LAUSR

Purpose The objective of this work was to optimize the anaerobic fermentation of a mixed waste stream, consisted of fruit and vegetables that have lost their marketing value and a disposable nappies’ hydrolysate. More specifically, the aim was to identify the optimal pH value for maximum hydrogen production and valuable metabolites such as volatile fatty acids and ethanol. Methods A wide range of pH values was tested (from 4.5 to 7.5 with 0.5 increment) using an automatic controller system, in batch fermentations that took place in mesophilic temperature conditions (37 °C). The first set of experiments was carried out with the fruit and vegetables mixture, diluted with water (2:3 v/v) and subsequent trials followed using the fruit and vegetable mixture with the disposable nappies’ hydrolysate at the same ratio (2:3 v/v). Results The maximum hydrogen volume was produced at pH 6.0 (1.34 L H 2 /L Reactor ) for the fruit/vegetable stream whereas, the maximum concentration of ethanol and volatile fatty acids (15.60 g/L) was reached at pH 6.5 for the same substrate. Regarding the mixed waste stream, both hydrogen production and metabolites concentration reached a maximum at pH 7.5 with 4.09 L H 2 /L Reactor and 17.16 g/L respectively. Conclusions Different optimum pH value for bio-hydrogen production was observed between the anaerobic fermentation of the two substrates (fruit/vegetables waste and mixed waste stream). Higher overall yields and concentrations of the metabolic products were obtained with the fermentation of the mixed substrate. Graphic Abstract