## 4 Conclusions  
The concept of the circular economy for the treatment of sewage sludge generated in WWTPs is a topic of great interest as it offers a variety of benefits on social, environmental, and economic levels. Currently, sewage sludge is considered a waste that requires special treatment before disposal and generates the highest costs in the management of wastewater treatment plants. These conditions are exacerbated by the increasing expansion of residential and industrial areas, creating more significant quantities of sewage sludge yearly. However, sewage sludge can be considered a relatively cheap, readily available and abundant raw material for producing energy and recyclable materials. Among the possible alternatives which utilize the surplus sewage sludge, the production of biodiesel could be profitable given the high lipid content of the specific sewage sludge, namely sewage scum and primary sludge. In this work, a process for the utilization of lipids in sewage scum and primary sludge was proposed and investigated. The utilization strategy consisted of first extracting the lipids from these two raw materials and then converting them into biodiesel and fine chemicals through sustainable chemical processes. In addition to free fatty acids, (R)-10-hydroxystearic acid and estolides were detected and quantified in the extracted \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sewage Sludge** & **Reagents Costs** & **Energy Demand** & **Product Revenues** & **Profits** \\ \hline
**Primary sludge** & & & & & \\ \hline Hexane & 2.5 € & & Biodiesel & 6.8 € & \\ H\({}_{2}\)SO\({}_{4}\) & 0.4 € & 154.5 MJ & Me-10-HSA & 1.5 € & \\ MeOH & 0.4 € & & Methyl & & \\ AlCl\({}_{3}\)-6H\({}_{2}\)O & 0.1 € & & & & \\
**Total** & **3.4 €** & **0.8 €** & & **8.6 €** & **4.4 €** \\ \hline
**Sewage scum** & & & & & \\ \hline HCOOH & 3.2 € & 425.7 MJ & Biodiesel & 53.6 € & \\ MeOH & 3.6 € & & Me-10-HSA & 2.9 € & \\ AlCl\({}_{3}\)-6H\({}_{2}\)O & 0.6 € & & Methyl & & \\  & & & & & \\ \hline
**Total** & **7.4 €** & **2.1 €** & & **86 €** & **76.5 €** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Analysis of manufacturing costs, energy demand and product revenues derived from chemical conversion of primary sludge and sewage sum in biochemicals. lipids. The direct esterification of the two extracted lipids with methanol was successfully carried out under mild conditions. Finally, the high-quality compounds can be purified and recovered by distillation in a vacuum. The extraction of biodiesel and the simultaneous recovery of (R)-10-hydroxysteratic acid methyl esters and methyl estolides from sewage sludge could significantly impact the current scenario of sewage sludge treatment. It would promote a change in perspective and transform sewage sludge from waste to be disposed of into a new raw material with valuable resources. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app14124974/sl](https://www.mdpi.com/article/10.3390/app14124974/sl)]([https://www.mdpi.com/article/10.3390/app14124974/sl](https://www.mdpi.com/article/10.3390/app14124974/sl)), S.1 Materials and methods; S.2 Characterization of reaction products obtained from esterification process, S.3 Estimation costs of biodiesel production from primary sludge and sewage scum, S.4 Analysis of biodiesel isolated by distillation process. References [63; 64; 65; 66; 67] are cited in the Supplementary Materials. Conceptualization, C.P. and L.d.B. ; methodology, L.d.B., C.P. and A.A.; validation, L.d.B., C.P. and A.A.; formal analysis, L.d.B. and C.P. ; investigation, L.d.B., C.P. and A.A.; resources C.P. ; data curation, L.d.B. and C.P. ; writing--original draft preparation, L.d.B., C.P. and A.A.; writing--review and editing, C.P., L.d.B. and A.A.; supervision, C.P. ; project administration, C.P. ; funding acquisition, C.P. All authors have read and agreed to the published version of the manuscript. This work was financially supported by European Union--FESR \"PON Ricerca e Innovazione 2014-2020.