Research On Domestic Wastewater Treatment By Establishing The Partial Nitrification-anammox Process In Constructed Wetlands

Constructed wetlands（CWs）as ecological engineering that simulate the physical,chemical,and biological processes of natural environments,can significantly reduce operating costs and carbon footprint.Moreover,CWs are characterized by flexible site selection,minimal environmental impact,and aesthetically pleasing landscapes.These features make them suitable for implementation in economically underdeveloped villages,small communities,ecological parks,and other areas in China.This aligns wi th the current development needs.However,traditional CWs rely on the nitrification-denitrification process for nitrogen and carbon removal,resulting in limited effectiveness in total nitrogen removal.Partial nitrification-anammox（PNA）is a comprehensive autotrophic nitrogen removal process that efficiently removes nitrogen without the need for a carbon source.In this study,we structed two different configurations of vertical subsurface flow constructed wetlands（VSFCW）with partial nitrification(VSFCW_PN):an upflow system with a"bottom inflow-artificial aeration"design and a downflow system with a"drip irrigation-tidal reoxygenation"design.These systems were compared in terms of the stability and performance of the PN process.After conducting comparative research,it was determined that the upflow VSFCW_PN configuration was more suitable for achieving the PNA process.To further investigate the establishment of the PNA process in the integrated VSFCW and provide theoretical support for the efficient treatment of rural domestic sewage,acrylic panels were installed in the integrated VSFCW.This allowed for the separation of the system into VSFCW_PN and VSFCW_AN.In the VSFCW_AN,NH4⁺-N and NO2^--N were removed through the anammox process using the effluent from the VSFCW_PN.Additionally,in-depth research was conducted on the relevant influencing factors and the long-term stability of the PNA process.The specific research results are as follows:Most microorganisms in the VSFCW system adhere to the substrate’s surface and are challenging to wash out the reactor.Consequently,NH₂OH is utilized as an inhibitor of nitrite-oxidizing bacteria（NOB）.In the upflow VSFCW_PN configuration with"bottom inflow-artificial aeration",the concentrations of dissolved oxygen（DO）and NH₂OH were maintained at（1.2±0.2）mg/L and 5 mg/L,respectively.The system maintained an average nitrite accumulation rate（NAR）of 88.24%,and the effluent NO2^--N/NH4⁺-N ratio was stabilized at（1.26±0.05）,establishing a stable PN process.Intermittent aeration consumes less energy compared to continuous aeration.However,operating under intermittent aeration mode leads to the continuous growth of heterotrophic denitrification bacteria,which affects the stability of the PN process.Similarly,the decrease in hydraulic retention time（HRT）results in increased diversity of microbial community structure and a gradual decrease in the abundance of autotrophic bacteria compared to heterotrophic bacteria.To optimize the relationship between reaction time and idle time in the downflow VSFCW_PN with the PN process,we conducted experiments with a reaction time of 5h and an idle time of 10h.Under these conditions,we achieved optimal reaction conditions with an ammonia transfer ratio（ATR）of 70.98%and a NAR of 80.74%.The NO₂^--N/NH₄⁺-N ratio was consistently maintained at approximately 1.30.During the operation of the downflow VSFCW_PN,we observed an increasing abundance of heterotrophic denitrification bacteria.As a result,the stability of PN was disrupted.Through 16S rRNA results,we discovered that the abundance of denitrification bacteria was approximately five times higher than that of ammonia-oxidizing bacteria（AOB）.This result indicates a significant presence of denitrification bacteria in the system.Comparatively,the performance of the downflow VSFCW_PN,operating with"drip irrigation-tidal reoxygenation"is more influenced by organic matter than the"bottom inflow-artificial aeration"upflow operation.When the influent C/N ratio in the VSFCW with anammox(VSFCW_AN)exceeds 0.84,the activity of anammox bacteria（An AOB）is low,resulting in the unsuccessful initiation of the anammox process.However,when the C/N ratio falls below 0.80,the activity of An AOB gradually recovers.Additionally,after the completion of biofilm,the VSFCW_AN develops resilience against adverse factors,allowing for the successful implementation of th e PNA process.In terms of influent concentrations,the average chemical oxygen demand（COD）,total nitrogen（TN）,and PO₄^3--P concentrations in the integrated VSFCW were measured at 120.75mg/L,60.02 mg/L,and 5.05 mg/L,respectively.The removal efficiencies for these pollutants were found to be 86.26%,90.22%,and 78.94%,respectively.It is worth noting that the plants removed 5.97%and 3.82%of the nitrogen and PO₄^3--P,respectively.After rural domestic sewage enters VSFCW_PN,approximately（32.28±4.46%）of NH₄⁺-N is oxidized to NO₂^--N,（71.36±10.50%）of COD,and（29.52±9.85%）of PO₄^3--P are removed.VSFCW_AN contributed（54.47±4.35%）to the TN removal efficiency（NRE）in the system.In addition,12.63±1.8%of COD and 53.33±6.08%of PO₄^3--P were removed.The substrate samples were collected in 10 cm（PN1,AN1）and 25 cm（PN2,AN2）height layers.The results of microbial community analysis indicate that the relative abundance of Nitrospira,a typical genus of bacteria with NOB function,is found to be less than 0.05%.NH₂OH has a significant inhibitory effect on NOB.As the height rises,the concentration of pollutants gradually decreases,and Ca.Brocadia decreased to 8%（from AN1 to AN2）.