Study On Nitrogen Removal Performance And System Stability Of Sponge Iron-based Bioretention Tank

Many studies have proved that zero-valent iron has strong denitrification and denitrification performance,but there is little practice in the application of sponge iron as zero-valent iron in biological detention ponds.The main concern is the potential harm of sponge iron to various elements of the bioretention system,such as the stress of plants,the destruction of microbial communities,and the excessive concentration of effluent iron.Based on the important denitrification and potential destructiveness of sponge iron,the growth law,physiological index and stress resistance of plants under sponge iron stress were compared with Iris and canna as biological detention pond plants.as well as the removal effect of ammonia nitrogen and nitrate nitrogen by the system,and the suitable proportion of sponge iron was determined.Secondly,through the construction of sponge iron-based biological retention tank,the denitrification enhanced nitrogen removal potential and nitrogen transformation relationship of the system under different influent conditions were discussed,and the decontamination effect,cleaning characteristics and interception capacity of the system under three kinds of pollutant loads were compared and analyzed.Finally,based on the changes of the factors of the sponge-iron-based biological retention pool under long-term operation,the sustainable use and operation stability of the sponge-iron-based biological retention pool are further determined.The main conclusions of the study are as follows:（1）The growth of Iris and Canna was affected by sponge iron stress.The higher the sponge iron content was,the more serious the plant peroxidation was,and the more obvious the damage to cell membrane,metabolic function and photosynthetic capacity was.In order to cope with the stress of sponge iron,the activities of SOD,CAT,APX and POD of Iris and Canna were increased,so 14%content was the maximum amount that could be accepted by the two plants.（2）The addition of sponge iron significantly improved the nitrogen removal efficiency of biological retention tank.The removal rate of NH₄⁺-N in the sponge iron-based biological retention pool is more than 96.00%.The removal rate of NO₃^--N is more than 92%.Compared with the control group,the removal effect is significantly improved.Combined with the effects of different sponge iron content on plant MDA content,H₂O₂ content,chlorophyll content,SOD,CAT,APX and POD enzyme activity,it was determined that the amount of sponge iron less than 14%was the suitable reference for high nitrogen removal efficiency and low plant stress.（3）The higher the sponge iron content,the greater the denitrification potential.When C/N increased from 0 to 10,the NO₃^--N removal rate of sponge iron-based biological retention tank increased from 53.73%～76.6%to 93.92%～100%,which significantly increased the potential of chemical denitrification and improved the NO₃^--N removal rate of the system.The higher the sponge iron content in the system is,the higher the effluent pH is,and the lower the DO concentration is.Excessive sponge iron will reduce the removal rate of COD.（4）The system with 14%sponge iron shows better nitrogen removal performance.under the conditions of low,medium and high pollution load,the removal rate of NH₄⁺-N can reach 94.68%～98.62%,the removal rate of NO₃^--N is more than 85%,the removal rate of TN is more than 90%,and the iron content in the effluent does not exceed 0.1 mg/L.In the biological detention tank system,the pollutants NO₃^--N,TN and TP with poor removal efficiency are easily washed,while the pollutants NH₄⁺-N and COD with better removal efficiency are not easily washed.The addition of sponge iron improves the pollution interception effect of the system.（5）After 9 months of long-term use,the sponge iron-based bioretention tank has low loss and good sustainable use.Canna has good adaptability to the stress of 11%and 14%sponge iron content.After 0.5 years of operation,the content of iron in plants increased,but the content of H₂O₂decreased slightly,the content of chlorophyll increased slightly,and the peroxidation environment was improved,which proved that canna has adaptability to low iron stress.（6）The dominant phyla were Proteobacteria,Bacteroidota,Actinobacteria,Patescibacteria,Planctomycetes and Chloroflexi,etc.At the level of each system,the dominant genera were mainly Terrimonas,Acidibacter,Nitrospira and so on.The relative abundance of the flora in each system is similar,but there are also some differences.The growth of the microbial community is affected by environmental factors such as sponge iron stress,pH and DO.（7）The addition of sponge iron has a strong inhibitory effect on the growth of denitrifying bacteria.Proteobacteria,Bacteroidetes,Patescibacteria and Planctomycetes had a positive effect on nitrogen removal,but their proportion decreased in sponge iron-based bioretention.Thauera,Pseudomonas and Dechloromonas are common heterotrophic denitrifying bacteria,but their abundances are not high in the early and late stages of stabilization,which proves that the addition of sponge iron increases chemical denitrification and inhibits microbial heterotrophic denitrification in the system.Excessive sponge iron is not suitable for the long-term stable development of microbial communities in bioretention tanks.（8）In order to ensure the stable operation of sponge iron base for a long time,the stress degree of sponge iron on plants,the enhanced denitrification performance of sponge iron and the community structure of microorganisms were comprehensively compared.it is concluded that the most suitable proportion of sponge iron is 14%.It provides a reference for the proportion of sponge iron,and provides a useful reference for the engineering application of sponge iron-based biological retention pool.