Anaerobic Digestion of Kraft Pulp Mill Foul Condensate Under Thermophilic and Mesophilic Conditions

Foul condensate produced during the chemical recovery step of the kraft pulping process is a methanol-rich wastewater stream commonly treated through a steam-stripping process to extract volatile components before their combustion. Applying anaerobic digestion to treat this wastewater stream presents an opportunity to recover the inherent renewable bioenergy without the associated steam utilization and improve the overall energy balance of the mill. In this study, a comparison of the performance and microbiology of anaerobic digesters treating foul condensate under thermophilic and mesophilic conditions was performed. In terms of performance, both digesters performed similarly with an optimal organic loading rate and hydraulic retention time of 4 kg chemical oxygen demand (COD/[m3·day]) and 1.5 days, respectively, which produced similar methane yields of ∼200 L CH4 per kg COD loaded resulting in a methane productivity of ∼0.75 L/(L·day). The anaerobic digesters removed 95–99% of methanol-associated COD but only 16–25% of non-methanol-associated COD in the feedstock. Microbial community analysis showed that under mesophilic conditions, methylotrophic methanogenesis was the principal mechanism for methanol removal while a hydrogenotrophic methanogenesis pathway was dominant under thermophilic and favored under higher sulfur loading conditions.