# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 4 Discussion  
The present results show that FVR are a suitable source of substrate for biogas production. In general, food waste moisture content is in the range of 48-95% depending on the source and type of waste [56]. Apple pomace typically contains 66.4-81% of moisture [58; 59], and cabbage contains more than 90% of moisture [60]; however, [PERSON] et al. [61] reported that the moisture content of cabbage waste was equal to 86%. The moisture content of cabbage waste including cores and leaves was even lower and amounted to 75.12% [20]. Pumpkin, depending on the species and part of the vegetable (flesh, peel, seed), contains 2.8-96.8% of moisture [62]. Pumpkin peels and pulp contain typically more than 85% moisture [63; 64]. The high moisture content negatively affects the applicability for incineration and composting [56], while this is an advantageous feature in the case of biogas production. The TS values of the studied residues were in good agreement with the literature. A similar TS value of apple pomace to the one obtained in this study was reported by [PERSON] et al. [65]. The TS content of cabbage leaves was in the range reported for cabbage waste. The TS content of cabbage waste depends on the species and ranges from 5.64% for white cabbage to 16.3% for brussels sprouts [66]. The VS values found in studied residues were in good agreement with the results reported for food waste, which contains 73-98% of VS. Such a high level of VS is favourable for composting, biogas production and other biological technologies as well as incineration [66]. The TKN and TP content of all studied wastes is in the range of 13.0-32.5 g kg\({}_{\text{DM}}\)\({}^{-1}\) and 0.5-9.8 g kg\({}_{\text{DM}}\)\({}^{-1}\), respectively, reported in the literature for food waste [56]. The TK content of pumpkin, walnut and cabbage wastes was much higher than the maximum value (14.3 g kg\({}_{\text{DM}}\)\({}^{-1}\)) reported for the food waste [56]. The optimal C:N ratio in composting and biogas production should be 30:1 [67; 68; 69; 70]. The C:N ratio obtained in this study for pumpkin residue and cabbage leaves was too low for both treatments; therefore, the addition of a carbon-rich bulk agent in the case of composting and carbon-rich waste in the case of AD might be necessary. On the other hand, very high C:N ratios obtained for apple pomace and walnut husks indicate a need for the addition of other nitrogen-rich substrates for both composting and AD processes. The SMY of studied residues, except for walnut husks, was in the range of methane potential of fruit and vegetable waste which is reported between 160 and 350 NL kg\({}_{\text{VS}}\)\({}^{-1}\)[16]. The fruit and vegetable residues are very heterogeneous and may contain leaves, peels, skins, rinds, cores, pits, pulp, stems, seeds, etc. The variety of fruit and vegetable species also affects the potential methane production. Nevertheless, the methane yield from studied residues is much lower than that from the most popular feedstock for biogas production, i.e., maize which produces 345 NL CH\({}_{4}\) kg\({}_{\text{VS}}\)\({}^{-1}\) on average [71]. The SMY obtained for cabbage leaves is in good agreement with [PERSON] [72] who reported an SMY equal to 309 \(\pm\) 13 L kg\({}_{\text{VS}}\)\({}^{-1}\) and 291 \(\pm\) 12 L kg\({}_{\text{VS}}\)\({}^{-1}\), for cabbage leaves and stems, respectively. However, the SMY shown by [PERSON] et al. [73] was higher compared to the results of this study. Anaerobic digestion of whole cabbage gives contradictory results. [PERSON] et al. [39] reported an SMY equal to 204 L kg\({}_{\text{VS}}\)\({}^{-1}\) while [PERSON] et al. [74] obtained a much higher SMY value (370.84 L kg\({}_{\text{VS}}\)\({}^{-1}\)).