Second-generation International Space Station Total Organic Carbon Analyzer Verification Testing and On-orbit Performance Results

N support of a crew of six on board the International Space Station (ISS), wastewater and urine is reclaimed and reprocessed into potable water. The ISS regenerative water processing assembly (WPA) purifies wastewater into potable water for consumption and oxygen (O 2) generation on station. The total organic carbon analyzer (TOCA) takes samples of WPA water and analyzes them for the presence of total organic carbon (TOC). Measuring TOC in the water provides a general assessment of water quality. Low TOC indicates the water processor is likely functioning properly. The WPA, which also contains conductivity sensors to monitor water quality, directs re-processing if water quality parameters are not met. These conductivity sensors are the first and second controls for water quality on ISS, and TOCA serves as the third control to out-of-specification potable water. The TOCA was designed to be a standalone, re-deployable piece of hardware that performs off-line sampling of regenerated potable water from the WPA. TOCA is located close to the Water Recovery System (WRS) rack to enable direct sampling from the WPA. It can also receive samples via a sample bag, which is normally filled from the potable water dispenser (PWD) on orbit. Several principles of TOC analysis are listed below:  Total carbon (TC) = total inorganic carbon (TIC) + TOC  TIC interferes with the direct measurement of TOC and must be removed prior to measuring TOC  TOCA converts and removes TIC by creating a pH shift with a slightly acidic buffer that forces inorganic carbon species into carbon dioxide (CO 2) gas  TOCA oxidizes the organic carbon species remaining in the sample water to CO 2 gas  TOCA-produced CO 2 gas is measured using a very precise infrared (IR) CO 2 detector