Sterilization of Unibed® Multifiltration Media by Gamma Irradiation

Unibeds^®* are multimedia layered sorption beds baselined for use in the Space Station Freedom (SSF) water reclamation system. Unibeds^® must be sterilized prior to use to avoid the introduction of bacteria into the water reclamation system when the Unibeds^® are routinely changed out. In the past, Unibeds^® were autoclaved in an attempt to achieve sterility. Some sorbent media used in the Unibeds^® decompose when exposed to high temperatures for extended periods. Although no significant sorbent decomposition occurs during the routine autoclave time of 30 minutes, it is uncertain whether sufficient sterilization temperatures are achieved at the Unibed^® core. Gamma irradiation has been evaluated as a practical alternative method to achieve sterility and eliminate possible sorbent thermal degradation.

Sorbent media were inoculated with irradiation resistant spores (10⁶ CFU/ML) of bacterium Bacillus pumilus and subsequently exposed to radiation doses of 1.5, 2.0, and 2.5 megarads (Mrad). A dose of 1.9 Mrad is generally lethal for B. pumilus spores. However, the level of shielding provided by the sorbent media was not known. Bacterial survivors occurred in all media irradiated at 1.5 Mrad. At 2.0 Mrad survivors were isolated from polymeric media only. No survivors were isolated from media irradiated with 2.5 Mrad. Sorbents were performance tested before and after irradiation with no indication of performance degradation.