# Sustainable Italian Cities: The Added Value of Biomethane from Organic Waste
## 2 Literature Review  
Generally, the biodegradable fraction of MSW varies from 30% to 65% [46], and OFMSW can be recovered from AD because of its high volatile solids [47]. AD supports the valorization of organic waste by generating biogas; for this reason, it is a valid option for the environmental management of MSW [48]. Using energy derived from biogas (as opposed to energy generated from fossil fuels) reduces CO2 emissions by 200 to 300 kgCO2/ton of biowaste [49]. Using energy derived from biomethane (BIO-CNG) follows the same approach, and its positive impact can be defined as follows:  
* Reduced GHG emissions in vehicle power train systems of 119 gCO2 eq/km, 151 gCO2 eq/km, and 159 gCO2 eq/km, relative to compressed natural gas (CNG), diesel, and petrol, respectively [50]. * GHG emissions from biomethane varying from \(-\)36 to 10 gCO2/MJ, all of which are lower than the 27 gCO2/MJ produced by a conventional biogas plant. This reduction is even more significant, considering the value of natural gas in Europe is equal to 72 gCO2/MJ [51]. * GHG emissions from biomethane production ranging from 45 to 68 gCO\({}_{2}\)eq/kWh, all of which are lower than the 124 gCO\({}_{2}\)eq/kWh, which is linked to the production of natural gas [52]. The economic perspective can be evaluated from several points of view (Table 1). Some studies have measured the cost of biomethane production, which varies from 0.54 to 0.73 g/m\({}^{3}\) if it is injected into the gas grid or used as transportation fuel, respectively [17], and from 0.71 to 0.75 \(\upcts/\)m\({}^{3}\) if it is used for feedstock composition [53]. In particular, some authors have defined the cost of biogas production as mainly a function of the type of feedstock used (ranging from 0.38 to 0.56 \(\upcts/\)m\({}^{3}\)) [54]. The substrate and size also influence the cost of biomethane, which is estimated to range from 0.5 \(\upcts/\)m\({}^{3}\) (4.7 cent \(\upcts/\)kWh) to 1.5 \(\upcts/\)m\({}^{3}\) (15 cent \(\upcts/\)kWh) [55]. Other research has compared the costs of biomethane and natural gas. Currently, subsidies are needed in order to promote the development of the biomethane market [56]. The carbon price plays a key role in the decarbonization of energy systems, and competitiveness can be reached when the price falls within the following values: 70 \(\upcts/\)ton of CO\({}_{2}\)[57] to 99 \(\upcts/\)ton of CO\({}_{2}\)[58]. However, these values are significantly lower than the ones registered in another work, which found the costs to range from 180 to 260 \(\upcts/\)ton of CO\({}_{2}\)[59]. Consequently, the analysis of biomethane plants is extremely complex, because multiple variables must be considered. Some researchers have framed the difference between biogas and biomethane plants in terms of profitability. In fact, initially, several policymakers adopted subsidies only for the production of electricity from AD. Biomethane can be more convenient when there is both a high retail price and a low biogas upgrading cost [60]. Within existing biogas plants, the profitability of biomethane is typically reached when a centralized upgrading system is constructed. However, this is only valid when the biogas plants are in relatively close proximity to each other [45]. Finally, the profitability of biomethane plants has been investigated in several other works. In one study, the co-digestion of food waste and sewage sludge to produce biomethane was found to be financially feasible (NPV equal to 5667 KSUS), while that of food waste and pig slurry was found to be unprofitable (NPV equal to \(-\)585 KSUS) when no subsidies were provided [61]. Another study confirmed this result--NPV was found to vary from 89.8 MUR to 132.7 MUR when the organic waste had a consistent weight in the mix of waste recovered. However, if only pig slurry, manure residues, cattle slurry, milk processing waste, and slaughterhouse waste were used, the economic performance ranged from 0.49 MUR to 19.1 MUR. The profitability was determined by the application of subsidies [62].