Study On Distribution Characteristics Of Microplastics In Wastewater Treatment Plants And Its Effect On Activated Sludge

Plastic products have been widely applied in various areas of human production and life due to their excellent physicochemical characteristics and low cost.However,about 60%of used plastics are buried or abandoned,and degraded into microplastics（MPs,size<5 mm）through abiotic or biotic processes..MPs have been found in various environments and even in food chains because of its wide application and sources.At present,more and more attention has been paid to the occurrence of MPs in wastewater treatment plants（WWTPs）.However,the distribution characteristics of MPs were different in different WWTPs,which were related to location,service population,influent quality and treatment processes,etc.In addition,the influence of abundant MPs on activated sludge is still not clear.Therefore,this study investigated the distribution status of MPs in three typical WWTPs（labeled as W1,W2 and W3）in Wuxi city by two different sampling strategies.Then,the effects of polyamide（PA）-MPs,which are widely detected but rarely studied,on sludge microorganisms（Escherichia Coli and Paracoccus Denitrificans）and activated sludge were studied.The main conclusions are as follows:（1）Wastewater samples were collected from WWTPs by two sampling strategies.The abundance and characteristics of MPs in wastewater were analyzed.Random grab-sampling result showed that the MPs concentrations in influents of W1,W2 and W3 were 392.2,38.3and 71.2 items/L,respectively.Approximately 90%of MPs in W1 influent were small white PE granules,which was the most significant feature of MPs in influent of W1.Whereas,the concentrations of MPs in W2 and W3 influents were at the medium levels among present studies.Fibers and granules accounted for the majority shapes of identified MPs in the three WWTPs.The main polymer types included polyethylene oxides,polypropylene,PA,polyester and polyethylene terephthalate.During the dense grab-sampling of W1 influents within 24 h,we observed surprising large fluctuation（from 29.1 to 4618 items/L）of MPs concentrations,indicating the limitation of random grab-sampling.The source,instead of proportion,of industrial wastewater plays an important role in contributing to the severe MPs pollution in W1.Considering the wide sources of plastic microbeads,it is necessary to further strengthen the source control of the plastic manufactures.Despite the high concentrations of MPs in the influents,the effective removal performance of wastewater treatment processes in three WWTPs successfully reduced MPs to less than 4.0 items/Lwith low ecological risk.（2）The effects of PA-MPs with different sizes and concentrations on two sludge bacteria were studied.To E.coli,PA-MPs with highest concentration（100 mg/L）and minimum size（20μm）showed longest inhibition time and highest inhibition rate.Besides,intracellular reactive oxygen species（ROS）levels increased the most,the range is between 20.4%and 29.2%.The inhibition effects of PA-MPs on P.denitrificans in the initial growth stage were stronger than those on E.coli,with higher intracellular ROS increasing level（82.5%-212.7%）.However,the nitrogen removal efficiency of P.denitrificans was not affected by exposure of PA-MPs.ROS generation induced by MPs exposure was the molecular initiating event of its toxicity.The dissolved plastic additives could also be responsible for toxicity of PA-MPs.（3）The toxic effects of PA-MPs with different sizes and concentrations on activated sludge were further studied.PA-MPs with highest concentration（100 mg/L）and minimum size（20μm）had the most obvious acute inhibition effect on sludge activity.After continuous injection of PA-MPs into the influent of SBR reactor,the removal performances of nitrogen,phosphorus and COD,and microbial activity of activated sludge did not change much at low dosing stage（4 mg/L）.Instead,it increased slightly.However,the effluent NH₄⁺-N,PO₄^3--P and COD continuously rose since medium（40 mg/L）and high（200 mg/L）dosing stages.The specific oxygen uptake rate,specific ammonium oxidation rate,and specific phosphorus release rate also decreased.Moreover,more EPS was secreted by activated sludge microorganisms to resist MPs stress and sludge flocs got smaller.After being exposed to PA-MPs,intracellular ROS levels of activated sludge decreased continuously.The activity of superoxide dismutase（SOD）was greater than that of the control at low and medium dosing stages but lower at high dosing stage.The activity of catalase（CAT）was always greater than control,indicating that oxidative stress induced by PA-MPs at medium and high concentrations was responsible for toxicity of PA-MPs.Exposed to PA-MPs at medium and high concentrations,the alpha diversity indices of microbial communities decreased,and the structure and composition were also affected.The content of Proteobacteria,ammonia oxidizing bacteria and phosphorus accumulating organisms decreased.The content of bacteria with EPS secretion ability increased.