The **water recycling system** onboard the **International Space Station (ISS)** is a highly advanced system designed to recover and purify water for astronauts living in space. Since resupplying water from Earth is expensive and limited, the system aims to recycle as much water as possible, reducing the need for resupply missions. It allows the ISS to operate for extended periods by turning wastewater (including urine, sweat, and cabin humidity) back into drinkable water. ### Overview of the ISS Water Recovery and Management System The ISS uses the **Water Recovery System (WRS)**, which is part of the larger **Environmental Control and Life Support System (ECLSS)**. The WRS is capable of recovering water from a variety of sources, and it is key to maintaining life aboard the space station by ensuring a reliable supply of clean water. The system recycles about **93-94% of the water** onboard, making the ISS a highly efficient system in terms of water usage. ### Sources of Water on the ISS The WRS collects water from the following sources: - **Crew urine** - **Sweat** (via humidity collected from the cabin air) - **Exhaled moisture** from breathing - **Wastewater** from handwashing and oral hygiene - **Condensed water vapor** from plant growth systems, experiments, and the atmosphere The water recycling system is divided into two main subsystems: 1. **Urine Processor Assembly (UPA)** 2. **Water Processor Assembly (WPA)** ### Water Recycling System Stages and Technologies #### 1. Urine Processor Assembly (UPA) The **UPA** is responsible for recovering water from the crew's urine. This is one of the most critical steps in maximizing water recovery aboard the ISS. The UPA consists of multiple stages: - **Distillation via a Rotating Drum**: Since water doesn't boil properly in microgravity, the UPA uses a specialized **vacuum distillation process**. It employs a **rotating drum** that helps separate water vapor from urine. The vacuum reduces the boiling point, allowing water to evaporate at a lower temperature. As the drum rotates, it prevents solids from sticking and clogging the system. - **Filtration and Chemical Treatment**: After distillation, the water vapor is passed through a **filtration system** and chemically treated with substances like **phosphoric acid** and **hydrogen peroxide** to remove any remaining organic contaminants, salts, or other impurities. These processes help eliminate harmful components such as **urea** and **ammonia** that would otherwise remain in the water. - **Brine Processor**: A portion of the leftover concentrate from urine processing (called **brine**) is still rich in water. NASA is developing a **brine processor** to extract even more water from this byproduct, which will further increase the water recovery rate. This brine processor could eventually boost water recovery from urine beyond 93-94%. #### 2. Water Processor Assembly (WPA) The **WPA** is responsible for purifying all recovered water from multiple sources, including urine, cabin humidity, and sweat. This assembly is more complex, involving several purification steps to ensure that the water is potable (drinkable). The process includes: - **Multifiltration Beds**: The first stage is a series of **filter beds** that trap particulates and remove organic compounds, such as volatile organic chemicals (VOCs). These beds are designed to absorb any contaminants that could be harmful if consumed. - **Catalytic Oxidation**: Next, the water goes through a **catalytic oxidation reactor** where any remaining organic compounds are broken down. In this stage, **hydrogen peroxide** is introduced into the water and then broken down using a catalyst, creating oxygen that oxidizes (destroys) organic contaminants, ensuring that the water is safe for long-term consumption. - **Ion Exchange**: The WPA also includes **ion exchange technologies** to remove salts and other dissolved ions, such as calcium and magnesium, that might build up in the water. This stage ensures that the water does not accumulate excess minerals over time. - **Final Chemical Disinfection**: Before being stored for use, the water is subjected to a final **chemical disinfection** step. A small amount of iodine is typically added to prevent bacterial growth in the water tanks. This is similar to the iodine used to purify water for drinking in camping or wilderness situations on Earth. The water is tested regularly onboard the ISS to ensure that it meets strict standards for human consumption. ### Water Recovery Rate and Storage Capacity The ISS water recycling system has a recovery rate of about **93-94%**, meaning that nearly all water produced from the crew’s metabolic processes and cabin humidity can be recovered and reused. This high rate of recovery is critical for minimizing the amount of water that needs to be resupplied from Earth. - **Water Consumption**: Each astronaut uses about **11 liters (3 gallons)** of water per day. This includes drinking water, hygiene activities (handwashing, brushing teeth), and food preparation. - **Water Volume Onboard**: At any given time, the ISS carries around **400 liters (105 gallons)** of water, including recycled water and reserves brought from Earth. This reserve ensures that the station has enough water to support the crew for a period even if the recycling system encounters a problem. ### Movement of Water Through the ISS Water System Water on the ISS follows a **circular path** as it moves through different systems: 1. **Crew usage**: Water is consumed for drinking, rehydrating food, hygiene, and other daily activities. It then leaves the body as **urine**, **sweat**, or **exhaled water vapor**. Water used for cleaning or experiments may also be returned to the recycling system. 2. **Collection of Wastewater**: Urine is collected through specialized toilets and sent to the **Urine Processor Assembly (UPA)**, while **sweat** and **exhaled water vapor** are captured by the **Condensing Heat Exchanger** from cabin humidity. This humidity is routed to the **Water Processor Assembly (WPA)** for purification. 3. **Water Purification and Reuse**: Once processed, clean water is returned to the station's potable water system. It is stored in water bags and tanks, where it is readily available for crew use. A portion of the water is always kept as a backup supply in case of system malfunctions. 4. **Repeat Cycle**: The water continually moves through this cycle, allowing for long-term habitation with minimal resupply. ### Innovations and Future Developments While the ISS water system is already extremely efficient, NASA and its partners are constantly working to improve the **water recovery rate**. The upcoming introduction of the **Brine Processor Assembly** could increase the urine water recovery rate to almost **100%**, further reducing dependency on external resupply missions. The lessons learned from the ISS's water system are also being applied to future missions, particularly for longer-term exploration of **Mars** and **deep space**, where total self-sufficiency will be essential. ### Conclusion The ISS's water recycling system is one of the most advanced life support technologies in human spaceflight. With a recovery rate of around **93-94%**, the system ensures that almost all water used by astronauts is reclaimed and purified, making long-term space missions possible without a continuous supply of water from Earth. This system, which includes the **Urine Processor Assembly (UPA)** and the **Water Processor Assembly (WPA)**, uses a combination of **distillation, filtration, ion exchange, and chemical treatments** to ensure the crew has a continuous supply of clean, potable water, all while operating in the extreme environment of space.