Black liquor increases methane production from excess pulp and paper industry sludge

Abstract The aim of the study was to use black liquor produced during the soda pulping process in a pulp and paper mill to increase methane production during pulp and paper industry sludge treatment and decrease the treatment cost. The effects of black liquor on sludge solubilization and methane production were assessed and the economic feasibility of the process was evaluated. Black liquor and NaOH were found to be equivalent in the thermochemical pretreatment process to solubilize sludge and disintegrate flocs. However, adding black liquor increased the background chemical oxygen demand and volatile fatty acid concentration and increased the amount of methane produced by approximately 7–30%. A start-up delay was emphasized by first-order kinetics model due to black liquor addition while methane production remained stable. Economic assessments of five scenarios were performed. It was found to be economically feasible to use black liquor to replace NaOH for the thermal pretreatment process. The surplus methane generated suggested that co-digestion of sludge and black liquor allows surplus bioenergy to be produced during the thermochemical pretreatment anaerobic digestion process.