Heavy Metal Removal From Sewage Sludge By Bioacidification And Fenton-like Oxidation

Wastewater treatment leads to the generation of a large amount of excess sludge. Treatment and disposal of excess sludge is difficult and costs up to 60% of the total cost of wastewater treatment plants. As the sustainable utilization, there is a great interest in using treated sludge as fertilizer or soil amendment material for land. Sludge, however, is contaminated with a high quantity of toxic heavy metals, and this fact has become an obstacle to sludge reuse. Therefore, heavy metal removal from sewage sludge is a big problem to be solved.Bioacidification and Fenton-like oxidation was combined to remove the heavy metals in the sewage sludge. The orthogonal bioacidification results show that the factor order of influencing pH reduction is: FeSO4·7H2O dosage > Sludge solid content > Sulfur powder dosage. The optimal bioacidification condition is attained as the sludge solid content of 2%, Sulfur powder dosage of 3 g/L and FeSO4·7H2O dosage of 15 g/L. Under this condition, pH reduces to 2.52 in 2.5d, which fits for the pH requirement of Fenton-like oxidation. Low pH, long oxidation time and high H2O2 dosage benefit the further heavy metal solubilization. Zn and Cd are mostly leached during the bioacidification and their solubilization denpends on the pH reduction, which may be mainly caused by the reason that the main fraction of Zn and Cd is unstable exchangeable, carbonate-bound, Fe-Mn oxide forms. Cu and Pb are mostly leached by Fenton-like oxidation sludge, because a large part of Cu and Pb exists in stable organic and sulfide and residual forms. After bioacidification and Fenton-like oxidation the content of Cu, Zn, Pb and Cd can reach 131, 134, 574 and 4.8 mg/kg respectively, and meets the national control standards for pollutants in sludge for agricultural use GB18918-2002.This paper mainly researched the effects of sludge solid concentration, the concentration of sulphur powder and the ferrous salt and the sludge reflux ratio four factors on excess sludge bioacidification, and optimized conditions of excess sludge bioacidification, pH and ORP were main examined indexes. The results show that the optimized conditions of bioacidification are: the excess sludge solid concentration 4%, the concentrations of sulphur powder and FeSO4?7H2O are both 8g/L, the sludge reflux ratio 0.3. Under the optimized conditions to carry on the experiment of bioacidification, the cycle was greatly shortened to only 1.25d. Compared original sludge with treated sludge's suspended solid (SS) concentration, volatile solid (VSS), sludge subsidence ratio (SV), discovered that SS didn't change; SV increased to 98% from original 70%, the sludge's free setting performance became worse; VSS (expressed by mass percent) declined to 39.34% from original 62.56%, its elimination rate reached 37.12%, the stability of excess sludge improved greatly.