## 2 Materials and Methods
### Theoretical Assessment
#### 2.2.4 GHG Emissions Reduction Potential of Biogas  
In biogas-based power generation plants, measurable emissions, such as methane (CH\({}_{4}\)), nitrous oxide (N\({}_{2}\)O), etc., are not produced, making estimations unnecessary [52; 53]. Nonetheless, some CH\({}_{4}\) leakage may occur from the bioreactor; hence, this study assumed a 5% leakage rate from the biodigester [54]. To estimate GHG emissions from these leakages, Equation (6) was applied [13; 20; 54]. Equation (7) was used to calculate the equivalent of CH\({}_{4}\)[12; 54]. The global warming potential of CH\({}_{4}\) compared to CO\({}_{2}\) is 25 kg CO\({}_{2}\) per kg CH\({}_{4}\)[55]. The potential for reducing GHG emissions from biogas has been evaluated concerning fossil fuels, particularly natural gas. _The Bangladesh Economic Review_ states that  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Components** & **N (\%)** & **P (\%)** & **K (\%)** & **References** \\ \hline \multirow{4}{*}{Large animal manure} & 2.7 & 0.624 & 0.6 & [42] \\  & 1.79 & 1.68 & 6.17 & [43] \\  & 0.92 & 0.33 & 0.66 & [44] \\  & 0.55 & 0.90 & 0.50 & [45] \\ \hline \multirow{4}{*}{Small animal manure} & 1.94 & 0.99 & 0.38 & [43] \\  & 1.82 & 0.59 & 1.11 & [46] \\  & 1.04 & 0.28 & 1.01 & [44] \\ \hline \multirow{4}{*}{Poultry manure} & 4.52 & 1.68 & 2.12 & [43] \\  & 2.7 & 1.32 & 1.45 & [44] \\  & 1.65 & 2.40 & 1.7 & [45] \\ \hline \multirow{4}{*}{Biofertilizer/Compost} & 3.3 & 0.92 & 2.1 & [47] \\  & 2.1 & 0.94 & 3.67 & [48] \\ \cline{1-1}  & 0.72 & 0.16 & 0.29 & [49] \\ \cline{1-1}  & 6.1 & 2.7 & 5.5 & [24] \\ \cline{1-1}  & 2.36 & 2.37 & - & [50] \\ \cline{1-1}  & 0.22 & 0.012 & 0.03 & [51] \\ \hline \end{tabular}
\end{table}
Table 2: The NPK content in livestock manure and biogas digestate/compost.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Nutrients** & **N** & **P** & **K** & **References** \\ \hline Loss of nutrients during composting (\%) & 40 & 10 & 20 & [40] \\ Nutrient bioavailability for agricultural plants (\%) & 40 & 80 & 90 & [40] \\ \hline \end{tabular}
\end{table}
Table 3: Loss of nutrients during composting and nutrient bioavailability to plants.  
Banglades achieved its highest electricity generation from natural gas in 2024, accounting for approximately 50.32% of total grid production [18]. This study used Equation (8) to estimate the CO\({}_{\text{2 eq}}\) emissions reduction potential from substituting natural gas with biogas [54; 56]. Here, the specific emission factor (SEF) for natural gas is 2.75 kg CO\({}_{\text{2}}\) per liter [54]. Natural gas is composed of about 90% CH\({}_{\text{4}}\)[57; 58]. The ratio for volume of natural gas and CH\({}_{\text{4}}\) content is 1:1.11. Thus, the volume of natural gas corresponding to CH\({}_{\text{4}}\) in liters is determined by multiplying the total amount of CH\({}_{\text{4}}\) in biogas by the equivalent factor (1.11). The net reduction in GHG emissions was calculated using Equation (9).  
\[\text{CH}_{\text{4}}\]  
emissions (from leakages) = 0.05 \[\times\] Total amount of CH\({}_{\text{4}}\) production \[\times\] CH\({}_{\text{4}}\) density (0.717 kg/m\({}^{3}\))  
\[\text{GHG Emissions (in CO}_{\text{2 eq}}\) = CH_{\text{4}}\]  
emissions (from leakages) \[\times\] Global warming \[\text{potential of CH}_{\text{4}}\] (7)  
\[\text{Avoided CO}_{\text{2 eq}}\]  
emission = Total quantity of natural gas equivalent to CH\({}_{\text{4}}\) \[\times\] SEF  
\[\text{Net reduction of GHG emissions = Avoided CO}_{\text{2 eq}}\]  
emission \[-\] GHG Emissions (9)  
from biogas (in CO\({}_{\text{2 eq}}\))