Secondary sludge achieved the lowest lipid yield, a meagre 6.3% and 5.1% for acidified and non-acidified sludges, respectively. The high difference between the values obtained for primary
and blended sludges, with and without acidification owns to the fact that municipal wastewater contains fatty acids from commercial soaps, potassium and sodium from household cleaning products,cosmetics, lubricant and coatings. During primary treatment, the physico-chemical process leads to a rapid formation of relatively insoluble calcium and magnesium salts precipitating during the primary wastewater treatment, which remain in the primary sludge [5]. For this reason, the acidification was responsible of the conversion of insoluble soaps into FFAs that are soluble in the extract solvent, increasing the lipid yield and the saponifiable matter [5]