Biochemical Methane Potential of a Biorefinery’s Process-Wastewater and its Components at Different Concentrations and Temperatures

A sustainable circular bioeconomy requires the side streams and byproducts of biorefineries to be assimilated into bioprocesses to produce value-added products. The present study endeavored to utilize such a byproduct generated during the synthesis of 5-hydroxymethylfurfural as a potential feedstock for biogas production. For this purpose, biochemical methane potential tests for the full process-wastewater, its components (5-hydroxymethylfurfural, furfural, levulinic acid, and glycolic acid), together with furfural’s metabolites (furfuryl alcohol and furoic acid), and phenols (syringaldehyde, vanillin, and phenol), were conducted at mesophilic and thermophilic temperatures to assess their biodegradability and gas production kinetics. 0.1, 0.2, 0.3, and 0.4 g COD of the test components were added separately into assays containing 35 mL of inoculum. At their lowest concentrations, the test components, other than the process-wastewater, exhibited a stimulatory effect on methane production at 37 °C, whereas their increased concentrations returned a lower mean specific methane yield at either temperature. For similar component loads, the mesophilic assays outperformed the thermophilic assays for the mean measured specific methane yields. Components that impaired the anaerobic process with their elevated concentrations were phenol, vanillin, and 5-hydroxymethylfurfural. Poor degradation of the process-wastewater was deduced to be linked to the considerable share of 5-hydroxymethylfurfural in the process-wastewater governing its overall characteristics. With excessive recalcitrant components, it is recommended to use such waste streams and byproducts as a substrate for biogas plants operating at moderate temperatures, but at low rates.