On Earth, water that passes through animals' bodies is made fresh again by natural processes. Microbes in the soil break down urea and convert it to a form that plants can absorb and use to build new plant tissue. The granular soil also acts as a physical filter. Bits of clay cling to nutrients in urine electrostatically, purifying the water and providing nutrients for plants.

Water excreted by animals also evaporates into the atmosphere and rains back down to the Earth as fresh water -- a natural form of distillation.

Water purification machines on the ISS partly mimic these processes, but they do not rely on microbes or any other living things.

"While you try to mimic what's happening on Earth -- which is so complicated if you really think about it -- we have to use systems that we can control 100 percent," said Monsi Roman, chief microbiologist for the ECLSS project at MSFC. ECLSS depends on machines -- not microbes -- because, "if a machine breaks, you can fix it.

The water purification machines on the ISS will cleanse wastewater in a three-step process.

The first step is a filter that removes particles and debris. Then the water passes through the "multi-filtration beds," which contain substances that remove organic and inorganic impurities. And finally, the "catalytic oxidation reactor" removes volatile organic compounds and kills bacteria and viruses.