# Sustainable Italian Cities: The Added Value of Biomethane from Organic Waste
## 3 Materials and Methods
### Definition of Plant Size  
A strategy is a plan of action taken by decision makers to achieve a final goal. Section 2 underlined that plant size assumes a key role in the economic analysis. Decision makers aim at reducing the environmental damage produced by inadequate waste management, and, at the same time, maximizing economic opportunities. Underestimating plant size prevents a share of the collected waste from being recovered, and its transfer causes higher emissions in the transport phase. In addition, this choice is not coherent with the principles of self-sufficiency and proximity (Directive 2008/98/CE). Overestimating the plant size can lead to economic losses when plant saturation is not achieved; consequently, the benefits of economies of scale must be balanced [68]. Biomethane plant size (S\({}_{\text{biomethane}}\)) is defined according to three variables (see Equation (1)), namely: (i) the quantity of feedstock (Q\({}_{\text{OFMSW}}\)), (ii) the unitary potential of biomethane (p\({}_{\text{b}}^{\text{u}}\)), and (iii) the number of operative hours (n\({}_{\text{oh}}\)). \[S_{\text{biomethane}}=\big{(}Q_{\text{OFMSW}}*P_{\text{b}}^{\text{u}}\big{)}/n _{\text{oh}} \tag{1}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Total MSW (tons)** & **Collection Rate (\%)** & **Total OFMSW (tons)** & **OFMSW (kg/capita)** \\ \hline Torino & 427,741 & 42.1 & 53,312 & 60.1 \\ Milano & 679,278 & 57.6 & 139,740 & 103.4 \\ Verona & 138,787 & 55.3 & 252,23 & 98.0 \\ Venezia & 166,601 & 57.0 & 35,904 & 137.1 \\ Padova & 124,588 & 55.2 & 26,460 & 126.1 \\ Trieste & 94,104 & 40.2 & 8570 & 42.0 \\ Genova & 287,287 & 33.5 & 17,802 & 30.5 \\ Bologna & 223,011 & 46.0 & 22,834 & 58.8 \\ Firenze & 240,573 & 50.3 & 46,518 & 121.7 \\ Roma & 1,689,206 & 42.0 & 255,161 & 88.8 \\ Napoli & 519,421 & 31.3 & 58,017 & 59.8 \\ Bari & 191,328 & 36.7 & 16,309 & 50.3 \\ Palermo & 347,008 & 7.2 & 8458 & 12.6 \\ Messina & 113,442 & 11.2 & 2139 & 9.0 \\ Catania & 218,213 & 10.3 & 4457 & 14.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistical data in 15 Italian municipalities in 2016 [67]. Figure 2: Map of 15 Italian municipalities. The amount of feedstock can be easily defined by the value reported in Table 3, but, at the same time, this table also reports different values for the separated collection rate (%SC). An increase in this last variable towards the final goal of 65% proposed by the European Commission also determines an increase in the collected OFMSW. The definition of the specific yields of SC for each fraction of waste was proposed by the authors of [69]. Four alternative scenarios are analyzed in this work, namely:  
1. Scenario \"As Is\", in which Q\({}_{\text{OFMSW}}\) is determined by the 2016 values. 2. Scenario \"To Be--65%SC\", in which the Q\({}_{\text{OFMSW}}\) is calculated according to the recycling target of 65% and the %SC linked to the OFMSW is equal to the current mix. 3. Scenario \"To Be--65%SC 40%OFMSW\", in which the Q\({}_{\text{OFMSW}}\) is defined according to the recycling target of 65% and the %SC of the OFMSW is equal to 40% (because this value is close to 41.2%, which was the 2016 average value %SC of the OFMSW in Italy). The percentage mix of separated waste is significantly diversified across the 15 municipalities, varying from 39.2% in the north to 45.7% in the south, and 41.7% in the central regions [67]. 4. Scenario \"To Be--110 kg/inhabitant\", in which Q\({}_{\text{OFMSW}}\) is evaluated as a function of the number of citizens. Some authors have assumed a yearly OFMSW production of 150 kg per inhabitant [70], considering good waste management practices. However, as discussed in Section 1, new policies that reduce the amount of waste generated also lower this value. A hypothetical reduction of 75% is considered, and, consequently, a yearly OFMSW production of approximately 110 kg per inhabitant is assumed. For example, considering Torino as a case study, the Q\({}_{\text{OFMSW}}\) is equal to 53,312 tons in the \"As Is\" scenario (Table 3).