Temporal variation of heavy metals in sewage sludge in typical cities in Gansu Province, northwest China

To investigate the temporal behaviors of heavy metals in sewage sludge in typical cities of industrial, industrial-agricultural, agricultural, or energy focused. Samples were collected every 10 days for a period of 1 year in four types of cities of Lanzhou, Tianshui, Qingyang, and Zhangye. The average annual values for all four cities were Cd (1.59-3.16 mg/kg), Pb (41.9-55.1 mg/kg), Cr (63.8-92.0 mg/kg), Cu (75.7-92.6 mg/kg), Zn (498-612 mg/kg), and Ni (3.66-4.25 mg/kg). The highest values were observed in June for Cd, Cr, and Zn, at Lanzhou and Tianshui. At Qingyang and Zhangye, the Cd, Cr, and Zn contents were stable throughout the year. There was a similar monthly change among the four cities regarding the levels of Ni content, and it was far below the background value. The monthly fluctuations in Cd, Pb, Cr, and Zn are mainly due to street dust effect. For cities with a developed industry, the impact of street dust during the first rains of the year on sewage sludge's heavy metal content must be highlighted as being of particular importance.