| Activated sludge is the inevitable solid waste pollution during biological wastewater treatment.Addressing the environmental concern of excess sludge is a key challenge in municipal wastewater treatment plants(MWTP).For sludge dewatering and reduction issuess,performance and mechanism of the acidification and ultrasound(A/US)treatments,the hydrodynamic cavitation and Fenton(HC/Fenton)reaction and the zero-valent iron activated Fenton oxidation and electrolysis(ZVI/H2O2/E)for activated sludge was investigated in this study.We first investigated the effects of process parameters on the sludge dewaterability represented by moisture content,CST and SRF.The content and species change of organic compounds such as polysaccharide,protein and humic substance were extracted and monitored.To reveal the synergism of three pre-treatment experiments,a suite of analysis were implemented:the three-dimensional excitation emission matrix(3-DEEM)spectra of extracellular polymeric substances(EPS)such as proteins and polysaccharides,Zeta potential and particle size of sludge flocs,and SEM/TEM imaging of sludge morphology.The results were as followed:The optimum dewatering conditions for the A/US system were 3 for pH,2.5 W mL-1 for US power density,with moisture content,CST and solid content decreased by 9%,18.2 s and 54.2%,respectively.UV254 and DOC of sludge supernatant increased by 61.3%and 29.2%,respectively.S-EPS content increased by 52.8%,LB-EPS and TB-EPS content decreased by 24.9%and 49.5%,respectively.The synergistic dewatering mechanisms were elucidated with a suite of macro and spectroscopic evidence.The characterization of treated sludge revealed that US-induced thermal,mechanical shearing force,and radical oxidation effects disrupt floc cells and accelerate the decomposition of EPS,releasing bound water into the bulk phase.In addition to enhancing hydrolysis of EPS,the acidic pH environment caused the protonation of functional groups on EPS,facilitating the reflocculation of US decomposed sludge for improved filterability.The optimum dewatering conditions for the HC/Fenton system were 3 for pH,0.14 g L-1 for[Fe2+],0.5 g L-1 for[H2O2]and 0.45 L s-1 for flow rate,with moisture content,CST and SRF decreased by 10.9%,30.2 s and 2.71×1011 m kg-1,respectively.S-EPS,LB-EPS and TB-EPS content decreased by 107,68 and 343 mg L-1,respectively.After hydraulic cavitation,the organic compounds such as protein and humic acid in sludge were released into the bulk phase,and the particle size of sludge decreased from 44.67 m to 8.02 ?m.Fenton reaction produced OH effectively degraded various organic compounds in sludge,which reduced the total area fluorescence intensity(?T,n)of EPS in each layer by 34.3,37.4 and 14.9(×106 A.U.nm2),respectively.The characterization results indicate a three-step mechanism,namely HC fracture of different EPS in sludge flocs,Fenton oxidation of the released EPS,and Fe(III)re-flocculation,that is responsible for the synergistically enhanced sludge dewatering.The optimum dewatering conditions for the ZVI/H2O2/E system were 3 for pH,0.14 g L-1 for[Fe0],0.42 ml L-1 for[H2O2]and 15 mA cm-2 for current density,with moisture content and CST decreased by 17.3%and 38.1 s.The content of fluorescent organics in S-EPS,LB-EPS and TB-EPS three layers decreased significantly,and the values of ?T,n decreased by 82.3,90.2 and 119.6(×106 A.U.nm2),respectively.By adding potassium iodide(KI),isopropanol(i-PrOH)and tert butyl alcohol(t-BuOH)two free radical trapping agents that free hydroxyl radical(·OHfree)and adsorbed radicals(·OHads)were conducive to improve the performance of sludge dewatering.ZVI could be oxidized to Fe(?)rapidly by acid and realize the recycle of iron(Fe2+?Fe3+?Fe2+)under eletro-field.Through infrared spectrum analysis and microscopic observation,it is fully proved that residual ZVI plays the role of skeleton builders in sludge,and flocculation of small granular sludge flocs can improve sludge dewatering and filtration performance. |
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