## 7 Conclusions  
The production of biogas from residual agro-food biomasses obtained through AD represents a promising tool for alternative production of energy from renewable sources. The use of livestock manure, and particularly that from ruminant species, may provide significant environmental benefits by reducing GHG emissions related to manure management. Manure AD may also provide a supplementary source of income for farmers as renewable energy is produced and the residual digestate has high fertilizing properties. Similar advantages can be also attributed to AD of fruit and vegetables waste in the perspective of a true circular economy. Currently, the most promising aspects to consider in order to improve AD process for biogas production include advanced AD plant technologies, such as the combination of anaerobic membrane technology with specific systems (e.g., coagulant addition, adsorbent addition, use of granular materials improving aeration) leading the membrane to work with high flow rates and reduced fouling problems. In addition, on-line monitoring of the process (e.g., through NIRS spectroscopy) should be performed to control the main process parameters and allow efficient biogas production. As for manure from ruminant, improvement of biogas yield may be achieved by feeding ruminants with diets containing high levels of concentrate, due to the generation of higher levels of soluble organic-C during anaerobic digestion. However, co-digestion of fruit and vegetable matrices with livestock manure seems to be the best strategy to improve the AD of these high lignocellulosic containing substrates. Alternatively, these substrates can be subjected to pretreatment with non-derivativizing solvents (e.g., N-Methylmorpholine-N-oxide) breaking the intermolecular interactions in cellulose without toxic pollutant residues. Another advantageous technique to increase biogas yield in high lignocellulosic containing substrates seems to be the bioaugmentation with anaerobic ruminal fungi and a pool of hydrogen-producing fermenting bacteria with an enhancement in CH\({}_{4}\) production, using a two stage configuration. The improvement of AD efficacy by inoculating specific microorganisms for biomass degradation, can be achieved either with naturally occurring microorganisms or with genetically modified microorganisms. Both the approaches have the advantages of enhancing the efficiency of the AD process, as these microorganisms are able to reduce the duration of fermentation. However, further research focusing on the development of engineered microorganisms with increasing degradation activity is needed.  
Future perspectives to improve biogas production from AD of agro-food waste should take into account the following aspects. Due to the origin and composition diversity of agro-food waste, which gives specific properties to them, an improved and specific characterization is needed to combine them in the correct proportion to enhance biogas production and process stability. To this aim the cooperation between biogas plants and laboratories specialized in waste characterization may promote AD efficiency. The seasonality of agro-food waste availability existing in areas characterized by small farms may cause input variability, which can be solved by the cooperation among farms dealing with different products. Communal plants could also contribute to the reduction of the economic and environmental transport costs.  
This paper was built on ideas by each of the authors. All authors contributed to the writing and editing of the manuscript.  
This research was carried out in the framework of the project \"Smart Basilicata \"(Contract n. 6386 - 3, 20 July 2016). Smart Basilicata was approved by the Italian Ministry of Education, University and Research (Notice MIUR n.84/Ric 2012, PON 2007--2013 of 2 March 2012) and was funded with the Cohesion Fund 2007-2013 of the Basilicata Regional authority.  
The authors declare no conflict of interest.