Enhanced Transformation Of Nitrogen And Phosphorus In Vertical Flow Constructed Wetland Filled With Construction Wastes Under Micro-Oxygen Regulation

Water pollution treatment has become a serious problem that restricts the country's economic and social development.Especially decentralized domestic wastewaters discharged from the vast rural areas and scattered residential areas lack effective treatment.With advantages of low costs,maintenance requirements and eco-friendliness,constructed wetlands(CWs)are acknowledged as a promising technology for in-situ treatment of decentralized domestic wastewaters.It has been documented that traditional CWs offer good performance in removing suspended solids and organic matters,but have poor removal efficiency for eutrophication elements such as nitrogen and phosphorus.By constructing lab-scale intermittent aerated vertical subsurface flow constructed wetlands(VFCWs),the present study investigated the influence of intermittent aeration and different construction wastes(gravel,red brick,fly ash brick)on the long term treatment performance in VFCWs for treating simulated domestic wastewater.The physicochemical properties of the three substrates were evaluated.The contribution of different removal pathways to nitrogen and phosphorus removal in VFCWs were quantified.Finally,the microbial response mechanisms in intermittently aerated constructed wetlands were obtained by using microbial analyses.The main results were as below:(1)Fly ash brick and red brick have a porous and rough micro-environment which were suitable for the growth of microorganisms.' Their physicochemical properties determine that these two kinds of substrates are suitable for removing nitrogen and phosphorus.The ammonia and phosphorus adsorption process of the substrates were fitted by the Langmuir isotherm model,and the order of adsorption capacity is fly ash brick>red brick>gravel.(2)Compared with non-aerated constructed wetlands,intermittent aeration can significantly improve the removal efficiency of pollutants in constructed wetlands.Through continuous monitoring,the removal efficiency of COD?NH4+-N and total nitrogen(TN)were as high as 94.71%,99.11%,93.91%,respectively.The mass removal rate of total phosphorus(TP)in intermittent aerated wetlands has also increased significantly.The long-term operation of the three aerated wetland systems remained stable for COD and nitrogen removal.But the TP concentration in the effluent increased in the second year,due to the gradually saturated adsorption.(3)Different substrates affected the performance of wastewater treatment in constructed wetlands.Fly-ash brick had the highest removal efficiency,followed by red brick and gravel.The results indicated that fly ash brick is potential substrate for enhanced nitrogen and phosphorus removal in constructed wetlands,followed by red brick.(4)Due to the limitation of nitrification,nitrification/denitrification were the major nitrogen removal pathways in the aerated wetlands(accounted for 36.79-55.44%).The main nitrogen removal pathways in non-aerated systems was the media storage(8.47-48.64%).Less than 16%of nitrogen was removed by plants uptake,and N2O emissions contribute 0.08-0.39%.Intermittent aeration also promoted plant growth,and thusly increased plant uptake of phosphorus by 0.57-1.45 times.Media storage was still the main phosphorus removal pathway,accounting for 23.92-59.47%.The phosphorus absorbed by microorganisms in the aerated systems accounted for about 30.93%,and only 14.84-26.47%in non-aerated systems.This result indicated that the contribution of microbial uptake to phosphorus removal,and intermittent aeration significantly changed the transformations of nitrogen and phosphorus in constructed wetlands.(5)Microbial analyses found that intermittent aeration increased functional bacteria associated with nitrogen and phosphorus removal in constructed wetlands.It revealed the intrinsic relationship between microbial community structure and water purification effects.The above research results provided theoretical basis and technical support for in-depth understanding of the mechanism of nitrogen and phosphorus transformation in intermittently aerated CWs with construction waste,and improving the ability of simultaneous removal of pollutants from scattered domestic sewage in CWs.