Production of biodiesel is one of the most important European targets within renewables for the future. To consider biodiesel a feasible alternative to fossil fuel, unconventional resources need to be exploited. This review aims to provide up-to-date knowledge on the existing reuse of lipids from urban wastewater to produce biodiesel. Lipids are readily removed by mixed microbial populations during wastewater treatments in sewage plants. Assessment results on potential annual European market supply indicate 3 – 414 104 tons (min for activated and max for grease trap sludge) of potentially extractable biodiesel from wastewater and an expected biodiesel demand of 14.8 106 tons. Considering the prospect of transforming sewage plants into biorefineries, we may cover on average 1.5, 6.2, 6.7 and 24.4% of activated, primary, scum and grease trap sludge respectively, of the European biodiesel market from wastewater-derived lipids. In addition, by implementing an optimized biotechnology selector, the overall biodiesel yield could be higher due to increased lipid incorporation into microbial biomass. This is not an insignificant amount and, if efficiently implemented, could represent an exploitable resource for biofuel production, an important and desired step towards a circular economy. The technology readiness level is still very low. There are several challenges and possible drawbacks, e.g., biogas yield loss, substrate depletion, or formation of floating sludge. Finally, no definitive legislative barriers towards wastewater-derived lipids have been identified; however, quality criteria as well as waste status have to be defined
