Ionic Silver and Iodine Biocide Losses from Potable Water in ISS-Material Tubes under Stagnant Conditions

Combinations of multiple different biocide-treated potable water solutions in multiple tube materials that are used in the International Space Station water distribution system were evaluated to determine the fate of the biocides during storage of water within the tubes. This phase of testing utilized the geometries, surface materials, passivations, and cleaning procedures used for tubes launched with ISS (�legacy tubes�). Results cover the first phase of testing, which used the weekly draining and refilling of tubes with fresh potable water solutions to simulate how the potable water�s biocide interacts with tube walls and how those interactions change with repeated stagnant exposures. Key findings from the test are presented. A comparison of silver loss rates to iodine loss rates indicates that on a molar basis, the loss of iodine concentrations in the 1000 to 4000 ppb range were almost an order of magnitude more than the silver loss rates from the target biocidal range of 200 to 400 ppb. Forward work to maintain ionic silver concentrations at target biocidal concentrations for potable water is discussed, focusing on optimizing the potable water formulation and the procuring, passivation, cleaning, and conditioning of tube materials.