Recovery of biofuels, biobased products and nutrients in agricultural biogas plants.

This PhD thesis focuses on the conceptualization and laboratory-scale implementation of a biorefinery model for the simultaneous production, purification, and recovery of high-value compounds and biofuels from Opuntia ficus-indica (OFI) and agricultural digestate. A biorefinery integrates biomass conversion processes to produce bio-based products, such as biofuels, biochemicals, biomaterials, and bioenergy, utilizing renewable biomass instead of fossil resources. The proposed biorefinery model is centered around anaerobic digestion (AD), a well-established biotechnology in the European Union, which converts organic carbon into biomethane, volatile fatty acids (VFAs), and fertilizers. The selected feedstocks were: (i) OFI cladodes, considered a third-generation energy crop, and (ii) agricultural digestate, an AD by-product. OFI cladodes were tested under various operational conditions to optimize the production of biogas, VFAs, and middle-chain fatty acids (MCFAs). The best VFAs and MCFAs outputs were purified using membrane filtration technologies to remove unwanted organic matter. Concurrently, agricultural digestate, rich in macronutrients, was processed through sequential filtration steps to recover nitrogen and phosphorus compounds. These purified VFAs and MCFAs were then combined with the recovered nutrients to create optimal conditions for polyhydroxyalkanoates (PHAs) synthesis using Thauera spp. This biorefinery model has multiple potential applications. First, it can contribute to energy security by producing domestic biomethane, reducing reliance on Russian natural gas, as outlined in the REPowerEU Plan. Second, it enables the valorization of agricultural digestate by recovering essential nutrients for bio-fertilizer production. Additionally, this thesis extends the biorefinery concept beyond biogas and nutrient recovery, demonstrating a flexible system for VFAs, MCFAs, and PHAs production, adaptable to various economic sectors.