Study On The Occurrence And Leaching Characteristics Of Heavy Metals After Sewage Sludge Heat Treatment

Along with the rapid development of population and urbanization, activated sludge is a byproduct of the sewage treatment process will the inevitably produce, treatment and disposal of excess sludge has become a closely watched environmental problems. The production of sewage sludge (SS) has increased sharply as a consequence of more stringent criteria for wastewater treatment plant effluent. The SS contains moisture content, heavy metals, organic compounds, macronutrients, organic micro-pollutants, microorganisms and eggs of parasitic organisms. Therefore safe and cost-effective disposal of sewage sludge from waste treatment plants is still a worldwide environmental challenge. According to the above literature review, we can conclusion that limitations of previous research on pollution of heavy metals in sewage sludge, ignore the bearing metal, just only based on the leaching of heavy metals toxicity of solid waste, however safety analysis does not essentially reaction the existence form of heavy metals in sewage sludge, especially heavy metals in the sludge after heat treatment and the relationship between the security can not ensure. Therefore, we use the sequential extraction procedure developed by Tessier et al. in sludge to determine the heavy metal speciation. Through the TCLP, the HMs migration characteristics of sludge after heat treatment (potential secondary pollution) can be studied.The fate of heavy metals in sewage sludge after thermal treatment, release and transformation rule of organic combine, clarify different sludge treatment methods on the activity of heavy metal pollution, and establish the dynamic correlation between them. Finally realize sludge harmless and resource recovery. The experimental studies have found that aerobic sintering can be more desirable to stabilize heavy metals in sewage sludge by the relatively stable state of (F3+F4).The total content of Cu in the state of (F3+F4) is about 85%, Cr and Pb are similar in a relatively stable state, these three heavy metals presence in the form of particulate matter. When the temperature reaches 900℃ is the maximum of content of Cr in the state of (F3+F4). Zn、Cd and Ni with characteristics by migration and leaching ability are in the presence of soluble state. More attention should be paid to the potential bonded fractions (F3+F4) as the changes of environmental and thus become more rapidly toxicity and bioavailability.Compared to aerobic sintering, pyrolysis can be stabilization more heavy metals in fractions of residual. In anaerobic pyrolysis process, organic carbon is generated through pyrolysis by organic matter in sewage sludge. It plays a solidification role to enhance the stability of heavy metals in sewage sludge. In addition to the state of Ni and Cd is still relatively unstable, the others present in the fractions of residual.Thermal treatment is regarded as environmentally friendly and has been used for sewage sludge treatment. To optimize this means, we studied the synergistic effect of oxidizing material addition on thermal treatment of sewage sludge. Mixed with Cao, the concentration of heavy metals leaching will be reduced; however, when the ratio is greater than the dosage of 5%, pH value greater than 11, these reasons is not conducive to the subsequent processing and use. In this study, CaOand SiO2 were selected to act as an additive at the same time. As the decrease of the ratio, the leaching concentration of heavy metals decreases; when the ratio is less than 1, the leaching concentration of heavy metals increases.Thermal treatment at 1000℃ was applied to immobilize heavy metals and the effect of different ratios (0%、5%、10% and 20%) of rice husk (RH) addition was investigated based on total heavy metals concentration, fractionation and leaching tests. This observation is in agreement with the results, in which RH possessed a lower heavy metals binding capacity with Ni, and a relatively high level of Ni contained in both sewage sludge and RH used in this study may cause some difficulty for its immobilization during the handle process. The other heavy metals in fractions of residual show no toxicity because of its stably bounded state after this process.The above results imply that thermal method can improve the solidification ability of heavy metals in sewage sludge with the high energy required. However, the RH addition has a synergetic effect on the risk reduction or enhanced immobilization of heavy metals in sewage sludge through the HTT process.