| Mi Microplastics refer to plastic particles with a particle size of less than 5 mm,which are widely found in rivers,lakes and the atmosphere,and even detected in the human body and other organisms.Due to the characteristics of easy use and low cost,plastics are massproduced and used,causing plastic pollution to become one of the inevitable pollutions of human society.As the final gathering place of urban sewage,the sewage plant plays an important role in decontamination and emission reduction,and is the last hurdle for waste water to enter the environment.Flocculation is an important treatment process in sewage treatment plants.Due to changes in the size and density of microplastic particles,the flocculation effect will also vary,and the use of different types of flocculants will also affect the flocculation removal effect.Therefore,flocculation experiments were carried out to analyze the flocculation The mechanism is imminent.At present,the research on flocculation reaction mainly focuses on the single factor research,but the single factor experiment cannot explore the interaction between different factors.This study takes the sewage treatment plant of Hohhot,an inland city in the north,as the research object.Through the whole process analysis of the treatment process of the sewage plant,the occurrence form of microplastics in different structures is explored,and the removal effect and migration law of microplastics in different treatment processes are analyzed.,to qualitatively analyze the shape differences of microplastics of different sizes.Orthogonal test and gray relational analysis were used to evaluate the influence of multiple variables and their interaction on flocculation effect.Response surface and regression analysis are used to predict the optimal reaction conditions and explore the interactive relationship between various factors,in order to provide a theoretical basis for reducing the discharge of microplastics from sewage plants.The main conclusions of this study are as follows:(1)Fibrous microplastics have the highest abundance and widest distribution in sewage treatment plants,and are the main form of existence,accounting for 62%,followed by fragments,accounting for 26%,and film-like microplastics,accounting for 12%.Among the four particle sizes,microplastics with a size of 0.25-0.50 mm are the most abundant,accounting for 32.9%,followed by microplastics with a size of less than 0.25 mm,accounting for 28%,and those with a size of 0.50-1.00 mm,accounting for 26%.Microplastics with a size between 1.00-5.00 mm are the least,accounting for 13%.Among the detected components,polyester substances(PET,PBT)are the main components(25%),followed by cellulose(21%).The influent abundance of the sewage plant is 73 n/L,the effluent abundance is 14 n/L,and the microplastic abundance in the final sludge discharge is 67 n/10 g(dry weight).Among them,fibrous microplastics are the main shape in the sludge,and the size is mainly concentrated Between 0.25-0.50 mm,according to the daily water inflow of 180,000 m3,it is estimated that 2.5×108 microplastic particles are still discharged into the environment every day.(2)The overall removal rate of the sewage plant is 80.8%.The sewage plant has the best removal effect on fragmented microplastics,reaching 89%,followed by fibrous ones.The sewage plant has the best removal effect on microplastics with a size between 0.25-0.50 mm,removing the overall trend of decreasing as the size increases.Among the four sizes divided,the proportion of fibrous shape is about 70%,and the size of fragments is mainly concentrated between 0.50-5.00 mm,and the proportion starts to increase after it is greater than 0.50 mm.The ratio is 50%,and it becomes the main form exceeding the fibrous existence.The film-like size is mostly concentrated in the range of less than 0.50 mm,accounting for more than 10%.The pollution load index(PLI)method was used to assess the risk of microplastics.The microplastics in sewage plant drainage have a high ecological risk and will pollute the environment.(3)Among the three treatment stages of the sewage plant,only the primary treatment has played a removal role,and the removal rate is 62.5%.%.In the primary treatment,the removal efficiency of film was the highest at 61.5%,followed by fragments at 52.6%,and the removal rate of microplastics with a size less than 0.25 mm was the highest at 57.8%.The number of microplastics in the primary treatment showed a decreasing trend with the progress of the process,and the removal efficiency increased with the decrease of size.Secondary treatment did not have a significant removal effect on microplastics and released a large number of microplastic particles into the water.In the secondary treatment,the secondary sedimentation tank plays a key role in the removal of microplastics,with a removal rate of 92.9%.The removal rate of the secondary sedimentation tank for thin films is 88.2%,and the removal efficiency of microplastic particles smaller than 0.25 mm is the highest.Up to 90% or more.The newly added biological turntable process in the second phase of the sewage treatment plant has a higher removal efficiency for microplastics than the biological pool.(4)In the single factor experiment,the highest removal rate of microplastics was when10 ml of PAC was added,among which the removal efficiency of PP was the highest,which was 57.8%,and the removal effect of PAM on PP was the best,which was 63.4%.Under the same conditions,the flocculation effect of organic flocculants on microplastics is better than that of aluminum-based flocculants and stronger than that of iron-based flocculants.In the simulation of composite flocculants,adding PAM to PAC can significantly improve the flocculation effect,and the removal rate can reach more than 80%,and the removal rate is related to the particle size of microplastics.The smaller the size,the higher the removal rate.In the wastewater simulation at the multi-factor level,it is concluded through orthogonal experiments that p H is the main factor affecting the flocculation effect,followed by stirring rate,dosage ratio,and stirring time.Under the same conditions,the removal effect of the PAC-PAM flocculation system on microplastics is better than that of the PAFC-PAM flocculation system.(5)The experiment was optimized and validated by combining the multiple response optimization method(RSM)with the orthogonal experiment,and the individual effect and interaction effect of the main influencing factors were evaluated.According to the results of the gray relational degree analysis,it can be known that in the PAFC-PAM flocculation system,the influence degree of the four factors on the removal of microplastics is above 0.5,indicating that the four factors have a significant impact on the flocculation effect,and the results of the gray relational degree analysis It is consistent with the results of the orthogonal experiment. |
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