Closed-Loop Valorization of Urban Sewage Sludge for Sustainable Biodiesel Production

The environmental and economic challenges stemming from fossil fuel dependence and climate change highlight the urgent need for sustainable biofuels. Wastewater treatment byproducts, such as sewage sludge (SS), represent a promising, yet underutilized, feedstock for biodiesel production within a circular economy. This research proposes a closed-loop valorization strategy for wastewater treatment byproducts, integrating solventless lipid extraction with the pyrolysis (at 400, 550, and 750 °C) of lipid-free residual biomass to produce the relevant biochars (BC). BCs can be functionalized (sulfonation run at 175 °C, 24 h), characterized (CHNS, FTIR, XRD, SBET, TPD, SEM), and used as catalysts for the esterification reaction of the previously extracted lipids with methanol to produce biodiesel. The extracted lipids were reacted with methanol (1:10 molar ratio, 80 °C, 5 h) using 3 wt.% of sulfonated catalysts per gram of grease, producing fatty acid methyl esters in high yield and conversion (>96%). Catalysts were easily and efficiently recoverable through centrifugation, rendering them stable and reusable for five cycles without losing their catalytic properties. This approach aims to transform SS into a valuable resource, fostering a more sustainable and efficient biofuel production system.