Removal Of Heavy Metals From Sewage Sludge Leached Organic Acids And Cleanup With Acids By Anaerobic Microbials

Sewage sludge contains some metal elements, such as As, Cd, Pb and Zn, and it is critical to removal metals from sewage sludge in order to reuse it. In this study, the contents and environmental behavoirs of heavy metals in sewage sludge from three cities, such as Changsha, Zhuzhou and Changde in Hunan province were investigated. Then the effect of pH value, leaching time and solid-to-liquid ratio on leaching heavy metals from sewage sludge with the low molecular weight organic acids was studied through a series of flask experiments, providing data basis for the sludge acid self-purification. And the production of volatile fatty acids (VFAs) in anaerobic system was investigated and the conditions, such as pH, inoculums, C/N ratio, temperature, sludge concentration and inhibitor CHC13, were optimized. Finally, the leaching rate of heavy metals in sewage sludge was studied. The main conclusions were as follows:The total content of Cd in sewage sludge ranged from1.43to260mg/kg, the concentrations of Cd in part sludge samples were beyond the permitted values of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002). The contamination of sewage sludge was mainly by Cd. The speciation of As and Pb in sludge mainly existed in reducible form, while that of Cd was mainly in reducible form, of Cu was mainly in reducible and oxidizable forms, of Ni and Zn was mainly in acid soluble and reducible forms, respectively. The results showed that heavy metals in sewage sludge were active. Based on the potential ecological risk index (RI) value for heavy metals in sewage sludge from industrial wastewater treatment plant, which it is higher than1200, the potential ecological risks are serious and should be innocuity.The results of the organic acids leaching experiments showed that the leaching effect of heavy metals in sewage sludge is available. With conditons, including2mol/L citric acid solution and solid-to-liquid ratio of1:20for48h, the extraction percentages of As, Cd, Ni, Pb and Zn from sludge reached20.8%,72.6%,82.0%,93.8%and98.1%, respectively. Based on the acetic acid orthogonal experiment, the leaching precentages of As, Ni and Zn in sludge were mainly affected by pH and solid-to-liquid ratio, while that of Cd was mainly affected by pH. Under the optimized condition, which is pH1, solid-to-liquid ratio1:30, leaching time24h and acetic acid concentration1mo/L, the leaching precentages of As, Ni, Pb and Zn reached28.5%,66.6%,17.1%and91.6%, respectively, and showed that acetic acid could effectively removal the heavy metals from sewage sludge.The experiment of acidogenic fermentation showed that the concentration of volatile fatty acids(VFAs) was arrived to0.087g/gVS(volatile solid) at intial pH10.0,0.118and0.082g/gVS was saved at the optimum contraction of inhibitors CHCl30.05mmol/L and Fe2+2mg/L, respectively. The results of Plackett-Burman test deeply showed that pH had a more significant effect on the acidogenesis, followed by C/N ratio and mass of inoculums. Based on the behalf of the optimized parameters, pH11.2, C/N ratio93.5:1and inoculums25.4%according to response surface analysis, the concentration of VFAs was arrived to0.184g/gVS, and the leaching precentage of Ni and Zn reached22.0%and32.0%, respectively. Therefore, the heavy metals in sewage sludge was proved could be removed with anaerobic acidification, but it should be carefully optimized in future.