Biomass acid pretreatment impacts on metabolic routes and bacterial composition of dark fermentation process.

Complex organic matter represents a suitable substrate to produce hydrogen through dark fermentation (DF) process. To increase H2 yields, pretreatment technology is often required. The main objective of the present work was to investigate thermo-acid pretreatment impact on sugar solubilization and biotic parameters of DF of sorghum or organic fraction of municipal solid waste (OFMSW). Biochemical hydrogen potential tests were carried out without inoculum using raw or thermo-acid pretreated substrates. Results showed an improvement in sugar solubilization after thermo-acid pretreatments. Pretreatments led to similar DF performances (H2 and total metabolite production) compared to raw biomasses. Nevertheless, they were responsible for bacterial shifts from Enterobacteriales towards Clostridiales and Bacillales as well as metabolic changes from acetate towards butyrate or ethanol. The metabolic changes were attributed to the biomass pretreatment impact on indigenous bacteria as no change in the metabolic profile was observed after performing thermo-acid pretreatments on irradiated OFMSW (inactivated indigenous bacteria and inoculum addition). Consequently, acid pretreatments were inefficient to improve DF performances but led to metabolic and bacterial community changes due to their impact on indigenous bacteria.

Impact of microbial inoculum storage on dark fermentative H2 production.

Complex organic substrates represent an important and relevant feedstock for producing hydrogen by Dark Fermentation (DF). Usually, an external microbial inoculum originated from various natural environments is added to seed the DF reactors. However, H2 yields are significantly impacted by the inoculum origin and the storage conditions as microbial community composition can fluctuate. This study aims to determine how the type and time of inoculum storage can impact the DF performances. Biochemical Hydrogen Potential tests were carried out using three substrates (glucose, the organic fraction of municipal solid waste, and food waste), inocula of three different origins, different storage conditions (freezing or freeze-drying) and duration. As a result, H2 production from glucose with the differently stored inocula was significantly impacted (positively or negatively) and was inoculum-origin-dependent. For complex substrates, hydrogen yields with the stored inocula were not statistically different from the fresh inocula, offering the possibility to store an inoculum.

Role of indigenous bacteria in dark fermentation of organic substrates.

Hydrogen production by dark fermentation of complex organic substrates, such as biowaste, can naturally take place with indigenous bacteria or by adding an external microbial inoculum issued from various natural environments. This study aims to determine whether indigenous bacteria associated with thermal pretreatment could impact dark fermentation performances. Biochemical hydrogen potential tests were carried out on seven organic substrates. Results showed a strong influence of the indigenous bacteria which are as effective as thermally pretreated exogenous bacteria to produce H2 and metabolites. High abundance in Clostridiales and/or Enterobacteriales was associated with high H2 yield. This study shows that no inoculum nor pretreatment are required to achieve satisfactory dark fermentation performances from organic waste.