Green solvent extraction of lipids from sewage sludge of wastewater treatment plants

This study reports the analysis of lipids extraction from sewage sludge using ethyl esters of volatile fatty acids (EEVFA). EEVFA are naturally occurring compounds, representing ones of the main constituents of flavour of fruits. They can be considered bioderivable solvents, since industrially obtainable through direct esterification of ethanol and VFAs, which can be both produced through fermentation of dedicated or exhausted feedstocks. In addition, they are easily hydrolysable in an aqueous environment, by regenerating species of origin which are completely biodegradable. For all these reasons, the use of EEVFA as extracting solvents would minimize the environmental impact respect with conventional solvents (hexane is typically used). A theoretical thermodynamic study of the lipids extraction was performed with an experimental matrix that emulated the chemical characteristics of a sewage sludge. This theoretical study was performed to obtain the phase diagrams for the lipids extraction and to identify the optimum operating conditions. A comparison of the lipids extraction using these green solvents and hexane was also performed. Then, the optimum conditions were tested and validated on real samples of sewage sludge, sampled from different municipal wastewater treatment plants. Theoretical thermodynamic studies, as well as experimental evidences confirmed that ethyl butyrate was an alternative and effective green solvent to extract lipids from sewage sludge.