Capillary structures could provide lower risk water recycling

Rows of small structures

Capillary Evaporator prototype with transparent capillary structures filled with test fluids. Credits: IRPI LLC

Human use a lot of water for drinking and hygiene. Recycling is a key strategy to make the water that is launched into space last longer. Existing water recycling methods in space use harmful chemicals or considerable energy, and do not recycle 100% of the water. Reliability is crucial as well. So the search continues for new approaches to improve the water recycling process.

NASA is considering capillary structures for water recycling. Capillary action involves electrostatic forces literally pulling water through small tubes, similar to how drops of water will hang on objects despite the force of gravity pulling them away. NASA’s capillary structures investigation studies “a new method of water recycling and carbon dioxide removal using structures designed in specific shapes to manage fluid and gas mixtures in microgravity.” The capillary structures equipment is made up of small, 3-D printed geometric shapes and sizes sizes (see above image).

This investigation also involved evaporation. “If you could do controllable evaporation in space, you could do all kinds of things” said Mark Weislogel, one of the project’s principal investigators.  “You could evaporate urine and recover all of the water. All of it. If you had a way of holding the liquid in a passive, no-moving-parts way like a puddle does on earth, but in space, then you could do a lot of unique processing, safely and with no maintenance.”

Just as with the capillary investigations, the evaporation “structures are set up to have different geometries, different angles, different heights, all these different parameters that we are varying across these structures to get quantitative data of evaporation in low gravity,” according to Kyle Viestenz, co-investigator for the project.

“If you could do controllable evaporation in space, you could do all kinds of things” said Mark Weislogel, one of the project’s principal investigators.  “You could evaporate urine and recover all of the water. All of it. If you had a way of holding the liquid in a passive, no-moving-parts way like a puddle does on earth, but in space, then you could do a lot of unique processing, safely and with no maintenance.”

Contractor held in hand.

Capillary Sorbent contactor with channels to expose liquid to ambient air. Credits: IRPI LLC

Another part of the investigation demonstrates the use of fluids in a carbon dioxide removal system, called the Carbon Dioxide Liquid Sorbent System. This system uses a network of “water falls” to bring a material used to absorb gases, into contact with air, allowing the carbon dioxide to be carried away by the liquid. In a microgravity environment, the liquid does not “fall,” but is driven by surface tension forces generated passively by the unique surface geometry of the capillary structures.

It is unknown if or when this technology would be deployed.

Further Information:

tanks and components mounted on three racks

NASA ISS water recovery equipment at Marshall Space Flight Center. Credit: SustainSpace.

Airbus ESA Advanced Closed Loop System (ACLS)

Two technicians point to large instrument box.

ACLS technology demonstrator generates oxygen and water in a closed system

The Advanced Closed Loop System (ACLS) is an advanced life support system that has been developed by Airbus for the European Space Agency (ESA) to be used as a technology demonstrator on the ISS, in the Destiny module, from summer 2018. The ACLS will be installed in the HTV-7 space transporter at the Tanegashima Space Center in Japan and is due to be transported to the ISS in August 2018. It is set to be operated for a period of one year.

The ACLS will purify air and produce oxygen for the International Space Station (ISS). Specifically, the ACLS extracts a portion of the carbon dioxide in the cabin atmosphere. Then, using hydrogen obtained from splitting water molecules, it will convert the carbon dioxide into methane and water in what is known as the Sabatier process. Oxygen is then produced from this water using electrolysis. Airbus asserts that this will increase overall system efficiency and hence reduce the need for supplies from Earth.

The main advantage claimed for the ACLS is the use of the adsorbent Astrine, a solid amine resin, which has a high adsorption capacity even at the carbon dioxide levels in the cabin air. The ACLS will be contained in an International Standard Payload Rack (ISPR).

A future use for the ACLS may be for the Deep Space Gateway / Lunar Outpost. NASA is reported to be looking to the ESA for part of the habitat.

Sources:

NASA Targets Reduced Water Usage for Long Duration Missions

NASA Water Recovery System

Water Recovery System (credit: NASA)

Water is essential for human survival on Earth and in space. A typical person requires between 3.5 to 15 liters per day. Yet launching large quantities of water up to the International Space Station (ISS) is terribly expensive, and would be a major impediment to future space settlement. So NASA has made water recycling a vital part of closing the life support “loop”. Meanwhile, water is often in short supply even on the Earth, especially in a clean, drinkable form.

First Steps

Russia’s space station Mir recycled cosmonaut sweat. An earlier NASA method of water re-use involved separating waste water into hydrogen and oxygen. The oxygen could be used for breathing and thus reduce the amount of oxygen that needed to be transported to ISS (NASA 2008).

NASA’s Current System on the ISS

NASA’s current water recycling system on ISS is the Water Recovery System (WRS), part of the Environmental Control and Life Support System (ECLSS). WRS water purification machines on the ISS cleanse wastewater in a three-step process. WRS first filters out particles and debris. Then, urine is processed though vapour compression distillation (VCD), while humidity condensate passes through multi-filtration beds to remove organic and inorganic impurities.

Products of the vapour and multi-filtration are mixed together and passed to a catalytic oxidation reactor that removes volatile organic compounds (VOC) and kills bacteria and viruses” (Carter at al. 2008). This system is designed to recycle about 90% of ISS waste water. ISS occupants are supplied with hydrated food that makes up the remaining 10%.

Watertech International device

(credit: Watertech International)

Similar technology is also being used in the field. Waterlife International produces a unit for local use. They have engaged in projects in Kenya, Rwanda, South Sudan and Cambodia. They are in talks to supply a provincial government in South Africa. Also see the Water Security Corporation.

Next Generation System

To do even better, the Alternative Water Processor (AWP) is currently being developed by NASA’s Next Generation Life Support Project, under the Game Changing Development Program.  It will support a crew of four each with 11 liters of water per day on a long-duration space flight mission. On such missions, food will likely be freeze-dried rather than hydrated, so it will not be available as a source of water. Hence, the newer AWP must have an even higher recovery rate than the older WRS.

AWP has a membrane-aerated bioreactor to destroy organic contaminate and a forward osmosis secondary treatment (FOST) system to remove dissolved solids. FOST uses saltwater as the drawing solution, then reverse osmosis is used to remove the salt, according to Michael Flynn, a research engineer at the NASA Ames Research Center.  (Also see NASA 2013)

Water Recovery System

FOST water processor (image credit NASA)

Construction of the first generation FOST was recently completed at the NASA Ames Research Center. The  system recently shipped to the NASA’s Johnson Space Center. FOST will undergo integrated testing with the membrane-aerated bioreactor, designed by Texas Tech University and constructed by NASA Johnson Space Center.  (NASA 2013)

“Inside the water recovery system is an evolving set of technologies with great promise,” said  Flynn. “Ultimately, these systems will continue to evolve and become increasingly more complex, integrated and smaller.”  (NASA 2013The new system has two major advantages. First, it allows for 95% recycling of waste water. Part of this improvement is due to the use of bioreactor to treat urine, which will avoid the need for highly toxic chromic acid. Second, the new system also processes a higher volume of waste water, so that crew will be able to engage in a greater range of water usage, such as hand-washing and laundry. Also, this new system is lighter, takes up less space and requires less energy per gallon processed, so it is better-suited for long-duration missions.

Bringing It Back to Earth

Sustainability Base water recycling system

Sustainability Base water recycling system

NASA’s FOST system is already finding use on Earth. NASA researchers installed a larger version of FOST in its Sustainability Base, which in combination with other water-saving technologies integrated into the building, is expected to reduce greywater consumption by more than 90 percent. (Total tap water usage is reduced by 40%.) Flynn points out that additional capabilities of FOST in the Sustainability Base include the ability to do long duration testing and failure prediction. It is certainly safer to do this on Earth than in space.

The Future

NASA’s next step may take inspiration from living systems, according to Flynn. Biomimicry is a form of engineering that imitates living systems. For example, in the human body, the small intestine is a highly efficient water absorption and filtering system that works reliably for many decades. It is also self-repairing. NASA is looking towards designing a system that emulates the positive qualities of the small intestine.

A Brief Lesson in Water Filtering

Knowing a few terms can help understand NASA’s technology. Water is typically recycled from several sources. Greywater is waste water from showers and sinks, and can include water from humidity. Blackwater contains fecal matter and urine, such as from toilets.  Reverse osmosis (RO) uses physical pressure to push water through a filter. In contrast, forward osmosis (FO) draws water through a filter using an ionic medium such as sugar or salt water, and has several advantages, such as reduced pore clogging.