Urban Sewage Sludge, Heavy Metal Reduction To Strengthen

It is a tough problem that much municipal sludge which is solid, half solid or liquid could be produced during the process of sewage treatment, and the sludge always contains pollutants such as heavy metals. Along with the advancement of modernization, the sludge increasing more and more disturbs the sewage plant. If wrongly treated, it also could jeopardize the eco-safety and human health, and result in the waste of resources and economy. There are lots of nutrients such as nitrogen, phosphorus and kalium, which make it possible for the sludge to reuse. But there are also large concentrations of heavy metals in the sludge restricting the sludge reuse. Therefore, it is urgent to solve the problem that how to reduce the heavy metals in the sludge and make the concentrations fulfill the reuse criteria. The experimental study is conducted to remove and reduce the poisonous heavy metals from municipal sludge, especially those elements surpassed the national criteria.Three organic acid were used as the extraction liquid in this experiment. The removing effectiveness of heavy metals including cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) in sewage sludge by applying citric acid, oxalic acid and acetic acid was examined, speciation changes and bioavailability of these heavy metals before and after acid application was analyzed, and concentrations of NO₃^--N and NH₄⁺-N in extracted liquid and sludge was also investigated.The results showed that the removing rate of these heavy metals excepting Cu in sewage sludge increased with an increase in reaction time and the concentration of acid solutions. Among the three organic acids used in this experiment, citric acid was more effective to extract heavy metals from sewage sludge. After a 24-hour reaction, 76.0% Pb and 92.5% Zn could be extracted from the sludge by citric acid. The removing rate by oxalic acid and acetic acid was small. And after 7 hours, citric acid extracted a majority of Pb and Zn, up to 52% and 74.2%, respectively. The fraction extracted is mostly the metals in the phase of stability, and the percentage of the metals in the phase of instability increased at different levels. After the reaction, lots of NO₃^--N and NH₄⁺-N resolved into the extracted liquid. Compared to the control which sludge reacted just with distilled water, the concentration of NH₄⁺-N in extracted liquid increased much by applying organic acid, while the concentrations NO₃^--N decreased. Because of the transformation of nitrogen, the concentrations of NO₃^--N and NH₄⁺-N in sludge was still high after reaction. There was 2.8mg·L^-1 NO₃^--N and 888.1 mg·L^-1 NH₄⁺-N in the extracted liquid after sludge reacting with 0.5 mol·L^-1 oxalic acid while the extracted rate of heavy metals is low, so it is a good"liquid fertilizer"for the extracted liquid to reuse.