Study On Deep-dewatering Technologies And Mechanism Of Sewage Sludge By Chemical Conditioning With Fenton's Reagent And Physical Electro-dewatering Process

Sewage sludge,a by-product of municipal wastewater treatment,is very difficult to be dewatered due to the highly hydrated nature of extracellular polymeric substances(EPS),which limited sludge treatment and disposal.Recently,many studies have reported that an advanced oxidation process(AOP)with Fenton's reagent can effectively degrade EPS,and improve sludge dewateribility.However,most of the current stuides focused on investigating Fenton process for wastewater treatment,with its feasibility on sludge dewatering and the related mechanism remained less clear.Moreover,the sewage sludge(water content of about 80 wt.%)with mechanically dewatereing(centrifuge or belt filter press)still needs to be further dewatered,which necessitates the investigation of an advanced technology for sludge deep-dewatering.Therefore,this thesis focuses on investigating the mechanism of Fenton process for the thickening sludge(water content of 97 wt.%),and the feasibility of electro-dewtering(EDW)technique for mechanically dewatered sludge(water content of 80 wt.%).The main findings are as follows:1.The optimum dosages of Fenton's reagent and qucik limeSewage sludge of the wastewater treatment plant in Wuhan was used in this study.The dosages of composite conditioners,i.e.initial pH of sludge,Fe(?),hydrogen peroxide(H2O2),and quick lime(CaO),were optimized through response surface methodology(RSM)aimed at controlling the water content of sludge cakes less than 55 wt.%and pH of filtrate was 6.5-7.0.The results showed for the raw sludge(pH of 6.76),while the water content of the dewatered sludge cakes was 55.82 ± 0.59 wt.%,and pH of the filtrate was 6.6 ± 0.2,the optimum dosages were 47.93 mg Fe(?)/g dry solids(DS),34.29 mg H2O2/g DS and 43.21 mg CaO/g DS.In order to optimize the dosage mode of Fenton's reagent,namely based on dry solids(DS)or organic matter contents(volatile solids/total solids(VS/TS)),three different types of sludge characterized with different organic matter contents(22.9,34.0,and 44.4 wt.%)were tested.The resulte indciated inconsistency of optimum Fenton's reagent existed if dosage based on DS,however,dosing Fenton's reagent based on organic mater contents can achieve consistent results with optimum dosages of 107-110 mg Fe(?)/g VS and 86-88 mg H2O2/g VS.Meanwhile,Fenton's reagent based on organic mater contents can more effectively degrade loosely bound EPS compared to dosage based on DS.2.The mechanism of Fenton's reagent conditioning for sewage sludgeScanning electron microscopy(SEM)analysis indicated organic hydrophilic substances of external sludge floc were reduced,and porous structure and agglomeration were observed in Fenton's reagent conditioned sludge.Fenton's reagent conditioning can be divided into two steps:(1)Fenton oxidation:oxidation in Fenton's reagent can degrade EPS into soluble protein(PN)and polysaccharde(PS),disintegrate dense sludge flocs,decrease sludge particle size,and slightly reduce bound water content of sludge;(2)Fe(?)coagulation:Fe(?)generated in Fenton process can increase sludge particle size,reduce small pore volume(1?10 nm)and increase large pore volume(>10 nm),and reduce specific surface area of sludge and bound water content.Through quenching experiment,47.4%ferrous was oxidized by hydroxyl radical for sludge conditioned with Fenton's reagent(the ratio of ferric iron content in total iron content reduced from 72.6 to 25.2%).91%dewatering performance can be attributed to ferric coagulationfor Fenton's reagent conditoned sludge.Oxidation effect can further improve sludge dewaterability,with water content of sludge reduced from 60.23 to 54.75 wt.%.3.Electro-dewatering technique for sludge deep-dewateringHigh voltage electrochemical workstation was adopted to replace the commonly used direct-current(DC)power supply,enabling in-situ linear sweep voltammetry analysis during EDW.There were ohmic polarization and activation polarization in EDW,and ohmic polarization was dominant in the polarization curves,which suggested the applied voltage was mainly used for EDW.EDW mode can be explained by the Ohm's law in a simple circuit.During the constant-voltage EDW mode,the electric current was opposite to the sludge electrical resistance.High filtrate flow rate was associated with low resistance of sludge and vice versa.Total amount of electric charge during the dewatering process was linearly proportional to the mass of the dewatering filtrate,suggesting the applied voltage was the determing factor for the energy consumption of EDW.The results gained from this thesis can help to better understand Fenton process for sludge dewatering.Meanwhile,energy-efficient EDW technology can be applied for deep dewatering of mechanically dewatered sewage sludge(water content of 80 wt.%).