Is the Anaerobic Digestion (AD) sustainable from the energy point of view?

Highlights: • Energy Paradigm: Proximity (resources), Adequacy (carrier), Vitality (technology). • Multi-level sustainability evaluation of the AD technology (ESA: ESI, EROI, EPT). • Considered feedstocks: organic residues vs energy crops. • EROI: livestock residues (8.1–11.9) > OFMSW (4.5) > energy crops (1.8–3.6). • No Field Spreading scenario for the digestate: 10–40% EROI reduction. This article aims to apply the Energy Sustainability Analysis (ESA) to the process of Anaerobic Digestion (AD), considering five different feedstocks: the organic fraction of municipal solid waste (OFMSW), agro-industrial products (energy crops) such as maize and sorghum silages, and agro-zootechnical wastes (slurry and manure) from cattle and pig livestock. The case study focuses on an AD bioreactor which is fed with a constant flow (50 ton/d) at a fixed total solids (TS) concentration (15% TS w/w). The processing chain is divided into three phases: pre-digestion operations, AD and the cogeneration of biogas, and post-digestion operations. The ESA methodology uses dedicated metrics and considers the entire technology chain from the energy resources up to useful energy. Energy crops present important energy footprints that must be spent for their production, while in the AD process, the pre-digestion and post-digestion phases present different energy costs for each analyzed feedstock. The results indicate that although the produced energy depends on the energy potential of the biomasses, the pre-digestion and post-digestion phase have an important weight in the overall energy sustainability of the system. In addition, in the case of the use of agro-zootechnical wastes and energy crops as feed, the availability of sufficient land to spread digestate on the field is essential to ensure the sustainability of AD technology, in its absence the energy sustainability can be critical as shown for the case of maize silage.