Influence of substrate pre-treatment by steam explosion on the acidogenic fermentation of lignocellulosic substrate by mixed microbial cultures

The energetic and material valorisation of biomass resources and especially those with a lignocellulosic structure is gaining increasing attention. Among various processes within the biorefining concept, anaerobic digestion has been extensively implemented for the production of methane from organic waste fractions. Acidogenic fermentation (acidogenesis) is an intermediate stage during anaerobic digestion (following hydrolysis and followed by acetogenesis and methanogenesis) that could be investigated as an individual process for the production of "green" chemicals. In this study, we are investigating the acidogenic fermentation of three complex lignocellulosic substrates, namely beet pulp, wheat bran and miscanthus by mixed microbial cultures. Next to that, we investigate the effect of substrate pre-treatment by steam explosion on the fermentation profiles. The parameters that were monitored are the pH, the production and composition of biogas, the concentration of the soluble organic matter and the concentration of metabolites like volatile fatty acids and ethanol. Three identical experiments were performed, one for each substrate. Each experiment included five series of reactors, representing different combinations of untreated/pre-treated substrate with inoculum; each series consisted of three replicates. An anaerobic mixed culture inoculum, impoverished in methanogens through pH adjustment and heat treatment, was developed from activated aerobic sludge. Experiments were conducted under anaerobic conditions for 22 days, in batch reactors of 100 ml working volume, at a constant temperature of 35 °C, with the pH left uncontrolled during fermentation. The pH stabilised after 7 days of fermentation in the area of 4.3-5.2 and methanogenesis was inhibited in all cases, with the exception of untreated miscanthus in the presence of inoculum. Results show that the fermentation profiles are substrate-dependent and no common conclusion concerning the effect of steam explosion can be drawn. While steam explosion solubilised part of the organic matter of all substrates, it did not necessarily improve their digestibility. Indeed, miscanthus is virtually non digestible both in the pre-treated and untreated forms, wheat bran shows similar behaviour in both forms and only beet pulp shows a different profile, with the untreated form being more digestible. Acetic acid and butyric acid where the most dominant metabolites with concentrations reaching 10.5 and 9.4 g_CODeq/kg_mixed liquer, respectively.