Beach-Cast Posidonia oceanica as a Feedstock for Biomethane Production: Technological Challenges and Ecological Perspectives Within the Calabria Case Study

Posidonia oceanica (P. oceanica) is a key marine seagrass that provides essential ecosystem services but managing its stranded biomass (banquettes) poses a challenge to local authorities due to high disposal costs. This review examines the ecological and bioenergetic potential of P. Oceanica, focusing on the Calabria region. Through the analysis of regional monitoring data (ARPACAL) and a comprehensive review of the literature on anaerobic digestion (AD) processes, the study analyzes the transition from waste to resource. The results highlight a worrying regression of Calabrian meadows due to siltation and high burial-induced mortality. Chemically, the biomass is characterized by high recalcitrance (lignin > 34%) and an unbalanced C/N ratio (~153), which limits the efficiency of standard AD. However, the review demonstrates that acidic thermochemical pretreatment (132 °C with 0.4% HCl) can increase methane yields by up to 575%, while co-digestion with nitrogenous substrates further stabilizes the process. The study concludes that, although technical challenges such as reactor corrosion and sediment loss remain, the transformation of P. oceanica into biomethane represents a technologically potential route toward a circular Blue Economy, provided that operational and economic trade-offs are carefully balanced. Adopting the “Ecological Beach” model could enable sustainable coastal protection while unlocking prospective renewable energy potential.