Research On The Distribution Characteristics And Removal Rules Of Microplastics In Urban And Township Sewage Treatment Plant

Microplastics（MPs）are plastic particles less than 5 mm in diameter,with small volume,low density,large surface area,etc.They are one of the most concerned"new pollutants"and have been clearly included in the national action plan for new pollutants.Due to the characteristics of large specific surface area and strong adsorption performance,MPs are easy to become carriers of persistent organic compounds,heavy metals and pathogenic bacteria,thus posing a greater risk of environmental pollution and endangering human health.Wastewater Treatment Plants（WWTPs）,as an important"sink-source"of MPs,play an important role in the migration and transformation process of MPs in human-natural systems.WWTPs have been considered as one of the important sources of MPs in the water environment.At present,a series of studies have been carried out in China on the fate and removal efficiency of MPs in different processes of wastewater treatment plants in eastern coastal cities,but less attention has been paid to urban and township wastewater treatment plants with different processes.Rural wastewater treatment plants were selected as the research objects to analyze the characteristics of MPs abundance,particle size and composition in urban and rural wastewater treatment plants in southwest China,and to investigate the effects of the scale of wastewater plants,process characteristics,regional features and other factors on MPs assignment characteristics and removal efficiency in wastewater treatment plants.The main conclusions are as follows:（1）The abundance,morphology,size and color of MPs in water samples and sludge from different processes（W_afor SBR process line,W_bfor CAST process line and W_cfor A²O process line）in a municipal wastewater treatment plant（WWTP-1）were analyzed to reveal the fugitive state and removal efficiency of MPs from different processes in the homogenous influent water.The results showed that the MPs abundance in WWTP-1 influent was 32.5±1.0 n/L and the final effluent abundance was 5.0±0.4 n/L,with a combined removal rate of 84.6%,and the tertiary treatment process+wetland treatment had a relatively good removal effect on MPs.The primary and secondary treatment processes were the main stages of MPs removal,with cumulative removal efficiencies（Wa,Wb and Wc）accounting for 64.8%,62.8%and 75.1%of the total removal efficiency,respectively.The secondary stage sequencing batch activated sludge（SBR）,cyclic activated sludge（CAST）and anaerobic-anoxic-aerobic（A2O）full process removal rates were 73.0%,75.6%and83.9%,respectively.Most of the MPs were transferred to the sludge,and the concentration of MPs in the dewatered sludge was 27.2±3.1 n/g.The characteristics of MPs in municipal wastewater treatment plants were as follows:the morphology was mainly fragments,the color was mainly black,the main size range was 30-100μm,and the main chemical compositions were polypropylene（PP）,polyethylene(Polyethylene（PE）and polyethylene terephthalate（PET）;（2）MPs analysis of inlet and outlet water samples from different wastewater treatment plants（WWTP-2-7）in townships revealed the differences in MPs fugacity and removal efficiency in township wastewater treatment plants.The results showed that the influent abundance of MPs in rural WWTPs ranged from 3.8±0.1-8.2±0.2n/L,the effluent abundance ranged from 3.1±0.03-8.8±0.4 n/L,and the removal rate ranged from 18.8-54.4%,and the influent abundance and removal efficiency were lower than those of municipal WWTPs.WWTP-7（SBR）with higher loading rate had the highest removal rate and WWTP-5（A²O）had the lowest.The physical and chemical characteristics of MPs in the township compared to the urban WWTPs showed that the main morphology,size and color were almost the same,but the percentage of film and transparent MPs increased significantly.The main types of chemical properties are:polyethylene（PE）and polyvinyl alcohol（PVA）;（3）By comparing the removal effect of each wastewater treatment process on MPs in urban and rural wastewater treatment plants,the factors leading to the existence of differences in the removal rate of MPs were investigated.It was found that the sedimentation time and mode of sludge settling were the main factors affecting the removal rate.The physical retention method with reduced grid spacing in the primary treatment stage can effectively intercept MPs,and tertiary flocculation sedimentation and wetlands can further remove MPs particles to optimize water quality;（4）The calculation of annual emissions from WWTPs reveals that there are differences in MPs emissions from urban and rural WWTPs:approximately 1.82×10¹¹MPs particles per year from urban WWTPs are discharged into the external environment through the tailwater and approximately 4.88×10¹¹MPs particles are transferred through the sludge.six rural WWTPs（WWTP-2-7）discharge into the external environment each year MPs particles are between 1.23×10¹⁰-4.03×10⁸.