## 4 Discussion  
Organic waste management poses a significant challenge for the animal husbandry sector, which produces substantial quantities of residues and wastes, both liquid and solid, necessitating proper treatment and disposal [25]. Inadequate waste management can result in greenhouse gas emissions, environmental pollution, health hazards, and economic losses [24]. Hence, there is a pressing need for sustainable and efficient solutions to mitigate the environmental impact of waste while extracting value from it. Anaerobic digestion is a promising technology for the valorization of organic waste, which is widely distributed across various regions. The biogas generated (mainly methane) can be utilized for multiple purposes, including electricity generation, vehicle fuel, natural gas substitution, heating, and cooking fuel [27, 28]. These facilities are complex technical systems requiring meticulous design and operation. The actual biogas yield from a full-scale facility is influenced by several factors, such as digester design (e.g., hydraulic residence time, residence time distribution, mixing intensity, presence of dead volumes/flow short-circuits), the conditions inside the digester (e.g., temperature, pH, salinity, concentration of inhibitors, like ammonia and sulfides), the presence of micronutrients and trace elements, sludge activity and the accumulation of non-digestible vegetable fibers [20, 29]. Properly designed and executed BMP tests can estimate the methane yield under field conditions. Additionally, operating a pilot-scale facility with continuous substrate feeding can provide valuable data on long-term process performance, inhibitor accumulation and the composition of both biogas and digestate [29]. Sustainability, in terms of technology, economics and the environment, remains an open question that requires further analysis and evaluation. New technologies, based on high-rate digestion reactors, with low capital expenditure and low technological complexity, are crucial, especially for small-scale projects. Energy sustainability, as expressed by the Energy Sustainability Index (ESI)--the ratio of electricity produced to the electricity consumed by the technology--is of particular importance [28]. The ESI for small-scale \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Dairy Cattle** & **Poultry** & **Swine** & **Sheep/Goats** \\ \hline Waste (tn yr\({}^{-1}\)) & 20,000 & 960 & 11,000 & 1200 \\ VS content (kg m\({}^{-1}\)) & 70 & 555 & 15 & 420 \\ BMP (L kg\({}^{-1}\) VS) & 280 & 310 & 160 & 220 \\ CH\({}_{4}\) generated (m\({}^{3}\) yr\({}^{-1}\)) & 390,000 & 175,000 & 26,000 & 110,000 \\ Electricity produced a (kWh yr\({}^{-1}\)) & 1,500,000 & 660,000 & 90,000 & 400,000 \\ CHP b (kW) & 200 & 80 & 20 & 50 \\ Digester CAPEX & 1,000,000 & 550,000 & – & 300,000 \\ Annual CAPEX (\(\xi\) yr\({}^{-1}\)) & 100,000 & 55,000 & – & 30,000 \\ Annual OPEX (\(\xi\) yr\({}^{-1}\)) & 100,000 & 60,000 & – & 40,000 \\ Total cost (\(\xi\) yr\({}^{-1}\)) & 200,000 & 115,000 & – & 70,000 \\ Total income (\(\xi\) yr\({}^{-1}\)) & 390,000 & 170,000 & – & 104,000 \\ Total profit (\(\xi\) yr\({}^{-1}\)) & 260,000 & 93,000 & – & 55,000 \\ Annual turnover (ATO) & 1,200,000 & 2,400,000 & 1,800,000 & 300,000 \\ Profit c (\% ATO) & 22\% & 4\% & – & 18\% \\ \hline \hline \end{tabular}  
* The electrical efficiency of the CHP engine was considered between 3.8 and 3.5 kWh-el m\({}^{-3}\) CH\({}_{4}\); b CHP capacity was determined considering 8000 h of anaerobic digester operation per year; c Profit was expressed as a percentage of the enterprise annual turnover (ATO).  
\end{table} 
Table 3: Summary of techno-economic assessment for different livestock waste anaerobic digestion facilities, considering a FIT of 0.26 \(\xi\) kWh\({}^{-1}\) and a 70% investment subsidy for CAPEX. The case of the swine enterprise was not further assessed since the capacity of the CHP required was below the range examined in the study (50 to 400 kV).