Production of biohydrogen by an anaerobic digestion process using the residual glycerol from biodiesel production as additive to cassava wastewater

Abstract

Despite the significant increase in biogas and methane production provided by the addition of glycerol to agro-industrial waste when submitted to anaerobic biodigestion, just a few studies are directed to hydrogen production using this procedure. Thus, the present study evaluated the anaerobic biodigestion of the cassava wastewater with the addition of as much as 5% of residual glycerol mass. Wastewater inoculum from swine farming was used, which was characterized through the isolation of anaerobic microorganisms, DNA extraction and genetic sequencing, revealing that this inoculum contained bacteria from the genus Brevundimonas, from subgroup Bacillus subtilis and Bacillus cereus, and from species Bacillus licheniformis. Factor planning 32 was established in order to evaluate the effects of temperature (between 37 and 43 °C) and glycerol concentration (between 1 and 5%) on hydrogen production to find the optimal values that would result in higher volumetric production of this gas. Control treatments without the addition of glycerol were also performed. The kinetics of biohydrogen production was experimentally determined and described by the Gompertz model. It was observed higher values of organic load removal for the treatments with the lowest concentrations of glycerol, despite this, the addition of glycerol provided the production of high volumes of hydrogen in concentrations ranging from 25.3 to 46.7%. The production of this biofuel in the treatments without the addition of glycerol ranged from 215.1 to 243.5 mL, while in the treatment with the addition of 3% of glycerol 1106.7 mL of hydrogen were obtained, an increase higher than 400%. On the other hand, hydrogen production per volume of treated waste was between 168.0 and 864.6 mLH2/Lresidue. The optimal conditions obtained in this study for the biodigestion process were 4% glycerol and a temperature of 38.5 °C. In these conditions, the total volume of hydrogen predicted by the model was 1093.5 mL or 854.2 mLH2/Lresidue, while the experimental value was 1102.6 mL or 861.4 mLH2/Lresidue. The analysis of the organic acids produced and the removal of the organic load showed the need for additional treatment of the effluents produced, which can be carried out with the use of physically separated acid and methanogenic phase reactors, also providing the obtaining of methane.