Global Harmonization of Life Cycle Assessments for Small- and Medium-Scale Wastewater Treatment.

Wastewater management (WWM) is vital for protecting human health and the environment, yet conventional centralized systems are resource-intensive and unsuited to growing urban pressure. Small and medium WWMs offer a potentially more sustainable alternative, but the lack of standardized life cycle assessment (LCA) approaches limits robust comparison and informed decision-making. In this study, we conduct a global harmonization of life cycle assessments of small- and medium-scale systems across 96 LCAs and 9 treatment technologies. By harmonizing life cycle inventories, we eliminate variability in methods, assumptions, and biases, thereby providing a consistent setting to compare environmental impacts. The harmonization process shows that GHG emissions have been underrepresented in inventories. The results suggest that direct emissions to air are a key contributor to climate change, and decarbonization strategies should focus on minimizing these emissions. Sequencing batch reactors showed the lowest average phosphorus and low nitrogen emissions to water with minimal chemical dosing and low direct GHG emissions to air. Our findings support the development of environmentally sustainable small- and medium-scale water-treatment solutions for municipal settings.