Environmental and Cost Life Cycle Analysis of Different Recovery Processes of Organic Fraction of Municipal Solid Waste and Sewage Sludge

Purpose: Two biological treatments for biodegradable substrates were assessed: (i) conventional co-digestion of source sorted organic fraction of municipal solid waste (SS-OFMSW) and sewage sludge (SS); (ii) preliminary dark-fermentation pre-treatment of the mixture of SS-OFMSW and SS, followed by a second step of anaerobic digestion. The produced biogas and hydrogen-rich gas are assumed to be used in an internal combustion engine to produce electricity and thermal energy. Methods: Life Cycle Assessment and Life Cycle Costing were implemented to assess the two biological treatments, which were also compared with the current treatments (separate aerobic composting of SS-OFMSW and anaerobic digestion of SS). The selected functional unit is the total annual amount of entering waste (189,000 t/y of SS-OFMSW and 15,500 t/y of SS) to an actual Italian plant, which was used as case study for this work. Results: The Life Cycle Assessment results show that the co-treatments of SS and SS-OFMSW provide general environmental improvements with respect to the reference case study of separate SS anaerobic digestion and SS-OFMSW aerobic composting. The case based on the preliminary dark fermentation of the mixture of SS-OFMSW and SS followed by digestion always shows better indicator values than the co-digestion one. This result is mainly influenced by the higher energy recovery, which, in turn, is due to the improved specific gas production of the digestion step, after the co-fermentation pre-treatment. The Life Cycle Costing shows that both the studied systems are economically sustainable, however the case based on the preliminary dark fermentation of the mixture of SS-OFMSW and SS followed by digestion has a shorter time of return of investment and a higher net present value than the co-digestion one. Conclusions: For the considered study case, the preliminary dark-fermentation pre-treatment of the mixture of SS-OFMSW and SS, followed by a second step of anaerobic digestion is preferable to their simple co-digestion process both from the environmental and economic points of view