# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 4 Conclusions  
This review aimed to summarize the main knowledge about the use and production of biogas and biomethane particularly from zootechnical slurry. Renewable energy has, in fact, become a priority for organizations worldwide, as the currently employed fossil fuels will soon run out. Furthermore, bioenergy supply can meet climate change objectives. By reviewing the literature, it emerged that AD is an efficient process for the production of biogas from waste of different origins. Animal waste products provide nutrient-rich and highly valuable raw material for renewable fuel generation through the AD process. Co-digestion of multiple feedstocks is an excellent way to improve biogas production. Contrary to the mono-digestion of manure alone, it has recently been suggested that the combination of agricultural and livestock wastes is an advantageous alternative that fully exploits the qualities of both feedstocks and, as a result, increases the biogas yield. Through alkaline pretreatment, tested with NaOH, the optimization of the digestion process was found to be able to increase the production of biogas and thus biomethane. However, further research on substrate mixing is needed to advance process design and optimization and to keep process costs down. Various upgrading technologies have also been developed to purify the biogas produced. With the improvement in biogas production and biogas upgrading in the future, the demand for biogas as an alternative energy source is expected to be met by replacing conventional fossil fuels. The by-product of the AD process, known as digestate, can be further used as a natural fertilizer in agriculture, providing plants with organic matter and ensuring the replacement of synthetic fertilizers.  
In conclusion, the road to the green transition of the industry and the diversification of energy supply sources is still uphill. Biogas is one of the solutions to global greenhouse gas emissions, and its importance in waste management is well established and will continue to develop in the future. What is certain is that renewable sources represent a pillar in this process, which will be strengthened through targeted investments. Environmental protection is an urgent and common problem that must be solved by all parts of society. On this front, much has been achieved, thanks to a mix of technological progress and incentives that have made renewable energies an increasingly convenient choice, and much more will be performed through further research and significant investigations that will perfect the best use of these energies.  
Conceptualization, D.M. and P.A.; methodology, D.M. and P.C.; software, C.C. and G.C.; validation, D.M. and P.C.; formal analysis, C.C.; investigation, D.M.; resources, P.C. and G.C.; data curation, D.M.; writing--original draft preparation, D.M.; writing--review and editing, P.C. and P.A.; supervision, P.A.; funding acquisition, P.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by an agreement between University of Molise and Italian National Research Council (Istituto sull'Inquinamento Atmosferico) under the grant \"L'utilizzo delle Biomasse emergenti nel processo di transizione ecologica: caratterizzazione e confronto degli impatti ambientali rispetto alle fonti energeticiche tradizionali e ninnovabili\".  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors wish to thank the CNR for funding the Ph.D. course and [PERSON] (ENEA) for his helpful and important contribution to the discussion.  
The authors declare no conflict of interest.