Microbiological Characterization and Concerns of the International Space Station Internal Active Thermal Control System

Since January 1999, the chemical and microbial state of the International Space Station (ISS) Internal Active Thermal Control System (IATCS) heat transport fluid (HTF) has been monitored by analysis of samples returned to Earth. Key chemical parameters have changed over time, including a drop in pH from the specified 9.5±0.5 to ≈8.4, an increase in the level of total inorganic carbon (TIC), total organic carbon (TOC) and dissolved nickel (Ni) in the HTF, and a decrease in the phosphate (PO₄) level. In addition, silver (Ag) ion levels in the HTF decreased rapidly as Ag deposited on internal metallic surfaces of the system. The lack of available Ag ions coupled with changes in the HTF chemistry has resulted in a favorable environment for microbial growth. Counts of heterotrophic bacteria have increased from <10 colony-forming units (CFUs)/100 mL to 10⁶ to 10⁷ CFUs/100 mL. The increase of the microbial population is of concern because uncontrolled microbiological growth in the IATCS can contribute to deterioration in the performance of critical components within the system and potentially impact human health if opportunistic pathogens become established and escape into the cabin atmosphere. Microorganisms can potentially degrade the coolant chemistry; attach to surfaces and form biofilms; lead to biofouling of filters, tubing, and pumps; decrease flow rates; reduce heat transfer; initiate and accelerate corrosion; and enhance mineral scale formation. The microbiological data from the HTF, and approaches to addressing the concerns, are summarized in this paper.