The three primary wastewater sources aboard manned spacecraft are: 1) humidity condensate, 2) hygiene waters, and 3) urine.
Humidity condensate is reclaimed from the air by condensing heat exchangers which control cabin humidity levels. In comparison to the other wastewater streams it is lightly contaminated with ammonia and with water soluble organics such as low molecular weight carboxylic acids and alcohols which are present at low concentration in the air.
Hygiene waste waters arise from routine activities such as hand washing, showering, clothes washing, oral hygiene, etc. These waste streams contain soap, particulates, and variable concentrations of dissolved salts and organics.
Urine, in general, contains high concentrations of dissolved salts and lower levels of excreted organic metabolic byproducts. Urea, the primary water soluble product of nitrogen metabolism, may occur in significant quantity. Urea hydrolyses to form ammonia and carbon dioxide.
Steps must be taken to prevent the entry of ammonia gas into the cabin atmosphere.
Aboard the Russian Mir Space Station, each of these three wastewater streams are reclaimed separately. Water reclaimed from urine is used to generate oxygen by electrolysis, reclaimed hygiene water is recycled for use in oral hygiene, hand wash, etc. Reclaimed humidity condensate is used as drinking water, food preparation water, etc. Because each stream is used for a different purpose, different standards of purification are also involved, with the most rigorous standards applied to water for human consumption. In contrast, the American water processor for the International Space Station Alpha purifies a composite wastewater stream to potable standards.
The primary means of water purification in both American and Russian life support systems is a process termed Multi filtration. In the Multi filtration (MF) process, particulate material is removed by filtration, and dissolved salts and organics are removed by adsorption onto ion exchange resins, activated carbons, and other sorbent media. Low molecular weight polar nonionic species such as alcohols and urea which are not effectively removed by adsorption are destroyed by a catalytic oxidation post-treatment of the MF effluent. In the Russian system this is done using beds of ambient temperature catalyst which are an integral part of the MF canisters. In the American system this is done using a low temperature aqueous phase catalytic oxidation reactor, termed the Volatile Removal Apparatus (VRA).
Due to the high salt content of urine, a preliminary treatment step is required. This involves one of several distillation/evaporation processes which all involve a phase change. Phase change processes are very costly from an energy perspective because the latent heat of vaporization of water must be supplied. Technologies which reclaim as much as possible of this latent heat are needed. These processes produce a urine distillate which then is treated by MF.
Author: Tugrul Sezen
[email protected]
Date Last Modified:7/26/1999
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