More on Water Issues

There is, of course, water used in the Hab for things other than washing. On Mars, there is water that one drinks, water that one uses to wash or cook, and water in the air that one breathes. Completely dry air is unpleasant to breathe, although one doesn't want it too damp, either. My textbook HSMAD (Human Spaceflight Mission Analysis and Design - the go-to handbook for manned mission design) suggests a humidity range of 50 +/- 20%. This sounds reasonably comfortable to me.

Of course, water is put into the air by humans when they breathe out (some water is lost due to evaporation from the inner surfaces of air passages, and some water is produced metabolically), so usually this water has to be removed from the air before the air is recirculated. In a habitat at an analogue site, usually the doors are not sealed airtight, and the internal humidity will approach the local outside humidity. Some places on Earth have unpleasantly low levels of humidity, but these are not completely intolerable, so there is no provision in the analogue habitats to remove water from the air. That is, we have no evaporate condensers to regenerate our water supply.

There is also water in food, or in food trimmings, and these we either throw on a compost pile in the GreenHab (our greenhouse, where we grow lettuce and vegetables) or simply throw away. The water that goes into the compost ends up nourishing the plants, and we also channel all our graywater to the GreenHab as well. It passes through a series of settling tanks, goes to the plants, and then passes into our system again, where it fills our toilet. After being used by the toilet, our water is sent into a septic system and not recycled. This way, we use water at least three times before it is finally dumped into the septic tank: once to cook, clean, or wash; once for the plants; and once for the toilet. The water we drink is used only that once, although most of it probably ends up in the sewage system as well (some of it is, of course, respirated or sweated and lost to the local environment).