Our proposed process is shown in Figure 1.
In the first step of the process, astronaut’s feces are collected into a storage tank using a vacuum toilet. Feces are then transferred into another tank and left to ferment for three days with natural gut flora to increase the concentration of volatile fatty acids (VFAs) that are later consumed by engineered E. coli to produce PHB. Next, the liquid containing VFAs is separated from solid particles using centrifugation and filtration. The resulting liquid containing VFAs is then passed to another storage tank. From there, VFAs are added to a fermenter inoculated with our engineered PHB-producing and PHB-secreting E. coli. Lastly, the resulting PHB is extracted from the liquid harvest stream. The PHB can then be used in a Selective Laser Sintering (SLS) 3D printer without the need for additional processing (Pereira et al., 2012).
We continuously improved our process design based on feedback received from space industry professionals including astronauts Dr. Robert Thirsk and Col. Chris Hadfield, Dr. Matthew Bamsey (a Chief Systems Engineer at the German Aerospace Center), and Dr. Pascal Lee (a Principal Investigator of the Haughton-Mars Project at NASA Ames Research Center and a co-founder of the Mars Institute). We used Equivalent System Mass guidelines, a method advised by NASA, to evaluate the feasibility of various process designs and followed NASA’s Life Support Baseline Values and Assumptions. Furthermore, we considered applications for solid human waste that NASA is studying and whether the byproducts of our process have any useful applications. Lastly, we considered the safety of our design as it was identified as an important design criterion by many of the experts we interviewed. More detailed description of the applied design can be found here.
Pereira, T., Oliveira, M., Maia, I., Silva, J., Costa, M. and Thiré, R. (2012). 3D Printing of Poly(3-hydroxybutyrate) Porous Structures Using Selective Laser Sintering. Macromolecular Symposia, 319(1), pp.64-73.