Life cycle assessment of prospective sewage sludge treatment paths in Germany

• Published in: Journal of environmental management Vol. 290; p. 112557
• Main Authors: Mayer, Felix, Bhandari, Ramchandra, Gäth, Stefan A.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15.07.2021
• Subjects: energy; eutrophication; Germany; Hydrothermal carbonization; Incineration; Life cycle assessment; Phosphorous recovery; phosphorus; Pyrolysis; Sewage sludge; toxicity; wastewater treatment; Germany; Life cycle assessment; Pyrolysis; Hydrothermal carbonization; Incineration; Sewage sludge; Phosphorous recovery
• ISSN: 0301-4797; 1095-8630; 1095-8630
• DOI: 10.1016/j.jenvman.2021.112557

Due to the amendment of the sewage sludge ordinance, both a thermal post-treatment and a phosphorous recovery from sewage sludge will become mandatory for large-scale wastewater treatment plants in Germany. This study analyzed four prospective treatment paths for sewage sludge by means of life cycle assessment. In the realm of a gate-to-cradle approach, environmental impacts were quantified for all compartments of the ReCiPe Midpoint (H) 2016 method. The spreading of digested sludge on agricultural soils was considered as the base case (system: AD + spreading). It was compared to the centralized incineration of sludge (system: AD + I), a decentralized hydrothermal carbonization followed by centralized incineration (system: AD + HTC + I) and a decentralized pyrolysis of sludge followed by centralized incineration (system: AD + P + I). For all cases, phosphorous recovery from the ash was included. A comparative evaluation showed that AD + spreading resulted in least environmental impacts in most categories but was subject to a high local immission potential due to sewage sludge spreading. It was found to be only justifiable, if toxicity and eutrophication were not compromised. Alternatively, a thermal post-treatment step is required. Hereby, AD + I and AD + HTC + I showed the overall least environmental impacts, while AD + P + I was characterized by similar or higher environmental impacts throughout all impact categories. Alongside the comparative analysis, a hotspot analysis was carried out and mitigation potentials were identified. For all thermochemical post-treatment paths, it was derived that (i) the share of fossil external energy must be kept to a minimum, (ii) primary or secondary measures to control N2O emissions during the incineration and pyrolysis should be implemented and (iii) the technological approach to recover phosphorous must be carefully selected. •Four sewage sludge treatment options were analyzed by means of life cycle assessment.•Sludge spreading was opposed to thermal post-treatment including phosphorous recovery.•Spreading was preferential, if toxicity and eutrophication were not compromised.•Hydrothermal carbonization and direct incineration resulted in similar impacts.•Pyrolysis resulted in equal or higher impacts throughout all impact categories.