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.