Kinetics of combined thermal pretreatment and anaerobic digestion of waste activated sludge from sugar and pulp industry

• Thermal pretreatment of waste activated sludge from sugar and pulp mills.
• Growth rate of methane yield revealed the kinetic variation after pretreatment.
• Share of the rapidly biodegradable matter is estimated by methane yield percentage.
• Simplified Reaction Curve model provides a quick assessment of methane yield.

The purpose of this work was to evaluate the effects of thermal pretreatment temperature (from 100 °C to 200 °C) and time (15–60 min) on the characteristics and the anaerobic biodegradability of wasted activated sludge from sugar and pulp mills. Reaction Curve model and First-order kinetics were used to obtain a better understanding of the data from the anaerobic biodegradability tests. Sludge characteristics and methane production prior and after thermal pretreatment confirmed that temperature exerted the major influence on sludge solubilization and biodegradability. The optimal methane yield (182 ml CH4/g VSFeed) and ultimate methane production rate (72.1 ml CH4/g VSFeed d) was obtained at 165 °C for 30 min. However, the negative effects of melanoidins and Amadori compounds appeared and the rising trend of biodegradability reversed at 200 °C. The growth rate of methane yield topped out at 476% at 100 °C, and the maximum methane yield (range from 28.9 to 72.1 ml CH4/g VSFeed d) were also observed at the first day, showing the radical kinetics improvement after pretreatment. This remarkable initial methane yield was attributed to the raising share of the rapidly biodegradable organics generated in thermal pretreatment, which were estimated with methane yield percentage of day 3. Both Reaction Curve model and First-order kinetics fitted with the experimental data well, and a Simplified Reaction Curve model without lag time provides a feasible tool to estimate methane yield of the rapidly biodegradable substrates.