Comparative life cycle assessment of sewage sludge drying by solar and thermal drying technologies

Abstract

Sewage sludge management is a critical challenge requiring sustainable treatment solutions. Drying is an essential pre-treatment step for utilizing sludge in waste-to-energy systems. Although drying is an essential step in sludge management, it has often been represented in a simplified manner using general assumptions in life cycle assessment (LCA) studies aimed at evaluating environmental impacts and greenhouse gas emissions. In many assessments, sludge drying has been identified as the most environmentally burdensome stage of sludge management due to its high energy demand, with solar drying frequently proposed as a low-energy alternative. Given these findings, optimizing the environmental performance of the sludge drying process is crucial for reducing the overall life cycle impacts of sludge management. Accordingly, this study aims to compare the environmental impacts of thermal drying technologies (belt, fluidized bed, and rotary) and solar drying using a gate-to-gate LCA approach. Additionally, impact reduction scenarios were assessed, including renewable energy integration and adjusting the final solids content of sludge. The LCA was performed using the ReCiPe 2016 Endpoint (H) method. Results indicate that using photovoltaic electricity and waste-incineration steam can significantly lower the environmental impacts of belt and rotary dryers, making them comparable to solar drying. On the other hand, solar drying's high land demand remains a major limitation. These findings provide key insights for decision-makers, helping optimize sludge drying processes with lower environmental impacts. The study highlights the importance of technology-specific strategies and renewable energy integration in wastewater treatment plants.

Keywords: Greenhouse gas emissions; Life cycle assessment (LCA); Renewable energy integration; Sewage sludge drying; Solar sludge drying; Thermal drying technologies.