Development and Verification of an Electrode System for Electrolytic Generation of Silver Ion Biocide for the Space Station Internal Thermal Control System Fluid

The Space Station Internal Thermal Control System (ITCS) fluid formulation is composed of a buffer solution consisting of a mixture of tri-sodium phosphate and sodium borate that produce a pH of approximately 9.5. A silver ion concentration is also specified in the formulation to assist in the control of microorganisms. The specification for the fluid places tight control on the allowed total organic carbon (TOC) concentration to further control possible food sources for microorganisms. Various ground tests conducted on the Laboratory and Airlock modules have demonstrated that the TOC specification is hard to maintain in a test environment. Test data also demonstrate that silver ion depletes as it comes in contact with various metals in the ITCS loop. When the silver is depleted, the microorganism populations can rapidly increase to a range of 10⁵ to 10⁶ colony forming units (CFU) per 100 milliliter.

The potential for microorganisms to form biofilms that are known to cause heat transport and corrosion problems has raised concern and initiated a search for a way to introduce silver ions into the ITCS loop. Electrolysis was chosen as a viable approach for continuous slow injection of silver ion into the ITCS fluid.

A silver electrode system has been developed and tested to demonstrate effectiveness for ITCS fluid. The electrode system controlling circuit was designed to operate with pulsed polarity switching to minimize contamination buildup on the electrodes. A desired silver generation rate is achieved by precise control of the voltage and circuit duty cycle. The data from this paper demonstrate the utility of the system for rapid 6-log reduction of large populations of microorganisms to less than detectable levels within 12 hours.