Experimental Study On The Removal Of Nitrate From Water By Iron Autotrophic Denitrification Filte

Nitrogen pollution is one of the most common types of water pollution,and wastewater nitrogen removal has always been a problem that cannot be ignored in the water treatment industry.Heterotrophic denitrification is the main biological denitrification method commonly used at present,but it cannot achieve the ideal effect on the treatment of low C/N sewage.Nitrate-Reduction ferrous/iron-Oxidation provides a new treatment idea for the low C/N sewage.Nitrate-Reduction ferrous/iron-Oxidation has the characteristics of easy access to raw materials,low price,and safety of products,which has a good development prospect.In this study,using low C/N ratio sewage as the simulated sewage,by building upflow quick-soluble ferrous（ferrous sulfate）autotrophic denitrification filter and slow-release ferrous（siderite）autotrophic denitrification filter,the effects of reactor start-up process and different factors on nitrogen removal efficiency were studied,and the structure and morphological characteristics of microbial community in different reactors were investigated.This thesis provides a reference for the mechanism analysis and application process of NRFO.The main conclusions of this study are as follows:（1）Experimental study on nitrogen removal by instant ferrous autotrophic denitrification filter.Using activated carbon as the base filler and ferrous sulfate as the electron donor,a biological denitrification filter was built by inoculation of anaerobic sludge,which was recorded as reactor 1.In order to slow down the oxidation of ferrous iron,the reactor 1b with additional zero-valent iron was used as the experimental group to compare the nitrogen removal effect of the two reactors.The study found that the amount of ammonia nitrogen generated in the reactor 1b with 5 g iron powder was higher than that in the reactor without iron powder,and the total nitrogen removal rate was 26.10%,which was lower than that of reactor 1（56.68%）.In the condition of 50 mg/L of influent nitrate nitrogen,400 mg/L of ferrous sulfate,and no added organic carbon,after 60 days of continuous operation,the microbial film on the filler was good,and it could run continuously and stably.There was a certain correlation between the ferrous iron consumption and the removal of nitrate nitrogen in the system,and the ferrous iron autotrophic denitrification system was established.After stable operation of the instant ferrous iron system,the average removal rate of nitrate nitrogen was 55.68%,the removal rate of total nitrogen was 53.99%,the reactor was in stable operation,the p H of the effluent dropped to about4,and the gas formation was observed,which was presumed to be N₂,NO and other gases.The effects of HRT,Fe/N,p H and other factors on nitrogen removal efficiency of the system were investigated.When the system was continuously operated to stability,The instant iron autotrophic denitrification system with HRT=16 h had the best nitrogen removal efficiency and the fastest nitrogen removal rate.After stabilization,the removal rate of nitrate and total nitrogen was 52.70%and 49.34%,respectively.The average removal rate of nitrate and total nitrogen in the effluent was 0.041 kg/（m³·d）and 0.038 kg/（m³·d）,respectively.The best nitrogen removal effect was achieved when Fe/N=3,with 98.07%nitrate nitrogen removal rate and 81.24%total nitrogen removal rate.p H affected both biological and chemical reactions,and it was found that the removal effect of nitrate nitrogen and total nitrogen was better when p H=6～7.Increasing the influent nitrate concentration can improve the nitrogen removal rate to a certain extent,but has a great influence on the effluent nitrogen concentration,resulting in a decrease in nitrogen removal effect.Therefore,the influent nitrate nitrogen is controlled at 50 mg/L,and the effect is better.（2）Experimental study on nitrogen removal in slow-release ferrous autotrophic denitrification filter.Using siderite and activated carbon as combined fillers,a biological denitrification filter was constructed by inoculation of anaerobic sludge.Under the condition of no organic carbon in the influent,the influent nitrate nitrogen concentration was 50 mg/L,HRT=24 h,ferrous sulfate 400mg/L,and the reactor ran stably after 60 days of continuous operation.The removal rate of nitrate nitrogen and total nitrogen of the system reached 93.28%and 52.38%,respectively.The system could operate continuously and stably,and a slow-release iron autotrophic denitrification system was established.The effects of HRT,influent nitrate concentration and C/N on nitrogen removal were investigated.Under the premise of continuous and stable operation,HRT=26 hours had the fastest total nitrogen removal effect and removal rate.After stabilization,the average nitrate nitrogen removal rate was 0.052 kg/（m³·d）,and the average total nitrogen removal rate was 0.026kg/（m³·d）.The nitrate nitrogen removal rate was 98.20%,and the total nitrogen removal rate was50.93%.When influent nitrate nitrogen was 50 mg/L,the removal rate of nitrate nitrogen was98.63%,which was almost all removed,accompanied by partial formation of ammonia nitrogen,no nitrite accumulation occurred,and the total nitrogen removal rate was 50.95%.The NRFO test of changing the C/N ratio of the influent showed that the slow-release iron autotrophic denitrification reactor could obtain good nitrogen removal effect under the condition of low C/N.The total nitrogen removal rate reached 0.049 kg/（m³·d）when C/N=3:1,and 94.95%when the total nitrogen removal rate was stable,and the ammonia nitrogen in the effluent was 2.97 mg/L.The nitrate nitrogen and nitrite nitrogen in the effluent were almost undetectable,and the ammonia nitrogen and total nitrogen in the effluent met the standard A of the effluent.（3）Analysis of microbial community structure and morphology in Nitrate-Reduction ferrous/iron-Oxidation system.NGS sequencing revealed that the dominant microbial species in both reactors were Proteobacteria,Bacteroidetes,Acidobacteria and Firmicutes.All of them belonged to the phylum iron-oxidizing microbes.Proteobacteria accounted for 62.65%of the total bacteria in the instant ferrous autotrophic denitrification system,which was significantly higher than that of the inoculated sludge.Proteobacteria and Firmicutes accounted for 35.13%and 16.54%in the sustained-release ferrous autotrophic denitrification system.SEM and EDS analysis of the granular sludge and filler in the two reactors showed that the microorganisms in the granular sludge in the two reactors were mainly rod-shaped bacteria,filamentous bacteria and coccoid bacteria.There were a large number of gullies and holes in the inside and outside of granular sludge compared with the inoculated sludge,presumably because organic matter was consumed and heterotrophic bacteria died in a large number.A large amount of Fe（Ⅲ）oxide attached inside and outside granular sludge,resulting in uneven gullies;The gas produced during the reaction results in the formation of voids inside the granular sludge.The Fe content in the granular sludge of the ferrous+activated carbon reactor increased significantly,indicating that the increased inorganic matter was mainly iron compounds generated by iron autotrophic denitrification.The granular sludge in the siderite reactor had a reduced C content and an increased O content.It is speculated that the organic matter in the sludge is consumed,the carbon source is mainly provided by CO₂and Fe CO₃in siderite,and the adsorption of Fe₂（CO₃）₃,Ca CO₃and other compounds inside and outside the granular sludge results in the decrease of C/O ratio.