| Sewage sludge is a by-product generated during municipal wastewater treatment,it is very difficult to be dewatered,thus enormously limiting its subsequent treatment and disposal due to the highly hydrated nature of extracellular polymeric substances(EPS).Recently,iron-based materials to catalyze ozonation have been widely applied for wastewater treatment.However,iron-based catalysts activated ozonation to improve waste activated sludge dewaterability and its mechanism have rarely been reported.Therefore,this thesis focuses on investigating the mechanism of different types of iron catalytic ozonation to enhance dewaterability of sewage sludge.The main contents are as follows: 1.Combined ozone and zero-valent iron treatment to enhance sludge dewaterability and pathogen inactivationSewage sludge was drawn from Shahu municipal wastewater treatment plant in Wuhan in this study.The initial p H of sludge,the dose of ozone,the dose of zero-valent iron(ZVI)were optimized aimed at minimizing the capillary suction time(CST)of the sludge.The results showed that at p H of 2,when the doses of ozone and ZVI were respective 0.03 g/g total solids(TS)and 63 mg/g TS,the CST of the conditioned sludge was lowest,and it reduced from 10.45 to 1.63 s L/g TS.At the optimal conditions,the results of the laboratoryscale diaphragm filter press dewatering experiments showed that the water content of dewatered sludge cake conditioned by ozone and ZVI was 52.67%,which was much lower than the water content of dewatered sludge cake of the sewage sludge(77.13%).These results implied that the combined ozone-ZVI treatment could significantly promote sludge dewaterability.To further investigate the mechanism of the combined ozone and ZVI conditioning on sludge dewaterability and the related effects on pathogen inavtivation,sludge physiochemical properties and the inactivation of pathogens were investigated.The results showed that:(1)Total organic carbon(TOC),protein(PN),and polysaccharide(PS)contents and three-dimensional excitation-emission matrix(3D-EEM)intensity in tightly bound EPS(TB-EPS)of the conditioned sludge were significantly decreased compared with those in raw sludge(RS),while TOC,PN and PS in soluble EPS(SB-EPS)remarkably increased,indicating that EPS were degraded into dissolved organic compounds and released into the liquid phase.The results of Pearson's correlation showed that humic acid-like compounds and soluble microbial by-products in TB-EPS were the key organic compounds affecting sludge dewaterability;(2)The particle size of the conditioned sludge was increased,indicating that the sludge flocs were coagulated by the coagulation effects of Fe3+ generated from ozone-ZVI conditioning.The zeta potential changed from negative to near zero,indicating that the surface charge of the sludge particles may be altered by the charge neutralization effects of Fe3+;(3)The results of electron paramagnetic resonance(EPR)showed that the main free radicals acted in ozone-ZVI conditioning were ·OH;(4)The sludge treated by the combined ozone-ZVI pretreatment met the requirements of Class A biosolid of the US standards for pathogens.2.Combined ozone and iron-based biochar treatment to enhance sludge dewaterabilityBased on our previous study on optimizing the doses according to the organic matter contents(volatile solids(VS)),the obtained sludge cake from the laboratory-scale diaphragm filter press dewatering tests was pyrolyzed under the argon flow at 800 °C,and the pyrolytic biochar was named as iron-based biochar.The combined ozone and iron-based biochar treatment to enhance the sludge dewaterability was investigated,and the effects of initial p H,doses of ozone and the iron-based biochar on sludge dewatering were optimized aimed at minimizing the CST of the sludge.The results indicated that at p H of 2,when the doses of ozone and iron-based biochar were respective 0.03 g/g TS and 0.45 g/g TS,the CST of the conditioned sludge was lowest,reducing from the 10.53 to 2.44 s L/g TS.At the optimal conditions,the results of the laboratory-scale diaphragm filter press dewatering experimental study showed that the water content of dewatered sludge cake conditioned by ozone and iron-based biochar was 53.13%,which was much lower than the water content of dewatered sludge cake of the sewage sludge(83.26%),which implied that the combined ozone and iron-based biochar treatment could significantly promote the sludge dewaterability.The X-ray diffraction(XRD)analysis implied that the main component of the iron-based biochar was Fe3O4.The study on combined ozone and iron-based biochar to enhance sludge dewaterability may shed more lights on the subsequent discovery and application of the pyrolytic biosolid char as a recycled and reused sludge conditioning reagent.3.Comparison of catalytic ozonation by different types of iron(Fe0/Fe2+/Fe3+) treatments on enhancing sludge dewaterability and their economic cost analysisOzone-Fe2+ and ozone-Fe3+ pretreatments to enhance sludge dewaterability were kept at similar reduction levels of CST similar to that of the ozone-ZVI pretreatment.The results showed that ozone-Fe2+ and ozone-Fe3+ pretreatments can remarkly improve the sludge deep dewatering ability.Comparing the costs of reagents of ozone-ZVI with the Fenton,ozoneFe2+ and ozone-Fe3+,the results showed that the reagents of ozone-ZVI was 198.6 RMB/tonne sludge with 80% water content,less than those of ozone-Fe2+(203.8 RMB/tonne sludge with 80% water content)and ozone-Fe3+(269.6 RMB/tonne sludge with 80% water content),and much more less than Fenton's reagent(476.5 RMB/tonne sludge with 80% water content). |
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