Simulated human feces for testing human waste processing technologies in space systems

Handling and processing human feces in space habitats is a major concern and needs to be addressed for the Crew Exploration Vehicle (CEV) as well as for future exploration activities. In order to ensure crew health and safety, feces should either be isolated in a dried form to prevent microbial activity, or be processed to yield a non-biohazardous product using a reliable technology. During laboratory testing of new feces processing technologies, use of "real" feces can impede progress due to practical issues such as safety and handling thereby limiting experimental investigations. The availability of a non-hazardous simulant or analogue of feces can overcome this limitation. Use of a simulant can speed up research and ensure a safe laboratory environment. At Ames Research Center, we have undertaken the task of developing human fecal simulants. In field investigations, human feces show wide variations in their chemical/physical composition. However, under controlled experimental conditions using healthy adults (e.g. astronauts) fed a standard diet, the variations are likely to be minimal and within statistically acceptable levels. We have prepared a number of simulants using organic chemicals, soy paste ("Miso") and other materials - particularly those capable of representing the water-holding capacity (WHC) of feces. The chemical composition of this simulant was a better approximation to human feces than previously used analogues. Rheological studies of the simulant are planned to ensure that it simulates fecal material. The emphasis on rheology and WHC is based on the assumption that in space habitats feces will be compacted to reduce volume and/or the water will be removed to safen the fecal waste.