Life cycle assessment of sewage sludge pretreatment for biogas production: From laboratory tests to full-scale applicability

• Published in: Journal of cleaner production Vol. 322; p. 129056
• Main Authors: Mainardis, Matia, Buttazzoni, Marco, Gievers, Fabian, Vance, Charlene, Magnolo, Francesca, Murphy, Fionnuala, Goi, Daniele
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-11-2021
• Subjects: Economic analysis; Enhanced anaerobic digestion; Life cycle assessment; Pretreatment; Sewage sludge; Life cycle assessment; Economic analysis; Enhanced anaerobic digestion; Pretreatment; Sewage sludge
• ISSN: 0959-6526; 1879-1786
• DOI: 10.1016/j.jclepro.2021.129056

In this work, different pretreatment technologies (low temperature thermal treatment, ultrasonication, alkali treatment, combined alkali-thermal treatment, icing-thawing) were investigated together with biochar addition to increase biogas yield from sewage sludge. The aim of the work was to develop a standardized protocol, including physicochemical characterization, biochemical methane potential (BMP) tests, digestate characterization, life cycle assessment (LCA) and economic analysis, to verify the techno-economic suitability and the related environmental impacts of upgrading conventional full-scale anaerobic digesters. Low-temperature (65–85 °C) thermal pretreatment showed the best performances in enhancing sludge solubilization and methane yield (up to 110%), followed by ultrasonication (up to 53%) and biochar addition (about 16%). The solubilization rate obtained after each pretreatment was similar for municipal and industrial sludge; however, it did not correspond to a proportional increase in methane yield. LCA showed that ultrasonication led to the worst environmental impacts at laboratory scale, due to the huge electricity request; however, at full-scale conditions, green-house gases (GHG) emissions could be reduced by 22%, fossil depletion by 25% and stratospheric ozone depletion by 18%, when compared to the baseline scenario (raw sludge anaerobic digestion). The economic analysis proved that the high ultrasonication capital costs could not be recovered in the considered timeframe (15 yr), while biochar price should be significantly lowered (down to 6.5 €/ton) to give an overall positive outcome. Finally, the thermal scenario would be convenient only when considering waste heat recovery. The proposed protocol, including physicochemical sludge characterization, BMP tests, digestate characterization, LCA and economic analysis, could be extended to other existing digesters to compare alternative pretreatment strategies, enhancing renewable energy generation in biogas form.
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•Pretreatment technologies were investigated to enhance sludge methane production.•Low temperature thermal pretreatment (75 °C) showed the highest CH4 increase.•LCA highlighted a significant difference between laboratory and full-scale results.•Ultrasonication led to annual net profits despite the high capital costs.•Biochar breakeven cost to yield profit was calculated as 6.5 €/ton.