Urban sewage scum and primary sludge as profitable sources of biodiesel and biolubricants of new generation

Lipids of sewage scum and primary sludge taken from several wastewater treatment plants were quantified and characterised. In sewage scum, lipids represented 36–50% of total solids and were primarily composed of free fatty acids (45–60%) and calcium soaps (27–35%). In primary sludge, total lipids were 20–24% of total solids, and 71–82% of these were calcium soaps. Estolides and 10-hydroxystearic acid (prevalently present as R enantiomer, with an enantiomeric excess >92%) were also identified and quantified. A scheme of valorisation was then specifically designed and positively tested for both the sludge. Lipids were first recovered (92–99%), activated and finally reacted with methanol and AlCl3∙6H2O (343 K, 2 h, yield >96%). Besides biodiesel, methyl estolides and methyl 10-hydroxystearate were efficiently isolated and purely separated in different fractions. A preliminary feasibility study was finally conducted and a possible integration of processes into a wastewater treatment plant was proposed and positively evaluated.