Assessment of Silver Based Disinfection Technology for CEV and Future US Spacecraft: Microbial Efficacy

This work describes the microbiological assessment and materials compatibility of a silver-based biocide as an alternative to iodine for the Crew Exploration Vehicle (CEV) and future spacecraft potable water systems. In addition to physical and operational anti-microbial counter-measures, the prevention of microbial growth, biofilm formation, and microbiologically induced corrosion in water distribution and storage systems requires maintenance of a biologically-effective, residual biocide concentration in solution and on the wetted surfaces of the system. Because of the potential for biocide depletion in water distribution systems and the development of acquired biocide resistance within microbial populations, even sterile water with residual biocide may, over time, support the growth and/or proliferation of bacteria that pose a risk to crew health and environmental systems. This work reports the efficacy of [500 ppb] silver fluoride in potable water as a biocide against each of five bacterial species previously isolated from spacecraft and the materials compatibility of the biocide with each of three heritage materials used in US spacecraft potable water systems (i.e., Inconel 718, 21-6-9 Stainless Steel (aka Nitronic 40), and 316 Stainless Steel). Silver concentration, total microbial load and materials analysis of submerged metal coupons were assessed at six time-points over the course of replicated, short-duration 21-day experiments using two surface area-to-volume coupon ratios representing those in the shuttle potable water bellows tank and water distribution system. These experiments are part of a multi-phase evaluation of silver-based biocide technology for CEV applications through development of preferred materials lists, investigation of silver biocide forms and delivery methods, down-selection of silver biocide technologies, and integrated system testing.