## 2 Materials and Methods
### Techno-Economic Evaluation  
According to Equation (1), the CAPEX of the anaerobic digester decreased from 6000 to 4500 \(\epsilon\) kW\({}^{-1}\) (or from 800 to 700 \(\epsilon\) m\({}^{-3}\)) as the plant capacity increased from 50 to 400 kW, reflecting economies of scale (Figure 2a). Similarly, operational expenses decreased from 700 to 350 \(\epsilon\) kW\({}^{-1}\) with increasing plant capacity. Electricity and maintenance each accounted for 32% of the total OPEX, followed by labor (26%) and consulting services (10%) (Figure 2b). \begin{table}
\begin{tabular}{c c c c c} \hline
**Waste** & **TS (g kg\({}^{-1}\))** & **VS (g kg\({}^{-1}\))** & **VS/TS** & **BMP (mL CH\({}_{4}\) g\({}^{-1}\) VS)** \\ \hline Dairy cattle & 83.1 \(\pm\) 4.6 & 70.1 \(\pm\) 4.1 & 0.84 & 276 \(\pm\) 3 \\ Poultry & 692.5 \(\pm\) 4.1 & 554.6 \(\pm\) 6.5 & 0.80 & 320 \(\pm\) 19 \\ Swine & 22.3 \(\pm\) 2.5 & 15.1 \(\pm\) 3.5 & 0.68 & 172 \(\pm\) 2 \\ Sheep & 352.2 \(\pm\) 14.4 & 236.3 \(\pm\) 8.9 & 0.67 & 252 \(\pm\) 6 \\ Goats & 717.0 \(\pm\) 3.4 & 606.0 \(\pm\) 22.4 & 0.85 & 227 \(\pm\) 9 \\ \hline \end{tabular}
\end{table} 
Table 1: Concentration of total solids (TS), volatile solids (VS) and biochemical methane potential (BMP) values for waste samples collected from different animal husbandry enterprises. Figure 1: Cumulative methane production curves from the batch anaerobic digestion of different livestock wastes samples. Considering the above, it was possible to calculate the costs per m\({}^{3}\) of biogas (methane) produced and per kWh of electricity generated (see Figure 3). The data indicate that the cost of biogas (methane) production decreased from 0.55 to 0.35 \(\epsilon\) m\({}^{-3}\) CH\({}_{4}\) and the cost of generating electricity decreased from 0.16 to 0.10 \(\epsilon\) kWh\({}^{-1}\), as the digester capacity increased from 50 to 400 kW. These values fall within the same range as natural gas and electricity prices, particularly for larger CHP installations. Therefore, low prices of natural gas and electricity represent significant barriers to the penetration of anaerobic digestion into small-sized animal husband enterprises. Figure 3b show the energy sustainability index (ESI) of the examined anaerobic digestion facilities, representing the ratio of produced  
Figure 2: (**a**) Capital expenses (CAPEX) in \(\epsilon\) per kW installed CHP electric power, and (**b**) operational expenses (OPEX) per kW installed CHP electric power, for small-size anaerobic digestion facilities, as a function of the CHP installed electric power. to consumed electricity. The data indicate that the ESI ranged between 4 and 8. The energy consumed accounted for 26% to 13% for the anaerobic digestion facilities with installed CHP electric power capacities of 50 to 400 kW, respectively. Figure 4 depicts the investment payback period of anaerobic digester investments as a function of CHP installed electrical power for various scenarios of financial support. In the case of a feed-in tariff (FIT) equal to 0.21 \(\xi\) kWh\({}^{-1}\), the payback period decreased from 15 to 5 years as the digester capacity increased from 50 to 400 kW. Consequently, facilities below 250 kW become less attractive from an investment perspective. With an increase in the FIT to 0.26 \(\xi\) kWh\({}^{-1}\), the payback period for facilities under 250 kW remains  
Figure 3: (**a**) Capital expenses (CAPEX), operational expenses (OPEX) and total cost (TOTAL) per m\({}^{3}\) of methane produced and kWh\({}^{-1}\) electricity generated, and (**b**) energy sustainability index (ESI) and electricity consumption as a percentage of produced electrical energy, for small-size anaerobic digestion facilities, as a function of the CHP installed electric power. between 5 and 7 years. Similar performance was observed when subsidizing 70% of the CAPEX for construction instead of electricity FIT.