Study On The Application Of Hydrothermal And Zero-valent Iron Activated Persulfate In Municipal Sludge Dewatering

Hydrothermal treatment technology（HT）has been widely concerned in sludge conditioning due to its characteristics of effectively improving sludge dewatering performance,biodegradability and high treatment efficiency.However,the high temperature（170-180℃）was often required when HT was used to conditioning sludge,which limited its application.The coupling of HT with acid,alkali,advanced oxidation and other chemical methods could effectively reduce the HT temperature while ensuring the effect of sludge dewatering.In this study,hydrothermal treatment combined with zero-valent iron activated persulfate（HT-ZVI/PDS）was used to conditioning sludge.HT temperature,persulfate（PDS）and zero-valent iron（ZVI）dosing and the way the coupling effect on the performance of sludge dewatering was explored.the process of organic matter degradation regularity and mechanism,improvement of the key factors of sludge dewatering was revealed.This study will contribute to a novel strategy for sewage sludge reduction and utilization.The main research contents and results are as follows.The conditioning effect of HT,ZVI/PDS and HT-ZVI/PDS processes was compared with the sludge from a municipal sewage treatment plant in Nanjing.The results showed that the dewatering performance of sludge deteriorated with the decrease of hydrothermal temperature in HT conditioning process,and the sludge dewatering performance was worst when hydrothermal temperature was 120℃,when the content of bound water in sludge increased from 0.81 g/g TSS to 1.03 g/g TSS and the proportion of hydrophilic functional groups in sludge increased from 49.14%to 52.55%.In ZVI/PDS conditioning process,specific resistance of filtration s（SRF）decreased from 5.95×10¹² m/kg to 2.61×10¹² m/kg,capillary suction time（CST）decreased from 190.6 s to 104.55 s,and sludge dewatering performance was improved.In the HT-ZVI/PDS conditioning process,the CST value and SRF value of sludge were reduced by 86.72%and 72.35%,respectively.With the increase of PDS and ZVI dosage,the SRF and CST decreased continuously,and the dewatering performance of sludge increased continuously.The degradation of organic matter showed that HT caused the solubilization of organic matter in sludge floc to the liquid phase,and the concentration of total organic carbon,protein and polysaccharide in hydrolysate increased sharply.After adding ZVI/PDS conditioning,sludge floc was further oxidized and decomposed,resulting in a further increase in the concentration of polysaccharide and ammonia nitrogen,but a decrease in the concentration of protein.Under the synergic conditioning of HT and ZVI/PDS,the organics in sludge were mineralized,resulting in the reduction of the concentration of extracellular polymer（EPS）,humic acid and fulvic acid were further decomposed.In this study,the coupling mode of HT and ZVI/PDS was further changed to compare the influence of coupling ZVI/PDS conditioning on sludge dewatering performance at different stages of HT（initial stage and constant temperature stage）,so as to determine the best coupling mode of HT-ZVI/PDS process.The results showed that the SRF decreased by 72.19%and the combined water content in the sludge decreased by 32.10%at the initial stage of HT coupled with ZVI/PDS conditioning.In addition,theα-helix content in protein decreased,indicating that the protein molecular structure was more loose,which was beneficial to improve sludge dewatering performance.Therefore,setting the conditioning temperature at 120℃,the optimal operation condition of HT-ZVI/PDS process was coupled with 200 mg/g TSS PDS and 100mg/g TSS ZVI at the initial stage of HT.Based on the above research results,the influence of hydrothermal temperature on ZVI/PDS treatment efficiency was analyzed by sampling in the heating process of the reaction under the optimal operating conditions of HT-ZVI/PDS process,and the key factors affecting the sludge dewatering performance were revealed.The results showed that the concentration of tightly bound EPS（TB-EPS）decreased from 203.15 mg/L to 9.91 mg/L and the concentration of free amino acids increased from 4.34 mg/L to 34.60 mg/L when the reaction temperature rose from25℃to 60℃.The fluorescence intensity of fluorescent compounds in TB-EPS decreased rapidly,and the O-P-O chemical bond related to nucleic acid broke.When the reaction temperature rose from 60℃to 80℃,the concentration of dissolved EPS（S-EPS）increased from 488.1 mg/L to 649.85 mg/L,and the concentration of polysaccharide in TB-EPS increased from 3.17 mg/L to 25.47 mg/L.The concentration of protein decreased from 14.30 mg/L to 9.58mg/L,and the fluorescence intensity of humic acid and fulvic acid in loosely bound EPS（LB-EPS）decreased.When the reaction temperature rose from 80℃to 100℃,the concentration of LB-EPS increased from 29.25 mg/L to 115.55 mg/L,the concentration of polysaccharide increased sharply from 46.02 mg/L to 508.54 mg/L,leucine disappeared,arginine appeared,humic acid and flavanolic acid begun to accumulate.The C-N bond associated with the protein was broken.When the temperature rose from 100℃to 120℃,the concentration of S-EPS increased from 937.45 mg/L to 1957.05 mg/L,and the polysaccharide concentration in LB-EPS increased from 8.06 mg/L to 130.84 mg/L.The concentration of free amino acids increased from 18.53 mg/L to 48.05 mg/L,and the fluorescence intensity of humic acid increased.The results of structural equation model showed that the protein in sludge was significantly correlated with sludge dewatering performance（p<0.01）.The protein residues containing tryptophan,tyrosine and phenylalanine were further analyzed by three-dimensional fluorescence spectroscopy.It was found that there was a strong correlation between tryptophan,tyrosine and soluble microbial metabolites in TB-EPS layer and sludge dewatering performance（p<0.01）.Furthermore,the fluorescence components in TB-EPS were identified and analyzed by parallel factor model,and the tryptophan protein in TB-EPS was confirmed to be the key factor affecting sludge dewatering performance（p<0.01,r=0.839）.