The Evaluation About Chemical Removal Methods And Effect Of Bioavailability Heavy Metals In Sludge

With the rapid growth of population and development of economic, the problem of environment pollution is becoming more and more serious. Although a lot of wastewater treatment plants have been constructed, which alleviate the urban sewage and industrial wastewater pollution effectively, the sludge produced in the process of sewage treatment is easy to cause secondary pollution. Due to sewage sludge contains abundant nitrogen, phosphorus, potassium and other nutrients, the agricultural utilization has become a hot spot of research. However, the heavy metal elements in the sewage sludge has become the key factor that restrict agricultural utilization of sludge. Many scholars carry out extensive reserch on how to reduce the toxicity of heavy metals in urban sludge, but their systemicness, economy and practicability still can not meet the requirements. So it is significant to figure out systemically methods which can remove heavy metals in sludge and benefits agricultural utilization.In this paper, the composition and chemical properties of municipal sludge from Dongguan were analyzed basically; the distribution of chemical speciation of heavy metals in sludge was determined in detail; the bioavailability of heavy metals was discussed, and provide a basis for further carrying out different methods to remove heavy metals in sludge. Finally, selected methods of treatment were evaluated from different angles.After the experiment of speciation analysis, it can be concluded that Cr and Cu mainly exist in oxidisable forms and residual forms; Zn, Ni and Cd mainly exist in acid-soluble forms and reducible forms. By compareing the content of bioavailability of heavy metals in sludge, it can be found that bioavailability of heavy metals is strong in five kinds of sludge. So, there must be targeted treatment means for resource utilization.Based on the speciation analysis, it chose 30% hydrogen peroxide and ammonium acetate to extract the oxidizable fraction and chose hvdroxylamine hydrochloride to extract reducible fraction. In the process of extraction, concentration of extractant, extraction time and solid-liquid ratio are main influence factors. Under the condition of solid-liquid ratio is 1:30 and extraction time is 16h, the extraction effect of these five heavy metals is the best. However, consider the possibility of industrialized operation, the condition of solid-liquid ratio of 1:10 and extraction time of 4h has been chosen.This study evaluates the experimental results by the reduction of total amount of heavy metals, morphological changes, TCLP test and economic analysis before and after the experiment. It showed that the removal of the five elements are exceeded in more than 50%, which means this method can effectively reduce the amount of heavy metals. At the same time, TCLP test may reflect the environmental effects better by showing the toxicity of heavy metals is greatly reduced. Moreover, Heavy metals transform from unstable forms to stable forms and bioavailability is reduced make it suitable for utilization. Although the cost-effectiveness of this experiment and traditional treatment methods are both rather, this method may have more environmental benefits.