Enhancing green lipid extraction from sewage sludge using ethyl hexanoate: A step towards an effective lipid valorization

Sewage sludge is a challenging waste with potential for resource recovery and biorefinery applications. This study investigates, for the first time, the recovery of esterifiable lipids from primary sewage sludge using ethyl hexanoate as a green bioderived solvent. Sludge pretreatments (dewatering, drying, and acidification) and extraction parameters (extraction time, temperature, and stirring mode) were systematically explored to optimize lipids recovery, comparing ethyl hexanoate batch extraction with other green techniques, including scCO(2) extraction. Ethyl hexanoate showed exceptional performance in extracting lipids from acidified wet primary sewage sludge, achieving near-total lipids recovery (90-94 %) as fatty acids under the explored extraction conditions (wet sludge: ethyl hexanoate weight ratio = 1, 25 degrees C, 1-7 h, under orbital shaking). Conversely, extraction of wet centrifuged sludge without acidification (wet sludge: ethyl hexanoate weight ratio = 1, 70 degrees C, 3 h, magnetic stirring) resulted in the thorough recovery of esterifiable lipids as a mixture of insoluble calcium soaps and fatty acids. Additionally, a new sustainable procedure was designed and successfully tested to recover esterifiable lipids from ethyl hexanoate extracts as solid calcium soaps. Adding Ca(OH)(2) to the ethyl hexanoate suspension enabled efficient recovery of free fatty acids (recovery yield >95 %) at ambient temperature as insoluble calcium soaps through simple centrifugation. Ethyl hexanoate proved to be exceptionally stable (no hydrolysis during extraction and lipid recovery) and highly recoverable (91.8 wt%). The potential direct industrial application of calcium soaps as sustainable lubricants makes this process a promising route for generating value from residual biomass, aligning with Circular Economy principles.