Astronauts on the International Space Station do, in fact, drink water that was once urine. Every drop of crew urine, sweat, and even the moisture from their breath gets captured, processed, and purified back into drinking water. The system now recovers about 98% of all water on board, and NASA considers the finished product cleaner than most municipal tap water on Earth. The story of how that happens, and why it took decades to get right, is one of the more impressive feats of engineering that most people never hear about.
Why Recycling Everything Matters
Launching anything into low Earth orbit is expensive. Water is heavy, and a crew of six can go through liters of it per day for drinking, food preparation, and hygiene. Resupply missions from Earth can deliver fresh water, but every kilogram launched costs thousands of dollars. For the ISS, recycling urine and humidity isn’t a curiosity or a last resort. It’s the baseline plan that keeps the station running without constant water deliveries.
The math gets even more unforgiving when you think beyond the ISS. A round trip to Mars could last two to three years, and there’s no resupply vehicle that can catch up midway. If a crew can’t recover nearly all of its water from biological waste, the mission either carries an impossibly large water tank or doesn’t happen. That practical reality is what drove both NASA and the Russian space program to invest decades into urine processing technology.
How the ISS Turns Urine Into Drinking Water
The system that handles all of this is NASA’s Environmental Control and Life Support System, or ECLSS. It’s a collection of hardware spread across several racks in the station’s modules, and different pieces handle different waste streams. Urine gets its own dedicated piece of equipment called the Urine Processor Assembly, or UPA, which uses a process called vapor compression distillation. In simple terms, the UPA heats urine at low pressure until the water evaporates, leaving behind a thick brine of salts and contaminants. The vapor is then collected and condensed back into liquid water.
Vapor compression distillation was chosen because it works well in a closed system and doesn’t require large chemical inputs that would themselves need to be launched from Earth.1PubMed. Life testing of the vapor compression distillation urine processor assembly (VCD/UPA) at the Marshall Space Flight Center The UPA was developed over roughly twenty years of ground testing before it was sent to the station, and it has been the primary urine-processing technology on the American side of the ISS since installation.2SAE International. Development Status of the International Space Station Urine Processor Assembly
But the water that comes out of the UPA isn’t ready to drink yet. It gets routed into the Water Processor Assembly, or WPA, which handles water from multiple sources: the UPA distillate, humidity condensate pulled from cabin air, and wastewater from handwashing or other hygiene activities. Inside the WPA, the water passes through a series of filters and ion-exchange beds that remove particles and dissolved chemicals. The final and most critical step is catalytic oxidation, where the water flows through a reactor packed with a noble-metal catalyst. Oxygen is bubbled through the water in the presence of this catalyst, which breaks down any remaining organic contaminants into harmless byproducts like carbon dioxide and water.3SAE International. Catalytic Oxidation Model Development of the Volatile Reactor Assembly Unit of the International Space Station Water Processor After catalytic oxidation, the water is tested and iodine is added as a disinfectant before it enters the potable water supply.
From 93% to 98% Recovery
For years, the ECLSS achieved roughly 93% water recovery, meaning that for every 100 kilograms of wastewater that entered the system, about 93 kilograms came back as clean water.4Water Resources Research. Sustainable Water Systems in Space: A Review of Current Technologies and Future Prospects That sounds high, and it is, but the remaining 7% represented a real problem. Most of what was lost was the concentrated brine left over after the UPA finished distilling urine. That brine still contained recoverable water, but extracting it from the thick, salty residue required entirely different hardware.
NASA’s answer was the Brine Processor Assembly, or BPA. This device takes the leftover brine from the UPA and heats it further, evaporating additional water from what was essentially the system’s waste product. As of mid-2024, the BPA had completed 41 full operational runs over 612 days of active use and recovered an estimated 741 kilograms of water that would otherwise have been lost. NASA calculates the cost savings at over $80 million, based on the mass of water that didn’t need to be launched to the station or discarded. The BPA has recovered six times as much water as the total mass of the hardware itself and all its consumables combined, which is an extraordinary efficiency ratio for any piece of space equipment.553rd International Conference on Environmental Systems. Brine Processor Assembly 2023-24: Operational Successes and Challenges on the International Space Station
With the BPA online, NASA has announced that the ISS now achieves 98% water recovery. That figure is considered a milestone because it’s close to the threshold engineers believe is necessary for crewed missions to Mars and other deep-space destinations. Getting from 93% to 98% may sound like a marginal improvement, but over the course of a multi-year mission, those extra percentage points translate to hundreds of kilograms of water that don’t have to be carried from Earth.
Sweat, Breath, and Humidity
Urine gets the headline attention, but the ISS also recovers water from less dramatic sources. Crew members sweat, exhale moist air, and wash their hands. All of that moisture enters the cabin atmosphere as humidity. The station’s air-handling systems include condensing heat exchangers that cool the air until water vapor condenses on their surfaces, much like water beading on the outside of a cold glass. That condensate is collected and fed into the Water Processor Assembly alongside the UPA distillate.
This humidity condensate is actually a mix of everything the crew and their environment put into the air: exhaled water vapor, sweat that evaporates off skin, moisture released during food preparation, and even trace amounts of chemical off-gassing from equipment and materials inside the station. Early characterization work on spacecraft humidity condensate found that while the water itself is relatively clean compared to urine, it still contains enough dissolved organics and microbial contaminants to require full processing before anyone drinks it.6NASA Technical Reports Server. Characterization of spacecraft humidity condensate The WPA treats all incoming water the same way regardless of its source, so humidity condensate passes through the same filtration, ion exchange, and catalytic oxidation steps that urine distillate does.
The Zero-Gravity Problem
One thing that makes all of this harder than it sounds is the absence of gravity. On Earth, water recycling and purification rely heavily on gravity to separate liquids from gases. Water settles to the bottom of a tank. Air bubbles rise to the top. You can let contaminated water trickle down through a packed filter bed. None of that works in orbit. Water doesn’t run downhill, and bubbles don’t rise to the surface.7International Conference On Environmental Systems. Zero Gravity Phase Separator Technologies – Past, Present and Future
Engineers had to develop entirely new ways to handle two-phase flows, meaning situations where liquid water and gas exist together in the same pipe or chamber. Standard equipment like packed towers, spray chambers, and bubble columns that work beautifully on the ground are useless in microgravity.8International Conference On Environmental Systems. Microporous Hydrophobic Hollow Fiber Modules for Gas-Liquid Phase Separation in Microgravity Instead, the ISS uses technologies like rotary separators that spin fluids to create artificial gravity inside a small chamber, and microporous hollow-fiber membrane modules that use surface tension and capillary forces rather than gravity to keep gas and liquid apart. These components are threaded throughout the ECLSS, and each one had to be designed, tested, and validated for long-term use in an environment where the usual physics of fluid behavior simply don’t apply.
This is one reason space water recycling took so long to mature. The core chemistry of distillation and catalytic oxidation was well understood on Earth for decades. The hard part was making all of it work reliably inside floating, sloshing, bubble-filled plumbing that behaves nothing like plumbing on the ground.
Russia Got There First
While NASA spent years developing the UPA for the ISS, the Russian space program had already been processing urine in orbit since 1990. The Soviets and then Russians operated space stations continuously for decades, starting with the Salyut series and continuing through Mir, and the need for water recycling was acute on those long-duration missions. Their approach to urine processing used different technology than the American VCD system, but the goal was the same: reclaim as much water as possible from crew urine to reduce dependence on resupply from Earth.9International Conference On Environmental Systems. A Description and Comparison of U.S. and Russian Urine Processing Hardware for the International Space Station
On the ISS, both systems were originally planned to operate in parallel, with Russian hardware processing urine on the Russian segment and the American UPA handling the U.S. segment. In practice, the integration has been more complicated. The Russian urine processor has had various operational issues over the years, and there have been periods when crew urine from the Russian segment was transferred to the American side for processing. The fact that two independent space programs arrived at different engineering solutions for the same problem, and then had to make those solutions coexist on a shared station, is a detail that doesn’t get enough appreciation. It also provided valuable redundancy: if one system went down, the other could pick up the slack.
Water From Thin Air (Well, From COâ‚‚)
There’s another water source on the station that most people don’t think about: carbon dioxide. Crew members exhale COâ‚‚ with every breath, and the station has to scrub it from the cabin air to keep the atmosphere safe. Rather than just venting it into space, the ISS runs some of that captured COâ‚‚ through a Sabatier reactor. In this device, carbon dioxide reacts with hydrogen in the presence of a nickel-based catalyst to produce methane and water.10Nature Communications. A membrane Sabatier system for water recovery and rocket propellant production The water is a byproduct of the reaction, but it’s a welcome one. It gets fed into the water processing system just like everything else.11Fuel. Exploring water production dynamics in a Sabatier reactor: A comprehensive experimental investigation
The Sabatier process also closes part of the station’s oxygen loop. Oxygen on the ISS comes largely from electrolysis, which splits water into hydrogen and oxygen. The crew breathes the oxygen and exhales COâ‚‚. The Sabatier reactor takes that COâ‚‚ and some of the leftover hydrogen and turns it back into water, which can then be electrolyzed again to make more oxygen. It’s not a perfectly closed loop — the methane byproduct is currently vented overboard, which represents a loss of hydrogen atoms that have to be made up eventually — but it dramatically reduces the amount of water and oxygen that needs to come from Earth.
What the Water Tastes Like
Astronauts who have been asked this question almost universally say the recycled water tastes fine. Some have described it as tasting like any other purified water, and a few have said it actually tastes better than certain municipal tap water supplies they’ve encountered on Earth. This shouldn’t be surprising given how thoroughly the water is processed. By the time it reaches the potable water dispenser, it has been distilled, filtered through multiple stages, run through a catalytic oxidation reactor, and disinfected. The final product meets strict quality standards that are at least as rigorous as, and in some ways tighter than, EPA drinking water standards in the United States.
The psychological dimension is real, though. Early on, there was genuine concern about whether crew members would be willing to drink water they knew came from urine. NASA invested in public demonstrations, educational materials, and transparent communication about the purification process partly to address crew comfort and partly to build public support for the technology. Astronauts have been good-humored about it — there’s a famous quip, often attributed to Don Pettit, that “yesterday’s coffee becomes tomorrow’s coffee” on the ISS. The reality is that once you understand the level of purification involved, the water’s origin is more of a conversation starter than a concern.
Getting Water on the Moon and Mars
For future missions beyond low Earth orbit, water recycling alone won’t be enough. Crews heading to the Moon or Mars will need to supplement their closed-loop systems with water harvested from local resources, a concept NASA calls in-situ resource utilization. On the Moon, orbital surveys have confirmed the presence of water ice in permanently shadowed craters near the poles. The question is how to extract it efficiently.
Two approaches are under active development. One involves thermally heating lunar regolith — the rocky, dustite soil covering the Moon’s surface — to drive off water vapor, which can then be condensed and collected. Modeling work supports the viability of thermal extraction for future resource architectures on the Moon.12Planetary and Space Science. Thermal extraction of water ice from the lunar surface II – vapor yields for an improved regolith model A second approach uses microwave heating, which can penetrate deeper into icy regolith and liberate water from subsurface layers. Experimental work has demonstrated that microwave heating can produce substantial quantities of water from simulated icy lunar soil.13PubMed Central. Massive Water Production from Cryogenic Icy Lunar Regolith by a Microwave Heating Method
Mars presents a different set of opportunities. The atmosphere is roughly 96% carbon dioxide, which means a Sabatier reactor or similar system could generate water directly from the Martian air if a hydrogen source is available. Mars also has water ice at its poles and possibly as subsurface deposits at lower latitudes. A crewed Mars mission would likely combine aggressive onboard recycling — building on the 98% recovery now proven on the ISS — with local water harvesting to create a sustainable supply. The recycling systems currently being perfected in orbit are, in a very real sense, dress rehearsals for the technology that will keep crews alive on the way to Mars and after they land.
Why the Water Is Probably Cleaner Than Yours
One persistent misconception is that recycled urine water must be somehow inferior to “fresh” water. In practice, the ISS water system produces extremely pure water precisely because it uses distillation followed by catalytic oxidation and disinfection. Most municipal water treatment plants on Earth rely on different processes — coagulation, sedimentation, sand filtration, and chlorination — that work well for large volumes but leave behind trace contaminants that the ISS system would catch. The station’s water undergoes regular quality testing, and if any sample falls outside specifications, the entire batch is reprocessed before it’s released into the potable supply.
There’s a useful comparison to think about here. On Earth, every glass of water you drink has been recycled by the planet’s own hydrological cycle. The water in your tap was once rain, which was once evaporated from rivers and oceans that contain, among other things, the urine and waste of every animal on the planet. The ISS just runs a much tighter, faster, more controlled version of the same basic loop: contaminated water goes in, energy and engineering separate the water from the contaminants, and clean water comes out the other side. The process on the station is more deliberate and more thoroughly monitored than anything that happens in a watershed.