Pretreatments for lignocellulosic biomass dark fermentation: impact on hydrogen and metabolite production

Over the past decade, increasing interest has been addressed to anaerobic fermentation. These bioprocesses have been investigated to not only produce hydrogen but also other high-value by-products, such as carboxylic acids (mainly acetate, butyrate, lactate), ethanol and other solvents. Indeed, hydrogen is characterised by a clean combustion process and is an efficient energy carrier with a considerable calorific value (122 MJ/kg), but represent only 20% of the total COD in fermentation, with more than 80% of valuable metabolites. Interestingly, anaerobic dark fermentative processes imply complex mixed microbial consortia able to operate in non-sterile conditions and to use various kinds of waste as feedstocks. Nevertheless, hydrogen yields from solid waste are directly correlated to the initial content in soluble carbohydrates (Guo et al., 2010; Monlau et al., 2012). In this context, lignocellulosic residues, rich in carbohydrates, constitute relevant feedstocks for dark fermentation but pretreatment is required to solubilise hemicelluloses and cellulose and make carbohydrates more available to microorganisms. High release of soluble carbohydrates can be obtained by thermal-acid pretreatment, enzymatic hydrolysis, combined or not with alkali upstream pretreatment. This paper will present a synthetic view of several studies combining thermo-chemical and/or enzymatic pretreatment of lignocellulosic biomass (wheat straw (WS) and sunflower stalks (SS)) with the purpose of H2 production by dark fermentation. As main findings, enzymatic hydrolysis of WS with an enzyme cocktail secreted by Trichoderma strain showed the necessity of working under sterile conditions to avoid a re-consumption of solubilized sugars by the endogenous bacteria naturally present in the substrate. A two-fold hydrogen production from 10 to 20 mL/g VS was obtained after addition of 5 mg enzymes/ g wheat straw. Interestingly, same results were obtained by adding enzymes directly into the fermenter, leading to a simpler and cheaper process (Quemeneur et al., 2012). The combination of thermal-alkali pretreatment (55°C, 4% NaOH (w/wTS) for 24 h) and enzymatic hydrolysis (cellulose 50 FPU/gTS; glucosidase 25 U/gTS and xylanase 50 U/gTS) on SS led to a 21 fold increase in hydrogen production from 2.4 to 49 mL/gVS (Monlau et al., 2013b). In comparison, thermal acid (170°C, 4% HCl for 1 hour) of SS led to a decrease of hydrogen production which was explained by the release of inhibitory by-products during pretreatments (furfural, 5-HMFand phenolic compounds). Interestingly, performing glucose fermentation with increasing amounts of acid pretreatment hydrolysate showed a metabolism shift from acetate/butyrate/hydrogen to lactate/ethanol and finally ethanol production with a yield of 2 mol ethanol/mol glucose consumed (Monlau et al., 2013a).