# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 1 Introduction  
Fruit and vegetable production and processing generate biomass losses which are inevitable and may pose serious environmental risks but also mean the loss of valuable nutrients [1]. The fruit and vegetable residues (FVR) and wastes are highly fermentable and biodegradable and can be a source of GHG emissions when disposed in landfills [2, 3]. Losses in agricultural production due to mechanical damage and/or spillage during harvest operation and during the post-harvest stage including losses due to spillage and degradation during handling, storage and transportation between farm and distribution are equal to ca. 20-25% [4]. Agro-industrial processing of fruit and vegetables during the processing of raw plant material in the transformation process to food product generates residues such as peel, pulp, pomace, shells, stalk, foliage etc. [1, 5]. These materials are not intentionally generated during food production and are not considered as waste but rather as residues which constitute 20-30% of fresh material [6]. Poland is known as one of the main producers of temperate climate fruit and vegetables, and their production and processing together with export are a very important part of agriculture [7]. In 2021, Poland produced ca. 4,000,000 tonnes of apples [8], 687,000 tonnes of cabbage [8], 289,900 tonnes of pumpkin [9] and 6100 tonnes of walnuts [10] in 2020. In the last 10 years, ca. 56% of harvested apples in Poland were processed into concentrated juice [11]. Apple processing generates pomace, for many years regarded as waste, which is a heterogeneous mixture consisting of peel, core, seed, stem, calyx and soft tissue [12]. This inexpensive and abundantly available by-product of the apple cider and juice processingindustries accounts for even 30% of the original fruit mass [6]. The inedible fraction of cabbage and pumpkin is equal to 20% of fresh vegetables [13]. Walnut production generates wastes such as leaves and husk of the fruit. The husk is the green outer layer of the fruit which splits at full ripening and is removed during piking of the walnuts. In processing, the walnut shell is another waste. The shell and green husk constitute approximately 50% of the total fruit weight [14]. The management of agro-industrial residues and wastes is challenging since wastes are produced seasonally and both wastes and residues vary significantly in quantity and quality. Besides that, agricultural and agro-industrial production is scattered. Since bio-waste, including agro-industrial residues, is a key waste stream with a high potential for contributing to a more circular economy, the European Union is strengthening rules on waste prevention and setting new municipal-waste-recycling targets such as: (a) at least 55% of municipal waste by weight will have to be recycled by 2025; (b) in all EU Members, by 31 December 2023, biowaste will be collected separately or recycled at source [15]. Consequently, the disposal of organic wastes is banned in many countries [16]. The other two options such as incineration and composting can cause environmental problems. Thus, valorisation pathways of FVR to energy are a promising option and can be a part of the circular economy. The organic waste streams, including the FVR, can be used to produce bio-based energy carriers such as biogas, bioethanol or hydrogen gas (H\({}_{2}\)) [17; 18; 19; 20; 21; 22; 23]. Biogas production on the industrial scale is considered a part of the green economy [24]. From the agricultural point of view, biogas should be seen as a very promising source of energy. On one hand, anaerobic digestion (AD) can be an attractive way to manage waste; other disposal methods require significant financial outlays. On the other hand, the energy produced can and should be used as a substitute for conventional energy sources, e.g., to heat livestock buildings or greenhouses. Biogas is a renewable energy source which is implemented successfully in many countries.