| In recent years,microplastics pollution has become an environmental issue of increasing concern to countries around the world.As an important place for the collection and discharge of microplastics,studying the characteristics of the occurrence of microplastics in the influent of wastewater treatment plants will help to sort out the sources of microplastics,so as to control the production of microplastics at the source,studying the effects of treatment processes in wastewater treatment plants on microplastics.The reduction efficiency of microplastics will help to select a treatment process with better microplastics removal effect for new wastewater plants on the basis of ensuring that the conventional indicators meet the standards,research on the water source and land-source discharge of microplastics in wastewater treatment plants will help to master the basic data on the source effect of microplastics in the environment of wastewater treatment plants.In order to study the occurrence and removal of microplastics in wastewater treatment plants in inland cities in China,this study taked four municipal wastewate treatment plants included Jinyang to the west of the Fen River in Taiyuan City,and Beijiao(in the north),Yangjiabao(in the middle)and Fendong(in the south)to the east of the Fen River as examples.The microplastics were separated and extracted by the pretreatment methods of flotation and digestion on the samples collected from the wastewater treatment plants.A stereo microscope was used to visually observe the morphological characteristics of the microplastics,and combined with a Fourier transform microscope imaging infrared spectrometer to analyze the composition of the microplastics.Study the characteristics of microplastics in different wastewater sources,and the impact of different treatment processes on the removal of microplastics,and evaluate the source effect of the final effluent and sludge on the microplastics in the natural environment.The research came to the following conclusions:(1)The concentration of microplastics in the influent water of the Taiyuan urban wastewater treatment plant were 11.7 n/L in Jinyang,54.8 n/L in Beijiao,10.7 n/L in Yangjiabao and 21.0 n/L in Fendong.No obvious correlation was found between plastic concentration and population density.Fibrous form was the main shape of influent microplastics.For film-shaped microplastics,it accounted for less than 15%of the total influent microplastics in various factories.The common granular microplastics in daily chemical products only in Yangjiabao wastewater treatment plant was discovered(only 0.7%of the total).In the four wastewater treatment plants,the proportion of small and medium-sized microplastics smaller than 1000μm was greater than 55%,transparent and black were the main colors of the four wastewater treatment plants influent microplastics.There is no obvious correlation between the characteristic parameters(the color,shape and size of the microplastics,etc.).The difference in the concentration of microplastics in the influent of wastewater treatment plants may be related to complex factors including the service population,the area of the service area,the degree of land use,and the living habits of residents.(2)The overall analysis of the composition of microplastics in the wastewater treatment plant samples found that the polypropylene-polyethylene copolymer,cellulose,polyester and rayon components in the four sewage treatment plants accounted for more than 55%of the total.Among other components,the Jinyang wastewater treatment plant located in the west of the city had more abundant types than the three wastewater treatment plants in the east of the city,which may be related to the widest service area of the plant.The components of microplastics were related to the use of plastic materials by residents in the service area and laundry waste water.(3)The overall removal rate of microplastics in the four tertiary wastewater treatment plants ranged from 16.15%to 89.04%,with an average of 62.94%,which was low compared with other studies.Compared the removal of microplastics by various wastewater treatment processes,the treatment processes at all levels in the plant had a certain removal effect on microplastics,but the reduction effect mainly occurs in the primary and advanced treatment stages,and the removal rate of the primary treatment range was 42.35%-50.94%,and the advanced treatment is 9.95%-68.45%.Affected by the large amount of microplastics carried in the return sludge,the removal rate of microplastics in individual water plants will show a negative increase after the secondary treatment.As an advanced treatment process to ensure that the quality of the factory water was up to the standard,the advanced treatment process of each wastewater treatment plant showed a certain reduction ability on microplastics,but the removal efficiency of microplastics of different shapes was different.Among them,Yangjiabao’s coagulation precipitation process had the highest fibrous removal rate,reaching79.06%,while the non-fibrous removal rate showed a negative increase of-21.33%.Jinyang wastewater treatment plant system 2’s filtration process had the highest non-fibrous removal rate was 81.62%,and the fibrous removal rate was also as high as 61.56%.(4)Compared the removal of microplastics in different structures.In the primary treatment,the removal rate of microplastics in the grit tank was 19.23%-34.27%,and the removal rate,size and shape of the microplastics in the grit tank had no obvious relationship.On the whole,the contribution rate of the primary sedimentation tank to the removal of microplastics in the primary treatment was lower than that of the grit tank.In the advanced treatment,the fiber filter of Jinyang system 2 and the sand filter of Fendong system 2 had removal rates of 47.22%and11.91%for microplastics.The removal effect of fiber filters pool on microplastics was better than that of sand filters.In the removal of microplastics of different shapes in the high-efficiency sedimentation tank,the removal of fiber was better than that of non-fiber.(5)The concentration of microplastics in the effluent of Taiyuan wastewater treatment plant was 3.5-9.8 n/L,which was generally at a moderately low level compared with other domestic research on wastewater treatment plants.The concentration of microplastics in the remaining sludge range was 10.4-50.4 n/g dry weight,which was at a medium to high level in China.When the wastewater treatment plant was operating at full capacity and the treated wastewater is discharged,it was estimated that about 1.74×10~4 billion microplastics enter the natural water body every year and about 1.41×10~4 billion microplastics were transferred to the natural water body through outbound sludge into other environmental media. |
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