# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 4 Discussion
However, the concentrations vary significantly depending on the species and part of the fruit. The highest values were reported for seeds (28.26-33.35%\({}_{\text{DM}}\)) while much lower (6.44-7.09%\({}_{\text{DM}}\); 6.8-14.45%\({}_{\text{DM}}\)) for peel and pulp, respectively. In this study, pumpkin residue consisted of peels and fibrous strands; therefore, the lower protein content resulted in low NH\({}_{3}\) and H\({}_{2}\)S concentrations. The lowest protein content is reported in walnut (5%\({}_{\text{DM}}\)) [99] and apple pomace (1.2-9.81%\({}_{\text{DM}}\)) [100,101] with most common values between 3 and 5%\({}_{\text{DM}}\)[100,102, 103, 104, 59]. Maize as feedstock for biogas production has several advantages such as high biomass performance, good preservability of ensilage, steady yields and high specific methane yield. The AD process is steady and maize silage guarantees a year-round supply for biogas plants. Well-known production and preservation methods also make maize one of the most popular feedstocks. The high level of subsidies in many EU countries also increased the profitability of biogas production from maize. However, the increasing price of this feedstock together with limitations for renewable energy sources subsidies introduced in many countries and the RED II policy force the biogas plant operators to look for new feedstock [105]. FVR are characterised by a lower SMY than maize and may elevate the H\({}_{2}\)S concentration in biogas, and therefore, the co-digestion of FVR may have adverse effects on biogas production from maize [82]. However, the addition of fruit and vegetable waste to other substrates such as slaughter waste or municipal sewage sludge stabilized the process and enhanced biogas production [43,81]. Biogas production is a continuous process which demands the supply of the feedstock throughout the year. Unlike the agricultural wastes and energy crops, FVR generated from fruit and vegetable processing may be available throughout the almost whole year since fruits and vegetables are stored or imported for the whole year of production. Cabbage and apples can be stored from 3 to 6 months, and white cabbage even up to 12 months [106] depending on the storage method. Fresh pumpkin can be stored for 2-3 months [107]. In addition, cabbage leaves, apple pomace and pumpkin residue can be preserved by ensilaging [61; 82; 108; 109]. However, the supply of FVR still may be uneven throughout the year. The co-digestion of maize and FVR may also be logistically difficult. The coordination among several suppliers and biogas plant operators at a spatial scale is required to minimize the transport costs [110]. The advantage of partial substitution of maize with FVR through co-digestion is a decrease in GHG emissions. The life cycle assessment (LCA) of maize production as a substrate for biogas plants revealed GHG emissions equal to 3521 kg CO\({}_{2}\) ha\({}^{-1}\) with 43% resulting from machinery [40]. The boundary of the LCA included seed-bed preparation and fertilization, followed by drilling and sowing followed further by shredding during harvest, transportation of maize chips and ensilaging in a concrete bunker silo. The LCA calculations included direct energy input from fuel and indirect energy input from materials, machinery and labour [40]. Similar results were reported by [PERSON] et al. [111] and were higher than those shown by [PERSON] and [PERSON] [112]. The current use of agricultural land in Poland for the cultivation of apples, cabbage, pumpkins, and walnuts is ca. 186,000 ha. Therefore, the use of FVR as feedstock for biogas production would contribute to the reduction in GHG emissions by 43,682 t CO\({}_{2}\) eq.