# Greenhouse Residues' Potential for Biogas Production
## 1 Introduction
The exploitation of agricultural residues via anaerobic digestion becomes popular since, besides energy, the digestate from anaerobic digesters treating agricultural residues can be used as a growing medium or nutrient solution within novel cultivation schemes [13]; however, issues such as the environmental and economic cost of transfer and processing may surpass any benefit from valorizing this type of renewable resource [14, 15]. Therefore, the sustainable management of all of these crop residues becomes mandatory. Except for anaerobic digestion, processing methods applied to biomass for energy recovery are pyrolysis, gasification, and combustion. The main disadvantages of combustion, pyrolysis, and gasification are the high cost of the processes due to the high energy consumption in order to achieve the high temperatures [16], the high investment cost, and the operational cost to make these technologies environmentally safe due to the atmospheric pollution induced. In the case of biomass with high moisture (e.g., leaves and suckers from vegetable crops), the energy required is even higher, rendering this option unsuitable. Contrary to the thermal methods, anaerobic digestion is suitable for wet and biodegradable organic material (such as vegetable residues). Other advantages are the low energy consumption, low sludge output, and high organic load [17, 18], as well as the potential to remove or convert CO\({}_{2}\) to CH\({}_{4}\) and produce a methane-rich gas mixture (biomethane) that resembles natural gas [19]. According to the European Union Biogas Barometer, there are 17.4 thousand active anaerobic digestion plants in Europe [14, 20]. In addition, 367.0 upgraded biomethane units are in operation, enabling the production of biofuels together with electricity and heat. More than 70% of biogas plants are farm-based due to important incentive policies (mainly applied in Germany, Austria, and Italy) [21]. In 2019, 49 biogas plants were in operation in Greece [22], producing 73.6 MW of electrical power, more than half of which (ca. 40 MW) came from landfills and sewage treatment plants [23]. Depending on the characteristics of the crop residues and their composition, a pretreatment method may be required before anaerobic digestion to improve biogas production. Pretreatment is especially needed if the feeding waste consists of lignocellulosic. Distracting the recalcitrant structure of lignocellulosic material enhances the accessibility of cellulose enzymes [24, 25]. However, there is no such problem with crop residues from greenhouse crops such as tomatoes and cucumbers, as these residues mainly contain leaves. Accordingto [PERSON] et al. (2016), tomato waste (tomato stems and leaves) has relatively lower lignin and hemicellulose content than other plant waste, such as corn stover. Since fruits and vegetables are characterized by low cellulose, hemicellulose, and lignin content ([PERSON], 1998; [PERSON], 1998), the hydrolytic process is not a limiting factor, and a high VS content with a low TS content results in rapid hydrolysis ([PERSON], 1998). The utilization of energy crops for biogas production is not sustainable. According to [PERSON] et al. and [PERSON] et al. (2013); [PERSON] et al. (2013), expanded cultivation of energy crops leads to loss of biodiversity and increased competition for the land area for food and fodder. In many countries, the current biogas production system is based on feedstock from energy crop production, requiring significant financial and energy costs ([PERSON] et al., 2013). On the other hand, the organic wastes and residues from agriculture are preferable for biogas production due to their characteristics (wet, bulky) and low economic value compared to energy crops ([PERSON] et al., 2013).