Methane production from anaerobic co-digestion of posidonia oceanica and organic fraction of municipal solid waste

Posidonia oceanica (PO), a Mediterranean endemic seagrass, forms extensive underwater meadows whose dead leaves accumulate on shores as "banquettes." These deposits are often removed and landfilled, resulting in environmental harm. Some authors have reported the potential of PO waste (i) as a source of cellulose to strengthen conventional polymers; (ii) as a renewable and inexpensive adsorbent to remove heavy metals and organic compounds; (iii) as a suitable source of lignocellulosic fibers to produce a wide range of materials including textiles, paper and composites. This study evaluates anaerobic digestion (AD) as a sustainable valorization route for PO waste, either alone or in co-digestion with the organic fraction of municipal solid waste (OFMSW). Several biochemical methane potential (BMP) tests were carried out to assess the effects of Posidonia features on anaerobic digestion (i.e., high lignin content and the presence of saline residues). BMP tests were conducted following standard protocols and used batch reactors (37°C for 35 days). Each test used olive pomace digestate as inoculum. The PO was pre-treated by 2 mm milling to ensure adequate mixing and by washing to remove residual salts. Mono-digestion of PO was performed with different substrate concentrations: 8, 16, 32 and 64 gVS/L. Co-digestion tests employed OFMSW as co-substrate in two different mass ratios of volatile solids: 80/20 (VS OFMSW / VS PO) and 20/80 (VS OFMSW / VS PO). Key findings revealed: 1) PO waste dosage higher than 8 gVS/L produces a increasing methane production in a range between 20 and 30 NmL CH4/gVS; 2) mono-digestion yielded 35 NmL CH₄/gVS, suggesting low biodegradability, consistent with other literature studies on PO; 3) co-digestion methane production depend on the mass ratio of volatile solids (from 390 to 135 NmL CH₄/gVS respectively for 80/20 and 20/80 VS OFMSW / VS PO); 4) the duration of AD lag phase decreased with PO fractions due to feedstock composition and microbial adaptation. The study highlights PO-OFMSW co-digestion as a circular economy strategy, providing a dual benefit of reducing beach waste landfilling while generating renewable energy. In particular, the results demonstrate that, with proper optimization, PO-OFMSW co-digestion can be effectively integrated into existing biogas plants, particularly in coastal Mediterranean regions where both PO accumulations and OFMSW are significant. This technology could be particularly valuable for small island communities seeking sustainable waste management solutions. This approach aligns with EU directives on waste management by transforming a problematic biomass into a valuable resource. Further implementation would require policy support for beach waste collection systems and incentives for biogas plants to adopt co-digestion processes. Future research will investigate different aspects: the optimization of substrate ratios to balance C/N content (e.g., co-digestion with nitrogen-rich substrates); the evaluation of different pre-treatment methods (thermal or chemical) to enhance methane production; the selection of specialized microbial consortia adapted to lignocellulosic substrates.