## 6 Practical Cases of AD Biogas Plants
\begin{table}
\begin{tabular}{c c c c} \hline
**Study** & **Benefits** & **Drawbacks** & **Ref.** \\ \hline Pig farm, large-scale biogas plant CSTR & Digestate valorisation as organic fertiliser in local villages. & The potential for global warming and photochemical oxidation increased due to higher CO\({}_{2}\) emissions. & [53] \\
600 m\({}^{2}\), northwest China & Sustainable environmental practices in the production of clean energy and the reduction of pollution emissions. & Reduced efficiency in comparison to the large-scale biogas plant in terms of energy consumption and environmental effects. & \\ Pig farm, household biogas production & Sustainable environmental practices in the production of clean energy and the reduction of pollution emissions. & Higher potentials for acidification compared to large-scale biogas plants. & \\ \hline Farrow-to-wean piglet production, CSTR & Complete system on-farm technology. Energy and nutrient recovery. & Higher energy consumption in cold months. & [28] \\ \hline  & Small; lab-scale & & \\ \hline  & Allow the calculation of the amount of renewable energy in the form of electricity and heat to further extrapolate to a full-scale plant. & According to the findings, the effects of AD technology and achievable CH\({}_{4}\) yields on transferring laboratory-scale Guangdong to the farm scale one differ depending on the livestock farming methods used (due to variations in feed, waste management techniques, and/or seasonality). & [25] \\ Pig slurry has been shown to be a good substrate for methanogenesis. & & to variations in feed, waste management techniques, and/or seasonality. & [25] \\ \hline Swine farm, Schott Duran bottle 20 mL, South Korea & Rich in trace elements. & A low C/N ratio could lead to ammonia inhibition. & [56] \\ \hline Pig farm, CSTR 5 L, Spain & Results from this experiment allowed an estimation for a full-scale plant. & Low biodegradability and C/N ratio. & [43] \\  & High buffer capacity of pig slurry. & & \\ \hline Farm, CSTR 8 L, Germany & This type of study is important to decide the suitable operational parameters to implement at medium and small-scale biogas plants. & The investment to install a combined heat and power plant in medium or small-scale biogas plants. & [59] \\ \hline Swine livestock facility, CSTR, 4.8 L, Portugal & Easiness of implementing the studied pre-treatment at a full-scale plant. & Low C/N ratio. & [47] \\ \hline  & Results demonstrated the process’ adaptability and versatility. & & \\ \hline Swine livestock facility, CSTR, 4.8 L, Portugal & Changing feeding frequencies on increase bioenergy production’s flexibility and provide farmers with greater autonomy in selecting the optimum course of action. & Need to sieve the substrate to avoid dogging the reactor pipework of a laboratory unit. & [27] \\  & The digestate met the legal need for agronomic valorisation. & & \\ \hline \end{tabular}
\end{table} 
Table 3: _Cont._However, besides the implementation of policies and subsidies, the net energy balance of each AD plant is the principal factor for successful development of AD technology. One of the most important metrics for evaluating the operational, financial, and environmental success of AD plants is the net energy balance. While a negative balance suggests inefficiencies that need to be evaluated to overcome the drawbacks, a positive balance guarantees that the plant will provide significant energy savings and environmental advantages. Simultaneously, strategies like the installation of policies and subsidies can be investigated and considered [61]. Concerning the biogas plant presented by [PERSON] et al. [52], the authors state that its success could be attributed to several factors: construction planning (the biogas plant was constructed simultaneously with the rest of the farm), which reduced investment costs; the maintenance and supervision tasks performed by local technicians; and the global integrated vision of swine manure as a resource for energy and nutrients to the farm. In the work developed by [PERSON] et al.