In the frame of the Waste4Soil project, the Catalonia Living Lab has developed, among other technologies, a biogas production pathway to treat various local agricultural residues, including beer bagasse, wine vinasse, alperujo (a by-product of olive oil production), and swine manure. This process treats the residues through anaerobic digestion (AD), assisted by a bioelectrochemical (BES) reactor integrated into the recirculation system. The AD-BES system effectively processes agricultural residues, consuming organic matter to produce biogas and digestate. Digestate, rich in micro- and macronutrients, is typically used as fertilizer. Within Waste4Soil, it will be utilized for ammonium sulfate extraction, which will serve as a key ingredient for soil improvers.
To ensure the scalability of the process and to develop technologies with practical and realistic applications, a 530 L AD-BES pilot was designed and built to facilitate the transition from laboratory to industry. This pilot includes a 500 L thermally insulated steel AD reactor and a 30 L cylindrical BES reactor equipped with 4 anodes and 4 cathodes (carbon brushes). Both reactors are connected by a hydraulic circuit that enables continuous substrate recirculation. Additionally, a 400 L decantation tank is integrated to separate the liquid fraction of the digestate (from which ammonium sulfate will be extracted) and is connected to the AD reactor.
The pilot is designed to process 120 L of waste per week, with a hydraulic retention time (HRT) of 35 days. It includes a pH control loop with a sensor and a dosing pump to prevent acidification of the reactor. The system also has a heater and temperature sensor to maintain an internal temperature of 35 °C, optimal for AD. The pilot is sealed to maintain anaerobic conditions and has a gas outlet circuit with a flowmeter to record daily biogas production. The BES reactor is connected to a power source, applying 0.3 V, and the current production is monitored.
Construction of the pilot plant was completed in August 2024, followed by a nine-week stabilization period. During this phase, a mixture of beer bagasse and pig slurry was used as a substrate, as these are two annual residues in the project. Over the nine weeks, physicochemical parameters stabilized, achieving consistent biogas production.
Once the construction and estabilization were achieved, the testing phase with seasonal residues began in October, and the pilot has been operating at full capacity since. In the upcoming stages of the project, seasonal residues from the olive oil and wine industries will be included and tested, as these sectors are prominent in Catalonia and generate substantial by-products that can be repurposed for biogas production and soil improver formulation.
