Pre-treatment to enhance biogas yield from pulp and paper mill sludge

Abstract

The purpose of this work was to study the potential to enhance biogas production from pulp and paper mill sludge by the use of thermal pre-treatment in combination with chemical pre-treatment. Biogas from waste is a renewable fuel with very low emissions during combustion. To further reduce the use of fossil fuels, more biogas substrates are necessary. Pulp and paper mill sludge is a large untapped reservoir of potential biogas. Pulp and paper mill sludge was collected from a mill that produces both pulp and paper and as wastewater treatment has a modified waste activated sludge system. As pre-treatments were chosen heat (70 °C or 140 °C) combined with either acid (pH 2 or pH 4) or base (pH 9 or pH 11, obtained with NaOH or Ca(OH)₂). Biogas potential was tested by anaerobic digestion batch assays under   mesophilic conditions. All pre-treatments were tested in six replicates. Biogas volume was measured with a gas-tight syringe and methane concentration was measured with a gas chromatograph. The methane yield from sludge subjected to thermal pre-treatment at 70 °C did not differ from the untreated sludge, but thermal pre-treatment at 140 °C had a positive effect. Within the 70 °C thermal pre-treatment group, the pH 2 acid was the most successful chemical pre-treatment, and Ca(OH)₂ pH 9 had the least effect with no measurable improvement in methane yield. For the 140 °C thermal pre-treatment group, acid and NaOH impacted methane production negatively, while the Ca(OH)₂–treated sludge did not differ from sludge without chemical pre-treatment. In conclusion, thermal treatment at 140 °C improved methane yield with 170% and for this sludge additional chemical pre-treatments can be excluded.