Study On The Distribution Characteristics Of Microplastics In A Municipal Wastewater Treatment Plant

As the whole society pays more and more attention to the pollution of microplastics in the environment,microplastics,as a new type of water pollutants,have been widely studied.At present,microplastics have already been generally found in different kinds of natural environments and living organisms,and related researches have also figured out that microplastics will bring a series of negative effects on organisms,and in some serious cases,microplastics can even cause animal death.The living organisms are widely influenced by this problem,including fish,shellfish,algae,and cetaceans,which are all important components of daily food consumed by people.Microplastics may be enriched and accumulated in human bodies when people eat these contaminated animals or plants,which further poses a potential risk to human health.Human activities is the main reason leads to microplastic pollution;in this case,microplastic pollution in urban areas is more serious than that in rural areas,especially in sanitary sewage.The sanitary sewage in the city will be transported to the urban sewage treatment plant through the urban pipe network and pumping station.The urban sewage treatment plant is also responsible for the control and disposal of water pollutants,and it is the"filter"of the urban water cycle.Understanding and analyzing the problem of microplastic pollution in urban drainage systems,mastering the characteristics of distribution of microplastics in sewage treatment plants are the indispensable part of solving the problem of urban microplastic pollution through sewage treatment plants in future.This paper,which takes one year to study a city sewage treatment plant as the research object,analyzes the time characteristics of the microplastic abundance and characteristic composition in the process.The potential influences of water quality parameters on the microplastic abundance and removal rate are also being analyzed.The main results of this research are listed bellow:（1）Comparing 5 microsplastic pretreatment technologies（KOH,NaOH,H₂O₂,HNO₃·HCl and HNO₃）through the single factor test,orthogonal test,digesting effect analysis and characterization of surface morphology,it is found that:KOH,NaOH digesting methods have a small effect on recovery rate after digestion,whereas they are easy to cause thickening of the water sample,so that the time of leaching grows and some deformation effects on the surface of microplastics may occur;HNO₃·HCl,HNO₃acid digestion methods have a greater impact on the recovery rate of microplastics and have a serious corrosive damage to microplastics;H₂O₂ is the best digestion method considering the effect of microplastics digestion,the influence of recovery rate,and the damage of microplastics,while the best time for digestion is 60 minutes and the most preferred dose is 10 mL/100 mL.（2）Through the spectral analysis of the four suspected microplastics in the inlet,sand settling tank,secondary settling tank and filter outlet of the sewage treatment plant,the proportion of microplastics was 27.0%,24.0%,19.0%and 21.0%,respectively,with an average of 22.8%.Statistically,it shows that the average abundance of microplastics of size of more than 100μm in the plant inlet is 427.8 items/L,the effluent of primary treatment is 350.0 items/L,the effluent of secondary treatment is 43.1 items/L,the effluent of tertiary treatment is 30.3 items/L,and the overall removal rate of microplastics in the plant is 92.9%.The one which contributes most to the removal of microplastics in the treatment process is the secondary treatment stage,whose removal rate accounts for 79.0%of the total removal.It is found that the abundance of microplastics showed significant changes in the data throughout the year,and the removal rate also fluctuated.The annual analysis also found that the sewage plant has a better removal effect on black microplastics,fibrous microplastics,and microplastics in large size,while the sizes of most microplastics in the study are between 100-250μm.（3）By comparing relevant parameters and microplastic abundance analysis,it is found that there is a correlation between COD concentration,NH3-N concentration,SS concentration and microplastic abundance;it is also found that there is a clear correlation between the COD removal rate,NH₃-N removal rate,SV value,SVI value,MLSS and removal rate of microplastics via comparing related parameters with removal rate of microplastics.Therefore,the factors that have a greater influence on the removal rate of microplastics in sewage treatment plants are obtained,mainly the sedimentation of activated sludge and the biochemical effect of the biochemical system.The removal rate of microplastics is higher when the biochemical effect of the biochemical system and the settling performance of the activated sludge are good.The measures to improve the removal rate of microplastics are mainly concentrated on two aspects:improving the sedimentation of activated sludge and strengthening the biochemical effects of biochemical systems.