Characteristics and risk assessment of sewage sludge from urban wastewater treatment plants in Shaanxi Province, China

In this study, an investigation of important properties, including moisture content, pH, plant nutrients, organic matter, mineral oil, and the contents of heavy metals (HMs) in sewage sludge (SS) collected from 32 urban treatment plants in Shaanxi Province, China, was carried out. The test results showed that the pH and the moisture, organic matter, total nitrogen (TN), total phosphorus (TP), and mineral oil contents of the SS varied over different rainfall periods, and most of the indicators met the standard criteria for SS agricultural reuse in China. Principal component analysis (PCA) and correlation analysis indicated that the pollutant characteristics of the SS depended on time span and geographical distribution. The mean contents of Pb, Cd, Cu, Zn, Ni, Cr, Hg, and As in SS were 3.95, 16.38, 5.43, 7.70, 1.31, 1.53, 32.77, and 1.40 times higher than the soil background values, respectively. Speciation analysis showed that the forms of HMs in the SS were significantly different. Assessments based on the geoaccumulation index (Igeo), Nemerow integrated pollution index (NIPI), and potential ecological risk index (RI) suggested that HM pollution risk levels were either uncontaminated or moderately contaminated in some regions and that SS recycled for agricultural applications carried a low risk. In conclusion, certain potential ecological risks exist for SS agricultural utilization in Shaanxi Province, and it is necessary to reduce the HM content before SS resource utilization for land application.