Content, speciation of heavy metal in sewage sludge and its environmental capacity for agricultural use in Chongqing

The environmental risks for agricultural use of sewage sludge depend on the contents of pollutants in sewage sludge and the properties of soils. Sewage sludge was collected from ten urban wastewater treatment plants located in Chongqing. The contents and distributions of Cu, Zn, Pb and Cd were analyzed. The maximum application rates and annual application rates of sewage sludge were calculated on the basis of soil environmental capacity and the application of nitrogen fertilizer. The results showed that the contents of Zn,Cu, Pb and Cd were 362.12～725.32 mg/kg, 69.84～170.22 mg/kg, 21.25～107.24 mg/kg and 1.26～4.25 mg/kg, respectively. The proportions of unstable Zn, Cu, Pb and Cd in sewage sludge were 42.89%～44.7%, 2.69%～9.45%, 0.95%～12.06% and 61.05%～79.91%, respectively. According to the static environment capacity, the maximum application rates of sewage sludge were 384.62 t/hm² for purple soil and 98.90 t/hm² for yellow soil. The annual application rates calculated on the dynamic environment capacity were 66.10, 60.91 and 55.71 t/hm² for a 15, 20 and 50 application year terms for purple soil, respectively, and were 30.32, 28.99 and 27.53 t/hm² for a 15, 20 and 50 application year terms for yellow soil. The annual application rate of sewage sludge calculated on nitrogen fertilizer application was 3.04～6.40 t/(hm²?a), which is safer due to the lower heavy metal input than that calculated on the dynamic environment capacity.