Study On Mechanisms Of Coupling Effect Between Clogging And Phosphorus Accumulation In Substrate Of Subsurface Flow Constructed Wetland

Constructed wetland(CW)is one of the most common ecological water treatment technologies in the world.Compared with traditional sewage water treatment plants,it has the advantages of low construction cost,simple maintenance,and less secondary pollution.It has been widely used in integrated control of water pollution and water quality promotion of reclaimed water.After entering the "14th Five-Year Plan",CWs have been included in the water ecological environment construction plans of many provinces.In view of the wide application of CWs in protecting the aquatic ecological environment,maintaining the long-term stable operation of constructed wetlands has important economic and ecological values.CWs often face substrate clogging and phosphorus adsorption saturation problems during operation,which will lead to a decline in their treatment performance,thereby reducing their service life.Although there have been related studies on substrate clogging and phosphorus adsorption saturation in subsurface flow CWs,most of the current research focused on the independent mechanism of substrate clogging or phosphorus accumulation.Nevertheless,combined clogging-adsorption interactions in CWs are still poorly understood.Therefore,it is of great significance to study the coupling mechanism of matrix clogging and phosphorus accumulation in CWs to provide theoretical basis and technical support for the efficient and stable operation of CWs.In this study,by constructing visualized microflow field systems,the preferential flow field and pore structure in porous media were monitored in situ,and the influence of phosphorus on material accumulation and flow field changes in substrate clogging was revealed.At the same time,by systematically analyzing the spatial distribution characteristics of phosphorus accumulation in subsurface flow CWs with different clogging degrees,quantifying the composition and distribution of the microenvironment inside the wetland,and combining microbial analysis and substrate characterization,the coupling mechanism of substrate clogging and phosphorus accumulation was explored.The obtained results provided theoretical support for the efficient and stable operation of CWs.The main conclusions are as follows:(1)Phosphorus loading increased the uniformity of the water flow field in the CW,and reduced the heterogeneity of the pore structure and the area of dead water.Before the end of the experiment,the proportion of preferential flow area was 21.98%and the box dimension was 1.69 when the influent contained phosphorus,which was significantly higher than that of the influent without phosphorus(6.13%and 1.57,respectively).In addition,the effective porosity in the case of influent containing phosphorus was about 3 times that of the case without phosphorus.This was mainly because the presence of phosphorus increased the content of tyrosine substances and soluble microbial by-products in the extracellular polymer,weakens the accumulation of tryptophan,reduces the hydrophobicity of the cell surface and the aggregation of cells,resulting in the formation of artificial wetlands.The flow field was relatively uniform.(2)The increase of phosphorus load would easily lead to the complete clogging of wetland substrate pores.Generally,biofilms accumulated in areas with high seepage flow rates.Due to the interaction between biofilms and shear stress,the positions of preferential flow channels in CWs were continuously rearranged,and complete clogging was not easy to form.However,when the influent contained phosphorus,the content of phosphorus and calcium in the accumulated substances in the matrix increased significantly,the phosphorus-containing substances and calcium deposited accumulate in the low flow field strength area of the pores,and the clogging substances in the throat were under shear.Migration and aggregation into pores under force can easily cause complete clogging of porous media.(3)The increase of clogging degree would lead to the reduction of hydraulic retention time and the increase of adsorption sites at the same time.Among them,the phosphorus removal efficiency decreased from 85.79 ± 2.55%to 76.04 ± 2.40%under low clogging degree.However,the increase of adsorption sites offset the effect of the reduction of hydraulic retention time,and the phosphorus removal efficiency increased to 91.64±2.08%at high clogging degree.Substrate adsorption was the main way for phosphorus removal in subsurface CWs.The phosphorus absorbed by substrates in CWs with high clogging degree accounted for 77.86± 2.63%of the total input phosphorus in the system,which was significantly higher than that in the control group(60.08± 4.79%).(4)Substrate clogging increased the release risk of accumulated phosphorus in subsurface flow CWs.Phosphorus fraction analysis showed that the increased substrate phosphorus accumulation in CWs under high clogging degree was attributed to the higher ratio of Fe/Al-P accumulation.The main reason was that clogging reduced the redox potential in the system and promotes the release of iron ions.The newly generated iron ions combined with phosphate ions and flocculate on the substrate under anaerobic conditions,resulting in increased accumulation of iron-bound inorganic phosphorus.Fe/Al-P has good bioavailability and is easily released under the disturbance of environmental factors,which increased the risk of phosphorus release in wetlands.