ISS ECLSS
The Environmental Control and Life Support System is a critical component of the International Space Station, responsible for maintaining a safe and habitable environment for crew members, similar to that of Earth, with an air pressure equivalent to sea level. Maintaining an Earth-like atmosphere enhances crew comfort and safety, and is significantly safer than a pure oxygen environment.
The various subsystems of the ISS ECLSS regulate atmospheric pressure, control temperature and humidity, remove carbon dioxide, manage oxygen and nitrogen levels, provide ventilation, treat sewage, and generate potable water.
The system was jointly designed and tested by NASA's Marshall Space Flight Center, UTC Aerospace Systems, Boeing, Lockheed Martin, and Honeywell. In addition to its primary functions, the ECLSS serves as a proof of concept for future, more advanced life support systems intended for deep space missions.
As of the 2020s, the ISS ECLSS continues to operate as a long-duration, closed-loop life support system and has incorporated technology demonstrators and operational upgrades to support extended mission timelines. Data and operational experience from ECLSS are being actively used to inform life support designs for the Lunar Gateway and future crewed Mars missions.
Water recovery systems
The ISS has two water recovery systems. Zvezda contains a water recovery system that processes water vapor from the atmosphere that could be used for drinking in an emergency but is normally fed to the Elektron system to produce oxygen. The American segment has a Water Recovery System installed during STS-126 that can process water vapour collected from the atmosphere and urine into water that is intended for drinking. The Water Recovery System was installed initially in Destiny on a temporary basis in November 2008 and moved into Tranquility in February 2010.The Water Recovery System consists of a Urine Processor Assembly and a Water Processor Assembly, housed in two of the three ECLSS racks.
The Urine Processor Assembly uses a low pressure vacuum distillation process that uses a centrifuge to compensate for the lack of gravity and thus aid in separating liquids and gasses. The Urine Processor Assembly is designed to handle a load of 9 kg/day, corresponding to the needs of a 6-person crew. Although the design called for the recovery of 85% of the water content, subsequent experience with calcium sulfate precipitation has led to a revised operational level of recovering 70% of the water content.
Water from the Urine Processor Assembly and from waste water sources are combined to feed the Water Processor Assembly that filters out gasses and solid materials before passing through filter beds and then a high-temperature catalytic reactor assembly. The water is then tested by onboard sensors and unacceptable water is cycled back through the water processor assembly.
The Volatile Removal Assembly flew on STS-89 in January 1998 to demonstrate the Water Processor Assembly's catalytic reactor in microgravity. A Vapour Compression Distillation Flight Experiment flew, but was destroyed, in STS-107.
The distillation assembly of the Urine Processor Assembly failed on 21 November 2008, one day after the initial installation. One of the three centrifuge speed sensors was reporting anomalous speeds, and high centrifuge motor current was observed. This was corrected by re-mounting the distillation assembly without several rubber vibration isolators. The distillation assembly failed again on 28 December 2008 due to a high motor current and was replaced on 20 March 2009. Ultimately, during post-failure testing, one centrifuge speed sensor was found to be out of alignment and a compressor bearing had failed.
By the late 2010s, the Water Recovery System was consistently achieving water recovery rates of approximately 90% when accounting for humidity condensate and wastewater combined, significantly reducing the need for water resupply from Earth and demonstrating the feasibility of sustained closed-loop water recycling in orbit.
Atmosphere
Several systems are currently used on board the ISS to maintain the spacecraft's atmosphere, which is similar to the Earth's. Normal air pressure on the ISS is 101.3 kPa ; the same as at sea level on Earth. "While members of the ISS crew could stay healthy even with the pressure at a lower level, the equipment on the Station is very sensitive to pressure. If the pressure were to drop too far, it could cause problems with the Station equipment."The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station.
The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation canisters.
Carbon dioxide is removed from the air by the Vozdukh system in Zvezda. One Carbon Dioxide Removal Assembly is located in the U.S. Lab module, and one is in the US Node 3 module. Other by-products of human metabolism, such as methane from flatulence and ammonia from sweat, are removed by activated charcoal filters or by the Trace Contaminant Control System.
Continuous atmospheric monitoring and redundancy between U.S. and Russian systems have enabled long-term crew occupancy without major atmospheric incidents, even during periods of hardware failure or delayed resupply missions.
Air revitalization system
Carbon dioxide and trace contaminants are removed by the Air Revitalization System. This is a NASA rack, placed in Tranquility, designed to provide a Carbon Dioxide Removal Assembly, a Trace Contaminant Control Subassembly to remove hazardous trace contamination from the atmosphere and a Major Constituent Analyser to monitor nitrogen, oxygen, carbon dioxide, methane, hydrogen, and water vapour. The Air Revitalization System was flown to the station aboard STS-128 and was temporarily installed in the Japanese Experiment Module pressurised module. The system was scheduled to be transferred to Tranquility after it arrived and was installed during Space Shuttle Endeavour mission STS-130.Oxygen generating system
The Oxygen Generating System is a NASA rack which electrolyses water from the Water Recovery System to produce oxygen and hydrogen, like the Russian Elektron oxygen generator. The oxygen is delivered to the cabin atmosphere. The unit is installed in the Destiny module. During a spacewalk, STS-117 astronauts installed a hydrogen vent valve required to operate the OGS. The OGS was delivered in 2006 by STS-121, and became operational on 12 July 2007. From 2001, the US orbital segment had used oxygen stored in a pressurized tank on the Quest airlock module, or from the Russian service module. Prior to the activation of the Sabatier System in October 2010, hydrogen and carbon dioxide extracted from the cabin was vented overboard.In October 2010, the OGS stopped running well due to the water input becoming slightly too acidic. The station crew relied on the Elektron oxygen generator and oxygen brought up from Earth for six months. In March 2011, STS-133 delivered the repair kit, and the OGS was brought into full operation.
Advanced Closed Loop System
The Advanced Closed Loop System is an ESA rack that converts carbon dioxide and water into oxygen and methane. The is removed from the station air by an amine scrubber, then removed from the scrubber by steam. 50% of the is converted to methane and water by a Sabatier reaction. The other 50% of carbon dioxide is jettisoned from the ISS along with the methane that is generated. The water is recycled by electrolysis, producing hydrogen and oxygen. This is very different from the NASA oxygen-generating rack that is reliant on a steady supply of water from Earth in order to generate oxygen. This water-saving capability reduced the needed water in cargo resupply by 400 liters per year. By itself it can regenerate enough oxygen for three astronauts.The ACLS was delivered on the Kounotori 7 launch in September 2018 and installed in the Destiny module as a technology demonstrator. It was successful, and remains on board the ISS permanently.
Following its successful demonstration, ACLS data has been incorporated into ESA and NASA life support development programs, with particular emphasis on reducing consumables and increasing system autonomy for exploration-class missions.
ACLS has three subsystems:
- The Carbon dioxide Concentration Assembly uses an amine reaction to absorb and concentrate carbon dioxide from cabin air to keep carbon dioxide within acceptable levels.
- The Carbon dioxide Reprocessing Assembly. A Sabatier reactor reacts from the CCA with hydrogen from the OGA to produce water and methane.
- The Oxygen Generation Assembly, electrolyses water into oxygen and hydrogen.
NASA Sabatier system
Elektron
Elektron is a Russian Electrolytic Oxygen Generator, which was also used on Mir. It uses electrolysis to convert water molecules reclaimed from other uses on board the station into oxygen and hydrogen. The oxygen is vented into the cabin and the hydrogen is vented into space. The three Elektron units on the ISS have been plagued with problems, frequently forcing the crew to use backup sources. To support a crew of six, NASA added the oxygen generating system discussed above.In 2004, the Elektron unit shut down due to unknown causes. Two weeks of troubleshooting resulted in the unit starting up again, then immediately shutting down. The cause was eventually traced to gas bubbles in the unit, which remained non-functional until a Progress resupply mission in October 2004. In 2005, ISS personnel tapped into the oxygen supply of the recently arrived Progress resupply spacecraft when the Elektron unit failed. In 2006, fumes from a malfunctioning Elektron unit prompted NASA flight engineers to declare a "spacecraft emergency". A burning smell led the ISS crew to suspect another Elektron fire, but the unit was only "very hot". A leak of corrosive, odorless potassium hydroxide forced the ISS crew to don gloves and face masks. It has been conjectured that the smell came from overheated rubber seals. The incident occurred shortly after STS-115 left and just before arrival of a resupply mission. The Elektron did not come back online until November 2006, after new valves and cables arrived on the October 2006 Progress resupply vessel. The ERPTC was inserted into the ISS to prevent harm to the systems. In October 2020, the Elektron system failed and had to be deactivated for a short time before being repaired.