Flow analysis of major and trace elements in residues from large-scale sewage sludge incineration

Increase of sewage sludge (SS) has led to the construction of more incineration plants, exacerbating to the production of SS incineration residues. However, few studies have considered the mass balance of elements in large-scale SS incineration plants, affecting the residues treatment and utilization. In this study, flow analysis was conducted for major and trace elements in the SS, the fly ash (sewage sludge ash, SSA) and bottom ash from two large-scale SS incineration plants. The elemental characteristics were compared with those of coal fly ash (CFA), and air pollution control residues from municipal solid waste incineration (MSWIA), as well as related criteria. The results showed that the most abundant major element in SSA was Si, ranging from 120 to 240 g/kg, followed by Al (76-348 g/kg), Ca (26-113 g/kg), Fe (35-80 g/kg), and P (26-104 g/kg), and the trace elements were mainly Zn, Ba, Cu, and Mn. Not all the major elements were derived from SS. Most trace elements in the SS incineration residues accounted for 82.4%-127% of those from SS, indicating that SS was the main source of trace elements. The partitioning of heavy metals in the SS incineration residues showed that electrostatic precipitator ash or cyclone ash with high production rates were the major pollutant sinks. The differences in some major and trace elements could be indicators to differentiate SSA from CFA and MSWIA. Compared with related land criteria, the pollutants in SSA should not be ignored during disposal and utilization.