Anaerobic digestion of space mission wastes D.P. Chynoweth, J.M. Owens, A.A. Teixeira, P. Pullammanappallil and S.S. Luniya Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA Abstract The technical feasibility of applying leachbed high-solids anaerobic digestion for reduction and stabilization of the organic fraction of solid wastes generated during space missions was investigated. This process has the advantages of not requiring oxygen or high temperature and pressure while producing methane, carbon dioxide, nutrients, and compost as valuable products. Anaerobic biochemical methane potential assays run on several waste feedstocks expected during space missions resulted in ultimate methane yields ranging from 0.23 to 0.30 L g-1 VS added. Modifications for operation of a leachbed anaerobic digestion process in space environments were incorporated into a new design, which included; (1) flooded operation to force leachate through densified feedstock beds; and (2) separation of biogas from leachate in a gas collection reservoir. This mode of operation resulted in stable performance with 85% conversion of a typical space solid waste blend, and a methane yield of 0.3 Lg per g VS added after a retention time of 15 days. These results were reproduced in a full-scale prototype system. A detailed analysis of this process was conducted to design the system sized for a space mission with a six-person crew. Anaerobic digestion compared favorably with other technologies for solid waste stabilization. Keywords Anaerobic composting; anaerobic digestion; solid wastes; space missions Introduction This paper presents design and operational information of a proposed solid waste man- agement system based on high-solids leachbed anaerobic digestion. The function of the process is to reduce volume and weight of, stabilize, and recover inorganic nutrients, stabilized compost, carbon dioxide, and methane from biodegradable waste fractions. Focus was on a 600-day exploratory mission (e.g., to Mars) which would require growth of plants as a food supplement as well as for oxygen regeneration. A six-person crew would generate about 10.5 kg/d dw (7.5 kg organic matter) solid wastes, including 9% dry human wastes, 51% inedible plant residues, 5% trash, 19% packaging material, 10% paper, 2% tape, 3% filters, and 0.7% other miscellaneous wastes (Verostko et al., 2001). The focus of work presented here was to evaluate a new version of the patented high- solids process Sequential Batch Anaerobic Composting (SEBAC) (Chynoweth and Legrand, 1993), which was modified to operate under the hypo- and micro-gravity environments of space missions. The SEBAC process uses a combination of solid-phase fermentation and leachate recycling to provide a simple, reliable process that inoculates new batches, removes volatile organic acids, and concentrates nutrients and buffer. The process has been tested on a variety of high-solids feedstocks, including woody biomass, the organic fraction of municipal solid waste, yard wastes, and blends of yard wastes and biosolids (Chynoweth et al., 1992; Chynoweth and Legrand, 1993). Organic matter is decomposed primarily to methane, carbon dioxide, and compost over a residence time of 10–30 days. The process is very stable, does not require mixing or oxygen, and is resili- ent after months of being idle without feedstock addition. Since the reactors may be oper- ated at low (ambient) pressures, bulky, high-pressure vessels are not needed. For space applications, a five-reactor system was envisioned, including one for feed collection and compaction, three for anaerobic composting, and one for post-treatment processing (Figure 1). Feed would be collected, coarsely shredded, mixed with station doi: 10.2166/wst.2006.248 Water Science & Technology Vol 53 No 8 pp 177–185 Q IWA Publishing 2006 177 Downloaded from https://iwaponline.com/wst/article-pdf/53/8/177/432777/177.pdf by guest on 23 May 2020