| The vast use of batteries by human now leads to severe environmental problems and resource waste. Although people are trying to find nontoxic substitute to replace the batteries containing hazardous heavy metals, the already reclaimed batteries and many still in use ones such as Hg-containing batteries and Ni-Cd batteries have not been well disposed. The existing treatment methods are mainly pyrometallurgical and hydrometallurgical processes which have some limitations including high energy-consuming and secondary pollution, etc. In order to seek an effective, economical and environmentally friendly method to treat spent batteries, this paper introduced a biohydrometallurgical method which combined the sludge metal bioleaching and batteries treatment. The indigenous acidophilic thiobacilli in sewage sludge can grow and produce bio-sulphuric acid through the addition of energy source and the acid was used to leach the heavy metals in spent batteries. The metals in sludge were removed simultaneously to satisfy land application.The paper studied the chemical and physical characterization of spent Ni-Cd batteries through XRD, TGA and ICP-MS, etc. The results showed that the presence of diffraction lines corresponding to metallic nickel (Nio), Ni(OH)2 andγ-NiOOH in the cathode, and metallic cadmium (Cdo), Cd(OH)2 in the anode as well as some KOH. The content of Ni and Co in cathode was 41.7% and 5.1% respectively. The content of Cd in anode was 64.8%. The estimated Cd(OH)2 accounted for 62.2% in anode and Ni(OH)2 binding with minor Co(OH)2 accounted for 57.7% in cathode.The biological acid culture was obtained through the acidification of sewage sludge. The metals in sludge were leached simultaneously with microbial production of sulphuric acid. At the sludge solid concentration of 20～25g L-1, the solubilization efficiency of Cd, Mn, Cu, Zn, Mg, and Al was highest at 98～100%, and that of Ca was higher at about 90%, Cr 76.36%. As for Pb, the solubilization efficiency was lowest at 20～50%.The battery electrode materials were leached using the biological acid and ordinary 0.2mol L-1 H2SO4 respectively under the same conditions. The results showed that as for the dissoluble metal Cd, the leaching efficiency by biological acid and chemical acid was similar. But for the acid-insoluble materials, the biological acid seems better. Moreover, the flasks test showed that there are some residues are very difficult to dissolve even in strong acid.A continuous flow two-step batch leaching system consisting of an acidifying reactor and a leaching reactor was set up to achieve the continuous acid production and bioleaching of batteries. The acid supernatant produced in the acidifying reactor by the microorganisms was conducted into the leaching reactor to dissolve electrode materials.The optimizing test of process parameters showed that the optimum sludge retention time (SRT) in acidifying reactor was 4d and hydraulic retention time (HRT) in leaching reactor was 1～3d. The pH in acidifying reactor was below 1.86 and the population of thiobacilli was 2.8×10-7cfu mL-1. The pH in leaching reactor decreased from the initial 5.0 to equal to influent during the 30～40d leaching. The complete leaching of Ni, Cd and Co cost about 25, 18 and 30d respectively with HRT=1d. The leaching behavior of the metals Ni, Cd and Co was different. Cd and Co can be leached mostly in the begin 6 or 7d with pH 3.0～4.5. The leaching of Ni showed two stages. The maximum dissolution of Ni reached at the 6th day and the 15th day with the pH 3.0～4.0 and 2.5 respectively. The former was the dissolution of Ni(OH)2 and the latter was the dissolution of metallic Ni. When FeSO4·7H2O was the substrate, it was difficult to decrease the final pH to lower than 2.2, while with substrate S0 the final pH was below 1.0. But the iron-oxidizing system has a stronger buffering capacity of pH than the sulphur-oxidizing system. As for the process load, the complete leaching of metals Ni, Cd and Co in 8 spent Ni-Cd batteries required 30～40d with 1L d-1 acid.In this process, the activity of microorganisms and acidification rate were the key to increase metals leaching efficiency. Factors affecting the sludge acidification rate were studied through flasks experiments and the results showed that acidophilic thiobacilli have wide adaptability of environment and the favourable acidification can be obtained under appropriate conditions in each kind of sludge including primary settling tank sludge, secondary sedimentation tank sludge and mixed sludge. The amount of substrate addition depended on the sludge solid concentration. In general, the moderate sludge solid concentration for bioleaching was 20～25g L-1 and the corresponding S0 or FeSO4·7H2O addition was 1～1.5% (w/v) or 4～6g L-1 respectively. The growth lag phase was affected by surface area of sulfur particles and attachment of microorganisms.Comparison of acid production by sulfur-oxidizing bacteria in sewage sludge and pure culture was conducted. The results showed that sludge has a better ability to acclimatize itself to the change of ambient environment than a pure culture. The more disadvantageous the ambient environment is to the sulfur-oxidizing bacteria, the more obvious advantage of the sludge than pure culture. Even though the initial pH was 10.0, the sulfur-oxidizing bacteria began to grow after 7d lag phase while in pure culture it was inhibited completely. At low (10, 20℃) and high (50℃) temperature, the rate of pH decrease and SO42- production was higher in sludge than in pure culture. The addition of glucose made pH reduce from neutral pH to pH 3.0～3.5 in the first 2d but no sulfur was oxidized. The sludge acidification stopped and almost no SO42- produced when glucose addition was 300mmol L-1. But when it was 100mmol L-1, there was no effect on sludge and 50% inhibition on pure culture. The tolerance concentration of small molecules acids, chloride and nitrate was double in sludge than pure culture.The chemical and biological mechanisms involved in the acidifying reactor and the leaching reactor were analyzed. Only some benzene ring containing and long chain organic acids are found and short carbon chain ones can not be found from acidified sludge by GC-MS. It can be observed from the scaning electron microscopy of acidified sludge that the thiobacillus in it attached on the impurities. In the leaching reactor, there are great populations of live thiobacillus although the heavy metals concentration was high. When the HRT was 6d, the amount of thiobacillus was 6.2×106 cfu mL-1. Meanwhile, a strain accounting for the fast reduction of pH in the leaching reactor was isolated and sequenced. It was identified to be 100% similar to Acidithiobacillus ferrooxidans strain Tf-49 based on 16S rDNA sequence analysis. The relevant phylogenetic tree constructed indicates that the strain should be classified into genus Acidithiobacillus ferrooxidans.The recovery of heavy metals solution was achieved by producing ferrite through coprecipitation. The conditions were optimized and the results showed that under the conditions of normal or little higher temperature, pH=11～11.5, Fe/Cd (mol/mol)=16, 0.3% (v/v) of H2O2 (30%) added, the ferrite product had a preferable magnetism and the heavy metals concentration in effluent could be decreased to below the drainage standard.
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