Study On Migration And Transformation Behavior Of Typical Pollutants In Sludge Dehydration And Pyrolysis Carbonization Process

Activated sludge method is the mainstream process of sewage treatment at present.During the operation of the process,a large amount of excess sludge will be generated.The excess sludge generally has a high water content and is difficult to dewater.Therefore,the treatment and disposal of excess sludge has become a major problem in the environment.The sludge also contains a large amount of toxic and harmful substances,such as phosphorus,heavy metals,antibiotic resistance genes,new coronaviruses and microplastics,etc.The migration of pollutants will cause huge environmental risks,which is also an important problem restricting the utilization of sludge resources.In view of the difficulty of sludge dewatering and the mobility of pollutants in sludge,this paper carried out research on the migration and transformation behavior of typical pollutants in the process of sludge dewatering coupled with pyrolysis and carbonization,and deeply explored the changes in sludge physicochemical properties and the interaction of conditioners and extracellular polymers during sludge dewatering.The paper revealed the changes between the occurrence forms of phosphorus and heavy metals in the process of sludge dewatering and pyrolysis,analyzed the migration and transformation behavior of pollutants in the process of reduction and recycling,and provided theoretical support for the realization of sludge reduction and resource utilization.The main findings are as follows:（1）Both PAC and Fe Cl₃conditioning can significantly improve the dewatering performance of sludge.The optimum dehydration effect of PAC was basically achieved at the dosage of 0.06 g/g TSS;the optimum dosage of Fe Cl₃conditioning was 0.12 g/g TSS.PAC has a higher charge density,and the improvement efficiency of sludge dewatering efficiency is significantly higher than that of Fe Cl₃conditioning.The dehydration effects of PAC are mainly electric neutralization and compression of the electric double layer;Fe Cl₃conditioning has the effect of adsorption and bridging.Fe Cl₃can coordinate complex reaction with the organic matter on the surface of sludge particles.Fe Cl₃has a stronger effect on the transformation ofα-helix toβ-sheet in the secondary structure of proteins than PAC,thereby reducing the reduction of hydrophilic groups on the sludge surface,weakening the ability of water-binding of organic matter on sludge particles,and improving the dewatering performance of sludge.（2）During the sludge dewatering process,the organic phosphorus in the solid phase migrates to the liquid phase.In the solid phase,the proportion of apatite inorganic phosphorus in the sludge conditioned by PAC increased,and the addition of PAC resulted in a decrease in the p H value of the system,and phosphorus was more inclined to combine with Ca²⁺ions to form calcium phosphate compounds in an acidic environment.In the sludge conditioned by Fe Cl₃,the increase of inorganic phosphorus is mainly in the form of non-apatite inorganic phosphorus.Due to the addition of a large amount of iron ions,phosphorus preferentially combines with iron ions to form the corresponding complexes.During the dehydration process,the proportion of the eight heavy metal residue states were change basically unchanged,and the oxidizable heavy metals were transformed into exchangeable and reducible heavy metals.According to the PMI index analysis,only the potential mobility of Pb and Ni in the mud cake conditioned by Fe Cl₃decreased,and the potential mobility of other heavy metals and the eight heavy metals conditioned by PAC were enhanced.In the process of dehydration,the destruction and release of organic matter destroys the bond between heavy metals and organic matter.After the heavy metals are released,they are bound to inorganic ligands or exist in a free ionic state.The two inorganic conditioners do not have the effect of solidifying heavy metals during the dehydration process,but will lead to an increase in the environmental risk of heavy metals.（3）After the pyrolysis and carbonization of the sludge,the content of organic phosphorus is less than 10%.Since most of the organic matter is decomposed by high temperature during the pyrolysis process,the organic phosphorus is converted into inorganic phosphorus.In the pyrolysis process of sludge with two conditioners added,the proportion of non-apatite inorganic phosphorus increased,and phosphorus was mainly combined with iron and aluminum compounds during the pyrolysis process.During the pyrolysis process,with the increase of the pyrolysis temperature,the non-apatite inorganic phosphorus in the sludge biochar decreased.During high temperature pyrolysis,phosphorus was more inclined to combine with calcium to form more stable compounds.The eight heavy metals are basically in the form of residues in the process of pyrolysis and carbonization.The residual content of As,Cd,Cu and Zn at 600°C was higher than 800°C,and the excessive pyrolysis temperature caused a part of heavy metals to be transferred to the pyrolysis gas and lost.During the pyrolysis process,the solidification effect of Cr improved with the increase of the pyrolysis temperature.The solidification effect of Fe Cl₃for Cd element is better than that of PAC;the stabilization effect of PAC for Cu is stronger than that of Fe Cl₃during the pyrolysis process.The PMI of most metals at the optimum pyrolysis temperature is basically below 5%,and the two conditioners have a good curing effect on heavy metals in the process of pyrolysis and carbonization.