# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 1 Introduction
Biogas production has a direct impact and contribution to 12 of 17 sustainable development goals adopted by all United Nations Member States in 2015 [25; 26]. In addition to generally recognized advantages such as a decrease in GHG emissions, waste utilization and nutrient recycling [27], the energy obtained from biogas is non-intermediate and biogas can be relatively easily stored. Utilization of organic wastes and organic residues as feedstock, digestate management as fertilizer and biogas upgrading with purification of CO\({}_{2}\) from a useless waste product to, for example, a high purity gas for the food and beverage industry [28] or for geological storage [29] closes the CO\({}_{2}\) loop and plays potentially an indispensable role in the transition to a circular bio-economy [30]. The assessment of the carbon neutrality of biogas is based primarily on the feedstock used for its production. Since expanded cultivation of energy crops leads to loss of biodiversity, loss of wet habitats, and increased competition for land area for food and fodder [31; 32], the organic wastes and residues which are typically wet, bulky, and of low economic value [33] are perfect feedstock leading the biogas to be considered a carbon-neutral source of energy because carbon originating from organic wastes comes from existing CO\({}_{2}\) in the atmosphere [34]. Another benefit of biogas production is post-production slurry (digestate) which has been proved to be a good fertilizer with increased content of NH\({}_{4}\)-N compared to substrates. Numerous studies concern the biogas production from either food waste [35] or its co-digestion with other substrates [16; 36; 37; 38]. Anaerobic co-digestion of cabbage waste with potato waste was studied by [PERSON] et al. [19]. The methane yield of pumpkin peels and the energy and economic evaluation of anaerobic co-digestion of fluted pumpkin with poultry manure were evaluated by [PERSON] et al. [21; 22; 23]. Less is known about mono-digestion of fruit or vegetables. [PERSON] et al. [39] compared the methane production from wastes of 20 leafy species popular in China focusing mostly on herbaceous plants. The highest methane yield was obtained from the AD of cauliflower (249.61 NL kg\({}_{\text{VS}}\)\({}^{-1}\)), while the lowest yield was observed in the case of Schizonepeta (81.52 NL kg\({}_{\text{VS}}\)\({}^{-1}\)). The study revealed that the volatile solids to total solids (VS/TS) ratio, lignin content, and hemicellulose content exerted a combined influence on the methane yield [39]. Analyses of GHG emissions often emphasise the significant share of the crop cultivation used as a substrate for biofuels production in overall emissions. The GHG emissions from crop cultivation are related to fertilization, plant protection, seeds treatment as well as the production of agricultural machinery and equipment [40]. The land-use change also significantly increases the GHG emissions from biofuels production. Organic wastes used as feedstock for biogas production may significantly contribute to the reduction in the GHG emissions through the replacement of the energy crops and avoidance of GHG emissions related to their production and harvest. Most studies have focused on the AD of mixed fruit and vegetable waste [41; 42; 43; 44; 45; 46; 47] or co-digestion of fruit, vegetable and other feedstock [48; 49; 50; 51; 52]; therefore, there is a gap in knowledge on specific methane yield from individual fruit or vegetable species. The aim of this study was to determine the SMY from anaerobic mono-digestion of fruit and vegetable residues from four individual species such as: apple, pumpkin, cabbage and walnut. The residues investigated in this study were chosen because of their high availability. In addition, the biogas chemical composition leads to the assessment of the possibility of using studied residues as co-substrates in biogas plants without impairing the AD process.