A Wastewater Treatment Plant (WWTP) is a municipal service that any city should consider as part of its Water and Wastewater Management Plan. Further, it is well known that there are different technologies that allow to reuse and to discharge treated wastewater (WW) into surface water. Nonetheless, one of the main by-products of any WWTP is the so called sewer sludge, which has the potential to produce energy through anaerobic digestion or simply to be stabilized and converted into harmless matter for the environment. Therefore, pre-treatment of sludge has become an essential process in order to enhance the methane production, if an anaerobic digestion is part of the WWTP. Some of the sludge pre-treatments technologies comprise thermal, chemical and mechanical techniques, including their subdividing methods and combination among each other; these pre-treatments enhance the biogas production, by hydrolysing the organic matters present in sludge while reducing the total amount of solid residue of the anaerobic digestion process.