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Spacecraft Water Exposure Guidelines: For Selected Contaminants, Volume 2
threatening crew health; conversely, the presence of disinfectants such as chlorine, and possibly iodine, can form toxic compounds in the presence of organic carbon (Miettinen et al. 1996). These considerations are mitigated by the fact that TOC will be measured in situ soon after the sample is acquired. Hence, there will be insufficient time for these secondary reactions to occur before the measurement is taken. Thus, in creating a standard, we will not attempt to compensate for microbial growth due to the presence of organic carbon, nor will we be concerned with chemical reactions that could increase the toxicity of the water.
To meet the primary goal stated above, the WRS must be bounded, or known, in a number of ways. The input water, which can come from humidity condensate, urine, or makeup sources (e.g., water brought up from the ground or obtained through fuel cells), must be reasonably well characterized so that, knowing the elements of the processing system, one can predict the most likely organic components to break through to the product water.
We will assume that the WRS will be taken offline under conditions where the load may knowingly exceed capacity or might damage the processor’s capability. For example, the WRS would not process humidity condensate immediately after a serious fire or after leakage of certain air pollutants such as ammonia from the U.S. Laboratory Module of the International Space Station (ISS).
We will assume that microbial control is accomplished with iodine or silver and that there will be no mixing of the product water with other water that may have an organic residue from a biocide. Specifically, we will assume that none of the water for analysis has originated from an ethanol tincture of iodine.
We will further assume that any other treatment of the post-process water will not involve an addition of organically contaminated water. For example, we will assume that the process of adding minerals does not involve the use of an organic counter ion such as formate. We will assume that any mixing in of ground-supplied water will not provide contamination from unusual pollutants (for example, pesticides and chloroform). Such additions to the product water totally confound the interpretation of the TOC measurement. Alternatively, quantification of the confounding compounds would enable the calculation of a TOC measurement that could be compared to the standard.