Design and Delivery of Filter for Removal of Siloxanes from ISS Atmosphere

Dimethylsilanediol (DMSD) has been identified as a problematic organic on ISS. This contaminant was initially identified in the ISS condensate and in the Water Processor Assembly (WPA) product water in 2010 when the Total Organic Carbon Analyzer (TOCA) detected an increasing TOC trend in the water produced by the WPA. DMSD is not a crew health hazard at the levels observed in the product water, but it may degrade the performance of the Oxygen Generation System (OGS) which uses the WPA product water for electrolysis. In addition, it can prevent the effective operation of the WPA catalytic reactor, and necessitates early replacement of Multifiltration Beds in the WPA. An investigation into the source of DMSD has determined that polydimethylsiloxanes (PDMSs) are hydrolyzing in the Condensing Heat Exchanger (CHX) to form DMSD. PDMSs are prevalent on ISS from a variety of sources, including crew hygiene products, adhesives, caulks, lubricants, and various nonmetallics. These PDMSs are also known to contribute to degradation of the CHX hydrophilic coating, rendering it hydrophobic and therefore adversely affecting its ability to effectively transmit water to the condensate bus. Eventually this loss in performance results in water droplets in the air flow out of the Heat Exchanger, which can lead to microbial growth in the air ducts and can impact the performance of downstream systems. Design concepts have now been developed for removing PDMS in the air stream before it can reach the CHX coating, thus preventing degradation of the coating and decomposition of the PDMS to DMSD. This paper summarizes the current status of the effort to deliver filters to ISS for removing PDMSs from the atmosphere before they can adversely impact the performance of the CHX coating and the WPA.