Review and perspectives of enhanced volatile fatty acids production from acidogenic fermentation of lignocellulosic biomass wastes

Abstract

Lignocellulosic biomass wastes are abundant resources that are usually valorized for methane-rich biogas via anaerobic digestion. Conversion of lignocellulose into volatile fatty acids (VFA) rather than biogas is attracting attention due to the higher value-added products that come with VFA utilization. This review consolidated the latest studies associated with characteristics of lignocellulosic biomass, the effects of process parameters during acidogenic fermentation, and the intensification strategies to accumulate more VFA. The differences between anaerobic digestion technology and acidogenic fermentation technology were discussed. Performance-enhancing strategies surveyed included (1) alkaline fermentation; (2) co-digestion and high solid-state fermentation; (3) pretreatments; (4) use of high loading rate and short retention time; (5) integration with electrochemical technology, and (6) adoption of membrane bioreactors. The recommended operations include: mesophilic temperature (thermophilic for high loading rate fermentation), C/N ratio (20–40), OLR (< 12 g volatile solids (VS)/(L·d)), and the maximum HRT (8–12 days), alkaline fermentation, membrane technology or electrodialysis recovery. Lastly, perspectives were put into place based on critical analysis on status of acidogenic fermentation of lignocellulosic biomass wastes for VFA production.

Anaerobic Digestion of Lignocellulosic Waste Materials

“…Co-treatment with lacasse, peroxidase, AD at 37 • C, 30 days 250.5 mL CH 4 /g.VS [19] High holocellulose (cellulose and hemicelluloses) contents and high C/N ratio are the three main characteristics of lignocellulosic biomass wastes [20]. Forest and agricultural biomass and, in some cases, the marine biomass consist typically of the same chemical components of cellulose (30 to 60 wt.%), hemicelluloses (12-35% in wood, 20-40% in agricultural residues), lignin (23-40% in wood, 10-40% in agricultural residues), extractives (2 to 15 wt.%) and inorganic salts at different levels [6][7][8].…”

“…Cellulose presents the tendency to form strong intra-and inter-molecular hydrogen bonds, which make cellulose highly insoluble to common solvents and resistant to enzymatic hydrolysis (Galbe and Zacchi 2012). Hemicelluloses present a quite lower degree of polymerization (Saha et al, 2019), are highly hydrophilic and more amenable to hydrolysis [20]. Both can be fermented into sugars necessary in biogas production.…”

“…Generally, conifer species have a higher content of lignin compared to broadleaf species. Therefore, cellulose crystallinity, the strong bonds between cellulose and hemicelluloses, the polymerization and nonbonding interactions (steric hindrance) of lignin, all increase the difficulty of enzymatic hydrolysis of cellulose [20,22]. Lignin and hemicelluloses produce a protective sheath around the cellulose, inhibiting the depolymerization of hemicelluloses and cellulose to plain mono-sugars, necessary for productive conversion of biomass into biogas.…”

“…The single cost of pretreatments is usually similarly high to other production processes [20,24]. Unfortunately, no pretreatment technique has been found that is suitable for all types of lignocellulosic materials [23].…”

“…Unfortunately, no pretreatment technique has been found that is suitable for all types of lignocellulosic materials [23]. Typically, a combination of two or multiple pretreatment methods is more effective and economic, and thus a preferable solution compared to single treatments [20,23].…”