| Sludge are the solid content which settled during the wastewater treatment process, or the water movement process.In this paper, the sludge mainly refers to the sludge with concentrating and dewatering in the sewage treatment plant.This kind of sludge is the important environmental pollutants, it has the complex properties, large volume, and its toxicity is prolonged.The treatment of heavy metals in sludge is the important subject of the environmental pollution contral fields. Especially for the increasingly serious environmental safety problems of China. The technologies of the treatment for heavy metals in sludge is a nice topic which has strong vitality.The agricultural disposal of sludge is an important route for the resource utilization of sludge in our agricultural country, but, the environmental risk of heavy metals in sludge is the bottleneck of the agricultural disposal. The heavy metals can not be efficiently eliminated from sludge by the traditional echnologies such as the digestion, concentration, dewatering for sewage sludge. In recent years, the new process exploited for heavy metals are beset by many serious problem such as high cost, severe conditions, low removal rates, associated secondary toxicant, potential safety hazard.The biological process has received considerable attention as one of the most potential advanced technologies for environmental engineering field. The application of biological process in heavy metal processing mainly include bioleaching, plant extract process etc. Among these biological process, bioleaching has many excellent features such as extensive adaptability, high removal rates, simple operation conditions, low cost bio-additive, and the reaction conditions are mild, low acid consumption. Above all, biological process can be synchronization actualize with the original digestion process in the sewage treatment plant. On the other hand, which can replace part of the functions of the digestion process. Further, it can use the primary buildings or structures of the sewage treatment plant, which result in smaller construction cost. So, its a potential advanced technologies.In recent years, there are many studies about bioleaching process of sludge, but, the removal rates for some heavy metal failed to produce the desired effects. Furthermore, the time-consuming is too long. The start-up rate of bioleaching is conditioned by various factors such as the initial pH of the reaction system and the dissolved organic matter (DOM). These factors lead to the lower removal rates, also the total operating cost will increase. In terms of the mechanism research, the previous literatures have few research results about the mechanism of the sludge bioleaching, especially in the bioleaching mechanism of Cd,Ni,Cr,Pb. In terms of the microorganism community sort, mainly concentrate on a single Thiobacilli, they had not done much work on compound-bioleaching of multi-chemoautotroph. Furthermore, the research about the joint-bioleaching by heterotrophic bacteria and chemoautotroph are very few.Based on the study about the sludge bioleaching by dint of compound Thiobacilli bacteria, this paper took a new idea of joint-bioleaching by heterotrophic bacteria and chemoautotroph, in virtue of Acid-Proof Metallothionein genetic engineering bacteria(MT genetic engineering bacteria), promote the process of bioleaching using Thiobacillus thiooxidans(T.t) and Thiobacillus ferrooxidant(T.f). The ultimate goal is improve the efficiency of the process. Furthermore, the effect of heavy metal load factor on bioleaching, and mechanism of heavy metal speciation transformation during the joint-bioleaching by MT genetic engineering bacteria were investigated detailedly. This paper does useful researches and trials to deepen the mechanism of bioleaching. The main study contents and findings are as follows:(1) Three independing bioleaching test which inoculated with isolated predominance strains were carried out, that respectively used Thiobacillus thiooxidans(T.t), Thiobacillus ferrooxidant(T.f), and MT genetic engineering bacteria as its predominance strain. The results indicated that, both Thiobacillus thiooxidans(T.t) and Thiobacillus ferrooxidant(T.f) showed preferable treating effect of bioleaching for heavy metal in sludge, but, there presented relatively significant differences between two kind of strains. The bioleaching of Ni,Cd,Cr,Pb with T.f showed better treating effect than T.t. The bioleaching of other element with T.t are better than T.f. These differences are relevant to the chemical speciation of variants heavy metal elements in aborignal sludge, its also relevant to the mechanism of the sludge bioleaching.The inoculation amount of Thiobacilli particularly affected the acidification rate, acidification degree, and the change trend of oxidation reduction potential (ORP). The bioleaching test that inoculated with isolated MT genetic engineering bacteria indicated that MT genetic engineering bacteria has small removal rates to heavy metals. The appetency sequence of MT genetic engineering bacteria on six heavy metals are:Cd>Zn>Cr>Cu> Pb>Ni.(2) The leaching effect of heavy metals in sludge with compound Thiobacilli were investigated. The test results showed that the bioleaching realized the mutual promotion between T.t and T.f, on the premise of sufficient substrate. The compound bioleaching were better than the independing inoculated with isolated strains. Concretely speaking, the postponement stage in compound bioleaching were shortened than it with the isolated strains, the high efficiency stage in compound bioleaching were pushed up, the time to reach the highest leaching effect were shortened, the total removal rate were also improved. In the variants of substrate proportion, with the higher S(elemental sulfur) concentration, the time to reach the lowest pH were shortened. But when the S concentration were above4g/L, the contribution for the rate of decrease had not improved significantly. The test demonstrates, Fe2+can be used as the promotable substrate for S, or at least the increase of Fe2+can make up for the lack of S. For T.t and T.f, the value are different to the substrate S and Fe2+, but the coordinate application of two substrate can mutually promote each other. In these cases, they can produce the best possible results that the isolated substrate was difficult to achieve. The influences of the variants of substrate proportion to the sludge bioleaching are obvious.(3) The sludge bioleaching test based upon different additive proportion of MT genetic engineering bacteria were carried out, on the premise of fixation volume ratio (compound Thiobacilli). The test demonstrates that "compound Thiobacilli+6mL MT genetic bacteria" can use as the best inoculation amount for the test. The improvement for different heavy metal elements in the joint-bioleaching by compound Thiobacilli-MT genetic bacteria are discrepant, on the premise of the best substrate proportion and best inoculation amount. In general, the removal rates of bioleaching for most heavy metal elements were all improved, and the later stages of bioleaching showed a smoothly trend, the fluctuation phenomenon tended to weakened. This result indicated that, T.t and T.f produce more steady bioleaching function in presence of MT genetic bacteria. On the other hand, the acidification of T.t and T.f in return stimulates the activity of MT genetic bacteria and its complexing effect to heavy metals.(4) The leaching effect of every heavy metal under the different proportion of S(elemental sulfur) and FeSO4·7H2O were investigated, on the premise of the presence of MT genetic bacteria. The results indicated that the proper substrate proportion (S°and Fe2+) can improve the efficiency of the process effectively. It also promote the removal of obstinate heavy elements, and implement the save of substrate S(elemental sulfur).During the test about the change law of SO42-with variants of the substrate proportion, the results showed that the generation of SO42-was promoted when the S°dominates the substrate proportion, but the rate of transformation from MS to SO42-were circumscribed.The effect of inoculation amount (MT genetic bacteria) for the process effectively about six heavy elements were compared, on the premise of fixation volume ratio (compound bacteria liquid of T.t and T.f). The results showed that the promotability of MT genetic bacteria for bioleaching mainly shows in the initial stage and the middle stage, particularly for Cu,Ni,Cr, Pb, there effect in the initial stage were improved obviously.5%inoculation amount is the best amount for Zn, Cd.(5) A new index was presented to measure the heavy metal load factor that undertaken by unit mass of Thiobacilli or compound Thiobacilli bacteria, or joint-bacteria in this paper. The results indicated that, the bioleaching treatment rates will not be affected with the fitting heavy metal load factors. The removal rates in these tests had been improved significantly than the removal rates without MT genetic bacteria. If we estimate the treating effect according to two targets:reach the standard, and the time-consuming does not more than4days, the best heavy metal load factors which meet the requirements are:0.221,0.051,0.407.0.010,0.128,0.127mg/mg. The treating effects would be inhibited if the load factors were too high, the main manifestation was the longer postponement stage.(6) The promotability effect with EPS-e3+complex which combined with macromolecule EPS(MT) and Fe3+that were formed by MT genetic bacteria were investigated. The results showed that the binding capacity of macromolecule EPS(MT) and Fe3+is strong enough. Fe3+improved the promoting capacity for oxidizing, when the EPS-Fe3+complex was added in, and the removal rates of C,Pb received rises of1.48and1.25times respectively. And this promoting capacity is lasting, it can promote to maintain Fe3+in solution state for long time.(7) The distribution characteristics of Cu,Ni,Zn,Cd in EPS(extracellular polymeric substance) and the intracellular or extracellular features were investigated. In this paper, a concise standard for determining the distribution characteristics in and out of cells about heavy metals was put forward. Also, the metabolic effect of dissolved organic carbon (DOC) with MT genetic bacteria was discussed. The results showed that MT genetic bacteria could proliferate using the original nutrition in sludge, and DOC would be degraded accompanied by the metabolizing. As a result, this metabolizing relieved some of the restraining factor for the growing of Thiobacilli.(8) The mechanism of heavy metal speciation transformation during the joint-bioleaching by MT genetic engineering bacteria were investigated. A new classification method on orignal sludge according to the chemical speciation of sludge was presented. The sludge bioleaching mechanism of six heavy metals C,Cd,Ni,Zn,Cr,Pb were categorized in the paper. The study demonstrates that the bioleaching process of Cu,Ni,Cd are mainly dominated by direct mechanism, the bioleaching process of Zn is mainly dominated by indirect mechanism, the bioleaching process of Cr,Pb are mainly dominated by combined mechanism. Within the analysis of above mechanism, the corresponding relation between the mechanism, the classification of the orignal sludge and the dominant microflora had been clarified tentatively, the study fruit expanded the connotation of combined mechanism bioleaching.
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