Anaerobic digestion of pre-treated biological sludge from pulp and paper industry using heat, alkali and electroporation

Abstract: The biological sludge formed in the pulp and paper wastewater treatment constitutes a costlyproblem to dispose off due to poor dewaterability. It is often incinerated or used as soilconditioner improvement. By using anaerobic digestion of the biological sludge, thedewaterability can be increased. Thanks to the formation of biogas, the sludge volume isdecreased and energy can be recovered as methane. By pre-treating the sludge, the biogasproduction can be increased, making the anaerobic digestion more economically feasible. Eleven samples of biological sludges from six Swedish pulp and paper mills, chosen torepresent different types and sizes of mills available in Sweden, were pre-treated with alkali(NaOH, pH12), heat (80˚C, 1 hr) and electroporation (2000 pulses, 10 kV/cm). Initialmethane production rate and methane potential of all sludges and pre-treatments weredetermined using batch experiment. A combination of two sludges (from the same mill) pretreatedwith alkali and heat was further investigated in a semi-continuous digester experiment. The batch experiments showed that alkali pre-treatment had the greatest positive effect onmethane production. Heat treatment performed second best, whereas electroporation had no orlittle effect. Overall, pre-treatments increased the initial methane production rate, but withinsignificant effects on the methane potential. Heat pre-treatment showed no difference inbiogas production compared to the control in the semi-continuous digester experiment. Alkalitreatment was shown to inhibit biogas production and cause high accumulation of acetate. Itcould not be concluded whether it was an effect from hydroxide or sodium ion addition.Further analysis of the NaOH impact on floc structure, toxicity and bioavailability issuggested to determine the suitability of alkali-treated sludge for anaerobic digestion.