Lipid extraction from sewage sludge using green biosolvent for sustainable biodiesel production

Sewage sludge is a type of urban waste generated as a byproduct of wastewater treatment in quantities ranging from 9 to 9.5 million tons (dry substance) in Europe. This waste can be considered a source of lipids, which can be converted into biofuels. Recovery of lipids from sludge is a critical step in the overall process, and the choice of the extracting solvent should satisfy efficiency as well as sustainability criteria. This study reports experimental optimisation of lipid extraction from sewage sludge using ethyl butyrate, a green bio-derivable solvent. Extraction conditions were optimised using the desirability function applied to the response surface methodology analysis of a Box–Behnken factorial design of experiments. By carrying out the extraction at 70 °C for 7 h, using an amount of solvent equal to the wet sludge, and without using any acids, 93.7% of the starting lipids were recovered. The extracted lipids were then efficiently converted into biodiesel (fatty acid methyl esters) through direct esterification promoted by AlCl3∙6H2O (catalyst). Furthermore, the residual sludge obtained after the extraction, different to that obtained using hexane, was found to be anaerobically digestible without any inhibition.