Turning urine into drinkable water apparently isn't so easy. At least not in space. As the space shuttle Endeavour and International Space Station (ISS) crews gear up for tomorrow's scheduled seven-hour spacewalk, they're still wondering what to do with their malfunctioning $250 million water-and-urine recycling system. The processor ran for about two hours today before shutting itself down, reports SpaceFlight Now. This failure follows yesterday's tense moments, when the urine processor assembly set off an alarm on the space station as astronauts attempted to test it, according to Space.com.

The astronauts have yet to determine what triggered what they believe to be a false alarm,  because they didn't  detect any smoke or a combustible odor, the Associated Press reports today. NASA is playing down the incident, noting that systems installed on the space station are complicated and that troubleshooting to get to the bottom of problem is not unusual. It took several attempts in 2006, for example, to get the new oxygen generator to work properly.

As Scientific American.com reported in October, 2007, a water-recovery system collects urine from the astronauts and condensation from the cabin air and, through a series of chemical treatments and filters, turns that moisture into potable water. The first step involves filtering solid particles such as skin cells and hair out of the liquid. After that, contaminants are chemically dissolved and oxygen is added to the liquid to oxidize trace organics so that they, too, can be removed. Next, the liquid is "polished," meaning chemicals left over from the cleaning process are removed. Finally, iodine is added for microbial control, much the way municipal water authorities add chlorine to city drinking water. The resulting liquid is sent to a large storage tank, which can be tapped for drinking.

In case you're wondering, astronauts aren't planning to sample any of the recycled urine during this trip. Instead, they  plan to bring samples from each stage of the water recycling system, including the urine processor, back to Earth aboard Endeavour to conduct quality control studies, reports Space.com, which has a video in which NASA demonstrates how the machine works.

NASA wants to test and monitor the system for several months before giving astronauts the okay to drink from it. The successful recycling of sweat, urine and other moisture aboard the shuttles and space station are crucial to NASA's plans to double the ISS's crew to six people and eventually send manned missions beyond the moon. It currently costs about $5,000 per kilogram (2.2 pounds) to ship water in a space shuttle.

NASA's Web site notes that the station crew depends now on water carried up aboard a space shuttle or cargo rocket. But an operational water recycler is expected to cut that need by 65 percent by producing about 6,000 pounds (2,722 kilograms) of potable water each year.

(Images of astronaut and mission specialist Donald Pettit installing and configuring the Water Recovery System (WRS) rack in the ISS's Destiny laboratory are courtesy of NASA)