Research On Separation Of Heavy Metals From Sewage Sludge By Electrodialysis

In recent years,with the rapid development of China’s economy and the improvement of government and citizens’awareness of environmental protection,the construction of sewage treatment plants in China is fast,and the amount of sludge is increasing every year.Sludge is rich in nutrients and can effectively enhance soil fertility.Therefore,land use has become a hot topic in sludge disposal.Heavy metals in sludge are an important factor restricting the use of land.In this paper,three slot electrolysis cell was used to treat the sewage sludge,and the best process conditions of heavy metal removal by electrodialysis were explored.The experiment is divided into two parts,electrodialysis treatment of aerobiotic sludge and anaerobic pretreatment sludge.（1）the experimental results of electrodialysis treatment of aerobiotic sludge show that the removal rate of heavy metals will be increased by prolonging the reaction time,reducing the pH and increasing the reaction voltage,and the sludge acidification can make the heavy metals in the sludge from the stable form to the unstable state and increase the removal rate of heavy metals.In the treatment of acidified sludge by electrodialysis,after the preacidification in pH=2.0 for 2h,the unstable content of Zn increased by 34.64mg/kg,while the stable state content decreased by 103.80mg/kg.The removal rates of Zn,Cu,Cr and Ni were 58%,31%,34%and 53%respectively under the condition of voltage 30V and reaction time 30h.After the sludge was treated with 2h,pH=2.83 NaClO and HNO₃ mixed solution,the unstable state content of Zn increased by 32.18mg/kg,while the stable state content decreased by102.38mg/kg.The corresponding removal rates were 69%,34%,37%and 56%under the voltage 30V and the reaction time 14h.（2）the experimental results of anaerobic pretreatment sludge treated by electrodialysis found that prolonging the reaction time and reducing the sludge pH were beneficial to the improvement of the removal rate of heavy metals.The acidified anaerobic pretreatment sludge was treated by electrodialysis.After the sludge was acidified with pH=2.33 for 2h,the unstable content of Zn increased by 83.13mg/kg,while the content of stable state decreased by112.18mg/kg.Under the condition voltage 30V and 30h,the removal rates of Zn,Cu,Cr and Ni were 46%,13%,23%and 40%respectively,and the Zn content in the unstable and stable states decreased 112.35mg/kg and 48.47mg/kg respectively.（3）anaerobic digestion will increase the content of heavy metals in sludge and the proportion of stable heavy metals,which is not beneficinal to the removal of heavy metals,so anaerobic digestion is not suitable to be used as a pretreatment method for sludge.After the anaerobic pretreatment of 55d,the content of Zn,Cu,Cr and Ni in the sludge increased respectively 122.08mg/kg,42.19mg/kg,22.77mg/kg,23.48mg/kg,and the proportion of stable states increased by 9%,4%,5%,15%respectively.