Biofuels & Bioenergy

Biodiesel production is gaining attraction as an efficient alternative for the valorization of sewage sludge. This waste, which can be considered as a raw material, contains significant quantitities of free lipids (10-30 wt%) liable to be transformed into biodiesel. Nevertheless, the low-quality of this feedstock makes necessary the use of novel conversion technologies, such as heterogeneous acid catalysis. This contribution deals with the extraction of lipids from primary and secondary sewage sludge collected from a wastewater treatment plant and their use in the synthesis of biodiesel over highly poison-resistant heterogeneous acid catalyst based on SBA-15-supported zirconium. Catalytic tests were performed in a 25 mL stainless-steel autoclave. The sludge was treated with different organic solvents to extract the lipids which were subsequently transformed into biodiesel over Zr-SBA-15 (two-step process). Alternatively and advantageously, similar experiments were carried out using directly the dried and wet sludge, without previous extraction (in-situ process). Zr-SBA-15 catalyst provided high biodiesel yields when processing primary and secondary sewage sludge in both the ‘two-steps’ and the ‘in-situ’ processes, converting almost 95% of the saponifiable fraction (free fatty acids and triglycerides) into fatty acid methyl esters (FAME). It must be noted that such good catalytic results are obtained even in the presence of high amounts of unsaponifiable matter and other impurities, typically accompanying such wastes. Besides, the excellent results (92%) obtained with wet (non-previously dried) sludge in the in-situ process demonstrate the high water tolerance of the Zr-SBA-15 catalyst, allowing to avoid the necessity of a sludge drying pretreatment.