| Constructed wetlands（CWs）are widely used because of their good environmental,economic and ecological benefits.However,there is still a lack of systematic research on the treatment of low C/N wastewater and nitrogen and Cr（VI）/perchlorate complex polluted wastewater by CWs.The wetland substrate plays a key role in the decontamination performance of CWs.Therefore,it is essential to optimize the combination of wetland substrates and propose effective control measures for wastewater containing nitrogen compounds.In this paper,iron ore,manganese ore,and wood chips are used as the main substrates to build three sets of series vertical flow constructed wetlands（TS-VFCWs）,namely control CWs（A-CWs:gravel + gravel）and enhanced CWs（B-CWs: iron ore + wood chips,C-CWs:manganese ore + wood chips）.The long-term operation of the CWs was studied to investigate the treatment efficiency of different combinations of substrate CWs on nitrogen-containing composite polluted wastewater（nitrogen and Cr（VI）/perchlorate）,to reveal the distribution and transformation patterns of different pollutants in CWs,and to elucidate the possibility and microbial mechanism of the synergistic enhancement of nitrogenous composite polluted wastewater purification by iron/manganese ores and wood chips.The main conclusions of this study are as follows:（1）Nitrogen load elevation and salinity shock had significant effects on the nitrogen removal efficacy and microbial community of CWs.The addition of iron ore and wood chips significantly improved the removal of conventional pollutants such as COD,ammonia nitrogen and nitrate nitrogen in the tandem VFCWs.The B-CWs system had better removal of COD,ammonia nitrogen and nitrate nitrogen during the pollution load raising stage,while high loads inhibited the nitrogen removal performance of the wetland system.As the load was reduced,the B-CWs system showed good recovery potential.3% salt stress reduced COD and ammonia nitrogen removal,but enhanced nitrate-nitrogen removal and maintained relatively good nitrogen removal.Iron ore dosing led to a large enrichment of nitrifying and autotrophic denitrifying bacteria,while wood chips played a key role in heterotrophic denitrification and organic matter degradation.In addition,the prevalence of denitrifying bacteria under salinity conditions ensured excellent denitrification performance.Thus,B-CWs possessed superior denitrification performance and salt shock resistance.（2）The addition of low concentration of Cr（VI）had little effect on the denitrification performance of CWs,but high concentration of Cr（VI）significantly inhibited denitrification and could not be recovered.The addition of 2 mg/L Cr（VI）had little effect on the denitrification of B-CWs,but significantly inhibited the removal performance of A-CWs and C-CWs.Simultaneous and efficient removal of nitrogen and Cr（VI）can be achieved by B-CWs at low Cr（VI）concentrations.When the Cr（VI）concentration increased to 10 mg/L,the denitrification and Cr（VI）removal performance in three CWs showed a downward trend.When the Cr（VI）concentration was further reduced to 5 mg/L,3 mg/L and 0 mg/L,the denitrification performance in the three CWs was still not recovered.The denitrification and Cr（VI）removal performance of B-CWs was better than that of C-CWs,and both were better than that of A-CWs in Cr（VI）concentration reduction stage.XRD and EDS further confirmed the presence of a large amount of bound Cr on the surface of iron ore and wood chips,indicating that B-CWs had excellent Cr removal potential.In addition,the addition of iron ore significantly increased the relative abundance of Proteobacteria,Chloroflexi and Firmicutes.In conclusion,B-CWs can achieve the simultaneous removal of low concentration nitrogen and chromium-containing wastewater.（3）Perchlorate dosing significantly inhibited nitrate nitrogen removal in CWs,but had less effect on ammonia nitrogen removal.When 5 mg/L perchlorate was added,the removal of nitrate nitrogen and TN was inhibited.When the perchlorate concentration in the influent was reduced to 1 mg/L,the removal rates of nitrate nitrogen and TN gradually increased.The removal of nitrate nitrogen by CWs was all better than that of perchlorate,probably due to the failure enrichment of the perchlorate-reducing bacteria during relatively short period（<32 days）.In addition,the removal rates of perchlorate in the three CWs were all lower than 10%,and while the removal efficiency of B-CWs was the best（9.5%）.It can be concluded that BCWs can be used as a purification technology for nitrogenous and perchlorate wastewater,but short-term operation（<32 days）was not enough to enrich sufficient perchlorate-reducing bacteria,while the gradient increase strategy of perchlorate loading may facilitate efficient removal of nitrogen/perchlorate complex pollution. |
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