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METHODS OF WATER PRODUCTION

Aboard the Shuttle Orbiter water is generated using a hydrogen-oxygen fuel
cell which produces both electricity and very pure water. Water can also be
produced by reduction of CO2 by hydrogen over a suitable catalyst. The
Sabatier and Bosch processes are two Carbon Dioxide Reduction Systems
which have been studied extensively. The Sabatier method has been selected
for International Space Station (ISS).

Production of Water by the Sabatier Reaction

Current technology for the reduction of carbon dioxide to produce water aboard
the International Space Station (ISS) is embodied in the Sabatier Reactor.

For each mole of carbon dioxide reduced, this system produces two moles of
water and one mole of methane. The methane is vented to space. For this
reason, only half of the hydrogen consumed in this reaction can be recovered
by electrolysis. The Sabatier reaction utilizes an alumina supported ruthenium
catalyst and, in comparison to the Bosch reaction, has the advantage of
greater thermodynamic favorability and higher reaction rates.

The primary disadvantage of the Sabatier process is that twice as much
hydrogen is consumed as can be recovered by the electrolysis of the produced
water. This results in a substantial perturbation in the material balance for the
production of oxygen from carbon dioxide. Using the Sabatier process either
the hydrogen imbalance must be compensated for by resupply, or not all of the
carbon dioxide can be processed.

IODINE

Disinfection of the drinking water supplies aboard spacecraft is a requirement
for the maintenance of flight crew health. Current NASA specifications for
potability limit the maximum microbial population to less than 100 Colony
Forming Units (CFU)/100mL, with total coliforms less than 1 CFU/100mL.
Elemental iodine (I2) was first applied as a drinking water disinfectant on U.S.
spacecraft in the Lunar Module during the Apollo missions which began in
1969. Prior to each mission, Lunar Module drinking water storage tanks were
prefilled with water containing an I2 concentration of 12 mg/L. This level was
sufficient to maintain a residual disinfectant concentration of < 0.5 mg/L. During
the Skylab mission potable water tanks were iodinated to 12 mg/L I2 prior to
launch. Afterward aqueous elemental iodine concentrations were monitored
and maintained in the range between 0.5-6.0 mg/L by direct additions to the
water storage tanks during flight using a 30g/L stock solution containing
potassium iodide (KI) and I2 in a 2:1 molar ratio.

For the Space Shuttle program, a new device for the controlled release of I2
termed the Microbial Check Valve (MCV) was introduced. The MCV is a canister
containing iodinated strong base ion exchange resin. In aqueous solution in the
presence of excess iodide (I-), polyiodide anions are formed.

MCV resin consists of polyiodide anions bound to the quaternary amine fixed
positive charges of a polystyrene-divinylbenzene copolymer anion exchange
resin. The bound polyiodide anions release I2 to water.

Onboard the Space Shuttle, high purity water produced by the fuel cells flows
through an MCV canister which provides both a contact microbial kill and
imparts an elemental iodine residual ranging between 0.5-4.0 mg/L. Similar
devices are installed at the Galley Auxiliary Port and the Extravehicular Mobility
Unit Service and Cooling Umbilical for microbial control. The MCV has proven
effective as a means for maintaining drinking water potability, and in
ground-based tests has demonstrated a remarkable ability to inhibit the growth
of biofilm.

Aboard the Shuttle Orbiter all MCV canisters require periodic replacement. To
simplify resupply logistics for future longer duration missions, regenerative MCV
(RMCV) hardware has been developed. This technology utilizes a packed bed
of crystalline elemental iodine to produce a saturated aqueous solution ([I2] =
300 mg/L) which is used to replenish depleted MCV resin.

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WATER: PRODUCTION,
RECLAMATION, DISINFECTION
INTRODUCTION

Aboard current spacecraft such as the Mir, water is used for drinking, food
preparation, personal hygiene, oxygen generation, and scientific experiments.
In the future, water will be supplied to plant growth chambers for the production
of both oxygen and food. Water may also be used in the destruction of solid
wastes by techniques based upon supercritical water oxidation (SCWO) or
steam reforming reactions in Advanced Life Support Systems (ALSS).

In a closed loop life support system wastewater must be purified so that it can
be used again. The costs of resupply from the ground are prohibitive. Water
which has been transported to the cabin atmosphere through evaporation and
through breathing is recovered as humidity condensates using condensing heat
exchangers. Water which has been used for personal hygiene (containing a
variety of substances including salts, soaps, hair and other particulate matter) is
collected for purification. Urine is also collected for treatment. Currently, these
three sources form the primary inputs to life support water reclamation systems.
Disinfectants must be added to the reclaimed waters to prevent the growth of
pathogenic microorganisms. Water contained in feces and other solid waste is
lost from the system and must be made up by onboard water production
facilities or by resupply.

Three separate water purification loops are used onboard the Russian Mir
space station. Urine is purified as a feed to the electrolysis cells which generate
oxygen. Hygiene water is reprocessed for re-use only as hygiene water. All
potable (drinking and food preparation) water aboard Mir comes from the
purification of humidity condensate or from resupply. This is similar to the
original water reclamation scheme for the Space Station Freedom program.
Subsequent reconfigurations of Freedom resulted in a single water processor to
treat a composite stream containing the three sources (humidity condensate,
hygiene water, and urine distillate). This has carried over to the water processor
design for the International Space Station Alpha (ISSA), due to begin
construction in late 1997.
"Victoria crater" Image Credit: NASA/JPL/Cornell  SPACE