| Summary: | Upflow anaerobic sludge blanket reactors (UASBs) used in sewage treatment generate two useful byproducts: sludge and biogas. This study evaluated the sustainability of four different scenarios for the treatment and final destination of biological sludge and biogas in a medium-sized wastewater treatment plant (WWTP) in South Brazil. At this plant, the sludge is sanitized by Prolonged Alkaline Stabilization and applied to agriculture (base scenario). Scenario 1 is about biogas use to dry sludge, which is taken to be used in agriculture. In Scenarios 2 and 3 the heat of the sludge burning is used for drying and sanitation. Finally, in Scenario 3 the ashes are destined to landfills. An environmental impact assessment was performed through life-cycle assessment using the ReCiPe 2016 evaluation method. Social life-cycle assessment indicators, adapted and developed for WWTPs, were used for social assessment. Economic assessment was performed through the analysis of life-cycle costs. The dashboard of sustainability (DoS) method was used for global assessment of sustainability. For overall sustainability assessment, Scenario 1 had the highest score (678 points) (best scenario) in the DoS. The environmental dimension was what facilitated this scenario. For this dimension, the following indicators presented the highest points when compared to the other scenarios: soil acidification, ozone formation, terrestrial ecosystem. The base scenario had the lowest score (worst case scenario) (375 points).
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