Valorization of Residual Ligno-celluloses from Anaerobic Digesters Through Product Recovery

Abstract

Complete digestion of lignocellulosic biomass during anaerobic digestion (AD) is impossible due to recalcitrance created by lignin, consequently leaving behind a significant part of the feedstock unconverted to methane. The lignin within prevents access to other degradable lignocellulosic constituents, namely cellulose, hemicellulose, pectin, etc. lowering conversion efficiencies to ~60% for most feedstocks. Such residues could be valorized by converting them to value added by-products when AD process is appropriately tweaked/ interrupted. The nature of lignin and its degradability under AD conditions has been tested to show loss of pure monolignols, dilignols and polyphenols as well as native lignin of various biomass feedstocks and is reported for the first time. In micro-digesters, anaerobic bacteria were found to breakdown monolignols, dilignols and polyphenols when provided as a sole carbon source. In leaves lignin degradation varied from 40-70% in dicots, 15-40% in monocots and 15-25% in gymnosperms. FT-IR data confirms the ability of hydrolytic anaerobes in degrading and bringing changes to the core lignin structure. In dicot leaves S units, in monocots H and S units and in gymnosperms few S and G units were lost under AD conditions. In monocots the loss of lignin during AD could be explained by the loss of cell wall phenolics and equally spaced cementing bridges connecting vascular bundles confirmed by light microscopy. An attempt was made to use this phenomenon to control the AD process and recover fibers from various biomass feedstocks which results in three outputs of AD namely methane, fiber and compost. Depending on the type of feedstocks 0.15-0.5g/gTS of the substrate could be recovered as fiber within 14-20d SRT. Correlation of fiber yield and lignin loss showed that for each unit of fiber recovered 0.16unit of lignin should be lost from the feedstock which could be developed as marker for process control. Biogas digester residue (BDR) retains about 40% of lignocellulosics and 25% more accessible NH4+-N which is ideal for mushroom cultivation. A 30% BDRsupplementation was found to enhance the yield of Pleurotus spp. in various substrates by 20-30% and reduce growth stages (pin head, fruiting body and first flush) by 4-5d. Biogas digester liquid (BDL) rich in soluble C, N,P, micro-nutrients and polyphenols (low and medium molecular weight lignins) were used as supplement to grow Pleurotus spp. by sprinkling it on ready to fruit mushroom bags and was found to increase the yields from 20-77% in different species and also reduce growth stages by 2-10d. Polyphenol in BDL acted as a precursor in enhancing lignin degrading enzymes such as Laccase and Peroxidase by 15-50% in different Pleurotus spp. and such an enzyme cocktail recovered could be used to process biomass for 2G ethanol production.