Dark Fermentation of Fruit Juice Effluents: Effects of Substrate Concentration

This study explores the impact of initial substrate concentrations on biomass growth, hydrogen production, and acid generation during acidogenic fermentation of a synthetic fruit juice wastewater. Four substrate concentrations, within the range 0.30–2.12 C mol·L−1, were tested using a mixed acidogenic bacterial culture. Batch reactors were employed to conduct the study, and the results were analyzed to determine inhibition effects. The highest hydrogen production (6.1 L H2·substrate C mol−1) and hydrogen percentage in the gas phase (57%) were achieved at a substrate concentration of 0.30 C mol·L−1. Higher substrate concentrations reduced the hydrogen production due to substrate and product inhibition events. The maximum H2 potential production was 4.15 Nm3/m3 reactor at a substrate concentration of 0.91 C mol·L−1. Biomass growth and VFA production followed exponential trends at low substrate concentrations, 0.30 and 0.091 C mol·L−1, while high concentrations resulted in linear trends due to inhibition effects caused by the substrate. The main acids produced were lactic at low concentrations, and acetic when dealing with high concentrations. The highest final acid concentrations were obtained with the highest initial substrate concentration, but their yields were significantly lower due to the substrate and product inhibitions experienced by the biomass.