Answer on @Quora by @RobertFrost01 to How is water recycled aboard the ISS? Great read – Xochielt Sanchez
Answer by Robert Frost:
The Water Processing Assembly (WPA) and Urine Processing Assembly (UPA) work together to turn waste water into potable water. They do that using a process called Vapor Compression Distillation.
Vapor Compression Distillation is essentially a mechanical mimicking of the natural water cycle on Earth. Water from the ocean, soil, and lakes evaporates due to heat from the sun. The evaporated water rises and when it reaches cold air levels condenses again back into liquid form, at which point it falls back to earth as precipitation (rain or snow).
To do that on ISS, we use these pieces of equipment:
The process is relatively straightforward. If you have dirty water, you boil it so that the water and dirt are separated. The water is drawn away as steam. The steam is then recondensed as clean water.
Since the ISS is a microgravity environment, we can't use gravity to cause that separation, so we have to use a centrifuge.
The challenge is how to do this process economically, since it takes a lot of energy to boil water. The ISS gets to cheat, here, though. Because the ISS is moving around in a vacuum, we can take advantage of that exterior vacuum to lower the pressure in the water chamber. By lowering the pressure, we lower the boiling point of the water, meaning less energy is required. We can't cheat like that, on Earth, though. On Earth we would have to input more energy into the system to lower the pressure.
On ISS, we have large solar arrays to provide the energy that is needed. Even if we weren't cheating with the pressure, it would still be a bargain because the cost to get water to space is exorbitant. For an open-loop life support system, 89% of the required up mass is water. An individual needs around 10,680 kg (2827 gallons) of water per year. It can cost up to $40,000 to lift a gallon of water into space. But the WPA and UPA together make the six person crew much less reliant on water being delivered.