Effects of Pretreatments on the Solubilization and Theoretical Methane Production of Waste Activated Sludge from a Brazilian Eucalyptus Kraft Pulp Mill

Waste activated sludge (WAS) originating from kraft pulp mill effluent treatment plants represents an important environmental challenge for this industry. Anaerobic digestion is a promising option for WAS treatment, with the added benefit of biogas production. This paper presents the application of thermal, thermal-alkaline, and mechanical pretreatment methods in order to promote the solubilization of organic matter to enhance the anaerobic digestion of the WAS from a Brazilian Eucalyptus bleached kraft pulp mill. A total of 16 pretreatment operating conditions were compared. Chemical analyses showed an improvement in organic matter solubilization, with an increase greater than 7-fold for soluble chemical oxygen demand and 4-fold for biochemical oxygen demand. Nutrient solubilization showed an increase greater than 10-fold for total Kjeldahl nitrogen and 3-fold for total phosphorus. Theoretical biochemical methane potential was improved from 211 mLCH(4)/gVS for raw sludge to 333-343 mLCH(4)/gVS after mechanical pretreatment, 314-360 mLCH(4)/gVS after thermal pretreatment, and 373-378 mLCH(4)/gVS after thermal-alkaline pretreatment. In general, thermal-alkaline pretreatment showed the best results for all the evaluated parameters, with the advantage of requiring lower temperature and retention time when compared to thermal conditions.