Water in space. 2 Water and wastewater

On November 23th, Diego Pozo, a Spanish space engineer, offered to the Young Water Professionals network a fantastic webinar about “Water in Space”.

In this series of 4 articles Diego explains us the content of his webinar for the people who couldn’t assist and for the not YWP members. Was a great webinar and is a great series of articles, so, in the name of all YWP’s all over the world, thank you Diego!!

You can read the firt part here: Water in space. 1 Experiments in micro gravity

One of the main and most obvious needs for Humans in Space, are the hydrating and the toilet needs. Since teleportable vending machines are not in use yet, and sending a gallon of water to Space slightly too expensive, water and waste recycling has always been a must for any Space adventure. In the case of ISS, the solution has been found with Water Recovery and Management (WRM) System, which ensures availability of potable water for crew drinking and hygiene, oxygen generation, urinal flush water, and payloads for operational needs if/as required. The WRS is comprised of the Urine Processor Assembly (UPA) and Water Processor Assembly (WPA). Figure 3 shows a schematic of the system.

Figure 4: WRS on-board the ISS

Waste water [1][2] is collected in the form of crew urine, humidity condensate, and Sabatier product water, and subsequently processed by the Water Recovery System (WRS) to potable water.

The figures 4 and 5 describe the recycling and processing loops for the whole system as well as the hardware and schematics details for both modules. The following synthesis of its functioning has been extracted from the referenced papers [] and []. The waste water bus receives humidity condensate from the Common Cabin Air Assemblies (CCAAs) on ISS, which condenses water vapor and other condensable contaminants and delivers the condensate to the bus via a water separator. In addition, waste water is also received from the Carbon Dioxide Reduction System. This hardware uses Sabatier technology to produce water from carbon dioxide (from the Carbon Dioxide Removal Assembly (CDRA)) and hydrogen (from the electrolysis process in the Oxygen Generation System).

Figure 5: WRM Flowchart [4]

Figure 6: WPA (above) / UPA (below) schematics [3]

Before starting explaining the schematic, it may be reminded that the ISS consists of the US segment conformed by several modules, the Russian segment, the Japanese laboratory Kibo (with an external facility), and the European module Columbus (Figure 6). Main Command of the ISS (final word if needed), lays in the hands of Houston Mission Control Center (MCC), its FCT and lastly in its Flight Director (FD) (Figure 7).

Figure 7: ISS Configuration. Source: Wikipedia

Figure 8: Houston MCC positions and center. Source: NASA


Waste water is typically delivered to the WPA Waste Tank, though the Condensate Tank located in the US Laboratory Module is available in the event the WPA Waste Tank is disconnected from the waste bus. If this is required, the crew must manually connect the Condensate Tank to the waste water bus. Once the WPA Waste Tank is online again, the crew will disconnect the Condensate Tank from the waste water bus. Condensate collected in this scenario must subsequently be offloaded into a Contingency Water Container (CWC). The CWC can then be emptied into the WPA waste tank via a pump, transferred to the Russian Segment for processing by the Russian Condensate Processor (referred to as the SRV-K) or vented overboard (though venting is highly discouraged due to the loss of water consumables and use of propellant required to maneuver the ISS into an acceptable attitude). It must be reminded that the ISS must be kept within a certain attitude window, in order not to interfere with debris, satellites (launch and communications constraints or hinders), and for safety of the station. Weight is costly in space, and thus ADS developed and just put in orbit the first heavy Geostationary Electric Orbit Raising (EOR) satellite: electric propulsion does not use disposable propellant thus cheaper and allowing for more space for Payload needs, at a lower price. On the same price lowering effort, reusable launchers have/are been developed by SpaceX and ADS among other companies in the sector.

Back to our dispose recycling, Crew urine is collected in the Waste & Hygiene Compartment (WHC), which includes a Russian Urinal (referred to as the ACY) integrated for operation in the US Segment. To maintain chemical and microbial control of the urine and hardware, the urine is treated with chemicals and flush water. The pretreated urine is then delivered to the Urine Processor Assembly (UPA) for subsequent processing. The UPA produces urine distillate, which is pumped directly to the WPA Waste Water Tank, where it is combined with the humidity condensate from the cabin and Sabatier product water, and subsequently processed by the WPA.

After the waste water is processed by the WRS, it is delivered to the potable bus. The potable bus is maintained at a pressure of approximately 230 to 280 kPa (19 to 26.5 psig) so that water is available on demand from the various functions. Users of potable water on the bus include the Oxygen Generation System (OGS), the WHC (for flush water), the Potable Water Dispenser (PWD) for crew consumption, and Payloads.

The Waste Water Tank includes a bellows that maintains a pressure of approximately 5.2 – 15.5 kPa (0.75 to 2.25 psig) over the tank cycle, which serves to push water and gas into the Mostly Liquid Separator (MLS). Gas is removed from the wastewater by the MLS (part of the Pump/Separator ORU), and passes through the Separator Filter ORU where odor-causing contaminants are removed from entrained air before returning the air to the cabin. Next, the water is pumped through the Particulate Filter ORU followed by two Multifiltration (MF) Beds where inorganic and non-volatile organic contaminants are removed. Once breakthrough of the first bed is detected, the second bed is relocated into the first bed position, and a new second bed is installed. The Sensor ORU located between the two MF beds helps to determine when the first bed is saturated based on conductivity. Following the MF Beds, the process water stream enters the Catalytic Reactor ORU, where low molecular weight organics not removed by the adsorption process are oxidized in the presence of oxygen, elevated temperature, and a catalyst. A regenerative heat exchanger recovers heat from the catalytic reactor effluent water to make this process more efficient.

Then, the Gas Separator ORU removes excess oxygen and gaseous oxidation by-products from the process water and returns it to the cabin. The Reactor Health Sensor ORU monitors the conductivity of the reactor effluent as an indication of whether the organic load coming into the reactor is within the reactor’s oxidative capacity. Finally, the Ion Exchange Bed ORU removes dissolved products of oxidation and adds iodine for residual microbial control. The water is subsequently stored in the Water Storage Tank prior to delivery to the ISS potable water bus. The Water Delivery ORU contains a pump and small accumulator tank to deliver potable water on demand to users. The WPA is controlled by a firmware controller that provides the command control, excitation, monitoring, and data downlink for WPA sensors and effectors.


Pretreated urine is delivered to the UPA either from the USOS Waste and Hygiene Compartment (outfitted with a Russian urinal) or via manual transfer from the Russian urine container (called an EDV). In either case, the composition of the pretreated urine is the same, including urine, flush water, and a pretreatment formula containing chromium trioxide and sulfuric acid to control microbial growth and the reaction of urea to ammonia. The urine is temporarily stored in the Wastewater Storage Tank Assembly (WSTA).

When a sufficient quantity of feed has been collected in the WSTA, a process cycle is automatically initiated. The Fluids Control and Pump Assembly (FCPA) is a four-tube peristaltic pump that moves urine from the WSTA into the Distillation Assembly (DA), recycles the concentrated waste from the DA into the Advanced Recycle Filter Tank Assembly (ARFTA) and back to the DA, and pumps product distillate from the DA to the wastewater interface with the WPA. The DA is the heart of the UPA, and consists of a rotating centrifuge where the waste urine stream is evaporated at low pressure. The vapor is compressed and subsequently condensed on the opposite side of the evaporator surface to conserve latent energy. A rotary lobe compressor provides the driving force for the evaporation and compression of water vapor. Waste brine resulting from the distillation process is stored in the ARFTA, which is a bellows tank that can be filled and drained on ISS. The ARFTA has less capacity (approximately 22 L) than the RFTA (41 L), but the capability to fill and drain the ARFTA on ISS avoids the costly resupply penalty associated with launching each RFTA. When the brine is concentrated to the required limit, the ARFTA is emptied into an EDV, a Russian Rodnik tank on the Progress vehicle, or into the water tanks on the ATV vehicle. Next, it is refilled with pretreated urine, which allows the process to repeat. The Pressure Control and Pump Assembly (PCPA) is another four-tube peristaltic pump which provides for the removal of non-condensable gases and water vapor from the DA. Liquid cooling of the pump housing promotes condensation, thus reducing the required volumetric capacity of the peristaltic pump. Gases and condensed water are pumped to the Separator Plumbing Assembly (SPA), which recovers and returns water from the purge gases to the product water stream. A

Firmware Controller Assembly (FCA) provides the command control, excitation, monitoring, and data downlink for UPA sensors and effectors.

The UPA was designed to process a nominal load of 9 kg/day (19.8 lbs/day) of wastewater consisting of urine and flush water. This is the equivalent of a 6-crew load on ISS, though in reality the UPA typically processes only the urine generated in the US Segment. Product water from the UPA has been evaluated on the ground to verify it meets the requirements for conductivity, pH, ammonia, particles, and total organic carbon. The UPA was designed to recover 85% of the water content from the pretreated urine, though issues with urine quality encountered in 2009 have required the recovery to be dropped to 74%. These issues and the effort to return to 85% recovery are addressed in the discussion on UPA Status.

WRM Status

An average estimate of 3340 L of potable water a year is supplied to the potable bus for Crew use and for the OGS. Without such careful recycling 40,000 pounds per year of water from Earth would be required to resupply a minimum of four crewmembers for the life of the station [3]. Animals need to be taken into account too, for both their drinking and their urinating needs. It has been calculated that 72 rats equal one human’s consumption. Nothing should be wasted in order to maximize our options of becoming a multiplanetary species.

Management of the water mass balance has continued to be a challenge due to the need to maintain 1002L of potable water on ISS for crew reserve, limited storage life of potable water in CWC-Is, and the need to minimize the introduction of free gas onto the potable bus.

Free gas is a significant issue in micro-gravity, since it cannot be removed from the water without a gas separator. As mentioned previously, the MRF addresses the free gas issue by using the 0.2 micron filtration to stop free gas during a CWC-I transfer to the WPA product tank. Free gas is vented from the housing by the crew as it accumulates. This procedure reduces crew time required for CWC-I transfer, but not without issue. First, enough free gas accumulates in the MRF housing during the transfer that it became necessary for the crew to still perform a short degassing procedure on each CWC-I prior to a transfer, thus reducing the crew time savings. To degas a CWC-I, the crew spins the CWC-I to coalesce the gas in one location, and then manipulates the bag to move the free gas to the CWC-I outlet port, where it can be vented into the cabin. Second, MRFs were only certified for two months of use, impacting their resupply and availability for use on ISS. A test is currently underway to extend the certified life of the MRF.

For further information on the WRM System, please refer to the references papers from which the explication above has been synthetized. The applications of this technology to deserted areas, or to big infrastructures such as airports, skyscrapers, etc. could make our water and energy utilization much more efficient. It could be an open action, as follow-up of this Webinar, for the YWP to explain when/how has this expertise being reused. NASA/ESA should be able to respond.

Next article: Water in space. 3 Space exploration 


  1. Status of ISS Water Management and Recovery; L. carter, C. brown, N. Orozco, NASA Marshall Space Flight Center, for the American Institute of Aeronautics and Astronautics
  2. Upgrades to the ISS Water Recovery System; M. Pruitt, L. Carter, R. M. Bagdigian and M. J.. Kayatin, NASA Marshall Space Flight Center, for the  45th International Conference on Environmental Systems, 2015
  3. NASA Science: Water on the Space Station. Link: 
  4. ESA’s Moon Village article. Link:
  5. Austrian Space Forum for analog Astronauts. Link:
  6. ESA’s JUICE. Link: 
  7. Exoplanets list within the habitable zone in Wikipedia. Link: ]; as well as the quest for Exoplanets, link:
  8. ESA’s Rosetta main website. Link:
  9. Rosetta wakes up from hibernation. Link: 
  10. Rosetta unveils comet’s water cycle. Link: 
  11. ROSINA instrument and comet’s water cycle. Link:
  12. ROSINA instrument website. Link:
  13. Evolution of water production of 67P/Churyumov–Gerasimenko: an empirical model and a multi-instrument study; various authors; September 2016; Monthly Notices of the Royal Astronomical Society, Volume 462, Issue Suppl_1, 16 November 2016, Pages S491–S506. Link:
  14. GSA website. Link:

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