Investigation Into The Strengthened Dewaterability And Its Mechanism Of Sludge And Dredged Sediment By Hydroxyl Radical Oxidation And Iron Salt Re-flocculation Conditioning

With the rapid development of social economy and increase of population,a large number of industrial sewage and domestic sewage are discharged,and the sewage treatment process inevitably produces lots of sludge.In addition,some of the sewage is discharged directly into the river channel,which sharply deteriorates the ecological environmental quality of the water body,and the pollutants in the sewage gradually accumulate in the bottom mud of the river channel.At present,environmentally dredging process is the most effective method to solve the river polluted sediment.The wide application of this technology produces a large amount of dredged sediment.Sludge and dredged sediments not only possess a large number of pathogenic microorganisms,parasites and heavy metals,but also large volume,high water content,poor dewaterability.Without proper treatment and disposal,their would cause secondary pollution to the environment,threatening to human health.Therefore,it is essencial to effectively condition these typical sediments for their volume reduction and harmlessness.In particular,reducing water content and volume of sludge and reducing the pollutant content are the bottleneck in treatment and disposal of sludge and dredged sediment,therefore it is urgent to develop economical,effective but environmtal-friendly approach for the reduction of sludge and dredged sediment.Based on this,this paper mainly investigated the effects of a series of conditioning processes based on hydroxyl radical oxidation and iron salt flocculation on dewaterability of sludge&dredged sediment and then explored the relevant mechanisms.(1)This paper firstly explored the mechanism of Peroxide/Zero-valent iron(Fe⁰)promoting dewaterability of sludge.Dewatering results exhibited that under addition of 0.6 g Fe⁰/g TSS and 0.08 g peroxide/g TSS,capillary suction time,specific resistance to filtration,and time to filtration of sludge was 50.7%,41.4%,and 54.4%of that in the control,respectively.In this condition,water content of sludge cake decreased from 91.3%±0.5%(the control)to 68.6%±1.3%.The mechanism explorations illustrated that the enhanced dewaterability was mainly induced by function of·OH and Fe(II)/Fe(III)species during Peroxide/Fe⁰ pretreatment.·OH decreased the polysaccharides,proteins,and humic acids in extracellular polymeric substance(EPS),then injured the cytoderm&cytomembrane through the releases of lactate dehydrogenase and N-acetylglucosamine,and further facilitated the decrease of intracellular substances,which disengaged the water trapped in EPS/intracellular substances.In addition,the cell lysis caused by·OH facilitated forming macro-pores.Moreover,·OH modified the conformational structure in extracellular/intracellular proteins,which strengthened the hydrophobicity of extracellular/intracellular proteins,causing the discharge of unbound water and sludge flocculation.Meanwhile,Fe(II)/Fe(III)species facilitated aggregating the denatured sludge particles.(2)To solve the problem of unstable and difficult transportation of liquid hydrogen peroxide in hydrogen Peroxide/Zero-valent iron conditioning technology,this paper sequentially developed Fe(II)catalyzing sodium percarbonate to dewater sludge and unraveled its potential mechanism.The results showed that as sludge was conditioned with 20 mg/g TSS Fe(II)and 50 mg/g TSS SPC for 1 h,water content of sludge cake(press filtration)was merely 55.6%±1.4%,which was 35.6%lower than that in the control.The mechanistic insights suggested that four intermediates or products(i.e.,·OH,H₂O₂,Fe(II),and Fe(III))yielded in the Fe(II)/SPC process lie behind the advanced sludge dewaterability,and·OH and Fe(III)were the two major contributors.We found that·OH collapsed and fragmented extracellular polymeric substances,damaged cell wall and permeabilized cytoplasmic membrane,and transformed conformation of the proteins secondary structure via affecting the hydrogen bond maintaining?-helix and disulfide bond in cysteine residues while Fe(III),the oxidization product of Fe(II),decreased the surface electronegativity and water-affinity surface areas of sludge flocs.As a consequence,the bound water release,flocculability,surface hydrophobicity,and drain capability were strengthened whereas the compact surface structure,colloidal force,network strength,gel-like structure,and apparent viscosity of sludge flocs were weakened.In addition,Fe(II)/SPC process also reduced the recalcitrant organics and fecal coliforms,which facilitated land application of dewatered sludge.The findings acquired in this work not only deepens our understanding of Fe(II)/SPC-involved sludge treatment process but also may guide engineers to develop both effective and promising strategies to better condition sludge for dewatering in the future.(3)In order to further achieve the recovery of catalyst and reduce the possibility of hydroxyl radical quenching in the presence of large amount of ferrous iron,this paper further exploited sodium percarbonate(SPC)expediting zero-valent iron(ZVI)corrosion(SPC/ZVI)process to conditions sludge for dewatering and explored the possible mechanism.Results showed that SPC/ZVI conditioning decreased water content of sludge cake from 90.5%(control)to 69.9%in presence of 0.10 g/g TS SPC and 0.20 g/g TS ZVI.Mechanistic research indicated that the enhanced sludge dewaterability mainly resulted from·OH and Fe(III)/iron polymers yielded in SPC/ZVI.·OH disrupted extracellular polymeric substance(EPS),damaged cytoderm&cytomembrane,and lysed intracellular substances,unbinding the bound water.Meanwhile,the breakage and inactivation of microbe by·OH prompted the production of macro-pores in sludge.·OH adjusted the conformation of extracellular/intracellular proteins by intervening in the H-bonds and S-S bonds,availing the hydrophobicity and slight flocculation of sludge.·OH further advanced the despiralization of?-helix to?-sheet in sludge pellets,benefiting cell-to-cell aggregation.Additionally,Fe(III)/iron polymers from ZVI corrosion accelerated to gather sludge and maintained its floc structure.Consequently,SPC/ZVI conditioning not only adjusted the apparent natures of sludge and its EPS characterization but also the features of residual pellets,which increased drainable water and facilitated their discharge but weakened the resistance of flow&compressive forces under filtration.(4)After the above researches,the authors explored a series of sludge conditioning technologies based on hydroxyl radical oxidation and iron re-flocculation.In view of the huge generation of dredged sediments(DS)from dredging process,this work further apllied one of the promising SPC-based technologies to achieve the volume reduction of dredged sediments.The work first investigated Fe(II)/SPC processing DS to advance their solid-liquid separation from filtering feasibility,operational mechanism,technic reinforcement to potential implication.16 mg Fe(II)/TSS&60 mg SPC/TSS treatment elevated solid content of DS from 25.7%to55.7%.·OH and Fe(III)with their hydrolyzed polymers,from Fe(II)/SPC system,are mainly lying behind the improved solid-liquid separation.Detailedly,the dilapidation of extracellular polymeric substances(EPS)with the destruction of biomolecules in EPS was completed by·OH invasion,which propelled the decrease of EPS-bound water,hygroscopicity,surface energy,and app arent stickiness in DS.Additionally,the extracellular/intracellular protein configuration might be rearranged under the role of·OH,with the increments of?-sheet&random coil but the decrement of?-helices.Such a conformational status facilitates the loosing trend of protein,which elevated hydrophobicity,the escape of unbound moisture,and the cell-to-cell aggregation but declined surface energy in DS.Simultaneously,aggregation of DS was strengthened,as Fe(III)and their hydrolyzed polymers promoted the relief of electrostatic repulsive-forces and the squeezing of double-electric layers.Furthermore,the gathered DS could be held by integration of Fe(III)with-COOH and-OH.Additionally,Ca O strengthened the filtering velocity and extent of Fe(II)/SPC-treated DS.In summary,this work developed a series of hydroxyl radical oxidation&iron flocculation-based conditioning processes to elevate dewaterability of sludge&dredged sediments and explored the potential dewatering mechnisms,which may provide basic data reference and theoretical guidance to choose sludge&dredged sediments conditioning processes.