Digestate valorization is essential for sustainable waste management and circular economy strategies, yet large-scale adoption faces technical, economic, and environmental challenges. Beyond waste-to-energy conversion, digestate is a valuable soil amendment, enhancing soil structure and reducing reliance on synthetic fertilizers. However, its agronomic benefits depend on feedstock characteristics, treatment processes, and application methods. This study reviews digestate composition, treatment technologies, regulatory frameworks, and environmental impact assessment through Life Cycle Assessment. It analyzes the influence of functional unit selection and system boundary definitions on Life Cycle Assessment outcomes and the effects of feedstock selection, pretreatment, and post-processing on its environmental footprint and fertilization efficiency. A review of 28 JCR-indexed articles (2018–present) analyzed LCA studies on digestate, focusing on methodologies, system boundaries, and impact categories. The findings indicate that Life Cycle Assessment methodologies vary widely, complicating direct comparisons. Transportation distances, nutrient stability, and post-processing strategies significantly impact greenhouse gas emissions and nutrient retention efficiency. Techniques like solid–liquid separation and composting enhance digestate stability and agronomic performance. Digestate remains a promising alternative to synthetic fertilizers despite market uncertainty and regulatory inconsistencies. Standardized Life Cycle Assessment methodologies and policy incentives are needed to promote its adoption as a sustainable soil amendment within circular economy frameworks.

Nitrogen fertilizers have been identified in energy crops LCAs as the main contributors to global warming, as well as to many other environmental impacts. The distinct production process and application emissions of nitrogen fertilizer types for top dressing produce different GHG savings when energy crops value chains are compared to fossil energy alternatives. In this study, three types of fertilizers (calcium ammonium nitrate, urea and ammonium sulphate) at N top dressing rates of 80 kg N/ha are used to grow rye for electricity generation under the conditions of the Continental Mediterranean climate of central-northern Spain. Complete LCAs for the whole value chain based on real data were performed in conjunction with soil nitrogen balances (SNBs) to assess the accomplishment of European Union (EU) GHG savings sustainability criteria, as well as the sustainability of fertilization practices for soil nitrogen stocks. The results obtained can provide interesting insights for policy making, since calcium ammonium nitrate, the most common fertilizer for rye crops, led to 66% GHG savings, as opposed to the 69% achieved when applying urea and 77% when ammonium sulphate was used. Nevertheless, the three fertilizers produced annual soil deficits greater than 50 kg N/ha. In order to ensure savings above 80%, as required by the EU sustainability criteria, and sustainable SNBs, additional optimization measures should be taken at key points of the value chain.

Methanol is a key platform chemical and a promising alternative fuel for heavy-duty transport, particularly maritime shipping. Renewable methanol is commonly proposed either as biomethanol from biomass or biomethane reforming, or as e-methanol from CO₂ hydrogenation using electrolytic hydrogen. This work proposes a third concept: CO₂-self-sufficient biomethanol from biomethane, where biohydrogen and biogenic CO₂ are internally produced through biomethane reforming with carbon capture and utilisation. Four process routes are assessed for Spain, combining conventional CO/H₂ methanol synthesis and CO₂/H₂ hydrogenation. Unlike e-methanol schemes, the proposed routes use biohydrogen rather than electrolytic hydrogen; unlike conventional biomethanol routes, it can avoid external biogenic CO₂ supply chains. In route 1, fossil methane and CO2 in conventional methanol production are replaced by renewable alternatives. Route 2 produces hydrogen and CO2 through steam biomethane reforming with carbon capture and utilisation, generating a CO2 surplus. Route 3 uses a fraction of biomethane to produce CO2 and biohydrogen, supplying the CO2 required by route 1 and yielding excess hydrogen. Finally, route 4 integrates routes 1 and 2, using the CO2 surplus from route 2 as feedstock for route 1. Routes 1 and 4 produce exclusively methanol and achieve the highest yields (108.6 t and 102.7 t per GWh-HHV of biomethane, respectively). All routes except route 1 are CO2 self-sufficient. Detailed techno-economic assessment of routes 1 and 4 gives methanol costs of 379-792 €/t and 496-933 €/t, respectively, depending on biomethane feedstock. These results highlight route 4 as a deployable option for avoiding external biogenic CO2 logistics.

Renewable methanol is commonly proposed either as biomethanol from biomass or biomethane reforming, or as e-methanol from CO₂ hydrogenation using electrolytic hydrogen. This work proposes a third concept: CO₂-self-sufficient biomethanol from biomethane, where biohydrogen and biogenic CO₂ are internally produced through biomethane reforming with carbon capture and utilisation. Four process routes are assessed for Spain, combining conventional CO/H₂ methanol synthesis and CO₂/H₂ hydrogenation. The proposed routes use biohydrogen rather than electrolytic hydrogen. In route 1 (baseline case), fossil methane and CO₂ in conventional methanol production are replaced by renewable alternatives. Route 2 produces hydrogen and CO₂ through steam biomethane reforming with carbon capture and utilisation, generating a CO₂ surplus. Route 3 uses a fraction of biomethane to produce CO₂ and biohydrogen, supplying the CO₂ required by route 1 and yielding excess hydrogen. Finally, route 4 integrates routes 1 and 2, using the CO₂ surplus from route 2 as feedstock for route 1. Routes 1 and 4 produce exclusively methanol and achieve the highest yields (108.6 t and 102.7 t per GWh-HHV of biomethane, respectively). All routes except route 1 are CO₂ self-sufficient. A detailed techno-economic assessment of routes 1 and 4 gives methanol costs of 403–817 €/t and 496–933 €/t, respectively.