Synergistic Effect Of Bioleaching Combined With Ultrasonic Treatment In Sludge Dewatering And Heavy Metals Removal

With the rapid development of urbanization in China,the problems of sludge treatment and disposal is becoming more and more serious.Sludge is a colloid with high water content and complex properties,and it also carries many toxic and harmful substances such as heavy metals and pathogens.If it can not be disposed safely,it is easy to cause secondary pollution and harm to the environment.How to reduce the water content and heavy metals content of sludge economically and efficiently and realize resource utilization is a difficult problem in sludge treatment and disposal.The main factors affecting sludge dewaterability and heavy metals removal include sludge particle structure,acidification effect and extracellular polymeric substances(EPS).Ultrasonic technology is clean and efficient,which can promote EPS cracking,reduce the particle size of sludge,and release heavy metals,thus improving sludge dewaterability and release rate of heavy metals.The process is easy to be carried out under acidic conditions,however,a large amount of inorganic acid will make the operation cost expensive and easy to cause pollution.In order to solve this problem,bioleaching is used to produce acid,which provides a suitable low pH environment for ultrasonic treatment.Meanwhile,the cavitation effect of ultrasound can strengthen the acidification and oxidation process of bioleaching,which can further oxidize cell wall and EPS,trigger the decomposition and hydrolysis of EPS,promote the transformation of bound water into free water,and accelerate the process of heavy metals entering the liquid phase.Based on this,the effects of bioleaching combined with ultrasound treatment(B/US)on sludge dewatering performance and heavy metals removal were discussed with sulfur powder dosage,FeSO4·7H2O dosage,ultrasound treatment time and power density as influencing factors,and the mechanism was analyzed.The results showed that after B/US treatment,sludge dewatering performance and heavy metals dissolution rate were significantly improved.When the dosage of elemental sulfur and FeSO4·7H2O were 3g/L and 4g/L,ultrasound time was lmin,and the power density was 3.80w/mL,the cake moisture,the specific resistance to filtration(SRF)and the capillary suction time(CST)of sludge cake were reduced by(5.56 ± 0.34)%,(70.09± 0.28)and(30.92 ± 0.43)%,respectively.The dissolution rate of heavy metals such as Cu,Zn,Cr,Ni also increased gradually,and the contents of Cu,Zn,Cr and Ni in the sludge met the requirements of agricultural sludge.After B/US treatment,the content of protein(PN)in the tightly bound EPS(TB-EPS)fraction reduced,a large number of filamentous structures appeared in sludge flocs,the particle size of sludge decreased significantly,the surface of sludge was fluffy and porous,forming internal pores and channels for sludge dewatering,and the sludge dewaterability was remarkable improved.By analyzing the changes of the total organic carbon(TOC),the chemical oxygen demand(COD)and the ammonia nitrogen(NH4+-N),it was found that the increase of the concentration of the above substances in the soluble EPS(S-EPS)was conducive to the dissolution of more carbon and nitrogen sources.Analogical analysis showed that the order of dissolution of the four heavy metals was Zn>Cu>Cr>Ni.This indicated that the dissolution rate of heavy metals in sludge was not only related to the forms of heavy metals and sludge environment,but also affected by the pH and ORP value of sludge system,while Zn was mostly sensitive to the changes of pH and ORP.By analyzing the mechanism of B/US treatment,it was found that the biogenic acid production can induce the decomposition and hydrolysis of EPS,and had the effect of biological oxidation flocculation and heavy metal leaching;the ·OH generated by ultrasonic cavitation can strengthen the leaching process,further oxidize and break down the cell wall and EPS,the protein content in TB-EPS was significantly reduced,the sludge particle size was significantly decreased,and the structure of sludge flocs was more damaged.These were beneficial to the conversion of bound water to free water and the dissolution rate of heavy metals.Therefore,B/US had the synergistic effect,which could greatly improve sludge dewaterability and heavy metals dissolution rate,and is conducive to sludge reduction,reuse and recycle.