Study On Microplastics Pollution And Removal In Municipal Wastewater Treatment

Microplastics usually refer to plastic particles with particle size less than 5mm,which is a new type of pollutant and has been widely concerned.Municipal wastewater treatment plant is one of the important sources of microplastics in natural water.However,the research on the pollution and control of microplastics in municipal wastewater is lack.Therefore,In this study,the simulated municipal wastewater with microplastics was taken as the research object,and sequencing batch membrane bioreactor（SMBR）process was used to treat the simulated urban sewage.Through the cultivation and acclimation of activated sludge,the operation of the system before and after adding microplastics was compared,and the removal efficiency of microplastics was analyzed,the influence and removal characteristics of microplastics on SMBR system were clarified.The aim of this study wasto providing the scientific basis for controlling microplastics pollution from the source.The results were as follows:After the start-up stage of sludge cultivation and acclimation,the sludge concentration（MLSS）of activated sludge increased to 5106mg/L,the removal rate of COD_Crreached 88.2%,and the removal rate of NH₄⁺-N reached 87.9%.The microbial abundance and dominant bacteria in sludge changed significantly,indicating that the acclimated sludge has adapted to the wastewater quality and sequencing batch operation mode of this study.The results of activated sludge properties analysis showed that the addition of microplastics had a significant impact on the activated sludge performance in a short time.The sludge concentration decreased to 3848 mg/L,and the sludge volume index（SVI）increased to 173.2,resulting in sludge bulking.After 8 days,the sludge settling performance returned to the level before the addition of microplastics,and the sludge concentration returned to about 4500 mg/L,which was lower than that before the addition of microplastics.The results of SMBR system showed that the amount of microplastics added in the influent was 10 grains/L per day.After adding microplastics,the removal rate of COD_Crdecreased temporarily and recovered to the level before adding microplastics from the 7th day;The removal rate of NH₄⁺-N was above 90%,the removal efficiency of TN was significantly improved,and the effluent nitrite nitrogen concentration was significantly lower than that before adding microplastics.The results of microplastics removal analysis showed that SMBR system had good removal effect on microplastics,and the removal rate of microplastics could reach 99.9%.The results of microbial structure analysis showed that the abundance and diversity of microbial species in sludge decreased with the addition of microplastics,but the abundance of Proteobacteria increased significantly from 50.29%to 68.89%.At the subordinate level,the results showed that the presence of microplastics could change the dominant aerobic bacteria in activated sludge,but had no obvious adverse effect on the growth of most facultative or specific anaerobic bacteria;The growth of simplicispira with shortcut nitrification was significantly inhibited by adding microplastics,and its abundance decreased from 25.31%to 4.26%.The results of membrane fouling analysis showed that the addition of microplastics could accelerate the formation rate of membrane fouling in SMBR system and aggravate the degree of membrane surface fouling.The main reason for membrane fouling is that organic matter and inorganic salt scale block membrane pores,and the degree is more serious after adding microplastics.The organic fouling of membrane was more serious than before adding microplastics.After adding microplastics,the number of microorganisms with abundance more than 1%detected on the surface of microplastics enrichment membrane was more than that of mud water mixture;however,the abundance of microorganisms whose growth and reproduction were inhibited by microplastics decreased on the surface of membrane.