# Greenhouse Residues' Potential for Biogas Production
## 1 Introduction
Several studies have been conducted to investigate the utilization of various types of tomato waste, such as those from groceries, local marketplaces or restaurants ([PERSON] et al., 2013; [PERSON] et al., 2013; [PERSON] et al., 2013; [PERSON] et al., 2013; [PERSON] et al., 2013), processing plants ([PERSON] et al., 2013; [PERSON] et al., 2013), tomato sauce ([PERSON] et al., 2013), and tomato pure ([PERSON] et al., 2013). Nevertheless, there are few reports on using vegetable residues' green parts. For example, [PERSON] et al. (2013) reported that tomato residues, including stalks, leaves, and residual tomatoes, could serve as good co-substrates for dairy manure and corn stover, provide balanced nutrients, and increase biogas yields. Another study ([PERSON] et al., 2013) suggests that tomato branches/stems are potential feedstocks for biogas production but no more than rotten or green tomatoes. Due to its lignin content, the BMP of tomato branches/stems was half of the BMP of rotten or green tomatoes. Furthermore, [PERSON] et al. (2013) proved that the methane fermentation of fresh tomato and cucumber residues from the greenhouse is energy efficient. More specifically, they showed that conversion of greenhouse residues to biogas is a sustainable disposal option since it provides heat and carbon dioxide after biogas combustion for the greenhouse needs. In the same line, [PERSON] et al. (2014) stated that greenhouses could use their crop residues for biogas production and, in turn, meet their energy demand. [PERSON] et al. (2014) evaluated the potential of tomatoes' and cucumbers' leaves and suckers generated in a greenhouse to provide energy autonomy to the greenhouse via a CHP unit combined with a battery system. The hybrid renewable energy system could partially cover the facility's energy demands. Generally, much of the tomato plant remains unused during traditional or greenhouse cultivation, harvesting, and industrial processing, resulting in a large amount of waste (peels, seeds, stems, leaves), including even the whole vegetable if damaged, reaching 473,989 tons in 2017 in Mexico (Mexico, 2017). Furthermore, this type of waste is generated seasonally just after harvesting, meaning that a large amount of waste is accumulated quickly (Mexico, 2017). Studies on biogas production from tomato and cucumber crop residues are limited to wastes from industrial processing or from green parts of the plant that are uprooted at the end of the growing season (such as stems), which, in addition to the plant, may have fruits. On the other hand, many leaves and stems are removed during the growing season, especially in hydroponic greenhouses ([PERSON] et al., 2013), since the regular removal of leaves and suckers is necessary to promote plant growth. Therefore, there is a massive production of biomass throughout the cultivating period. Designating this type of residue for feed for gazing animals in neighboring fields is the currently practiced option, but it should be done in an organized way to avoid possible environmental impacts such as gas emissions or nitrogenous compound leaching ([PERSON] et al., 2013). The alternative option for energy production is studied in this paper, which aims to evaluate the methane potential of the various types of waste biomass generated during the hydroponic cultivation of tomatoes and cucumbers in a greenhouse and its variation in time. The study also involves a detailed recording of the waste mass generated within a year. In this way, an accurate estimate of the annual methane production can be made.