Effects of ultrasonic pretreatment on the solubilization and kinetic study of biogas production from anaerobic digestion of waste activated sludge

- Ultrasonic pretreatment to improve anaerobic digestion of waste activated sludge.
- Linear increase of the solubilization degree was observed with the specific energy.
- The maximum solubilization degree of soluble chemical oxygen demand was 26%.
- A biogas overproduction of 31.43% (219.5 mL/g VS) was achieved at 35000 kJ/kg TS.
- Pathogen inactivation is most effective at lower TS (2%) and continuous stirring.

Waste activated sludge (WAS) is a polluting waste with severe management problems that must be treated to prevent pollution and human health risks. Anaerobic digestion (AD) is the most used process to stabilize sludge; however, it must be improved because the biomethanation of sludge entails low biodegradability, pathogen inactivation, and biogas production. This study investigated the effects of ultrasonic pretreatment (USp) of WAS as a strategy to improve AD. Macromolecule solubilization, heavy metal behavior, pathogen inactivation and biogas production were evaluated. USp was applied at a range of 5000-35000 kJ/kg TS (total solids). The maximum solubilization degree of soluble chemical oxygen demand was 26%, and 22.9% of proteins at 35000 kJ/kg TS. The highest USp reduced only 2 log units of pathogens; nevertheless, a high inactivation was obtained when TS were reduced to 2% and continuous stirring was applied. AD of raw and sonicated WAS were compared with biochemical methane potential tests, and a biogas overproduction of 31.43% (219.5 mL/g VS) was achieved at highest USp. A modified Gompertz model was used for kinetic study of biogas production. The kinetic parameters at highest energy were: biogas production potential, G0 = 634.2 mL; maximum biogas production rate, Rmax = 57.23 mL/day.

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