[Sludge Gardening Utilization Potential and Composite Ecological Risk Assessment of Shanghai Sewage Treatment Plants]

Based on the Shanghai Ecological Space Special Plan （2018-2035）， this study explored the significant demand for soil resources in ecological construction. By 2025， Shanghai is expected to add 8 666.67 hectares of green space and 16 000 hectares of forest. As a critical pathway toward low-carbon sludge resource utilization， approximately 1.22 million tons of sewage sludge are produced annually by Shanghai's wastewater treatment plants. After stabilization， this sludge can serve as an ecological substrate for green space development. This study systematically monitored and analyzed the heavy metals， polycyclic aromatic hydrocarbons （PAHs）， and ecological nutrients in dewatered sludge from 10 representative sewage treatment plants in Shanghai. The results indicate that the organic matter and total nutrient levels in the sludge meet the Organic Fertilizer （NY 525-2021） standard， with an average comprehensive fertility index of 1.84， indicating high fertility and significant potential for improving impoverished soils. Further analysis revealed that the heavy metal content in the sludge complies with the Soil Environmental Quality-Risk Control Standard for Soil Contamination of Development Land （GB 36600-2018） and that PAH levels meet the Sewage Sludge Treatment and Disposal-Agricultural Sludge Quality （CJ/T 309-2009） standards. This compliance demonstrates that the sludge meets the safety requirements for green space construction in urban areas， making it highly viable for application. However， considering the potential cumulative effects of heavy metals， a composite risk assessment method， based on the analytical hierarchy process （AHP）， was developed in this study to perform a scientific and systematic assessment of the potential ecological risks of prolonged sludge application， particularly focusing on heavy metals and PAHs. The composite risk assessment results indicate that the overall ecological risk of dewatered sludge from Shanghai's sewage treatment plants is relatively low， suggesting that it is suitable for urban landscaping projects. This supports soil improvement goals and offers a viable pathway for low-carbon sludge resource utilization. Nevertheless， special attention should be given to the potential ecological risks associated with mercury （Hg） and nickel （Ni）， as their long-term accumulation may adversely affect ecosystems. Therefore， it is recommended that future sludge resource utilization initiatives enhance Hg and Ni content monitoring and continue risk assessment efforts to ensure the long-term sustainability of ecological construction projects.