Somewhere on the Moon. From: http://discoveryenterprise.wordpress.com/2008/06/24/reasons-to-go-to-the-moon/
As on earth, water is essential for life in space – while spaceships and fancy pressurised zoot-suits capture the imagination, water remains a key sticking point for those planning a space holiday or emigration to the Moon. Each astronaut on the International Space Station is allocated about two litres of water daily. They stretch the ration by collecting, cleaning and reusing wastewater, condensate in the air and urine. Water management in space might have lessons for those of us firmly grounded on earth.
The current Water Recycling System (WRS) in use on board reclaims waste water from the Space Shuttle's fuel cells, from oral hygiene and hand washing, and by condensing humidity from the air, and from urine. Without such careful recycling 40,000 pounds per year of water from Earth would be required to resupply a minimum of four crewmembers for the life of the station.
Slightly more complex than earthly water filters, the Sabatier process uses a nickel catalyst to interact with hydrogen and carbon dioxide at elevated temperatures and pressures to produce water and methane. The water is retained for recycling processes, and the methane is vented outside of the space station.
Finally, a little comic irony from those who claim to be using water filter technology developed by aliens.