Study And Application Of Advanced NO₃^--N Removal Using Sulfur Autotrophic Denitrification Process

With the increasing needs of human society,the nitrogen cycle on the earth is being destroyed by excessively artificial nitrogen emissions.NO₃^--N is the highest valence state of nitrogen in the nitrogen cycle which is easy to accumulate in water ecosystem and destry the ecological balance.Therefore,it is urgent to develop a new type of high-efficiency and low-consumption denitrification process.Traditional heterotrophic denitrification process requires an external carbon source,which causes a high cost,while sulfur autotrophic denitrification has a good application prospect with many advantages,such as the absence of organic source,lower sludge production,lower sludge treatment burden and the adaptability to more types of wastewater.In this study,different forms of electron donors(elemental sulfur powder,hot-melted sulfur flakes,N_a2S₂O_3,Na₂S)and the start-up methods?SBR and continuous?of sulfur autotrophic denitrification process were screened to determine best startup plan of the actual wastewater treatment process.Graphite wastewater-sulfur autotrophic denitrification process reactor was built to degrade the high-salinity and high NO₃^--N graphite production wastewater after pretreatment by chemical precipitation.The operation of the reactor in the process startup and operation stages was measured,and the structure of the microbial community in the reactor at each stage was analyzed using high-throughput sequencing technology.Finally,metagenomic technology was used to explore the nitrogen and sulfur metabolism processes in different stages,the relative abundance changes of related metabolic pathways,functional enzymes,functional genes and salt-tolerance genes in this study,the conclusions of this study are as follows.?1?A shaker-anaerobic flask experiment was used to explore the start-up time and NO₃^--N removal efficiency of the sulfur autotrophic denitrification system when elemental sulfur,hot-melt sulfur tablets,Na₂S₂O₃,and Na₂S were used as electron donors.The results show that the system using elemental sulfur was the first to be launched with the shortest start-up time,stable effect,lower nitrate accumulation and lower cost.The SBR reactor and the continuous reactor with elemental sulfur as the electron donor were established.The results show that the continuous reactor has shorter start-up time and higher NO₃^--N removal efficiency.Therefore,the continuous reactor with elemental sulfur as the electron donor was selected as the operation mode of subsequent actual wastewater treatment.?2?The graphite production wastewater-sulfur autotrophic denitrification process was started for continuous operation for 125 days,When the NO₃^--N concentration in the influent was 599.3±6.7 mg/L,the salinity was 15 g/L,the HRT was 30 h,the NO₃^--N concentration in the effluent of the sulfur autotrophic denitrification system was 5.20 mg/L,and the NO₃^--N removal efficiency was 99.1%,the NO₃^--N removal rate was 0.5 kg·N/?m³·d?and the effect was stable.According to high-throughput sequencing results,Proteobacteria had the highest relative abundance at the phylum level;Betaproteobacteria had the highest relative abundance at the class level,and Thiobacillus had the highest relative abundance at the genus level and the relative abundance was positively correlated with the increase of salinity and NO₃^--N.?3?According to the results of metagenomic analysis,the main denitrification processes in the nitrogen metabolism pathway are the nitrate reduction,nitrite reduction,nitric oxide reduction,and nitrous oxide reduction.With the increase of salinity and NO₃^--N concentration in the system,the relative abundance of function denitrification genes,sulfur oxidation function genes,and salt tolerance function genes in Thiobacillus was generally increasing.In order to resist the change of external osmotic pressure and maintain a high NO₃^--N removal efficiency,ensure the efficient operation of graphite wastewater-sulfur autotrophic denitrification system.