Study On The Mechanism Of Key Elements Of Nitrogen,Phosphorus And Iron During Thermal Treatment And Resource Recovery From Deep Dewatered Sewage Sludge

The treatment and disposal of waste activated sludge is a worldwide problem because of its high water content and huge production.Fenton conditioning can promote sludge dewaterability,however,a lot of iron-rich sludge was generated,which still needed to be treated.In order to reduce its environmental impact and reuse iron simultaneously,this thesis focused on the changes of key elements（Fe,N,P）during the thermal treatment process and their reuse as value-added products.The impacts of the thermal treatment of iron-rich sludge on the transformation of nitrogen and phosphorus,as well as on the control of nitrogen oxides precursors control and phosphorus recovery were studied.Finally,the feasibility of the reuse of iron-rich biochar as a dewatering aid and the related dewatering mechanism in terms of iron species were discussed.The main contents of this thesis include:1.Changes of Nitrogen During the Pyrolysis Process of Raw sludge and Fenton Conditioned SludgeThe migration and changes of nitrogen in char in the pyrolysis process under different pyrolysis temperatures（300,500,700,800 and 900 ^oC）of raw sludge and Fenton conditioned sludge were studied.The results showed that as the pyrolysis temperature increased from 300^oC to 900 ^oC,the nitrogen in the sludge was gradually transformed into the gas phase,which was supported by the increased yield of the precursor of nitrogen oxides（ammonia and hydrogen cyanide）.Compared with raw sludge,more nitrogen was observed in the biochar pyrolyzed from the Fenton conditioned sludge,along with more production of stable nitrogen-containing functional groups,while resulted in less presence of nitrogen in tar during the pyrolysis process,thereby reducing the yield of ammonia and hydrogen cyanide.At the pyrolysis temperature of 300 ^oC,the solid nitrogen content in the pyrolysis biochar of Fenton conditioned sludge was significantly higher than that of raw sludge（82.45%versus 64.51%）.At the pyrolysis temperature of 700 ^oC,the amine nitrogen present in the sludge pyrolyzed tar was degraded obviously.Meanwhile,its content in the pyrolyzed biochar of Fenton conditioned sludge was higher than that of raw sludge（6.10%versus 5.51%）,and the yields of ammonia and hydrogen cyanide were reduced by 13.56%and 33.37%respectively.The introduction of iron effectively alleviated the emission of nitrogen oxide precursors during the pyrolysis process.2.Changes of Phosphorus in Pyrochar and Hydrochar Produced from Raw Sludge and Fenton Conditioned Sludge and its BioavailabilityThe effect of iron introduced from Fenton conditioning on the form and content of phosphorus in biochar（300,500,and 800 ^oC）and hydrochar（100,200,and 280 ^oC）were explored.And a phosphate-solubilizing microorganism（Pseudomonas aeruginosa）was used to compare the effects of two thermal treatment methods on the subsequent utilization of biochar.The results showed that the increase of treatment temperature and the introduction of iron can promote the enrichment of phosphorus in biochar,and the total phosphorus content of pyrochar was higher than that of hydrochar.For example,the total phosphorus contents of the pyrochar and hydrochar obtained from Fenton conditioned sludge at 800 ^oC and 280 ^oC were 26.29 and 22.29 mg/g respectively.Na OH-P and HCl-P were the main phosphorus forms in the biochar,and the introduction of iron made Na OH-P content of pyrochar obtained from Fenton conditioned sludge higher than that obtained from raw sludge（i.e.,at 300 ^oC:75.03%versus 30.36%）,while the HCl-P content in hydrochar was higher than that in pyrochar,accounting for more than 50%of the total phosphorus content.The ³¹P nuclear magnetic resonance results found that the main form of phosphorus in the prochar and hydrochar was orthophosphate and pyrophosphate existed when the thermal treatment temperature was low（i.e.,300 ^oC for pyrochar and 100 ^oC for hydrochar）,and gradually disappeared as the temperature increased（i.e.,800 ^oC for pyrochar and 280 ^oC for hydrochar）.And the content of plant-available phosphorus of pyrochar obtained from Fenton conditioned sludge at 800 ^oC reached its maximum,which was slightly higher than that of hydrochar at 280 ^oC（9.61 versus9.19 mg/g）.The dissolved phosphorus of pyrochar and hydrochar after the inoculation with a phosphate-solubilizing microorganism after an incubation period of 30 days reached its maximum（5.20 mg/g）in the pyrochar obtained from Fenton conditioned sludge at 300 ^oC.Meanwhile,the microbial phosphorus in the dissolved phosphorus predominated after 7 days of incubation and gradually increased.The analysis of the solid phase phosphorus form after an incubation of 30 days revealed that Na OH-P was the main phosphorus species released by phosphate-solubilizing microorganism.Compared with pyrochar,HCl-P was more released in hydrochar,and these phenomena were more obvious in hydrochar obtained from Fenton conditioned sludge.3.Reuse of Pyrochar obtained from Fenton Conditioned Sludge to Enhance Sludge DewaterabilityEnhanced sludge dewaterability via oxone activated by pyrochar from iron-rich sludge,as indicated by more decreases of capillary suction time（23.00%）and specific resistance to filtration（8.50%）,was achieved at a low temperature of 300 ^oC（Fenton-300）compared to those at a high temperature of 800 ^oC（Fenton-800）.Multivalent iron species were identified in Fenton-300 pyrochar（including Fe O,Fe₂O₃ and Fe₃O₄）and Fenton-800 pyrochar（including Fe O,Fe₂O₃,Fe₃O₄,and Fe⁰）.The acidic p H of 3.75 for Fenton-300 pyrochar activated oxone,and p H of 3.97 for Fenton-800 pyrochar activated oxone were observed.Experiments with model compounds indicated that,at these acidic p H values,both Fe₃O₄ and Fe₂O₃ failed to activate oxone.In contrast,Fe O and Fe⁰ were capable to activate oxone.In contrast with Fenton-300 pyrochar activated oxone system,although Fe⁰ in Fenton-800pyrochar also effectively activated oxone,most of the newly generated Fe（III）was reduced to Fe（II）in Fenton-800 pyrochar activated oxone system,resulting in less Fe（III）present in the dewatered sludge cake（9.30 mg/g dry solids（DS）in the Fenton-300 pyrochar system）versus0.41 mg/g DS in the Fenton-800 biochar system).The superiority of Fe（II）activated oxone for enhanced sludge dewaterability than Fe⁰ activated oxone were further proved in changes of radicals generation,degradation of hydrophilic proteinaceous compounds,and surface thermodynamics.