Study On Removal Of Heavy Metals And Stabilization In Agricultural Utilization Of City Sewage Sludge

The main research objective in this paper is the sludge generated by UrumqiWater （Group） Co.Ltd in the summer and winter of2010. The approach to theremoval and stabilization of main heavy metals Cu、Zn、Pb、Cr leads to a discussionof the types and process of the removal agent and stabilizing agent. The resultsindicates that:（1） The sludge consists a high percentage of organic matter content which can beused as a soil improver according to the analysis of the its basic properties.（2） According to heavy metals content and chemical speciation analysis in sludge,we can find that the Cu, Zn, Pb, Cr are the main heavy metal pollutants whilethe highest content of Cu is700mg/kg, which is mainly distributed in the unstablestate; correspondingly, the highest content of Zn and Pb are1290mg/kg and400mg/kgrespectively, which is mainly distributed in the residual fraction. The content ofCr is not high, but mainly located in unstable state, so it has a high possibility to beabsorbed and utilized by plants that is greatly harmful. The content of Cd thatdistributed in the unstable state is low. The content of Cu, Zn, Pb in sludge exceed ourcontrol standards of pollutants in agricultural utilization of City sewage sludge, whilecontent of the other heavy metals are lower than the agricultural standard limit.（3） Different chemical removal agent, such as, ferric sulfate, phosphate, EDTA,citric acid, SDS, are chosen for the study on the removal of Cu, Zn, Pb, Cr in sludge.The results show that ferric sulfate and phosphate have the best effects.The order of extraction capacity of copper, Lead is: Ferric sulfate> phosphate>EDTA> citric acid>SDS; the order of the extraction capacity of Zinc is: ferricsulfate> phosphate> citric acid> EDTA> SDS; The order of the extraction capacityof Chromium is: phosphate> Ferric sulfate> citric acid> EDTA>SDS.The optimum conditions of Fe³⁺handling experimental sludge: concentration is6g/L, solid-liquid ratio is1:50, reaction temperature is45℃, reaction time is3d. Theremoval rate of copper, zinc, lead, chromium is76.22%、96.28%、30.65%、52.57%on the optimum conditions.The optimum conditions of phosphate handling experimental sludge: theconcentration is10%, solid-liquid ratio is1:12.5, liquid–solid ratio is1:1.2, reactiontemperature is40℃, and reaction time is24h. The removal rate of copper, zinc, lead,chromium is46.34%,64.86%,21.52%,72.67%on the optimum conditions.（4） The study on stabilization of Cu, Zn, Pb, Cr is conducted by using bentoniteand humic acid.The optimum conditions of Xinjiang summer street bentonite handlingexperimental sludge: Room temperature, duration of oscillation is12h, leaching liquorpH value is around6, dosage and dry sludge mass ratio （g/g）is1:12.5. the stable rateof copper、zinc、lead、chromium is92.67%、98.08%、100%、97.6%on the optimumconditions. After bentonite stabilizing, heavy metals in the sludge are mainlydistributed in the residual, a small amount of the distribution in sulfides and organicform, while the distribution in unstable state less content.The optimum conditions of humic acid handling experimental sludge: Roomtemperature, duration of oscillation is8h, leaching liquor pH value is around6,dosage and dry sludge mass ratio （g/g）is1:5. The stable rate of copper、zinc、chromium is89.4%、96.36%、89.4%on the optimum conditions. After humic acidstabilizing, heavy metals in the sludge are mainly distributed in the residual, a smallamount of the distribution in sulfides and organic form, while the distribution inunstable state less content.（5） Based on above research and economic analysis, it demonstrates that the bestsolution for processing heavy metals in experimental sludge is to use bentonite.Heavy metal stabilization process can be combined with sludge compost technology.Under the optimum conditions, the costs of processing1tons of sludge by bentonite isabout60RMB.