The effect of thermophilic anaerobic digestion co-substrate ratio on biogas yield for bioregenerative life support

Abstract. Organic solid waste streams in bioregenerative life support systems (BLSS) for Lunar and Martian surface missions primarily include byproducts from human metabolism and inedible biomass from food preparation and agricultural crop residues. Co-digestion of these substrates in an anaerobic bioreactor could generate mission critical consumables and increase the self-sustainability of a manned surface habitat. Co-substrates of swine feces and wheat straw were used as simulants of human feces and inedible biomass, respectively. To calculate a design volume of an anaerobic digester for a BLSS, batch experiments were conducted at bench scale. A control group of swine feces only was compared with three treatment groups containing co-substrate ratios of swine feces-to-wheat straw (1:3, 1:1, and 3:1) based on chemical oxygen demand. Each group contained the same mass of swine feces. Co-substrates were inoculated with a thermophilic microbial community, and each mixture was diluted with diH₂O to 3% total solids. Batch digesters were continuously stirred at 270 RPM and submerged in isothermal heat baths at 50°C for a digestion period of approximately 30 days. Volume of biogas yield was measured via fluid displacement throughout the experiment, and biogas samples were analyzed via gas chromatography. It was hypothesized that biogas yield would increase as SF:WS decreased. Mean values for total biogas, CO₂, and CH₄ were different for all treatments and control (p < 0.0001). These results support the hypothesis. A digester to accommodate inputs from three male and three female adult humans will be designed using kinetic data from biogas yield.