A focus on anaerobic digestion and co-digestion strategies for energy recovery and digestate valorization from olive-oil mill solid and liquid by-products

European Union policies focused on the acceleration of renewable energy systems take-up have recently set forward the minimum binding goal of contributing to the supply of at least 32% of the European energy demand in 2030, and by the hand reaching the task of reducing net greenhouse gas emissions by minimum 55%. Moreover, it is expected that at least 25% of the whole bioenergy shall be obtained from anaerobic digestion/co-digestion (AD/AcoD) of organic matter, comprising agro-food wastes, whole crop silages, and sewage sludge, among others. Within this review study, key state-of-the art of anaerobic digestion processes for the treatment of both liquid and solid residues of olive-oil industries, OMW and OMSW, with a focus on energy recovery and digestate valorization is reviewed and discussed, with a focus on proposed and most effective co-digestion co-substrates, bioreactor configurations and conditions (temperature, hydraulic retention time, organic loading rate), adopted sequences (comprising pretreatments, acidogenesis and methanogenesis stages), biogas upgrading (H2S purification, CO2 removal) and success case. Energy experts highlight compressed biogas as the best positioned candidate to become the fuel of the future, as a result of its virtue of being a clean, renewable and indigenous source of energy. Moreover, biogas is also expected to help reduce the cost of imports of natural gas. In the current scenario, in which worldwide new Circular Economy adoption is pursued, modernization and up-grading of industry, in particular the agri-food sector, is a very relevant and necessary challenge.