Design and Implementation of Combination Charcoal and HEPA Filters for the International Space Station Cabin Air Ventilation System
The International Space Station (ISS) was designed to have the cabin air filtered through High Efficiency Particulate Air (HEPA) filters which filter out particles and bacteria to protect the crew and downstream condensing heat exchangers (CHX). Currently, there are a total of 21 HEPA filters installed throughout the United States On-Orbit Segment (USOS) in Nodes 1, 2, and 3, US Lab, and the Airlock. The increase of Total Organic Compounds (TOC) within the Water Recovery System (WRS) and the Oxygen Generation System (OGS) throughout the life of the International Space Station (ISS) has been tied to the level of Dimethylsilanediol (DMSD) within the closed loop system. The likely source of the DMSD has been identified as polydimethylsiloxane (PDMS), which degrades in the air and goes through hydrolysis on the CHX within the Common Cabin Air Assembly (CCAA). The use of a charcoal media to scrub the air of PDMS was successfully used within the Node 1 cabin air system for ~2.5 years. Since Node 1 does not have a CHX there was no need for HEPA level filtration at that location. However, due to high fungal counts registered by the air samples taken by the crew, the charcoal-only filters were removed in lieu of HEPA filters. Review of the Air Quality Monitor (AQM) data following the installation of the charcoal-only filters proved that the station air side levels of volatile siloxanes were greatly reduced with only four filters installed in Node 1. To increase the filtering capacity on the air side, filters were designed to combine the volatile siloxane scrubbing capability of charcoal along with the filtering capability of HEPA filters. The following paper will detail the Charcoal HEPA Integrated Particle Scrubbers (CHIPS) and provide a status of their impact on the overall air and water quality on-board the ISS.