Removal Efficiency Of Microplastics In Constructed Wetlands And Its Impact On Constructed Wetlands

Microplastics refer to plastic with a size of less than 5 mm,which have attracted widespread attention due to their potential threats to the natural ecological environment.At present,researches on the removal of microplastics from sewage is mainly based on conventional sewage treatment systems.As a new sewage treatment facility,constructed wetlands are still in infancy.This study firstly took the typical constructed wetlands（consisting of subsurface flow and surface flow process sections）as the object to investigate the removal effect of microplastics in actual engineering,and selected wetland types for follow-up research based on the comparison of microplastics.Then,small-scale experiments were conducted to explore the effects of different factors on the removal of microplastics from underground constructed wetlands;finally,microplastics were mixed with substrates in different weight ratios to explore the effect of microplastics accumulation in wetlands on their water purification efficiency,plant and matrix effects.The impact of the microbial community;the main conclusions are as follows:（1）Two typical composite flow constructed wetlands in Suzhou were studied.The results showed that the abundance of microplastics in the influent of the two constructed wetlands（CW1 and CW2）were（10.20±1.97）items/L and（6.77±0.95）items/L,abundance of microplastics in the effluent was（0.78±0.11）)/L and（0.74±0.19）/L,respectively,and the total removal rate was 92.35%and 89.01%.The size of microplastics is mainly 30.8～500μm,the main types are polyethylene terephthalate（PET）,polystyrene（PS）,polypropylene（PP）,and the shape is mainly fiber.The subsurface flow process section plays a major role in the removal of microplastics,with a contribution rate of over 96%.（2）At the pilot scale,explore the effects of different factors（substrate size,plant and biofilm enrichment）on the removal efficiency and vertical distribution of microplastics in subsurface flow constructed wetlands（VFCW）.The experimental results showed that the 30-day average microplastic removal rate based on coarse sand was 100%for VFCW3 and VFCW4 with or without plants.Increasing depth shows an exponentially decreasing distribution.The average microplastic removal rate of VFCW1 30d with fine gravel as substrate was 96.45%,and the distribution abundance of microplastics in the substrate decreased gradually with the increase of depth.The 30-day average removal rate of microplastics in VFCW2 with coarse gravel was 93.55%,and the distribution abundance of microplastics in the second layer（3-6 cm）and the first layer（0-3 cm）was the highest.On the 30th day,the removal rate of microplastics by coarse gravel-based VFCW5（with less biofilm）was 79.36%,and the removal rate showed a further obvious downward trend.The above studies show that substrate size and biofilm enrichment can significantly affect the removal efficiency and vertical migration of microplastics in constructed wetlands.（3）In order to gain an in-depth understanding of the potential impact of the continuous accumulation of microplastics in constructed wetlands,different concentrations（0%,0.2%,0.4%,0.8%,1.6%）of microplastics were added to the constructed wetlands.Wetlands with coarse sand as the matrix.Microplastics built 5groups of artificial wetland systems,which continued to operate for 180 days.The experimental results show that with the increase of the concentration of microplastics,there is no significant difference in the removal rates of COD,NH₄⁺-N and NO₃^--N,and NO₂^--N does not accumulate significantly,indicating that the water purification efficiency of constructed wetlands is not affected by microplastics.With the increase of the concentration of microplastics,the plant height and fresh weight of tall fescue plants in the constructed wetland decreased,and the contents of superoxide dismutase,peroxidase and malondialdehyde in leaves increased significantly,indicating that microplastics have a negative impact on the ecological environment.At the OUT level,principal component analysis was performed on the wetland microbial community.With the increase of microplastic concentration,the microbial communities of different constructed wetlands were obviously separated（the total detection rate was 58.07%）.At the microbial phylum level,the relative abundance of Fimicutes showed a significant downward trend,and the relative abundances of Chloroflexi,Aidobacteria and Nanoarchaeota showed a significant upward trend.At the genus level,the relative abundances of Desulfobacter,Delftia,Magnetococcus,Hydrogenobacter,and Fusobacterium showed a significant decreasing trend,while the relative abundances of Denitrifier,Zoogloea,Sulfuritalea and Lautropia showed a significant downward trend.The relative abundance showed a clear upward trend.Affected by microplastics,the composition of wetland microbial communities showed significant structural changes.