Background and purpose: In activated sludge process large amounts of sludge are produced. The disposal and management of this sludge remains a big challenge due to environmental, economic, social and legal factors. Thus, selecting an appropriate process that could reduce sludge would be of great help in solving this problem. The aim of present research was to determine the effects of three processes including Fenton, ozonation and Fento-ozonation on excess sludge reduction in activated sludge process.

Materials and methods: A bench scale study was conducted on three processes including Fenton, ozonation, and Fento-ozonation for reduction of dry solids (DS) and sludge volume (SV). The effects of pH, time and ozone dosage were also studied.

Results: The results indicated that in ozonation process (ozone dose of 0.14 gO₃g_TSS^-1) the maximum efficiency of dry solids and sludge volume reduction were 79.32% and 87.67%, respectively. The results also showed that the effect of reaction time on reduction of sludge volume was significant in all three processes. Moreover, significant effect of increase in ozone dosage (from 0.07 to 0.14 gO₃g_TSS^-1) was observed on reduction of all parameters in ozonation and Fento-ozonation processes (P<0.05).

Conclusion: Ozonation alone was found to be more efficient than Fenton and Fento-ozonation in sludge treatment. However, the combination of Fenton and ozonation processes did not improve sludge reduction.