Sludge produced in wastewater treatment plants needs to be stabilized before it can be safely disposed. Sludge pre-treatment technologies can be categorized into mechanical, thermal and chemical treatments. The aim of sludge pre-treatment is to solubilize the particulate organics, thereby making them more accessible for subsequent microbial action. Ozonation has also been reported as an effective sludge pre-treatment technology but with a different disintegration mechanism. The level of improvement, however, seems to be better than thermal/thermochemical treatment at low pre-treatment temperatures and comparable at higher temperatures. The optimum combination sequence was determined and changes in the sludge characteristics after individual and combined pre-treatments were measured. Biogas production and solids concentrations in the digested sludge were monitored to assess possible enhancement with such pre-treatment. The energy balance was analyzed by subtracting the energy input during the pre-treatment from the energy recovered from the combined heat and power production of the produced methane.