Anaerobic mono-digestion of wheat straw in a three-stage semi-continuous system inspired by a beetle larva gut.

Based on the efficient digestive system of the larva of the sun beetle (Pachnoda marginata) in utilizing lignocellulose-rich biomass, a reactor system was designed to partially mimic processes that occur in the larval gut. We aimed developing a process with this system for anaerobic mono-digestion of wheat straw to produce volatile fatty acids (VFAs) and biogas. Three stirred tank reactors connected in series were customized with polyurethane foam and operated at 37 °C. The first two reactors were designed to mimic the midgut of the larva with enhanced VFA production and the third one to resemble the hindgut and optimized for methanogenesis. The system was fed in a semi-continuous regime with ground wheat straw in an alkaline medium. Enrichment cultures from midgut and hindgut of the larva were used as inocula and three conditions with increasing organic loading rate were tested. The highest biomass to methane conversion occurred at the lowest organic loading rate and highest retention time, when a methane yield of 148 mLNorm g-1VS was obtained and VS degradation reached 44%. The microbial community was enriched toward cellulose and hemicellulose degrading taxa, including Dysgonomonadaceae, Lachnospiraceae, Marinilabiliaceae and Ruminococcaceae. The methanogenic community shifted from predominance of Methanosarcina to Methanobacterium and Methanoculleus. A comparison with other studies investigating straw utilizing bioreactors confirmed that a core microbiome composed of the phyla Firmicutes, Bacteroidetes and Proteobacteria effectuates the anaerobic digestion of lignocellulose-rich materials, regardless of the source of the inoculum and the process conditions. The designed process was stable and capable to convert wheat straw into methane and carboxylates. Further adaptations are suggested to improve the anaerobic digestion and decrease assembly and operating costs.