If humans ever want to work and live in space, whether in habitats on the Moon or Mars or in stations far from Earth, a reliable source of clean drinking water is essential. This presents many challenges in space, where resources are limited, and resupply missions are costly, time-consuming, or both. For starters, humans cannot survive for more than three days without water. Water is also essential for oxygen generation, irrigating edible plants, and hygiene. Meeting these requirements requires a closed-loop system that can provide clean water for months to years without replenishment.
According to a recent study published in *Water Resources Research*, the Environmental Control and Life Support System (ECLSS) aboard the International Space Station is a good example of the progressing being made in this area. To date, the ECLSS has demonstrated the ability to recover 93% of the water lost by astronauts through urine, sweat, and humidity. However, the authors note that significant challenges remain and explore multiple approaches to realizing Sustainable Water Systems (SWS) that are energy-efficient, durable, and capable of providing a steady supply of clean water.
The review paper was led by David Bamidele Olawade, a public health researcher at the School of Health, Sport and Bioscience at the University of East London, the Medway NHS Foundation Trust, and York St John University. He was joined by James O. Ijiwade, a researcher of Environmental Science and Nanotechnology at the University of Ibadan, Nigeria; and Ojima Zechariah Wada, a postdoctoral researcher of water management and environmental biotechnology at Hamad Bin Khalifa University, Qatar.
Challenges
As they note in their review, the ECLSS system aboard the ISS provides a blueprint for closed-loop water reclamation, but improvements are needed for future applications. While the ISS can be resupplied with water within a matter of hours, the logistical challenges are considerable. Official estimates suggest that the process can cost tens of thousands of dollars per kilogram, and the cost increases exponentially for more distant missions. In addition to the high cost, matters are further complicated by limited payload capacity, which further restricts the cargo that resupply missions can carry.
Current systems like the ECLSS are too power-intensive for use beyond LEO and not efficient enough to be sustainable over indefinite periods. Moreover, extracting resources in off-Earth locations faces challenges such as microgravity, vacuum conditions, temperature fluctuations, weight limitations, and analysis and communication issues. In remote environments like the lunar South Pole or deep space, where access to solar power is limited by long periods of darkness, alternative energy sources must be developed.
There's also the issue of maintenance, since conventional water recycling systems are subject to corrosion and wear and tear over time. On long-duration missions, the ability to perform regular maintenance is limited, making system durability paramount. To address these challenges, Olawade and his colleagues considered recent advancements in filtration systems, disinfection methods, and autonomous technologies. Systems like the ECLSS on the ISS, they note, offer a blueprint for closed-loop water reclamation. However, future systems need to be more energy-efficient and designed to resist corrosion and other mechanical issues.
Living off the Land
In their review, the authors emphasize the importance of In-Situ Resource Utilization (ISRU), a vital aspect of all plans for future lunar and Martian exploration. Per the Artemis Program, NASA plans to establish a lunar base in the Moon's South Pole-Aitken Basin, a heavily cratered region. The same is true of China's International Lunar Research Station (ILRS) and the European Space Agency's plans to create an international Moon Village. This destination is favorable because of the abundant water ice located in craters - aka permanently shadowed regions (PSRs) - in the southern polar region.
Similar considerations inform the planning for future missions to Mars. For years, robotic missions have surveyed the surface for water sources, particularly in the mid-latitudes. However, extracting and purifying extraterrestrial water poses technical and logistical challenges, including the need for specialized equipment to access and process water reserves buried in regolith. On Mars, there's also the question of subsurface water quality, given the high levels of perchlorates and other harmful organic compounds.
This requires advanced extraction and purification systems to render these water sources acceptable for human consumption and life-support systems. They also require power systems that are similarly sustainable, durable, and well-suited to extreme environments.
Power Requirements
To summarize, water systems for space need to be closed-loop, efficient, and durable while requiring minimal power. To address the significant energy demands of extraction and purification systems, the authors consider various solar and solar-thermal energy applications. Such systems could provide clean energy for pumping, desalination (via reverse osmosis or electrodialysis), and powering purification methods like photocatalysis and filtration. They are also suitable for decentralized, distributed systems, which are ideal for habitats in extraterrestrial environments, where power plants and grid systems are infeasible.
In addition, photothermal systems convert solar radiation into heat, which can be used in processing ranging from solar distillation to desalination. Hybrid PV-thermal solutions offer additional efficiency by simultaneously generating power for pumps and filters while desalinating and disinfecting water supplies. However, solar power is limited in environments like the Moon's polar regions due to extended periods of darkness, while Mars receives less solar radiation than Earth (about 43% to 60%). To this end, the authors explore small modular nuclear reactors, which are currently under consideration for future lunar and Martian bases through NASA's Kilopower Reactor Using Stirling Technology (KRUSTY) program.
Biological Systems
They also consider recent advancements in bioreactors and biofiltration, which are vital parts of Bioregenerative Life Support Systems (BLSS). NASA's research into BLSS technology has demonstrated the potential of bioreactors for recycling water from human and plant waste. Applications include microbial fuel cells (MFCs), which generate electricity as bacteria break down organic matter, offering the dual benefit of clean water and electricity. This technology is also very efficient in low-energy environments and can be used to recycle nutrients, supporting water management and agricultural needs.
While ion-exchange, ultraviolet, and ozone systems are effective for desalination, filtration, and disinfection, they require significant energy. In contrast, bioreactors use microorganisms to break down organic pollutants more efficiently and require less energy. However, bioreactors are still limited in some respects, which requires that additional methods be considered. "When combined, these techniques provide a dependable, multi-phase strategy for preserving water quality in space," the authors conclude.
In addition, sand-substrate biofilters are an option, a time-honored method for purifying water in rural areas or during wilderness expeditions. The simple method could replace chemical water purification systems for space missions, reducing costs and energy use. Electrochemical treatment and photocatalytic purification are two additional options; the former relies on electric currents to break down impurities, while the latter uses light-activated catalysts to clean without chemicals. Any or all of these systems would suffice for purifying water and recycling wastewater, especially that produced through urination, defecation, and humidity.
Nanotechnology
The authors also explore advances in nanomaterials, which include nanofiltration (NF) membranes and nanoparticles. The authors highlight graphene oxide as one of the most promising nanomaterials, particularly in membrane form. These are incredibly thin, strong, and possess unique properties that make them ideal for filtration, such as high surface area, durability, nanosized pores, and reactivity toward water pollutants. They can also be engineered with a specific pore size to filter out specific contaminants and are less prone to fouling.
Autonomous Systems
Finally, the authors consider how advances in machine learning and AI could be leveraged to improve water quality in space. The future of space exploration is likely to rely heavily on autonomous systems capable of operating with minimal human oversight, and water management is one area that could benefit greatly from them. As the authors indicated:
Water management systems that can autonomously monitor, adjust, and optimize water quality and recycling processes in real-time will be essential for maintaining long-term space habitats. These autonomous systems, driven by advances in artificial intelligence (AI) and machine learning, have the potential to revolutionize water management by predicting system failures, reducing energy consumption, and improving overall efficiency.
To accomplish this, AI-powered systems will need to be capable of intelligent, data-driven control, rather than reliance on pre-programmed schedules. These systems must rely on real-time sensor data to understand and react to water quality and the hardware employed. As an example, they cite deep convolutional neural networks and their ability to analyze water images and identify pathogens autonomously. They further cite existing machine learning algorithms, such as Random Forests and Support Vector Machines, which can process spectrometer sensor data to classify water and verify the effectiveness of treatment processes.
In terms of ISRU, AI systems could ensure water and power resources are used efficiently by coordinating extraction, purification, and distribution. The predictive power of AI also offers greater resilience in the face of changing conditions, sudden decreases in available energy, and other unforeseen challenges.
Each of these options presents potential benefits and downsides, the authors note. However, an integrative approach that balances these options to achieve maximum effectiveness and efficiency could ensure that future missions to the Moon, Mars, and other deep-space locations have a sustainable water supply. Combined with in-situ methods for growing food, removing waste, generating power, and meeting other basic necessities of living, humans will be able to extend their presence throughout the Solar System.
Further Reading: Eos, Water Resources Research