Effects Of Different Operating Conditions Of Subsurface Flow Constructed Wetlands On Nitrogen Removal In Wastewater And Its Mechanism

With the rapid development of industrialized urbanization,a large amount of domestic sewage has been discharged into natural water bodies without treatment,resulting in a significant decline in the surface water quality of our country,a serious eutrophication phenomenon.The treatment of domestic sewage has been concentrated on more attention.Constructed wetlands(CWs)have been widely applied in domestic sewage treatment due to their cost-effective,easy-maintaining and environmentally-friendly performances.Microbial nitrification and denitrification in CWs are the main mechanisms on nitrogen removal in the systems.In order to increase the removal rates of pollutantants of N and COD in vertical flow constructed wetlands(VFCWs),this study was focuse on the practional conditions,such as COD/N ratio,intermittent aeration,and hydraulic retention time.The main research results and conclustions are as follows:(1)The combined effects of COD/N ratios(3,6,12)and intermittent aeration in VFCWs on pollutants removal,microbial communities and nitrogen function genes were studied.The increase of COD/N ratios from 3 to 12 enhanced the removal efficiency of TN,NO3–-N and COD.The removals of NH4+-N decreased as the COD/N ratio increased.The optimal removals of TN(87.65%),NH4+-N(93.20%),NO3–-N(80.80%)and COD(73.93%)were obtained in VFCW2(COD/N ratios was 6).Illumina Miseq High-throughput sequencing analysis showed that high COD/N ratios increased the richness and diversity of microbial communities.The absolute abundance of nir K,nos Z,nir S,amo A,nxr A,and anammox bacterial 16 S r RNA presented various changes under the different ratios of COD/N.The increase of COD/N ratios enhanced the copy numbers of nir S,nir K and nos Z,which participate in denitrification process.High COD/N ratios(6 and 12)were in favor of Actinobacteria,Firmicutes and Chloroflexi,which mainly play important roles in the process of denitrification.(2)The best removal of COD(74.16%),NH4+-N(93.56%),TN(86.88%),and NO3–-N(79.65%)were achieved in VFCW1(carbon source addition and intermittent aerated system).Illumina Mi Seq300 high-throughput sequencing indicated that carbon source addition and intermittent aerated system increased the diversity and richness of microbial community.Thecopy numbers of nitrification functional genes,denitrification functional genes,and anammox functional gene(anammox bacterial 16 S r RNA)presented various changes under the different aeration modes and carbon source addition.Among them,an increase of the DO concentration and addition of carbon source facilitated the absolute abundance of microbial nitrification(nxr A,amo A)and denitrification functional genes(nir S,nir K,nos Z),respectively.These results demonstrated that carbon source combined with intermittent aeration is valid to improve the pollutants treatment performance in this systems.(3)Intermittent aeration and different hydraulic retention time(HRT)could enhance the nitrogen removal effect of TN in VFCWs.This study strengthened the N removal efficiency of the system by combining dissolved oxygen(DO)and HRT.As the results showed,the longer HRT(HRT = 6)in intermittent aeration VFCWs had optimal removal rate of pollutants(TN 89.41%,NH4+-N 95.24%,NO3–-N 86.96%,COD 88.07%).Illumina Mi Seq300high-throughput sequencing demonstrated that longer HRT(HRT = 6)combined with sufficient oxygen increased the richness and diversity of the microbial community.In a system with intermittent aeration combined with HRT of 6,Proteobacteria,which participated in the conversion and circulation of carbon and nitrogen,and Acidobacteria,which played an important role in microbial heterotrophic denitrification,have the highest abundance under this operating condition.In addition,the absolute abundance of nitrogen functional genes in the system with longer HRT(HRT = 6)is higher,indicating that the operation of aeration and the extension of HRT increase the copy number of the functional genes related to nitrification,nxr A and amo A,and increase absolute abundance of functional genes(nos Z,nir K and nir S)related to heterotrophic denitrification.The present findings highlight the importance of the interaction between the intermittent aeration and the HRT in improving the pollutant removals in VFCWs.