Water forms Earth's oceans, lakes, and rivers, and is also present in the atmosphere. All Earth's creatures rely on water to sustain life. It comprises 90 percent of a human's body weight. Water transports nutrients and electrolytes from one part of the body to another, and washes out wastes and toxins from the body.
Because of water's essential nature, the mini-ecosystem of a spacecraft must provide water to sustain the crew and any live organisms. Based on the average quantities of water, food, and oxygen that people use everyday, water constitutes about 95 percent of the total mass of "consumables" required for human life support within a spacecraft.
Systems within a spacecraft also are required for the disposal of, or storage and return of, waste (urine) or "grey" (dirty) water (flush, laundry, dishes). This wastewater amounts to about 30 kilograms per person each day, or about 97 percent of the total output mass of human by-products. If water were not recycled by the crew, the mass of water required to be "launched" for a 2-year round trip to Mars with six crew members would be a phenomenal 129,473 kilograms, or 134,070 liters (35,375 gallons)!
Water is also needed for use in the spacesuit's liquid cooling garment, which circulates chilled water through about 91 meters (300 feet) of plastic tubing laced in stretchable spandex fabric long underwear. It can absorb up to 2,000,000 joules (about 478 food calories) of body heat per hour, a rate produced by extremely vigorous physical activity (approximately 160 joules, or 38 food calories, are released when burning a piece of newspaper one square centimeter, or 0.16 square inch).
Too much water in the atmosphere of the spacecraft's cabin is a problem. If relative humidity is too high (from 25 to 75 percent is desirable), as it tends to be in space habitats, condensation on surfaces can occur. Such moisture can damage electronics and increase fungal and microorganism growth that is difficult to control. Humidity also inhibits the cooling of the human body (in confined spacecraft habitats) through the evaporation of perspiration. In microgravity environments, water droplets stay on the skin without the presence of convection .
For long duration missions and especially if humans are to venture beyond the orbit of Earth, spacecraft and other space habitats must be self-sufficient by recycling air and water. Water available for recycling comes from three primary sources: humidity condensate, wash-water, and urine. Two different processes can be used to purify these three wastewater sources. The first process involves physicochemical technologies, which use distillation, filtration and phase change processes (for example, vapor compression distillation). The second process involves bioregenerative processes, which use photosynthetic plants grown in aquaculture.
Plants supply essentially pure water through transpiration. Typically, plants transpire 200 to 1,000 liters (50 to 260 gallons) of water for one kilogram of dry biomass . Such plants can take up wastewater (such as contaminated shower and laundry water) and the resulting transpiration water (water given off by the plants) can be condensed and collected from the plant chamber onto a condensing heat exchanger. This water, along with condensate from the crew chamber, is treated by ultraviolet light to remove bacteria and degrade trace organic compounds. Any surplus water is returned to the aquaculture system or the hydroponic nutrient solution.
Bioregenerative systems are slow, can operate with little electrical power, and are multifunctional (plants absorb carbon dioxide, generate oxygen, and can provide food). Physicochemical technologies for air and water regeneration tend to be fast-acting but power-hungry. By carefully combining both technologies, a hybrid design can be developed with distinct advantages over purely individual systems.
Schwarzkopf, Steven H. "Design of a Controlled Ecological Life Support System." Bioscience 42 (1994):526–535.
Waligora, James M., Michael R. Powell, and Richard L. Sauer. "Spacecraft Life Support Systems." In Space Physiology and Medicine. A. E. Nicogossian, C. Leach Huntoon, and S. L. Pool, eds. Philadelphia, PA: Lea & Febiger (1994):109–127.