Sludge Disintegration And Effects On Harmful Components In Excess Sludge Under Discharge Plasma

With the quick development of economic,lots of domestic sewage and industrial wastewater are discharged into wasterwater treatment plants.As a result,large quantities of excess sludge are inevitably generated during the biological wasterwater treatment process.Due to large amounts of harmful substances in sludge,the excess sludge would cause secondary pollution to the environment in case of improper treatment and disposal.Those traditional terminational treatment and disposal methods have been unable to meet the requirement of social development.Reduction of excess sludge from the source have received great attentions.Sludge disintegration is crucial to sludge reduction,and also beneficial to subsequent sludge treatment and disposal.The abundant organics,microorganisms and complex zoogloeal structures in excess sludge make the sludge disintegration difficult.Discharge plasma is an advanced oxidation process,and contains abundant reactive oxygen species(ROS)with strong oxidizing property,which could realize organics degradation,zoogloeal structure destruction and cell lysis.Thus,it is possible to realize sludge disintegration.Based on above analysis,the discharge plasma technology was proposed to sludge disintegration in this paper.The main work was conducted in terms of sludge disintegration performance,disintegration mechanisms,dissolution and transformation behavior of heavy metals,changes in antibiotics resistance genes(ARGs)in sludge during sludge disintegration.The main results are as follows:1.The sludge disintegration performance was evaluated in terms of disintegration rate,settling,dewatering and biochemical properties.It was concluded that discharge plasma effectively realized sludge disintegration,and significantly improved the settling,dewatering and biochemical ability,which was conducive to promoting the sludge reduction,improving the effluent quality and reducing the ecological risk and economic cost of the subsequent sludge treatment and disposal.At 20 k V,60 min,the total suspended solid(TSS)was reduced by 27.84%,the dissolved organic carbon(DOC)content was increased from 33.75 to 173.75mg L^-1,and the disintegration rate was achieved by 18.22%.At the same time,the SV₃₀ was reduced from 97%to 33%,the water content of filter cake was decreased from 81.92%to76.03%,the bound water content was decreased from 2.66 to 0.73 g g^-1,the BOD₅/COD increased by 41.33%.2.The sludge disintegration mechanisms were clarified in terms of cell lysis,extracellular polymeric substances(EPS),sludge floc structure and reactive oxygen species(ROS).It was found that the generated^·OH,O₂^·?and O₃ played important roles in the destruction of floc structure,cell lysis and EPS transformation,promoting the mass release of organics and bound water from floc and microbial cells.At 20 k V,60 min,the coliform bacteria numbers dramatically reduced 89.04%,and the relative lactate dehydrogenase(LDH)content increased1516%.The soluble EPS content significantly increased from 1.60 DOC mg/g TSS to 31.34DOC mg/g TSS,and its corresponding ratio enhanced from 16.10%to 58.51%.Large and integral sludge flocs were destroyed into smaller sludge fragments.Meanwhile,ROS could effectively degrade those macromolecular organic substances that are difficult to be bioavailable in water phase.3.The effect of discharge plasma on heavy metals in sludge were investigated in terms of dissolution performance,fraction transformance and leaching potential.It was found that discharge plasma treatment promoted the dissolution of heavy metals in solid phase,and reduced their ecological risks in sludge phase.At 20 k V,60 min,the heavy metals Cd,Cr,Ni,Pb and Zn in solid phase were decreased from 38.87,684.11,105.37,110.46 and 682.01 mg kg^-1 to 21.40,542.92,100.76,84.00 and 609.24 mg kg^-1,respectively.While,that in liquid increased.At the same time,discharge plasma treatment changed the fraction distribution.The oxidizable fraction(F3)contents of heavy metals Cd,Cr,Ni,Pb and Zn decreased from 18.48,562.35,36.96,38.66 and 107.27 mg kg^-1 to 3.16,422.49,26.79,25.87?19.88 mg kg^-1,respectively.These changes in fraction resulted in the reduction of calculated pollution index(GCF)and risk index(RI)of heavy metals in solid phase.Besides,the leaching content of metal Cr in solid phase decreased,while that for Cd,Ni,Pb and Zn increased,but all lower than the identification standard of leaching toxicity.4.The effect of discharge plasma on ARGs in sludge were researched in terms of the changes in ARGs content and microbial community structure.It was found that weak oxidative stress(low discharge voltage or short treatment time)was beneficial to the proliferation of ARGs.The contents of ARGs(aac(3)-?,bla TEM-1,Tet C and Tet W)and integron(Int I-1)increased.Strong oxidative stress dramatically decreased the ARGs contents in solid and water phase,and the reduction degree in water phase was greater.Furthermore,the Int I-1 displayed significant positive correlations to Tet C and Tet W,suggesting that the changes in Tet C and Tet W contents might be closely related to the horizontal gene transfer.Proteobacteria and Bacteroidetes were the most dominant bacteria both in solid and liquid phase at the phylum level,and Acinetobacter was the most dominant bacteria at the genus level.Proteobacteria and Acinetobacter could resist the oxidative damage from discharge plasma,and maintain a high relative abundance.The analysis of coorelation between bacteria and genes suggested that Firmicutes and Chlamydiae could significantly affect the changes in Tet C,Tet W and Int I-1.Romboutsia,Acidovorax,Parachlamydia,and Armatimonadetes?gp might be the potential host bacteria of Tet C,Tet W and Int I-1.In conclusion,discharge plasma treatment could realize cell lysis,EPS transformation and floc rupture,promoting the release of organics and bound water.As a result,discharge plasma treatment could efficiently disintegrate excess sludge,and improve the settling,dewatering and biochemical performance significantly.In addition,the discharge plasma treatment was beneficial to the dissolution and fraction transformation of heavy metals,and also reduced the ARGs contents,thus reducing the ecological risk of excess sludge.