Alkali-buffering effect to increase hydrogen production on co-fermentation of citrus agroindustrial waste

Fermentation operational pH is a critical parameter for hydrogen production and aiming to improve the pH stability in the process, supplementation with buffering agents can be an alternative. The effect of two buffering agents, sodium bicarbonate (NaHCO3) and calcium carbonate (CaCO3), in hydrogen production from citrus peel waste (CPW) and citrus processing wastewater (CPWW) was evaluated, in assays carried out in batch reactors. The calcium carbonate was identified the buffering agent with the most significant positive effect on H-2 production. The effect of CaCO3 concentration (1.0-4.0 g L-1) with variations in the citrus peel waste (7.0-25.0 gTVS.L-1) and citrus processing wastewater (1.0-3.0 gCOD.L-1) concentrations were evaluated through an experimental design. In the assays with CaCO3 and NaHCO3, the hydrogen production was 1099.7 mL L-1 and 517.3 mL L-1, respectively, representing an increase of 176.8 % and 30.2 % when compared to the control assay (397.4 mL L-1), and CaCO3 was identified as the best alternative for alkalinizing the process. Through experimental design assays the high importance of the adjustment between buffering capacity and substrate availability for hydrogen production was verified. Higher H-2 production (940.0 mL L-1) was obtained in the assay with 2.5 g. L-1 of CaCO3, 16.0 gTVS.L-1 of CPW, 2.0 gCOD.L-1 of CPWW, and 3.5 gTVS.L-1 of inoculum sludge, resulting in a gCaCO(3).g(-1)CHO ratio of 0.45. For the assay with the highest hydrogen production, butyric acid (1752.1 mg L-1) and acetic acid (1768.3 mg L-1) were the metabolites in the highest concentration and a relative abundance higher than 90 % of Clostridium sensu stricto was identified. A maximum energy potential of 634.91 MJ t(-1) of CPW was obtained with an estimated electricity generation potential of up to 3.2 million MWh per year.