Evaluation of significant factors in hydrogen production and volatile fatty acids in co-fermentation of citrus peel waste and processing wastewater

Solid waste and wastewater from citrus processing are potential raw materials for obtaining biofuels; however, the alternative of co-fermentation of these substrates has not been explored, and therefore a primary investigation is needed to outline which process variables should be better evaluated, aiming at higher H2 production. Plackett-Burman experimental design was used to evaluate the effects of the variables: pH (5.0, 6.5, and 8.0); citrus peel waste (CPW) concentration (5.0, 10.0, and 15.0 gTVS.L-1); citrus processing wastewater (CPWW) concentration (1.0, 2.0 and 3.0 gCOD.L-1); yeast extract (0, 1.0, and 2.0 g.L-1); particle size (10, 20, and 30 mesh); and autochthonous inoculum bioaugmentation (0.5, 1.5, and 2.5 gTVS.L-1). The variables identified with the most significant effect on the kinetic parameters of H2 production, and for the organic acids production, were CPW and CPWW concentrations, and particle size. The highest H2 production potential of 142.5 mL.L-1 and the highest maximum production rate of 29.8 mL.L-1.h-1 were obtained under the condition: pH of 5.0, 15.0 gTVS. L- 1of CPW, 3.0 gDOQ.L- 1of CPWW, 2.0 g.L- 1of yeast extract, particle size of 30 mesh, and bioaugmentation of 2.5 gTVS.L-1. Under such conditions, acetic (368.6 mg.L-1), butyric (1181.09 mg.L-1) and valeric (696.4 mg. L-1) acids were observed as the main metabolites at the end of the batch reactor operation. There was an important change when comparing the microbial community structures of the autochthonous inoculum and the higher hydrogen production assay, with a predominance of Lactobacillus, Megasphaera, Olsenella, and Prevotella genera. The metabolic interaction between lactic-acid bacteria and lactate-utilizing bacteria was identified as the main pathway of hydrogen production. The results made it possible to identify the technical viability of co-fermentation of waste from the citrus production chain, proposing that a better adjustment of substrate concentration variables can enhance energy recovery from H2 production.