**4. Conclusions**

The present study illustrates the intermediate results conducted in the framework of the Sustainability of the Olive oil System (S.O.S.) project, with a particular interest in the recovery of olive mill wastes for energy purposes. The findings up to now considering technical, economic, and environmental aspects are very promising, since laboratory experimental trials showed higher amount of biogas (80.22 ± 24.49 NL.kgSV−1) and methane (47.68 ± 17.55 NL.kgSV−1) when implementing a higher content of olive mill wastewater (30% of *v*/*v* in our case). Our results are lower than those obtained in other studies, due to the high content of OMWW in polyphenols; however, they suggest the possibility to explore furthermore AcoD of OMWW considering other farm, livestock, or food industry by-products to enhance biogas and biomethane production, and consequently increase the positive effects on the environment by limiting the impacts, as supported by LCA. In this regard, the analysis of environmental impacts has shown that the production of biogas from OMWW is advantageous from several points of view. In fact, through anaerobic digestion, multiple benefits are obtained such as: the management of a critical waste to be managed from an environmental point of view; the valorization of the biogas present in the digestate that, otherwise, would be dispersed in the environment, representing an environmental load; the production of renewable energy produced exclusively from waste; the production of an exhausted digestate that can be used as organic fertilizer and avoid the impacts generated by the production and distribution of synthetic fertilizers. This represents a strategy for implementing circular economy models, with benefits that go beyond just improving environmental performances. In fact, the economic analysis has demonstrated the profitability of this solution, which, due to the feed-in tariff guaranteed for 15 years, allows dealing with the investment for the plant without the risk of the aleatory energy selling price. Moreover, the recovery and valorization of thermic energy for district heating would allow a further advantage, both for the producer and for the community—using a source of heating with a low environmental impact and that is economic and independent from the public network and therefore free from fixed costs in the bill. Current policies and subsidies encourage the use of by-products instead of energy crops for renewable energy production, so this could be a further stimulus to the spread of small plants dedicated to the digestion of OMWW. In addition, global policies now focus on the adoption of sustainable and circular development models, so it is desirable that the force research should be concentrated on the valorization of waste, transforming it into by-products capable of producing benefits for the whole community.

**Author Contributions:** Conceptualization, S.B., G.F., T.S., A.I.D.L. and B.B.; data curation, S.B., G.F., T.S., A.I.D.L. and B.B.; formal analysis, S.B., G.F., T.S., A.I.D.L. and B.B.; funding acquisition, B.B., A.S., G.G. and G.Z.; investigation, S.B., G.F., T.S., A.I.D.L. and B.B.; methodology, S.B., G.F., T.S. and A.I.D.L.; project administration, A.S., G.G. and G.Z.; resources, A.S., G.G. and G.Z.; supervision, A.S., G.G. and G.Z.; validation, S.B., G.F., T.S., A.I.D.L., A.S., G.G., G.Z. and B.B.; visualization, S.B., G.F., T.S., A.I.D.L., A.S., G.G., G.Z. and B.B.; writing—original draft, S.B., G.F., T.S., A.I.D.L. and B.B.; writing—review and editing, S.B., G.F., T.S., A.I.D.L. and B.B. Please turn to the CRediT taxonomy for explanation of the term. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the AGER 2 Project (grant no. 2016-0105).

**Data Availability Statement:** Data is contained within the article. Raw data of the study results may be available on request from the corresponding author.

**Acknowledgments:** Raw material implemented in experimental trials was kindly provided by Olearia San Giorgio olive mill free of charge.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
