Aquatic Alchemy
Pulling Water from the Air
Every time an astronaut exhales, washes
up, or urinates, water is involved. In an effort to minimize
the amount of fluid lifted into orbit, the National Aeronautics
and Space Administration (NASA) is seeking ways to recapture
that water, clean it, and store it for reuse.
The ISS currently provides
clean water through the use of a water recovery system that reclaims
wastewater such as used oral hygiene water, urine, and cabin
humidity condensate. But the space environment offers unique
challenges to the provision of water. For example, water must
contain no dissolved gases, as gas doesn’t separate well
at zero gravity, either in tanks or in the body. Therefore, the
water must be purified to a level exceeding EPA standards for
Earth drinking water. The water recovery system consists of a
urine processor assembly (essentially a still that boils off
water, leaving a thick waste layer behind) and a water
processor assembly developed by researchers at NASA’s
Marshall Space Flight Center and Hamilton Sundstrand Space
Systems International.
Sources: Shiklomanov IA, State Hydrological Institute, United Nations
Educational, Scientific, and Cultural Organisation. 1999. World Resources
2000–2001—People and Ecosystems: The Fraying Web of Life. Washington,
D.C.: World Resources Institute; Harrison P, Pearce F. AAAS Atlas of Population
2001. Berkeley, CA: American Association for the Advancement of Science,
University of California Press. |
According to Dave Parker,
program manager for Hamilton Sundstrand’s water processor program, the
water processor assembly is a multistage system that uses
filters to remove particulate matter and salts. The water is
then run through a catalytic oxidizer to remove
low-molecular-weight organic molecules such as alcohols. This
part of the process takes place at a temperature of 275°F
and under pressure so it doesn’t flash to steam, Parker
explains. The system then removes any by-products and remaining
dissolved gasses. Finally, the fluid goes through an ion
exchange process to remove the oxidation products.
This process, according to Parker, can
produce about 1.5 gallons of treated water per hour, and uses
approximately 700 watts of power. To reduce maintenance times
and the volume of consumables that must be delivered from
Earth, the system has been designed with an 80- to 90-day
change-out schedule for particulate filters, and a 60- to
70-day schedule for chemical filters, with no more than 12
hours of maintenance time required per year.
The water processor assembly
is designed to provide limited amounts of highly purified water
with
minimal energy consumption and maintenance, but Parker believes
the system could be scaled up for Earth usage. “We’re producing as nearly pure potable water as
you’re likely to find anywhere,” he says.
“The question you’d have to answer on Earth is
whether you need water of that purity.”
Parker suggests the system
could perhaps be used in military applications, to protect crews
against
chemical or biological attack, or aboard naval vessels. Another
potential use is in hospitals, where high-quality water is
important. “We’re also working with the Army to
design a system to create potable water from diesel
exhaust,” he says.
There are also substances
often encountered in terrestrial water that you’ll never find
aboard the space station, such as arsenic, mercury, and heavy
metals, but Parker says the water processor assembly could be
adjusted to deal with these substances in a limited-application
water stream. “While you could plug this system into a
municipal water system, I suspect that the economics
wouldn’t work. The system operates to higher standards,
and avoids things that municipalities traditionally employ,
such as the addition of chlorine to sterilize water. NASA
doesn’t allow any chlorine aboard the ISS, so we use heat
instead. That wouldn’t be economical in a
multimillion-gallon-throughput municipal system.”
Further Out, Longer Stay
With NASA looking in more
detail at a manned Mars mission--which would involve a 40-million-mile
trip one way and 3-5 years--work has begun on a
fully regenerative water recycling system, one that can provide
a crew with adequate water for drinking and hygienic needs for
up to three years without recharging. Enter the Vapor Phase
Catalytic Ammonia Removal (VPCAR) system.
Michael Flynn, the project’s
principal investigator at NASA’s Ames Research Center,
says VPCAR has been designed to mimic the natural hydrologic
cycle. “On Earth, you open the tap, drink water, produce
waste, treat the waste, and discharge it back into the
ocean,” he explains. “The sun heats the water,
which evaporates and forms clouds. Those clouds are exposed to
ultraviolet radiation, which destroys organic contaminants, and
then rain falls to begin the cycle again. We’ve
integrated all of these processes into a single small machine.
We take in waste water, vaporize it, oxidize organic
contaminants, re-condense it, and the water is ready for
use.”
Precious cargo. A water recovery system developed by Hamilton
Sundstrand and NASA (right) processes fluids such as used oral hygiene
water, urine,
and cabin humidity condensate generated aboard the International
Space Station into potable water.
image(s): NASA; inset: Hamilton Sundstrand |
In general, says Flynn,
nonregenerative technologies (like the ISS’s water processor assembly),
are dominated by adsorptive technologies such as activated
carbon, which boast low power consumption. “You make the
trade-off of having a shuttle fly up every ninety days to
resupply expendables like filters because the system uses
relatively little power,” he says. On the other hand,
there will be no resupply opportunity on a manned Mars mission,
so it’s desirable to spend more on power than on
resupply. As an example, Flynn says, the nonregenerative
systems aboard the ISS only use an estimated 123 watt hours per
kilogram, compared to what he says is around 300 watt hours per
kilogram for the fully regenerative VPCAR.
VPCAR works by sending the waste stream
across a wiped-film rotating disk evaporator, which removes
inorganic salts and nonvolatile large-molecular-weight organic
contaminants. Lightweight organic molecules and ammonia, which
are volatilized in the evaporator, are oxidized by a
high-temperature catalytic oxidation reactor, converting these
organics into carbon dioxide, water, and nitrous oxide. This
high-temperature process also helps destroy any biologically
active organisms in the waste stream.
Full characterization studies of VPCAR
have been completed, and the system, Flynn says, meets all NASA
specifications. The next step will be a test aboard training
aircraft, followed by full-scale flight-testing during a
proposed lunar mission.
Flynn admits that VPCAR,
too, would have limited applications on Earth, although some
aspects of it are
being considered. The U.S. South Pole research station, for
example, has examined the evaporative portion of the system.
The research station wants to purify its sewage such that it
could be used to make an ice layer for its runway. “We
also have some rural Alaskan tanneries looking at using the
system to recycle their waste, and some oil companies are
looking at it as a technology to separate oil and water,”
Flynn says. “VPCAR is also being looked at as an
alternative method of distilling salt water into
fresh.”
A Hello to ARMS
While some researchers
are trying to make sure there are no bacteria in the water,
others are going out
of their way to welcome them. Tony Rector is a bioprocess
engineer with Dynamac Corporation at Kennedy Space Center,
where he and colleagues are working on a project called
ARMS--the Aerobic Rotational Membrane System.
ARMS consists of a clear
Plexiglas reactor vessel, filled with 115 tubes (dubbed “membranes”)
that are home to a community of bacteria. Oxygen moves from the
inside of each membrane to its outside surface, where bacteria
are present in colonies called biofilms. Contaminated water
flows past these biofilms, where the bacteria can use the
oxygen to transform undesirable compounds found in the
wastewater to less harmful compounds.
Biological treatment reactors
using membranes aren’t new technology, but this system
is innovative in that the membranes rotate. This exposes more
of
the bacteria to more of the infused oxygen and the contaminants
that provide their nourishment.
“Biological systems like these can
achieve high treatment efficiencies with low mass and energy
requirements,” says Rector. “By rotating the
membranes, we can enhance mass transfer, making the system as
efficient as possible.”
As living organisms, bacteria are subject
to many of the same stresses that will impact human astronauts.
If the biocommunity were to go down due to some shock (for
example, an unexpected radiation dose or loss of heat or
oxygen) it could be days before the bacteria could be
resuscitated.
“Part of our long-term plan is to
subject these organisms to a variety of shocks, and see how
they react and recover from such events,” says Rector.
“We’d also like more information on their ideal
living conditions, what bacterial species are most tolerant and
resilient, and other information of that sort.”
Rector’s research
group will also be examining different biochemical processes
in a variety of
bacterial species, including nitrification bacteria and
hydrogen-oxidizing bacteria. Potential candidate bacteria
will have to be evaluated for their tolerance to
pharmaceuticals, hormones, antibiotics, and other substances
that may be excreted by the astronauts using the system.
Space drinks. The Aerobic Rotational Membrane System (above) is a system
of tubes (“membranes”) containing biofilms. Contaminated water
flows past the biofilms, and bacteria use oxygen supplied from inside the
membrane to convert waste compounds in the water. The Vapor Phase Catalytic
Ammonia Removal system (right) mimics the natural hydrologic cycle and
could find a berth on a manned Mars mission.
image(s): Left to right: NASA Kennedy
Space Center; NASA Ames Research Center |
|
Because of the potential
sensitivity of the living organisms, and because bacteria are
unable to
process all of the materials in a typical waste stream, Rector
envisions using ARMS in space as a first phase in an overall
water treatment process, followed by a chemical/mechanical
process: “Our goal is to reduce the contaminant loading
on these processes and therefore reduce the size and energy
requirements for larger-scale physical and chemical
systems.”
“Bioregenerative research focusing
on water recovery has been conducted at NASA research centers
for many years. While we are in the initial stages of the ARMS
project, we are very encouraged by its current
performance,” says Rector. “Depending on mission
scenarios and timelines, it could be five or six years before
we can complete our investigation of the biological and
mechanical component aspects of this system.” He adds,
“I think there’s a lot of potential, but
there’s still a lot of research to be done.”
Rector believes ARMS could
well find a place in maritime applications, perhaps aboard cruise
ships,
which must thoroughly treat their waste before discharge. “This system will find its best use in a small
environment,” he says.
“Best use in a small
environment” seems to be a key descriptor of all of these
processes. None of the individuals involved in research on the
three systems envision them as replacements for large-scale
municipal water treatment facilities. But all agree they should
work--and work well--in a confined environment, where
it’s necessary to have very clean water and where all
that’s available as feedstock is what Flynn describes as
“the nastiest stuff imaginable.” The military,
medical facilities, any place with a need for high-quality
water supplies where conventional sources are
unreliable--all are potential down-to-Earth targets for
NASA’s spaceborne water systems.
Lance Frazer
Suggested Reading
Carrasquillo RL, Cloud D, Kundrotas RE. 2004. Status of the
node 3 regenerative ECLSS water recovery and oxygen generation systems.
SAE Technical Paper 2004-01-2384. Presented at: 34th International Conference
on Environmental Systems, 19-22 July 2004, Colorado Springs, CO.
Warrendale, PA: Society of Automotive Engineers International.
Dusenbury JS. 2003. Military land-based water purification
and distribution program. Presented at: Maintaining Hydration:
Issues, Guidelines, and Delivery, 10-11 December 2003, Boston, MA. Available: ftp://ftp.rta.nato.int/PubFullText/RTO/MP/RTO-MP-HFM-086/MP-HFM-086-11.pdf [accessed 11 January 2005].
Flynn MT. 2004. The development of the vapor phase catalytic
ammonia removal (VPCAR) engineering development unit. SAE
Technical Paper 2004-01-2495. Presented at: 34th International
Conference on Environmental
Systems, 19-22 July 2004, Colorado Springs, CO. Warrendale, PA: Society
of Automotive Engineers International.
Rector T, Garland J, Strayer RF, Levine L, Roberts M, Hummerick
M. 2004. Design and preliminary evaluation of a novel gravity
independent rotating biological membrane reactor. SAE Technical
Paper 2004-01-2463. Presented at: 34th International Conference
on Environmental Systems, 19-22
July 2004, Colorado Springs, CO. Warrendale, PA: Society of Automotive Engineers
International. |