Valorization of sewage sludge via non-catalytic transesterification

Abstract

To seek a way to valorize sewage sludge (SS), it was chosen as a raw material for biodiesel production. As such, non-catalytic transesterification of dried SS was carried out, to enhance its value. Note that picking a waste material such as SS as an inexpensive lipid feedstock for biodiesel production, without lipid extraction, greatly increases the economic viability of biodiesel. Also, to enhance biodiesel sustainability, ethanol (EtOH) was employed as the acyl acceptor, in this study, and this was experimentally justified by results showing that employing EtOH as an acyl acceptor provided an effective means for compensating for the lower heating value arising from the large amount of palmitic (C₁₆) acid in SS. This study experimentally proved that the fatty acid ethyl ester (FAEE) yield at 380 °C reached up to 13.33 wt% of dried SS. Given that the lipid content of dried SS is 14.01 ± 0.64 wt%, the FAEE yield of 13.33 wt% implied that 95.14 wt% of lipid in dried SS had been converted into FAEEs. The introduced SS valorization in this study offered an excellent opportunity to address diverse environmental hazards arising from SS and associated emerging contaminants. Given that the optimal temperature for the non-catalytic conversion for biodiesel production from SS was found to be 380 °C, emerging contaminants, such as microplastics and antimicrobials, were simultaneously degraded, due to their inferior thermal stabilities. Lastly, considering that the introduced biodiesel conversion process is thermally induced, the SS reside after the biodiesel conversion process can be further used in thermo-chemical processes as raw materials for gasification and pyrolysis (future work).

Keywords: Biodiesel; Fatty acid ethyl esters (FAEEs); Sewage sludge; Valorization; Waste management; Waste-to-energy.