## 4 Small Biogas Plant Concept  
Anaerobic digestion is a stablished process used for the management of food waste [48]. There are different established concepts for the anaerobic digestion of food waste, including batch and continuous reactors, submerged and solid-state systems, and biogas plants with single- or multi-stage fermenters [48,49]. Building new biogas plants for the fermentation of food waste is expensive and there are some issues that have limited this [49]. These include the use of foreign materials, which can lead to undesired sedimentation and an inefficient process, as well as process instability due to possible acidification [48]. As a solution for the decentralised production of biogas from food waste on the island of Tenerife, this paper also presents a new biogas plant concept, affordable for small producers, which can be fed with the selective and stabilised collected food waste at the municipal landfill or directly in the small towns. This concept is based on digesters made of high-quality textile materials, such as high-density polyethylene (HDPE) films. These films are impermeable to liquids, durable, and certified by the German Institute for Building Technology (DIBt). They have a typical lifespan of 25 years and are used in landfills [50,51]. Such films are also used in agriculture to prevent liquids from escaping, and are called plastic-sealing membranes. If the membrane is damaged, the leak detection system, which is installed in accordance with the general method approved by G-Quadrat Geosynthetics, will detect this and allow the damage to be repaired [51]. These plants are simple to build in a short space of time and in an economical way, as the construction is not based on concrete but on textile materials and a light metal construction that supports the textile materials that make up the reactor [52]. The biogas reactor is partly located underground; it acts as insulation and improves the sustainability of the plant, protecting against UV radiation [51]. This small biogas plant ranges in electrical power output from 25 to 50 kWel and meets all the legal and technical requirements to ensure a reliable and long-lasting plant operation [53,54,55]. All interfering substrates, such as stones or other foreign materials, are removed at the feeding line of the biogas process during the homogenisation process. These can damage the textile material or leave sediment in the system. To avoid any gas release, it is essential to ensure continuous compliance with the safety rules for biogas facilities with membrane seals and the resulting requirements [55]. Leakages should be detected quickly and reliably, and spilled water-polluting substances should be retained and disposed of accordingly [53,54]. The biogas obtained will be used to produce electricity and heat, reducing the use of fossil fuels on the island. According to the biogas yields obtained from food waste, shown in Figure 5, after 15 days, with approximately 674 NL/kg VSs and postfermentation with enough hydraulic retention time to obtain the remaining biogas yields of at least 55 days, the application presented can be operated directly in urban districts, accommodating approximately 20,000 tons of food waste per year [52]. This would be the equivalent to the potential food waste from the household waste collected in La Laguna in 2020, with an estimated potential of approximately 18,500 tons, or in Santa Cruz de Tenerife, with a potential of 36,850 tons in 2020 [17,23,56]. Indeed, 18,500 tons of food waste is equivalent to approximately 1.8 \(\times\) 10\({}^{6}\) Nm\({}^{3}\) biogas and to a biomethane potential of 10.88 GWh Hs per year. Figure 6 shows the prototype for the small biogas plant made of textile materials and its components. The initial substrate, with approx. 12% DM, will be mixed with recirculated reactor content, with a typical DM of 6%, in the homogenisation tank.