Integration of fermentative biohydrogen with methanogenesis from fruit–vegetable waste using different pre-treatments

Highlights: • An optimal pre-treatment for FVW used in two-stage fermentation is presented. • The spectroscopic characteristics of soluble metabolites were analysed. • The effluents were characterised by EEM with a parallel factor analysis. - Abstract: Fruit–vegetable waste was subjected to three different pre-treatments to enrich the two-stage biofuel production potential. The fluorescence excitation–emission matrix (EEM) spectra coupled with parallel factor (PARAFAC) analysis and fluorescence regional integration analysis were utilised to investigate dissolved organic matter degradation during two-stage fermentation process. The results showed that compared with that of alkali and enzyme pre-treatments, the acid pre-treatment resulted in the maximum biogas production rates and proportion in the hydrogenogenic stage (10.11 mL/h, 41.2% hydrogen) when combined with the methanogenic process (4.67 mL/h, 76.1% methane). In addition, the analysis of soluble metabolites composition indicated that both ethanol- and butyric acid-type fermentation processes had taken place as a result of acid pre-treatment, whereas only butyric acid-type fermentation resulted from alkali and enzyme pre-treatments. The PARAFAC analysis modelling of the EEM spectra revealed three fluorescent components in the effluents of three fermentation stages and assumed that the projected characteristic value may be used as a rapidly obtained indicator for substrate degradation and system stability of a two-stage biofuel production process