’s only generating a tiny trickle of electricity so far. In the laboratory, filtering one liter of urine in eight hours produced a few microcoulombs of electric charge. That’s about as much as the static electricity from rubbing a balloon on your hair. “Still,” says Nicolau, “our system is a proof of concept, and we are still working to increase the overall efficiency.” Eventually the system should at least generate enough power to run itself, he says.

A tradeoff remains, though: The system requires small amounts of oxygen to make electricity. And oxygen, of course, is something else you’re going to want in space. “We are using some breathable oxygen from the cabin,” Nicolau says, so the system would require backup oxygen generation. That could mean making oxygen from water via electrolysis, or using other chemical processes to make it. And that means another life-support system that can break. Ultimately, “we have to find a way not to use oxygen at all,” Nicolau says.

The new method does produce drinkable water, Nicolau says, though he hasn’t sampled any because it hasn’t yet been tested for bacteria and other pathogens. He promises a photo, though, once he and his team are able to gather around the bioreactor and toast with glasses of recycled urine.

We could even get energy-producing urine recyclers right here on Earth. “You could deploy this in developing countries where water is scarce,” Nicolau says, or the military could use it in remote desert locations.

If a future in which you drink water made from urine doesn’t sound like a future to look forward to, think of it this way: The water you drink now comes partly from the entire planet’s pee, just recycled a lot more slowly.

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