The Space Architect's Diary #007
Water Recovery Systems in Space
Water, air, and food are the big threes that all living things on earth need to survive. This article is an overview of our research on how water is maintained and recycled in space and on the International Space Station (ISS).
Until we devise the means to mine asteroids, moons, and planetary bodies for Ice, we still need to transport, store and recycle water. The most elaborate water reclamation system designed is the Water Recovery System (WRS) on the ISS. WRS is one of the main parts of the ISS’s Environmental Control and Life Support System (ECLSS).
Water Recovery System (WRS) Overview
Currently, the ISS can support six crew members for an extended period. On average, astronauts are limited to 11 Liters of water per day. This water is allocated for drinking (2L), food rehydration, hot rehydration, personal hygiene, and contingency reserve. The existing ISS WRS can recycle around 6,000L of water annually, making about 25% of the astronauts’ annual water needs. The rest of the water that the astronauts need is transported from earth on resupply missions – water transported from the earth to the ISS net costs $2,720 (Falcon 9) per Liter.
The WRS reclaims water from urine, wastewater, cabin humidity condensate, and hydration systems in the astronaut’s EVA suits. The Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) that make up the WRS fit in two adjacent International Standard Payload Racks (ISPR), the WRS-1 & WRS-2. Worth noting that the Oxygen Generation System (OGS) rack is always positioned adjacent to the WRS racks.
Currently, the WRS has an overall water recovery of 93.5%. In 2021-2022, a new Brine Processor Assembly (BPA) will be shipped to the ISS and tied into the existing WRS, which will raise the water recovery to 98%.
WRS Interaction with other ISS Systems
- Temperature and Humidity Control – Condensate to WPA
- Universal Waste Management System (Toilet) – Pre-treated Urine to Urine Transfer System to UPA
- Habitable crew Cabin – Recycled air from UPA to crew Cabin
- Disposable Waste Storage (To be shipped out of ISS) – UPA (Brine Filter)
- Habitable crew Cabin – Recycled air from WPA to crew Cabin
- Habitable crew Cabin – Recycled air from crew Cabin to WPA (Waste Water Storage Tank)
- Disposable Waste Storage (To be shipped out of ISS) – WPA (Multifiltration Bed)
- ISS Moderate Temperature Loop (MTL) – MTL to WPA (Heat Exchanger)
- Onboard Oxygen Tanks – Oxygen to WPA(Catalytic Reactor Assembly)
- Habitable crew Cabin – Oxygen from WPA to crew Cabin
- Water Storage – Potable Water from WPA to Storage tank to be used as needed.
WRS How does it work (From Urin to Potable Water)
Urine Processing Assembly (UPA)
The ISS Tranquility toilet has a fan-driven suction system that sucks the urine and stores it in 20L storage tanks. The urine then gets processed in the UPA. The first stage is Distillation. The urine is pumped to the Distillation Assembly (DA). The drum inside the DA rotates, creating artificial gravity via centrifugal force. The pressure inside the DA is lowered by 80%. That reduces the boiling point of water to 25 °C. So, when the urine enters the DA, it immediately starts evaporating; the water vapor is collected and condensed by a compressor in the DA and sent to the waste water storage tank in the WPA.
The Evaporation causes the air trapped inside the urine to release. Extra air affects the pressure, thus the boiling point, so the air is expelled from the DA to a purge pump with a heat exchanger (Cold flow) that condenses any water vapor in the air. Both air and condensed water go through a Gas Separator Unit; the air is then routed to an odor filter and released to the crew cabin as recycled air. The water is spun outwards to the inner edge of the separator then pumped to the waste water storage tank in the WPA.
The remaining urine in the DA, the brine, is pumped out into the Advanced Recycle Filter Tank Assembly (ARFTA), where it is stored until there is enough brine to be compressed and forced through the brine filter. The output liquid from the filter, which has less brine, is fed back to the DA. The Brine filter is removed and changed when it becomes fully saturated. The saturated filter is stored to be shipped out of the ISS as waste.
Water Processing Assembly (WPA)
The Waste Water Storage tank collects greywater from different ISS systems like the UPA distilled water, wastewater, cabin humidity condensate, and hydration systems in the astronaut’s EVA suits. The greywater first passes through a gas separator unit that separates the air from the water. The air is flushed back to the crew cabin as recycled air. The water, which is still full of particulates, is then pumped into a Particulate Filter, which removes particles up to 0.5μm, like dust particles. Then the water is forced through a Multifiltration Bed to remove dissolved contaminants like bacteria. The Bed, once full, is replaced with a new one.
Next, the water is sent to the Volatile Removal Assembly (VRA). The VRA removes volatile organic compounds, like Ethanol and Acetic Acid, through a catalytic reaction where the water’s temp is raised to 267°C. Then oxygen is introduced in the catalytic reactor to oxidize and neutralize the dissolved organic volatiles.
The water coming from the Multifiltration Bed passes through a heat exchanger that transfers heat from the ISS Moderate Temperature Loop (MTA) to make the process as efficient as possible. It then passes through a heat recovery unit then a heater to raise its temperature to 267°C before going through the catalytic reactor. After leaving the reactor, the hot water passes through a heat recovery unit that transfers the heat to the water leaving the heat exchanger, creating an efficient semi-closed heat transfer loop.
The water leaving the VRA is then pumped into a gas separator unit that separates the oxygen from the water. The oxygen is flushed into the crew cabin as recycled oxygen. The water remaining in the gas separator is pumped into the ion exchange bed, removing dissolved oxidized products and adding iodine to the water. The output is potable water stored in the Product Water Tank to be used by the crew as needed.
And the Cycle begins again.