Study On Bioleaching Heavy Metal From Urban Sewage Sludge

The water environment pollution caused by the discharge of industrial wastewater and urban wastewater in china has been controlled effectively with sewage treatment plants'building and operation in many cities of china. But another problem was arised with the increase of urban sewage treatment rate,that is how to treat sewage sludge safely.As we know, sewage sludge is the potential fertilizer source and soil amendment for agricultural production, because of the abundant Nitrogen, Phosphorus, Kalium and organic matters. At the same time, the exist of high content heavy metals in sewage sludge had severely restricted the popularization and application of the technology of municipal sludge agricultural resources use. Actually, the contents of Cu , Zn , Cd of sewage sludge were exceed the relevant limits of the control standards for pollutants in sludges from agricultural use(GB4248-84) in most cities of china .So it was an important and urgent mission for us to take effective measures to control the concentration and biotoxicity of the high content heavy metals. Most researches have been carried out for the problem, and many new technologies on removing heavy metals from municipal sludge were exploited. Among which the bioleaching was praised as a green sludge cleaning technology and become the research focus on heavy metal pollution control. Presently, bioleaching as a new technology on removing heavy metals from sludge was systematic studied rarely. Most researches on bioleaching were operated in glassware with pure strain, however, the systematic research based on the engineering application of bioleaching was in scarce. Therefore, to promote the application value of bioleaching, it is very important to implement research from the viewpoint of engineering application.The sewage sludge from Guilin sewage treatment plant was taken and studied. The research aims are to effectively remove the poisonous heavy metals and reduce the biological toxicity. Based on the studies of the growth characters of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxians, one systematic research about the engineering applying was conducted from the four aspects: the selection of bioleaching reactor, the design of bioleaching process, the optimization of bioleaching parameters and the potential toxicity assessment of treated sludge.The analysis of sludge components showed higher agricultural value for Guilin sewage sludge , and the contents of organic matter and total nutrition were 426g/kg and 99.1g/kg.respectively.Cu, Zn, Cd existed in Guilin sewage sludge were selected as the main removal objects in this study due to the contents of those elements exceeding the agricultural standard. The Tessier method was adopted to continually extract various chemical speciations of sludge heavy metals. The results showed that most Cu, Pb, Ni, Cr in sludge existed in their forms of sulfide, organic combination and residual, and the majority of Zn, Cd were bound to the unstable forms, as exchangeable, carbonates and Fe-Mn oxides. The heavy metal stability was decided as Cd>Zn>Ni>Pb>Cu>Cr.Through the status analysis of sewage sludge treatment facilities, the Sequence Batch Reactor(SBR) was adopted as the main reactor for bioleaching heavy metals from sewage sludge. The surplus sludge was the treatment object in this study. The major effect factors of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans were researched signally. The bioleaching optimum operation parameters for Acidithiobacillus ferrooxidans are as follows: substrate dosage,10g/L;temperature,25℃; aeration rate, 0.4L/(min·L); inoculation volume, 30%. The bioleaching optimum condition for Acidithiobacillus thiooxidans is that: substrate dosage was 3g/L, average particle size of elemental sulfur was 165um, temperature was 20~30℃,aeration rate was 1.0 L/(min·L),inoculation volume was 10% the further study on optimum operation period and stability of bioleaching under the optimum treatment condition indicated that the proper operation periods were 2 days for Acidithiobacillus ferrooxidans and 3days for Acidithiobacillus thiooxidans.In this experiment, the indigenous Acidithiobacillus ferrooxidans and indigenous Acidithiobacillus thiooxidans were attained from Guilin sewage sludge by enrichment and purification. The Michaelis-Menten Modle was introduced into derive the equations of bioleaching Acidophile, the kinetic constants of saturation constant (Ks) and specific growth rate(μm) for Acidithiobacillus ferrooxidans were 13.763 g/L and 0.169 h-1, that for Acidithiobacillus thiooxidans were 1.836 g/Land 0.075h-1.The characteristics of transfer and release of different speciations were discussed by the analysis of the content of Cu, Zn, Cd in fresh sewage sludge and treated sludge. The result showed that the residual contents of Cu, Zn, Cd all are accord with control standard for pollutants in sludges from agricultural use (GB4248-84). The organic combination and residual were the major speciations of Cu, Zn and Cd in bioleached sludge. The Sediment Quality Guidelines (SQGs) and Hakanson index method were used to assess the potential biological toxicity of fresh sludge and bioleached sludge. The assessment results showed that the detoxication efficiency of Acidithiobacillus ferrooxidans was better than that of Acidithiobacillus thiooxidans. The potential biological toxicities of main heavy metals were in the following order: Cd>Cu>Zn>Ni>Pb>Cr(fresh sludge)and Cd>Cu>Zn>Pb>Ni>Cr(bioleached sludge). The synthetic potential biological risk indexes of sludge were decreased by more than 90% through the bioleaching process. The sludge bioleached by Acidithiobacillus ferrooxidans was at the low potential biological toxicity, the synthetic potential biological risk index was 84.75. So the Acidithiobacillus ferrooxidans was considered as the proper bioleaching bacteria for removing heavy metals from sewage sludge.