Bioleaching Of Heavy Metals In The Sludge Of City Sewage Treatment Plant

With the development of domestic sewage treatment,heavy metal is main reason for hindering resource utilization of sewage sludge.Bioleaching is an economical and efficient method to remove heavy metal.According to the present situation of accumulation of heavy metals in sludge,this paper examines the effect of bioleaching by A.thiooxidans.The bioleaching mechanism was discussed by using modern analysis methods.And the impact on dewatering and settling performance of sludge was studies during bioleaching.Effects of sulfur dosage,inoculation dosage,initial pH and low molecular weight organic matter on bioleaching was studied.The optimum sulfur dosage is 5 g/L.The removal rate of Cu,Zn and Cr were 62.1%,91.9%and 30.0%respectively with 5 g/L sulfur dosage.The removal rates of Cu,Zn and Cr were 60.5%,82.4%and 32.0%respectively with 5%inoculum.The removal rate of heavy metals with 5%inoculum was similar to more inoculum.The 5%inoculum was selected as optimal parameters.With sludge of pH<7,the removal rate of heavy metals is ideal.The inhibition of bioleaching is temporary with pH=9 sludge.But after A.thiooxidans adaptation,heavy metals were also removed.Different low molecular weight organic matter have different effects on bioleaching,and the effect extent on heavy metas followed as Cr>Cu>Zn.The removal rate was followed as ethanol>control group>acetic acid>formic acid.The accumulationin of heavy metal in EPS and the morphological changes of heavy metals in sludge during bioleaching were researched.And leaching mechanism was studied with XRD and so on.The results showed the content of heavy metals in EPS increased at first and then was constant.The content of heavy metals in EPS followed as Zn>Cu>Cr.Heavy metals in EPS mainly originated from the adsorption of heavy metals in the liquid phase.Bioleaching could remove heavy metals and change its chemical form.EEM analysis showed that the changes of fluorescent material was complex.The tryptophan was main fluorescent material in original sludge.The fluorescence of fulvic acid increased first and then reduced.The humic acid was main fluorescent material in sludge in final.FTIR analysis showed the polysaccharides and proteins could be used by heterotrophic bacteria.Absorption peaks related to S-H stretching vibration and SO42-asymmetric variable angle vibration appeared after bioleaching.Because under the role of Thiobacilllus,compound containing S-H group and sulfate were generated.EDS and XRD analysis showed the sludge was mainly elements C,elements O and SiO2.After bioleaching the sludge composition unchanged.In addition SO42-and metal in sludge were formed a new mineral NaMg6Al3(SO4)2(OH)1812H2O.After bioleaching settling and dewatering performance of sludge improved.The dewatering performance of sludge with pH1.85 is best.At that time,the 24h natural sedimentation rate of original sludge increase from 22.8%to 40.6%,the dewatering rate of sludge centrifuged at 4400 r/min rised from 68%to 85%,and the specific resistance was from 10.8×1012 m/kg to 3.2×1012 m/kg.The Zeta potential wasincreased from-12.50 mV to 1.72 mV,because electrostatic repulsion between the sludge particles is reduced making the sludge easy to flocculate.SEM shows that dewatering performance of the sludge improves after bioleaching.The TP/TN concentration in leachate during bioleaching were higher than control group.indicating the acidic environment causes cell fluid release,increase of density of sludge,easy to settle.All of these cause improvement of settling and dewatering performance of sludge.But the sludge destruction and filamentous bacteria expansion led to deterioration of dewatering performance at pH<1.85.