Design of Anaerobic Digestion Systems for Closed Loop Space Biomanufacturing

Abstract

Space biomanufacturing has the potential to substantially reduce the launch mass of long-duration manned missions. Active research areas include enhancing methane conversion to polyhydroxybutyrate by methanotrophs to enable 3D-printed mission tools. Because anaerobically digesting mission wastes can recycle essential carbon, this process can provide necessary biomanufacturing feedstocks like methane. Thus, this paper analyzes the costs and benefits of adding anaerobic digestion to aid space biomanufacturing. First, stoichiometry determines theoretical yield from inedible food biomass (e.g., the straw and husk of rice) and human waste. Thereafter, this work examines how digester design and operating conditions, including operating temperature, solid loading density, digestion duration, and pretreatment impact methane yield. This paper also studies numerous combinations of possible wastes. The work assesses impact through a systems engineering analysis that optimizes performance and specifications of this two-step process via the Equivalent System Mass (ESM) metric. ESM augments traditional shipped mass costs with those of pressurized volume, demanded power and thermal control, and needed crew time. This analysis helps quantify the extent of loop closure for space biomanufacturing and its trade-off with ESM, and finds that it is possible to close as much as one third of the loop. This paper also incorporates a parametric sensitivity analysis to highlight the positive impact that mission horizon increase has on anaerobic digestion viability.