| Constructed wetland is known as a kind of green and environment-friendly wastewater treatment technology in the world.Constructed wetlands in unsaturated areas can efficiently remove ammonia nitrogen(NH4+-N)and chemical oxygen demand(COD)through extra oxygen supplementation.The height of unsaturated zone is an important factor that affects oxygen supplementation.Low height may cause the low reoxygenation rate while excessive height may cause the destruction of the anaerobic environment in the saturated area,influencing the removal of denitrification and then reducing the total nitrogen removal rate.Clearly,it is vital to choose suitable height of the unsaturated zone can be seen that the choice of coupling mode between the height of the unsaturated zone and the denitrification promotion in the saturated zone is crucial to the overall denitrification and phosphorus removal effect of the constructed wetland system.Against the above situation,this study optimized the height of the unsaturated zone and proposed a novel constructed wetland system that utilized the conductivity of carbon felt and iron-carbon micro-electrolysis to enhance electron supplyment.In the height optimization experiment of unsaturated region,different unsaturated zone heights(0,10,20 cm)were set.In the carbon felt experiment,the addition methods of evenly distribution,vertical connection and coupled microbial fuel cells were set up to investigate the possible effects of different distribution modes of carbon felt on electron self-supply.In the iron-carbon micro-electrolysis experiment,biochar scrap,iron and micro-electrolysis fillers were applied as functional substrates respectively to investigate the contribution of different functional fillers to the pollutant removal efficiency of wetland system.Water quality monitoring,quantitative polymerase chain reaction,high-throughput sequencing,microelectrode,phosphorus form analysis,and matrix characterization were used to monitor and analyze the data of each system,exploring the influence of different enhanced electronic supply methods on the removal of pollutants in wetland system and reveal the internal mechanism.The main conclusions are as follows:1.The elevation of the unsaturated region significantly promoted the removal efficiency of NH4+-N and COD.The accumulation of NO2--N was observed at the low height of unsaturated region,indicating that short-cut nitrification and denitrification occurred in the unsaturated zone.By comprehensive comparison of effluent water quality,the height of unsaturated zone is determined to be 20 cm,making for the removal of NH4+-N and COD.The removal rates were 96.8±0.8%and 90.4±0.8,respectively.A large amount of NO3--N was accumulated during nitrification.Meanwhile,the consumption in the unsaturated zone resulted into the decrease of C/N in the denitrifying zone,affecting the removal of TN.2.The addition of carbon felt promoted the removal of NH4+-N and TP in the wetland system.The vertical connection of carbon felt made the NH4+-N removal rate of the CWL system reach to 82.3±0.8%,which was nearly 50%higher than that of the CWu without carbon felt(57.1:±.7%).As the results of functional gene abundance and routine physical and chemical indexes displayed,the abundance of functional genes in the saturated zone of the carbon felt vertically connected constructed wetland system was not high.But the removal efficiency of TN was high and the microbial community structure changed significantly,suggesting that electrons positively transferred on the carbon felt.The electron transport improved the removal of NH4+-N in the saturation.Area.3.The addition of biochar could help enriched microorganism.The addition of scrap iron promoted chemical denitrification and chemical autotrophic denitrification,where the removal rate of NO3--N reached to 94.2±1.3%in the CWFI system.The invigorating effect of nitrogen and phosphorus removal in the wetland system by the addition of biochar or scrap iron alone was limited due to the release of nitrogen and phosphorus,microbial toxicity and dissimilated reduction(The removal efficiency of TN in the CWFB and C WFI system reached to 41.5±4.7%and 76.1±0.6%,respectively).Firstly,Fe-carbon micro-electrolysis could advantage of both.Secondly,Fe-carbon micro-electrolysis could reduce the content of single functional filler to alleviate oxidative stress on wetland plants,improving the removal of N and P,especially NH4+-N by plants.Thirdly,a large number of galvanic cells were formed to promote the decomposition of organic matter and denitrification(The removal rates of TN and TP in CWFME system were 86.5±1.7%and 98.8±0.7%).In this study,we used microfiber as filler to construct unsaturated zone in the wetland,which could realize the supplement of dissolved oxygen in the system.The increase of the height of unsaturated zone would promote the removal of NH4+-N,but reduce the removal rate of TN.The vertical connection of carbon felt enhances the electron transfer process and facilitates the removal of ammonia nitrogen and total nitrogen from the system.The addition of Iron-carbon micro-electrolytic fillers promoted the biological and chemical denitrification and improved the removal rate of TN and TP in the system. |
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