Biogas production from defatted seed cakes of second-generation oil crops: Chemical characterisation, methane yields, and process stability

Biodiesel production from second-generation oil crops is expanding due to their higher sustainability compared to first-generation sources. However, these crops alone cannot fully meet international greenhouse gas (GHG) reduction standards (e.g., Renewable Energy Directive (RED II)). Valorising biodiesel by-products can further enhance sustainability, yet this potential remains largely underexplored. This study assessed defatted seed cakes (DSCs) from second-generation oil crops as feedstocks for anaerobic digestion (AD). Three DSCs, Camelina sativa (CSC), Brassica carinata (BCC), and Brassica juncea (BJC), were characterised for proximate composition, and biochemical methane potential (BMP) assays were determined, showing that BJC produced the highest methane yield (366 ± 18 mL CH 4 g −1 TS), while thermophilic conditions generally yielded lower biomethane potentials (BMPs) than mesophilic ones. Continuous stirred tank reactor (CSTR) trials, conducted for all DSCs, at a hydraulic retention time (HRT) of 30 days and an organic loading rate (OLR) of 1.3 g VS L −1 d −1 , achieved biomethane production of up to 314.5 ± 8.9 mL CH 4 g −1 TS. Volatile fatty acids (VFAs) and total ammonia nitrogen (TAN) concentrations remained within non-inhibitory ranges, ensuring process stability. Biomethane yields reached approximately 85% of theoretical potential, demonstrating efficient conversion and highlighting the strong potential of DSCs for circular, sustainable biodiesel systems. While full-scale validation is still required, these results provide a preliminary demonstration of the suitability of DSCs as feedstocks for AD and biogas production within a circular valorisation strategy.