Enhancement And Stability Of The Complete Autotrophic Nitrogen Removal Over Nitrite Process In A Tidal Flow Constructed Wetland

Since the CANON process is conducive to realize effective nitrogen removal when treating wastewater with low C/N,the establishment of the CANON process in a CW system has been an issue in the current studies.In consequence,a modified single-stage tidal flow constructed wetland(TFCW)was conducted to study the regulatory mechanisms of CANON in the system.From the two points of macroscopical process performances and microscopical biological characteristics,improvement of CANON in the TFCW was carried out at ambient temperature and low NH4~+-N concentration,and the microbial characteristics in the system was also analyzed.Optimal control of a modified single-stage TFCW via CANON process had been conducted sequentially.Thereafter,according to the characteristics of wastewater,influences of organics on the performances of the TFCW via CANON process was also investigated,and metabolic characteristics of anammox cultivated in the system were focused at different organics concentrations.In conclusion,the project provides theoretic basis and data support for the low C/N wastewater treatment and the engineering application of new type of CWs.The main research contents and results were as follows:(1)This study was conducted to explore nitrogen transformation and associated microbial characteristics in a modified single-stage TFCW at five different shunt ratios,which were expected to explore an effective way to strengthen the CANON process in TFCW.Shunt ratio significantly affected nitrogen removal during operation of the TFCW with adoption of a modified step-feeding mode.When the shunt ratio was 1:2,TFCW had the best pollutant removal performance,the mean TN removal rate reached up to(80.20±5.67)%and(89.95±9.01)%for NH4~+-N correspondingly.The absolute abundance of the seven key functional genes(i.e.anammox 16S r RNA,nar G,nap A,nir S,nir K,qnor B and nos Z)involved in nitrogen removal were all increased as the shunt ratio was increased.Molecular biological analyses demonstrated that both denitrification and anammox were enhanced when the shunt ratio was greater than 0:1,which was conducive to the removal of TN in TFCW.Regarding the stepwise regression analysis equations,it was known that the removal of TN in TFCW relied largely on the partial nitrification/denitrification process and the CANON process.The results showed that the optimal shunt ratio of the modified single-stage TFCW for effective nitrogen removal was 1:2.(2)This study attempted to achieve a high-rate nitrogen removal via the CANON process in a modified single-stage TFCW with step-feeding,and nitrogen transformation pathways in the TFCW treating simulate domestic wastewater were explored under shunt ratio constraints.The experimental results showed that,the TFCW with the shunt ratio of1:1 had the best nitrogen removal performances,the mean TN and NH4~+-N removal rates were(127.00±13.78)and(124.77±4.37)mg/(L·d),respectively.Since the shunt ratio was greater than 0:1,the absolute abundance of anammox 16S r RNA increased.The anammox bacteria was enriched most effectively at the shunt ratio of 1:1,which was conducive to the removal of TN in TFCW.The absolute abundance of nar G,nap A,nir K,nir S,qnor B,and nos Z increased slightly,which are involved in denitrification.However,these changes were not significant because of the poor denitrification performance of the system,which was attributed to a lack of organics in the wastewater.Regarding the stepwise regression analysis equations,it was known that the removal of TN in TFCW relied largely on the CANON process.It could be concluded that autotrophic nitrogen removal via CANON process developed in the TFCW with optimized microenvironment that developed as a result of appropriate limited oxygen condition and concentrations of organics.The optimal shunt ratio of the TFCW for nitrogen removal was 1:1.(3)This study was conducted to explore nitrogen transformation and associated microbial characteristics in a TFCW with the CANON process under influent C/N constraints.The influent C/N increased from 0.0 to 10.0 via the addition of glucose in the influent as a source of organics.The results showed that,influent C/N significantly affected nitrogen transformation rates in the TFCW throughout the experiment.As the influent C/N increased from 0.0 to 6.0,the absolute abundance of functional genes involved in denitrification could be enriched as a consequence of the addition of organics in influent,and then the simultaneous nitrification,anammox,and denitrification(SNAD)processes occurred in the TFCW,resulting in the enhancement of nitrogen removal in the system.However,as the influent C/N was more than 6.0,the activity of aerobic ammonia-oxidizing bacteria(AOB)was inhibited and its quantity reduced,leading to the deterioration in nitrogen removal of the system.When the influent C/N was 6.0,the SNAD process was enhanced most effectively in the system owing to the development of multiple and complete nitrogen removal pathways in the TFCW.The TFCW respectively had the best TN removal efficiency and removal loading rate(93.3±2.3)%and(149.30±8.00)mg/(L·d),indicating that the results had been more than the maximal TN removal efficiency in a CANON process under ideal conditions.