Bioleaching Of Cu, Pb And Zn From Sewage Sludge

The production of excess sewage sludge increases substantially with the development of wastewater treatment facilities, which makes the proper disposal of sewage sludge become more and more urgent and essential. The high content of organic matter and nutrient concentrations suggest sludge use preferably land application However, high concentration of heavy metals concentrated in sludge is one of the most pressing environmental impacts. Bioleaching is cost-effective and promising solution for reducing the content of heavy metals in sludge. In mis research, the bioleaching process of raw sludge was investigated in six aspects, including the selection of optimal parameters, mechanism of bioleaching, transformation of microorganism, effecting factor on bioleaching, evaluation of sludge for land application after bioleaching, and operation of bioleaching reactor.The inocula of mixed sulfur bacteria inoculums obviously promoted the bioleaching of Cu, Pb and Zn from sludge and shortened bioleaching running time. With 5-6 days and inoculum of 2%, the bioleaching efficiencies of Cu, Pb and Zn were 96.5%, 41.4% and 82.9%, respectively, which was similar to bioleaching with high strength inocula. The pH decline was faster with the addition of higher sulfur concentrations. However, there was no significant difference in bioleaching efficiencies of Cu, Pb and Zn among sulfur concentrations of 3g/L, 4g/L and 5g/L. According to various factors such as cost and bioleaching efficiency, the inoculum of 2% and sulfur concentration of 3g/L were selected as the optimal parameters.The transformation among different heavy metals form during bioleaching could be described by continuous measurement, and the bioleaching mechanism could be indirectly presumed with variation of heavy metals forms. Cu at initial sludge mainly existed as sulfide precipitate form, most of which turned to exchangeable form during bioleaching, and .was solubilized mostly by direct mechanism. The original forms of Pb and Zn were mainly carbonate precipitate and organically bound. Most of the carbonate precipitate Pb transformed into exchangeable Pb but a little of organically bound Pb. Pb was dissolved from the sludgedue to transformation of carbonate precipitated and sulfide precipitated to exchangeable form by cooperation of both direct and indirect mechanisms. The majority of exchangeable Zn was transformed fiom organically bound and carbonate precipitated forms, and the bioleaching mechanism of Zn was mainly indirectThe partition of Cu, Pb and Zn in sludge cell was observed using the combination of transmission electron micrograph (TEM) and energy dispersive X-ray analysis (EDAX). The results showed that all the three kinds of metals were obviously found in the cell; however, the partition at different sites within the cell was not homogenous. The absorption ability of sludge cells to Zn was less than that to Cu and Pb, resulting in the substantially lower content of Zn in the sludge cells than that of Cu and Pb. The heavy metals adsorbed or combined outside the cell and absorbed inside the cell solubilize via the extracellar polymer substances (EPS) inevitably, and the content profile of different metals in EPS demonstrated various pattern. The highest concentrations for Cu, Pb and Zn during bioleaching were 14.7%, 20.3% and 24.2% of their total concentration in sludge, respectively.A great deal of heterotrophic microorganisms in sludge died during bioleaching, and its amount decreased fiom 2.5X109 cell/mL before bioleaching to 12xl07 cell/mL after bioleaching. The VFA and amylose in sludge leachate increased to a large degree during the running, while protein and EPS showed less increase. The microbial community was investigated using SEM and TEM. With time on, the microorganism in bioleaching sludge became much more and distributed densely than the control sludge, and the baciUiform bacterium showed an obvious tendency of increase. There were various sorts of bacillus and short bacillus in bioleaching sludge, and some dead bacterium with empty cell and damaged cell could be found at the same time.The initial sludge pH affected bioleaching efficiency largely, and the effect extent on heavy metals followed as Pb>Cu>Zn. But if the bioleaching time was prolonged, the heavy metals could be leached even with sludge of initial pH 10. There was much discrepancy in effect of different organic compound addition on bioleaching, where the effect extent followed as Pb>Cu>Zn. The negative influence on bioleaching was decreased with the increase of carbon...