The International Space Station (ISS) requires a constant and reliable supply of water for the crew’s survival, hygiene, and for generating breathable oxygen. On Earth, water is plentiful, but launching one gallon of water into low-Earth orbit can cost tens of thousands of dollars, making resupply prohibitively expensive for long-term operations. The high cost of launch means that sustaining a crew on the station relies heavily on conserving and recycling every drop of water used. This necessity drives the complex engineering solutions required to maintain a livable environment hundreds of miles above the planet.
External Water Resupply
While the ISS is designed to be largely self-sufficient, external resupply remains a consistent part of the water management plan. Cargo vehicles launched from Earth carry water tanks to supplement the station’s internal supply and make up for system losses. These robotic resupply missions, such as the Russian Progress, SpaceX Dragon, Northrop Grumman Cygnus, and Japanese HTV, deliver fresh water.
These cargo flights occur approximately six to eight times per year, ensuring the station’s potable water reserves remain at safe levels. The Russian Progress spacecraft often carries water tanks specifically designed to transfer their liquid contents directly into the station’s storage system. This resupply water is primarily used to balance the system, providing the necessary input for the Oxygen Generation Assembly (OGA) to produce oxygen via electrolysis.
Onboard Water Recycling Systems
The core of the station’s water supply is the sophisticated Water Recovery System (WRS), which drastically reduces the need for external resupply. The WRS is engineered to process all forms of wastewater, including moisture collected from the cabin air, hygiene water, and crew urine. This comprehensive system allows the station to maintain a continuous, self-sustaining water cycle.
One of the main inputs to the WRS is humidity condensate, which is water vapor collected from the crew’s breath and perspiration as it condenses on cooling surfaces. Urine is processed separately by the Urine Processor Assembly (UPA), which utilizes a vacuum distillation process to boil the urine at a low temperature, separating the water vapor from the concentrated brine waste.
The Water Processor Assembly (WPA) then takes the purified urine distillate, along with the humidity condensate and hygiene water, for a final purification sequence. This involves pushing the water through a series of filters, including a multifiltration bed containing adsorbent and ion exchange media to remove organic and inorganic contaminants. The next step is a Catalytic Reactor, which uses high temperature to oxidize any remaining volatile organic compounds into carbon dioxide and water.
The overall goal of the WRS is to achieve maximum closure of the water loop, consistently recovering over 90% of the water used by the crew. With the integration of technology demonstrations like the Brine Processor Assembly, which extracts even more water from the UPA’s leftover brine, the total water recovery capability can approach 98%.
Water Storage and Quality Control
After the water has been thoroughly recycled and purified, it undergoes a rigorous quality control process to ensure it is safe for consumption and use. The purified water is often cleaner than many municipal water sources on Earth. A biocide is added to the water to prevent microbial growth during storage and distribution.
The US segment of the ISS typically uses molecular iodine as this biocide, while the Russian segment often uses ionic silver. The concentration of these biocides is closely monitored to ensure it is high enough to kill microbes but low enough to be safe for the crew. Before the water is dispensed for drinking or food preparation, a Potable Water Dispenser (PWD) passes the water through a deiodination filter to remove the iodine.
The final product water is stored in various tanks and Contingency Water Containers (CWC-Is), ready for distribution across the station. This clean water is used for the crew’s direct consumption and for preparing freeze-dried food.