Assessing the ISO hierarchy validity in circular wastewater treatment life cycle assessments: A Portuguese case study

Abstract

This study evaluates the validity of ISO hierarchy in handling multifunctionality in life cycle assessments of circular wastewater treatment plants (WWTPs). The case study focuses on retrofitting a WWTP to produce Kaumera biopolymer, a potential substitute for sodium alginate. Various multifunctionality handling approaches—system expansion, zero-burden, economic, and mass allocations—were applied and various functional units were selected to calculate environmental impacts. The global warming (GWP), mineral resource scarcity (MRSP), and fossil resource scarcity (FRSP) indicators were examined. The results indicate that Kaumera offers significant environmental benefits (40 %–99.9 %) in GWP, MRSP, and FRSP compared to sodium alginate. System expansion provides a comprehensive assessment, making it the preferred approach. Economic allocation yields closer results to system expansion than other approaches, while zero-burden and mass allocation show 88–93 % and 100 % improvements, respectively, leading to misleading conclusions. We suggest that ISO should prioritize economic allocation over mass allocation in wastewater treatment studies.