Research On The Application Of Autotrophic Denitrification Technology For Composite Sulfur Substrates

To further improve the effluent discharge standard of urban wastewater treatment plants,reduce the total pollutant discharge and effectively control the eutrophication of water bodies,this study was conducted to construct a composite sulfur matrix autotrophic denitrification filter with sulfur/pyrite for deep denitrification and phosphorus removal in the secondary effluent of actual wastewater plants.The reactor start-up characteristics were mainly investigated,and the effects of different influent NO₃^--N concentrations and actual ambient temperature variations on the performance of the reaction system for nitrogen removal and phosphorus removal were examined.The thesis clarifies the operating characteristics of the system under different operating conditions by means of sludge extracellular polymers（EPS）secretion production,3D fluorescence spectroscopy results and reaction kinetic fitting analysis.It also combines SEM-EDS secondary mineral analysis on the pyrite surface and high-throughput sequencing to study the characteristics of microorganisms in the system,analyze the removal process of N and P and removal mechanism of composite sulfur matrix reaction,and provide corresponding theoretical and technical support for the design of actual secondary effluent depth denitrification and phosphorus removal in wastewater treatment plants.The main research results are as follows:（1）The complex sulfur matrix autotrophic denitrification filter was successfully started after 36 d of operation under continuous influent conditions with a feedwater NO₃^--N concentration of 20 mg/L and an HRT of 12 h.The system showed a stable denitrification capacity,with a stable NO₃^--N removal rate of about 63.2%and an effluent NO₃^--N concentration of 7.35 mg/L.（2）The pollutant removal characteristics of the system with different influent NO₃^--N concentrations were investigated.The NO₃^--N removal rate decreased from96.68%to 78.99%during the process of increasing the influent NO₃^--N concentration from 15 mg/L to 45 mg/L.Meanwhile,the NO₃^--N concentration of the reactor effluent all gradually increased from 0.45 mg/L to 10.2 mg/L,but the NO₃^--N removal load increased.The maximum removal loads of NO₃^--N and TN both occurred at the influent NO₃^--N 45 mg/L condition,which were 0.0813 kg-NO₃^--N/（m³·d）and 0.0944kg-TN/（m³·d）,respectively.The removal of PO₄^3-was mainly through the formation of precipitation with Fe³⁺released and oxidized by thionite in the denitrification process,and the system P removal remained stable throughout the feed water concentration change phase,and the removal did not increase significantly.（3）The effect of seasonal temperature change on the system operation performance was examined.When the ambient temperature decreased from 35°C to9°C throughout the process,the NO₃^--N and PO₄^3--P removal performance of the reactor were weakened to different degrees.The NO₃^--N removal rate was only50.61%at the low temperature range of 9-11°C,and the removal load was reduced to0.028 kg-NO₃^--N/（m³·d）.The PO₄^3--P removal rate,on the other hand,decreased from69.02%at 35°C to 25.31%at 9°C.The effluent PO₄^3--P concentration increased to0.53 mg/L,and the removal of PO₄^3--P decreased significantly.The decrease in microbial activity at low temperatures was the main factor for the decrease in system removal performance.（4）The complex sulfur matrix autotrophic denitrification reaction under the variation of influent NO₃^--N concentration and actual ambient temperature were in accordance with the primary reaction kinetic model,and the removal of NO₃^--N was mainly concentrated in the middle and lower parts of the reactor.Both LB-EPS and TB-EPS contents in the sludge increased with the increase of influent NO₃^--N concentration,and the ratio of PN to PS did not change much in each stage,and the sludge maintained flocculent state throughout the fluctuation of influent NO₃^--N concentration.（5）The SEM results of the siderite showed some short chain-like rods and medium-length rod-like morphological bacteria clearly visible on the surface of the particles,and the EDS results of the secondary minerals revealed spherical amorphous aggregates or amorphous Fe（OH）₃ colloids as well as more obvious irregular plate-like Fe PO₄ precipitates.Microbiological analysis showed that Proteobacteria and Chloroflexi at the phylum level and Thiobacillus,Sulfurimonas,and Ferritrophicum at the genus level were dominant before and after environmental temperature changes.The percentage abundance of each was reduced in the low temperature phase at 9-11°C compared to the high temperature phase at 30-35°C.All of these bacteria were closely associated with N and P removal.