Municipal wastewater treatment produces significant quantities of sludge, that need appropriate treatment so it can be used for another purpose without constituting a threat to the environment, contributing to a sustainable circular economy.

In this work, the efficiency of ultrasound, Fenton reagent and ultrasound-Fenton (US-Fenton) processes were evaluated separately, for the treatment of municipal activated sludge. Additionally, the effects of operational parameters such as pH, hydrogen peroxide and ferrous iron concentrations, and cavitation time were studied. During the experiments, the COD reduction and the volatile solids vs total solids (VS/TS) ratio were followed, trying to reach the optimal experimental conditions.

According to the results, pH = 4.0, hydrogen peroxide concentration of 30 mM, ferrous iron concentration of 2.0 mM and a reaction time of 60 min (cavitation: 3 s ON and 5 s OFF), were selected as optimal conditions. Analyzing the results, ultrasound and Fenton processes allowed to reach 17.3 and 25.9% of COD removal, respectively, while the combined US-Fenton was more efficient with a 94.8% COD reduction. Regarding the VS/TS ratio, the process that showed better results was US-Fenton once again, reducing the original value of 0.59 to 0.16. The ultrasound and Fenton processes showed a lower VS/TS ratio reduction to 0.26 and 0.22, respectively.

In conclusion, the combination of ultrasound-Fenton (US-Fenton) achieved high COD removal and a significant VS/TS ratio reduction of the municipal activated sludge, showing better efficiencies than both processes separately.