Research On Intensified Separation Of Heavy Metals From Sludge In Electro-dewatering Process

As the construction of wastewater treatment plant,the output of sludge increased dramatically.As a result,treatment and disposal of sludge and become a serious problem to be solved.The water content of sludge after mechanical dewatering is 80%,which cannot be used for subsequent treatment and disposal.Sludge contains abundant nitrogen,phosphorus,potassium and organic matters,and it is suitable for land utilization.However,the heavy metals in it restrict land utilization.This paper used two intensifying methods which were citric acid acidification and power supply change to improve removal effect of heavy metals(Cu,Cr,Cd,Zn,Pb)in sludge during electro dewatering process.By this way,sludge dewatering and heavy metal removal can be realized at the same time.This experiment researched current change,electro-dewatering effect,heavy metals removal effect,speciation changes of heavy metals under three conditions which were continuous power supply without acidification,continuous power supply after acidification and intermittent power supply after acidification,and did energy consumption calculation.The analysis of current change and dewatering effect showed that acidification and intermittent power supply could increase current and dewatering rate.Under the condition of low voltage gradient(30V/cm,40V/cm),current and dewatering rate increased as acidification concentration rose.But under the voltage gradient of 50V/cm,high acidification concentration led to fast decrease of current,which was unfavourable for sludge dewatering.The highest dewatering rate(68.9%)was achieved under the condition of intermittent power supply,voltage gradient of 50V/cm,acidification concentration of 0.2mol/L.The moisture content was 42.9%.The analysis of heavy metal removal showed that under the condition of continuous power supply without acidification,the removal rate of Cu,Cr,Cd,Zn,Pb were 10.22%,16.0%,12.7%,10.7%,18.2%.The removal rate increased substantially after acidification.The optimum operating conditions were: the maximum removal of Cu,Cd,Zn were 39.65%,42.36%,22.09% under intermittent power supply,voltage gradient of 40V/cm,acidification concentration of 0.4-0.6mol/L;the maximum of Cr,Pb were39.53% and 48.46% under continuous power supply,voltage gradient of 50V/cm,acidification concentration of 0.2-0.4mol/L.The analysis of speciation changes of heavy metals showed that in the untreated sludge,Cd and Cr had the highest percentage(72.3%,62.4%)of acid soluble and reducible speciation(unstable speciation).Pb was next,of 50.5%.However,the percentage of unstable speciation of Cu and Zn were extremely low.The percentage of stable speciation(oxidizable and residual speciaton)of Cu and Zn were 88.0% and 99.1%,meaning Cu and Zn had really weak migration ability and were not easy to remove.After acidification and intermittent power supply,the percentage of unstable speciation was increased and residual speciation was decreased.But,acidification concentration and voltage gradient could not be too high.The analysis of energy consumption showed that it was decided by both voltage gradient and acidification concentration.Under the condition of highest dewatering rate,energy consumption was 218 kWh per ton of wet sludge.