Research Progress on Occurrence, Removal and Fate of Microplastics in WWTPs

WWTPs are considered to be a major indirect source of MPs discharged into the environment. Characteristics of MPs in the influent and effluent water and sludge of WWTPs were analysed, including abundance, type, shape and size. Detailed comparisons were made to summarise the removal efficiency of each treatment process on MPs and the analysis of the factors affecting them, and to analyse the trend of MPs attribution in WWTPs. It is also concluded that the main shapes of MPs in the influent and effluent water of sewage plants are fibres and fragments, in which fibres account for an increased proportion in the outlet water, and the main polymer types are PP, PE, PA and PET; the shapes of MPs in sludge are mainly fibres, and the main polymer types are PP, PE and PET; and the removal of MPs mainly relies on the physical effects of adsorption, interception, flocculation, filtration, precipitation, and so on.


Introduction
Microplastics (MPs, <5 mm) [1] have received widespread attention as a new type of pollutant worldwide and have become one of the popular research areas in recent years.WWTPs, as pools of domestic sewage, industrial wastewater and surface runoff, are a major indirect source of poly pollutants discharged into the environment.Liu et al. [2] have statistically, China's sewage treatment plants will have 2.84×10 12 MPs discharged into natural water bodies every day, and the MPs entering the environment through sludge will be as high as 2.99 × 10 14 per year, according to which it is assumed that the Mps discharged from sewage plants into the environment every year is even more uncountable.Therefore, the research on MPs removal technology in needs to be strengthened urgently.
This paper reviews the Mps in the influent and effluent water of wastewater treatment plant, systematically introduces and compares the treatment efficiency of the existing treatment process of wastewater treatment plant on Mps and analyses its influencing factors, and analyses the trend of MPs in the influent and effluent water and sludge to provide a theoretical basis for the subsequent research on the removal of MPs.

Characterisation of Mps Assignment in WWTPs
The characterisation of MPs in wastewater treatment plants is influenced by a number of factors, including the level of local economic development, the geographic location of the plant, the population served, lifestyles, and climatic and seasonal conditions.The resulting MPs entering the WWTP are diverse in abundance, polymer type, size and shape.Table 1 lists the distribution characteristics of Mps in the influent and effluent water of the 13 domestic and international wastewater treatment plants, and Table 2 lists the distribution of MPs in the sludge of different domestic and international wastewater plants.
The influent abundance of Mps counted in Table 1 ranged from 0.20 to 1868.00 n/ L. The influent abundance counted varied due to the different minimum pore sizes of the filtration equipment and testing methods.The abundance of Mps MPs in the influent water of most WWTPs is positively correlated with the number of people served [16].Meanwhile, MPs abundance has some seasonal differences [17,18].Depending on the treatment technology, operating conditions and influent abundance used in the wastewater plants, there is a large difference in the removal efficiency of MPs in the wastewater plants (63.25%-99.9%),and consequently a large difference in effluent abundance.According to Table 1, the average value of MPs abundance in domestic effluent is 46.94 n/L, and according to the daily effluent treatment volume of 144 million m 3 in China's WWTPs in 2019, it can be estimated that China's WWTPs will discharge 6.76×10 12 MPs into the natural environment every day.Most of the MPs removed in sewage plants are retained in the sludge, and the average value of MPs abundance in sewage treatment plant sludge shown in the statistics of Table 2 is 3.80×10 4 n/Kg, which, combined with the national production of sludge, roughly estimates that MPs entering the environment through the sludge in our country every year will be as high as 1.89×10 14 MPs.
More than 30 polymers have been identified in effluent samples from different WWTPs [19].As shown in Table 1, the common MPs in influent are PP, PE, PA and PS, in addition to PVC and PET.The chemical composition determines the density of MPs and therefore directly affects the MPs removal efficiency [4].During wastewater treatment, the proportion of polymers with a lower density than the wastewater (e.g.PE and PE) increases, and these polymers make up a larger proportion of the effluent, as can be seen in Table 1; conversely, polymers with a higher density than the wastewater (e.g.PS, PA, PET, and PVC) are favoured for sedimentation, and therefore are more easily captured by the sludge, as can be seen in Table 2.
Fibres and Fragments are the most dominant forms of MPs in wastewater, and they occupy a relatively high proportion of both influent and effluent water.Wherein the high aspect ratio of the fibres allows them to remain in the water for a longer period of time, resulting in an increased proportion in the effluent.From Table 1, it can be seen that the particle size distribution of MPs entering the wastewater treatment plant is in the range of 0-1000 μm.According to the study, the proportion of microplastic particles with small particle sizes in the wastewater treatment process gradually increases [4], and microplastic particles with large particle sizes are easily removed due to gravitational settling and susceptibility to adsorption by suspended solids, etc., and MPs with smaller particle sizes can be suspended in the liquid without being remove.

3.1.Primary Treatment
Primary treatment of wastewater is the removal of suspended and particulate matter from wastewater by physical methods such as sedimentation, filtration or proper aeration.The treatment process may consist of screening, gravity settling and flotation in series.Table 3 shows the effectiveness of MPs removal at the primary treatment stage of eight domestic and international WWTPs.Note: "-" indicates that the original text is not labelled.
From Table 3, it can be seen that the removal rate of MPs in the primary treatment stage ranged from 33.8% to 95%, and the overall removal efficiency was good, which is consistent with the relevant foreign reports [21].Microplastic particles with relative densities less than 1 and adhering to suspended material (oils, etc.) are readily removed by flotation, whereas the removal of denser MPs occurs in the grizzly, grit and sedimentation stages.Among them, the grid can only remove MPs with larger particle size, and it is difficult to retain MPs with smaller particle size; and the water flow in the sand sedimentation stage is faster, the residence time is shorter, and it is difficult to play a good sedimentation effect on MPs, so the primary treatment mainly plays a role in the primary sedimentation tank.The use of flocculants had a positive effect on the removal of MPs and improved the removal of MPs to some extent.For example, Xuan et al. [15] showed that the removal of MPS mainly relies on the primary treatment process, because of its aeration biofilter process in the primary treatment of cyclone sedimentation tank + advection sedimentation tank + high efficiency coagulation and sedimentation tank, able to MPs for many times sedimentation, to a certain extent, to improve the removal rate.
In summary, for the primary treatment process the microplastic removal efficiency can be improved by combining the grill, sand sedimentation tank and settling tank; or increasing the hydraulic retention time of the primary sedimentation tank and optimising the treatment conditions of the settling tank to improve the microplastic removal efficiency of the wastewater plant.

3.2.Secondary Treatment
The secondary biological treatment processes commonly used in domestic and international wastewater treatment plants include: A 2 O, AO, CAST, SBR, OD and so on.In secondary treatment, adsorption, trapping and retention of MPs through the use of sludge flocs or microbial polymers are mainly concentrated in the secondary sedimentation tanks following the biological tanks.Sludge flocs or bacterial extracellular polymers grow and accumulate on the surface of MPs.Over time, the mass of the MPs increases, which enhances the removal effect during sedimentation and facilitates the sedimentation removal operation in the secondary sedimentation tank.Also, the settling properties of activated sludge and the biochemical effectiveness of the biochemical system are factors that have a strong influence on the removal of MPs in WWTPs.From Table 4, it can also be seen that the treatment efficiency of each secondary treatment process for MPs is around 60%, due to the different operating conditions of each plant, there are some differences in MPs treatment efficiency, but in general the gap is not large, and the removal efficiency of MPs is slightly higher than the efficiency of the primary treatment.
In summary, the three main areas to consider for initiatives that might improve the removal of MPs from secondary treatment are increasing the hydraulic retention time, improving the settling of activated sludge and enhancing the biochemical effectiveness of the biochemical system.

3.3.Tertiary Treatment
Tertiary treatment is a process that removes BOD, nitrogen, phosphorus, dissolved substances and bacteria from wastewater on top of secondary treatment.
Note: "-" indicates that the original text is not labelled.
From Table 5, it can be found that the MBR technology at the tertiary treatment stage is most effective in treating MPs, up to 99.9%.Membrane bioreactor technology (MBR) has been shown to be a proven process for the treatment of municipal and industrial wastewater due to its multiple functions such as filtration, retention and capture, and better removal of MPs.Membrane separation technologies, in addition to MBR, include microfiltration (MF) and ultrafiltration (UF), and Pramanik et al. [31] found that the removal efficiencies of MF and UF for MPs with sizes of 75 μm or more were 91% and 96%, respectively, but there are drawbacks such as high energy consumption, membrane contamination, and high cost.Both WWTPs studied by Jia et al. [9] used UV disinfection as tertiary treatment, and the results showed that the removal of MPs by this process was almost zero.
It was also shown that UV disinfection caused a slight increase in the concentration of MPs in water, which was mainly due to the fragmentation of MPs by UV irradiation, which increased the abundance of small particle size MPs [32].From the statistics in Table 5, it can be seen that traditional processes such as dissolved air flotation and sand filtration can remove most of the MPs in the wastewater treatment plant, but they are less effective in removing small particle size MPs.
In summary, coagulation air flotation, sand filtration, and membrane separation techniques are effective in the removal of MPs.The coagulation stage can be optimised to improve the MPs removal efficiency by optimising the coagulation conditions, such as the type and dosage of flocculant, pH and temperature, and also some technological improvements, such as electro-flocculation technology [33], and surface positively charged modified microbubbles technology [34]; the improvement of sand filtration process can be achieved mainly through the change of the filtration media; and for the membrane separation technology can be achieved through the change of the membrane material to improve the removal efficiency of MPs.

Trend Analysis of MPs in WWTPs
Figure 1.Fate of MPs in wastewater treatment plants.Figure 1 illustrates the fate of MPs in wastewater treatment plants.From a process flow point of view, the final fate of MPs after treatment in a wastewater treatment plant is effluent and sludge [35].From Table 1, the average removal efficiency of MPs in 12 WWTPs was 85.91%, and the combined Table 3, Table 4 and Table 5 give the following removal efficiencies of MPs in primary, secondary and tertiary treatment of wastewater plants: 33.8%-95%, 21.8%-92.4%,and 0.11%-99.9%.These removed MPs end up in sludge, so the way sludge is treated determines the pathway and amount of MPs that enter the terrestrial environment.Because sludge contains high levels of organic matter and nutrients, many countries encourage the use of sludge in agriculture as a soil conditioner or fertiliser, with up to 80% of municipal sewage plant sludge being used in agriculture in some countries.However, MPs in sludge are difficult to be degraded by microorganisms, and MPs will continue to accumulate in large quantities in the environment, altering soil physicochemical properties, inhibiting plant growth and seedling development to a certain extent [36], and can enter the edible parts of plants [37], and ultimately enter the human body through the food chain, posing a hazard to humans.With the increasing production of sludge from WWTPs across the country, the removal of MPs from sludge remains an urgent necessity.Considering the large amount of wastewater discharged in China, 14.09% of the MPs discharged with the effluent from the wastewater plant to the natural water bodies also need to be removed urgently, so how to improve the efficiency of removing MPs from the wastewater treatment plant is also a key research issue in the future.

Summary and Prospect
(1) The fugitive characteristics of MPs in the influent water of WWTPs are influenced by many factors, including the level of local economic development, the geographic location of the wastewater plant, the population served, the population served, the lifestyle, and the climatic and seasonal conditions.Understanding the basic information of wastewater plants is important for MPs removal studies.
(2) The main polymer types of MPs in the effluent of WWTPs are PP, PE, PA and PET, and their main shapes are fibres and fragments, so how to effectively remove these types of MPs and fibrous MPs from WWTPs is the focus of future research.In addition, it is also necessary to minimise the production of products based on the above polymers, or to find high-quality alternative raw materials.
(3) The main polymer types of MPs in sludge are PP, PE and PET, with fibres MPs accounting for the largest proportion.China's sewage plant sludge production is large, and it is urgent to solve the problem of microplastic pollution in sludge.Ultra-high temperature sludge composting is the main research direction to solve the problem of sludge microplastic pollution in the future.(4) Among the various methods of removing MPs, the role is mainly physical effects such as adsorption, retention, flocculation, filtration, precipitation and so on.Meanwhile, membrane separation technology also plays an important role in the removal of MPs, and its existence of membrane pollution, high cost and other issues can be studied in depth.

Table 1 .
Characteristics of MPs in water from different WWTPs in China and abroad.
Note: "-" indicates that the original text is not labelled.

Table 2 .
Occurrence of MPs in sludge from different WWTPs in China and abroad.

Table 3 .
Effectiveness of mfficroplastic removal at the primary treatment stage of WWTPs.

Table 4 .
Effectiveness of MPs removal at the secondary treatment stage of WWTPs.

Table 5 .
Effectiveness of MPs removal at the tertiary treatment stage of a WWTP.