## 3 Results
### Biogas Facilities' Location, Resources Allocation and Pre-Dimensioning  
The modelling results refer to adequate local potential for the implementation of ACoD biogas production facilities considering local conditions, resources' spatiotemporal availability and the technical indicators about ACoD conversion processes and biogas caloric value (Table 2). The referred data obtained in biogas production at a laboratorial/pilot scale were considered in a resource availability assessment, in location analysis and the pre-dimensioning of the potential biogas production facilities. Improving biogas production processes (biogas quantity and quality) through the ACoD of different substrates increases the potential return and competitiveness of these processes, relative to other alternative resource recovery technologies. The spatially explicit model indicates a unique centralized biogas plant as a possible solution (Figure 5b), but also allows us to explore several scenarios/simulations regarding the number of facilities, dimension and location according to a multiobjective framework. In this context, as an example, two different scenarios were developed, i.e., decentralized systems with eight biogas facilities (Figure 6a) and centralized systems with five biogas production facilities (Figure 6b). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Indicators** & **Value** & **Reference** \\ \hline Volatile solids/Total solids (\%) & 65 & [29] \\ Methane yield (mL gVS\({}^{-1}\)) & 48.5 & [29] \\ Biogas yield (mL gVS\({}^{-1}\)) & 93.3 & [29] \\ Average methane content (\%) & 52 & [29] \\ Biogas calorific value (MJ m\({}^{-3}\)) & 21.5 & [47] \\ Waste sludge production (kg TS capita\({}^{-1}\) year\({}^{-1}\)) & 16.4 & [43] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Indicators adopted in the case study. Figure 4: Population density (inhabitants km\({}^{-2}\), dairy cattle farms (number) and biogas production facility location suitability. Local urban sewage sludge availability (853 tonVS year\({}^{-1}\)) and dairy cattle manure (10,970 tonVS year\({}^{-1}\)) refer to a potential for biogas (1,102,773 m\({}^{3}\) year\({}^{-1}\)), methane (573,441 m\({}^{3}\) year\({}^{-1}\)), heat (7,046,717 kWh) and electric energy (2,205,545 kWh) production. The distributed system scenario revealed a maximum biogas production potential in the range of 70,000-320,000 m\({}^{3}\) per facility per year. Assuming the indicators described in Table 2, the referred biogas potential production is equivalent to approximately 447,300-204,000 kWh of heat energy or 140,000-640,000 kWh of electrical power (Figure 6) per facility per year. In contrast, the centralized scenario indicates a maximum biogas potential production between 265,000 and 390,000 m\({}^{3}\) per facility per year, which is equivalent to 1,600,000-2,600,000 kWh of heat energy or 500,000-800,000 kWh of electrical power. In the development, one of the centralized system scenarios (three biogas plants), which considered the existing WWTP biodigester (ETAR do Ave, Tougues), requires the construction of two new facilities (Figure 6). Figure 5: (**a**) Distance road analysis (m) to biogas plants; (**b**) Allocation analysis considering centralized (1 biogas facility); (**c**,**d**) distributed scenarios (5 and 8 facilities). These solutions considered different alternative projects and future biogas facility management models. Centralized system solutions reduce the initial investment, but reinforce the complexity and the costs of operation. According to [73], centralized treatment plants have a significantly higher overall payload distance, which supports the idea of a higher transport efficiency of a decentralized system compared to a centralized one [74], even in a territory with a high density of urban and peri-urban areas [73].