The Study Of Method And Mechanism On Simultaneous Removal Of Heavy Metals And Improvement Of Sewage Sludge Dewaterability During Bioleaching Employing Acidophilic Bacteria

With the rapid development of urbanization and industrialization and the stricter andstricter sewage discharge standards, the generation of sewage sludge increased drastically inour country during the last three decades. Proper treatment and safe disposal of sludge hasbeen being an important factor affects city developments, especially the control of heavymetals contamination and the improvement of dewaterability are the bottleneck issues.Considering that the conventional physical and chemical methods posses several drawbacks,such as consumption of high erergy and high operating cost, bioleaching method wasemployed in this study. Mixed cultures of acidophilic bacteria (MCAB) enriched from naturalbacterial resource (acid mine drainage: AMD) and the corresponding acclimated sludge wereused as inoculums during bioleaching to solubilize heavy metals and improve sludgedewaterability simultaneously. The best inoculum was employed to determine the optimizedconditions for sludge bioleaching. The meachism of sludge dewaterability improvementduring bioleaching process was explored on the basis of this experiment research andliterature research to provide experimental evidences for its engineering application. The mainconclusions are as follows:(1) MCABs were enriched with9K medium and modified Starkey medium, which werefurther empolyed to get acclimated sludge, and then MCABs and acclimated sludge were usedas inoculums to boost bioleaching process. The results indicated that all the MCABs andacclimated sludge can effectively solubilize heavy metals and improve sludge dewaterability,and the synergy effects between exogenous and indigenous ferrous/sulfur bacteria canobviously enhance bioleaching efficiency. The acclimated sludge made with MCAB enrichedby modified Starkey medium obtained the best leaching efficiency, of which thecorresponding leaching efficiencies of Cu, Zn, Cd were90.51%,92.84%,88.64%and thecentrifugal dehydration rate was89.75%.(2) In order to enhance bioleaching efficiency further, the best inoculum mentionedabove was used to determine the optimized conditions for bioleaching process, investigatingthe influences of inoculation rates, substrate species and addition doses, initial sludge pH onbioleaching efficiency. On the basis of single factor experiments, optimized the conditions forsludge bioleaching. With the optimized conditions of inoculation rate10%, Fe2+and Ssubstrates addition dose4g/L, initial pH5.0,97.11%、98.88%、95.11%of Cu, Zn, Cd weresolubilized and the centrifugal dehydration rate increased to90.50%. (3) The meachism of sludge dewaterability improvement during bioleaching wasspeculated with the combination of present experimental results and other literature reports.The results showed that the micro structure of sludge changed from incompact flocculationshape to compact granular shape, and the size of sludge particle also increased. Scanningelectron microscopy (SEM) observations showed that crystalline substances with regularshapes formed in bioleached sludge. Energy dispersive spectrum (EDS) analysis revealed thatcrystalline substances mainly consist of iron, oxygen and sulfur elements. These crystallinesubstances might be a kind of secondary minerals mixtures including schwertmannite andjarosite, which increased the density of sludge particle, hence improving its dewaterability.