Optimization Of Treatment Method Based On The Speciation Analysis Of Heavy Metals In Sludge

In recent years, with the rapid development of urbanization and economy, sewage and industrial wastewater were increasing, and the sludge, as the byproduct of sewage treatment, was correspondingly increased. Sludge was not only rich in organic matters and nutrients, such as nitrogen and phosphorus and so on, but it still more or less contained some heavy metals to limit the utilization of sludge. Four sludges of three categories were selected to study. According to the results of the total amount and speciation analysis of heavy metals in sludge, chemical method, hydrothermal method and stabilization method were selected to treat the slugde, and Toxicity Characteristic Leaching Procedure (TCLP) was used to test the effect of treatment.BCR method was used to determine the distribution of chemical speciation of heavy metals in four kinds of sludge, and the conclusion was drawn, which the chemical speciation of different heavy metals in different sludges varied widely. And suitable treatment methods were selected for the speciation of heavy metals in sludge: some weak acid or soluble salts, such as acetic acid, sulfuric acid, and ammonium acetate were selected to remove acid soluble fraction; Reducing agents like hydroxylamine hydrochloride were selected to remove reducible fraction; Oxidizing agents like hydrogen peroxide were selected to remove oxidizable fraction; And residual fraction was in stable state, so it was not considered to be removed. Because acid soluble fraction and reducible fraction were in unstable state, they could be stabilized by phosphate and other inert materials.In chemical experiment, concentration of leaching agents was chosen according to BCR method, and the optimal process conditions for a variety of sludges were chosen by regulating different solid-liquid ratios and extraction time:For S1, the best solid-liquid ratio and extraction time to remove Zn and Ni was1:30and16h; S2was oxidized by hydrogen peroxide firstly, and then extracted by ammonium acetate with solid-liquid ratio of1:30and extraction time of4h. S4was oxidized by hydrogen peroxide firstly, and then extracted by acetic acid with solid-liquid ratio of1:20and extraction time of8h. In hydrothermal experiment, by adjusting the solid-liquid ratios and pH values of extraction agent, we concluded that the smaller solid-liquid ratio, and the stronger the acidity of the leaching agent, the better removal of heavy metals. But overall, the effect of hydrothermal method was not very satisfactory. In stabilization experiment,phosphate was used as stabilizing agent. By controlling the time and temperature of stabilization, it could be concluded that with time and temperature increased, stabilizing effect of phosphate on heavy metals enhanced. However, the experiment that restabilization of sludge after hydrothermal method showed that it was not necessarily better treatment effect of heavy metals than single hydrothermal method and single stabilization method.It could be seen by TCLP that toxicity of Zn, Ni and Cd after hydrothermal and chemical treatment reduced, while toxicity of Cu and Cr increased; and after the stabilization process, toxicity of heavy metals in four sludges reduced.Through the speciation analysis of heavy metals after treatment, we found that most speciations of heavy metals converted to the expected speciation, indicating that it was a feasible idea to choose treatment method based on speciation analysis.