Microbial Characterization of Space Solid Wastes Treated with a Heat Melt Compactor

The purpose of the project has been to characterize and determine the fate of microorganisms in space-generated solid wastes before and after processing by candidate solid waste processing. For FY11, the Heat Melt Compactor (HMC) was the candidate technology that was assessed. Five HMC product disks were produced at ARC from either simulated space-generated trash or from actual space trash. The actual space trash was the STS 130 Volume F compartment wet waste. Conventional microbiological methods were used to detect and enumerate microorganisms in heat melt compaction (HMC) product disks and in surface swab samples of the HMC hardware before and after operation. In addition, biological indicators were added to the STS trash prior to compaction to determine if these spore-forming bacteria could survive the HMC processing conditions, i.e., high temperature (160 ϒC) over a long duration (3 hrs). The HMC disk surfaces were sanitized with 70% alcohol prior to obtaining the core saples to ensure that surface dwelling microbes did not contaminate the HMC product disk interior. Microbiological assays were run before and after sanitization and found that sanitization greatly reduced, but did not eliminate, the number of identified isolates. To characterize the interior of the disks, ten 1.25 cm diameter core samples were aseptically obtained from each disk. These were run through the microbial characterization analyses. Low counts of bacteria, on the order of 5 to 50 viable cells per core, were found, indicating that the HMC operating conditions might not be sufficient for absolute sterilization of the waste. However, the direct counts were 6 to 8 orders of magnitude greater, demonstrating that the vast majority of microbes present in the wastes were dead or non-cultivable. An additional indication that the HMC processing conditons were sterilizing the wastes were results from commercial spore test strips that had been added to the wastes prior to HMC operation. Nearly all could be recovered from the HMC disks post-operation and all were showed negative growth when run through the manufacturer’s protocol, meaning that the 1 x 10 6 or so spores impregnated into the strips were dead. Control test strips, i.e., not exposed to the HMC conditions, were all strongly positive. One area of concern is that the identities of isolates from the cultivable counts included several human pathogens, namely Staphylococcus aureus.