# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 4 Discussion
\begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Residues**} & **Cultivated Area of** & **Maize Field Area** & **Maize Field Area Equivalent** & **Reduction in GHG Emissions** \\  & **Crops in Poland** & **Equivalent to** & **to Area of Crop Cultivation** & **Resulting from the Use of** \\  & **1 ha of Crops** & **in Poland** & **Waste for biogas Production** \\ \cline{2-5}  & **[ha]** & **[ha]** & **[ha]** & **[t CO\({}_{2}\)]** \\ \hline Cabbage leaves & 14,599 & 0.041 & 598 & 2106 \\ Apple pomace & 161,948 & 0.072 & 11,660 & 41,066 \\ Pumpkin residue & 6700 & 0.020 & 134 & 472 \\ Walnut husks & 2700 & 0.004 & 11 & 38 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The GHG reduction due to maize silage replacement with FVR in biogas production. The SMY of pumpkin residue obtained in this study is much lower than that shown in the literature. [PERSON] et al. [74] reported an SMY of pumpkin equal to 372.41 NL kgvs\({}^{-1}\) while [PERSON] and [PERSON] [75] showed even higher SMY values (422 \(\pm\) 4 NL kgvs\({}^{-1}\)). Those differences between the values from the literature and the results of this study can be attributed to the differences in the substrate used in BMP. In this study, the pumpkin residue consisted of peels and fibrous strands while in studies of [PERSON] et al. [74] and [PERSON] and [PERSON] [75] the whole fruit was used. The differences in methane production concerned not only the SMY but also the kinetics of the process. Different peaks of daily methane production in the AD of studied residues denote the discrepancy in fermentation kinetics. Since fruits and vegetables are characterised with low cellulose, hemicellulose and lignin content [76; 77], the hydrolytic process is not a limiting factor and a high VS content with a low TS content results in rapid hydrolysis [78]. Therefore, the first peak may result from the conversion of pre-existing soluble organic matter. The second peak represents the further solubilization of the easy-to-digest substances lightly bound with particulate organics, while the last peak is the result of solubilization and methanization of tightly bound biodegradable substances [38]. Another important indicator for methane production is the rate of the decomposition of the biodegradable material. The time needed to obtain 50% (T50) of potential methane production for all studied residues was 8-13 days and was similar to the values obtained for the AD of ensilaged wetland plants, harvested as a vegetation management measure in nature-protected areas, considered as a problematic waste difficult to the utilization but useful for biogas production [79]. The T95 was much shorter for pumpkin residue than for wetland plants which needed 25-41 days to reach 95% of potential methane production [79]. In turn, the walnut husks and cabbage leaves produced 95% of methane in a similar period as was reported for _Carex elata_ and _Phalaris arundinacea_ reported by [PERSON] et al. [79] and for _Pliragmites australis_ reported by [PERSON] et al. [80]. The anaerobic mono-digestion of FVR promotes rapid acidification which results in the inhibition of methanogenic microorganisms and leads to nutritional deficiencies caused by an improper C:N:P:S ratio. Therefore, the co-digestion of selected FVR or the co-digestion of those residues with agricultural waste may stabilize the AD through avoidance of the scarcity or excess of nutrients and enhance the methane production [16]. Co-digestion of vegetable waste with slaughter waste increased the methane yield by 74.2% compared to the mono-digestion of vegetable wastes [43], while co-digestion of fruit and vegetable wastes with municipal sewage sludge led to an improvement in the stability of the process and enhanced biogas production [81]. The possibility of co-digestion of ensiled apple pomace and maize silage was studied by [PERSON] and [PERSON] [82]. The production of biogas from a mixture of these two substrates was ca. 15% lower than that of maize silage alone and ca. 17% higher than that of the apple pomace.