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A wee drink for thirsty astronauts
16 March 2007
NewScientist.com news service
Hazel Muir

Enlarge Waste not, want not

AT THE bar in any exclusive hotel, you expect the price of your drink to sting. But $3000 for a glass of water? Ouch. That's the price tag in the most exclusive hotel of all - the International Space Station. Ferrying fresh water up to the station's astronauts equates to a bar bill of millions of dollars every year.

Now NASA says enough is enough. No longer will it shell out big bucks to truck up all that water. As far as the agency is concerned, the astronauts can just drink their own wee.

No, really. From 2009, space station astronauts will drink their own urine, sweat, and even rat pee from the labs. Luckily, though, that's only after they've installed a high-tech machine to recycle this vile cocktail with other waste water to make purer drinking water than any you'd get out of city taps on Earth.

Astronauts say they're fine with the idea. "When you talk about drinking recycled urine, a lot of people get a little bit green just thinking about it," says Layne Carter, an engineer at NASA's Marshall Space Flight Center in Huntsville, Alabama. "But if you're comfortable being strapped to a rocket and launched into space, drinking a little recycled urine isn't going to bother you. The astronauts are very professional and they don't really have an issue with it."

From the start of the ISS project, engineers knew that supplying sufficient water would be a bugbear. Each astronaut uses around 4.4 litres of water a day for drinking, washing and so on, and sending up water from Earth on the shuttle or uncrewed supply ships costs roughly $11,000 per litre.

At the moment, a Russian-built water processor replenishes some of the drinking water by condensing humidity from the air. But spacecraft still have to supply 2200 litres of fresh water to maintain a crew of three for one year, at a cost of around $24 million. Storing fresh water also takes up precious room, and given that the astronaut crew is expected to increase to six in 2009, much more efficient water recycling will be imperative. "Over the course of a year they'll use up much more water than we can supply, so they'll have to recycle it," says Carter.

NASA's solution is the "water recovery system", for which Carter is the systems engineer. His team is putting the finishing touches to the machine, which should fly to the ISS in October 2008 on the shuttle Discovery. It will recycle 93 per cent of all water used on the ISS, reducing the annual demand of a six-person crew to about 1700 litres.

A key part of the system is a process that can salvage about 85 per cent of the water in urine. After taking a leak, astronauts will flush the toilet with a little water - 0.3 litres of flush water for every 1.2 litres of urine, to be precise. Then a chemical dispenser will add a dash of chromium trioxide and sulphuric acid. This prevents the chemical reactions and microbial growth that would otherwise make the loo smell like a public toilet on a Saturday night.

Then the mix is pumped into a distillation cylinder, which spins so that the urine forms a thin film on the walls. Without this spinning, the urine would simply form globs and float aimlessly about in the microgravity. The cylinder wall heats the urine to about 40 °C, and the pressure inside is kept so low - about 0.05 atmospheres - that the water evaporates.

A compressor then pressurises the water vapour in the space around the cylinder so it condenses on the drum's outer surface. The heat of condensation travels back through the cylinder wall to warm the incoming urine, a cunning design trick that saves energy. "With any process on the space station, you've got minimal resources in terms of power, weight and volume - you have to be extremely efficient," says Carter.

Distilling water off the urine leaves behind a revolting liquid that Carter generously calls "brine". The brine is recirculated through the distiller again and again until 85 per cent of the water in the urine is recovered. Astronauts will periodically remove a tank of spent brine and bin it in a Progress vehicle, the uncrewed modules used to ferry supplies up to the ISS. These serve as trash containers while they are docked with the station, and afterwards are jettisoned to burn up in the atmosphere.

Meanwhile, the more palatable distillate ends up in a 45-litre tank, where it mingles with other waste water - reclaimed moisture generated in the living area by the crew, via breathing, washing, tooth brushing and shaving, and pee from lab animal cages. It is now ready for a four-stage scrubbing in the "water processor assembly", designed and built by Connecticut-based aerospace company Hamilton Sundstrand.

First, a filter traps any particles bigger than about 0.5 micrometres wide. Then the water runs through a series of filtration beds to remove dissolved contaminants. Adsorbent materials such as activated carbon remove organic compounds like benzene and caprolactam, a contaminant released from the abundant Velcro used on the station, while an ion-exchange resin eliminates inorganic compounds like common salt. The resin exchanges positive and negative ions for hydrogen (H+) and hydroxyl (OH-) ions, which combine to create pure H2O. By this point, the only contaminants in the water are volatile organics such as ethanol and acetone. To remove these compounds, the water flows into a catalytic reactor that heats it to 130 °C and injects a little oxygen gas. This oxidises the volatile organics to carbon dioxide or organic acids, which are removed by further ion-exchange beds, while the heat kills off any bacteria, fungi or viruses.

Finally, the machine laces the water with 1 to 4 milligrams of iodine per litre, just to prevent any microbes colonising the water system later, then pumps it into a 57-litre storage tank. It can purify nearly 6 litres per hour, so ISS astronauts will only have to run it for around 5 hours a day. "They'll probably run it for 8 to 10 hours every other day to reduce the cycle life on the hardware," says Carter.

Carter is confident that the water recovery system will behave well in space, thanks to rigorous testing. He and his colleagues rode on NASA's "vomit comet" plane, which flies a series of parabolic arcs to produce half-minute bursts of microgravity, and tested components of the catalytic reactor and the distillation unit to make sure that gases and liquids moved through them as expected. Tests on shuttle missions were also successful.

Meanwhile, more than 120 staff at the Marshall centre queued up to donate their bodily waste for the cause. Each day for six months volunteers - from engineers to accountants - visited a mock-up of the ISS for an hour to run on treadmills, microwave their food and pee in the toilet. "They would exercise, take sponge baths, brush their teeth and shave, just like the crew on the space station," says Carter.

The water recovery system scrubbed up their mucky water rather well. Now something of a connoisseur of his colleagues' recycled waste, Carter says there's not even a hint of sewage, just a slightly "medicinal" tang from the iodine. The level of organic carbon compounds is less than 2 per cent of that found in typical US tap water. To make sure the astronauts don't drink harmful amounts of iodine during long stays on the ISS, they will install a filter that mops up the iodine just before it reaches the drinking tap.

How well the system fares when it is installed on the ISS next year is sure to influence future designs. Urine recycling will become essential when astronauts set up a base on the lunar surface, and that's something NASA is already designing.

"Right now, we're starting the development work for that, and I'm really excited about it," says Carter. After all, it will save NASA millions of dollars. When astronauts set foot on the moon once again, their pee may - quite literally - be worth its weight in gold.

From issue 2595 of New Scientist magazine, 16 March 2007, page 48-49

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