A systematic review on the occurrence and removal of microplastics during municipal wastewater treatment plants

Anvar Asadi; Faranak Khodadoost; Nebile Daglioglu; Setareh Eris

doi:10.4491/eer.2024.366

Environ Eng Res > Volume 30(2); 2025 > Article

Asadi, Khodadoost, Daglioglu, and Eris: A systematic review on the occurrence and removal of microplastics during municipal wastewater treatment plants

Review

Environmental Engineering Research 2025; 30(2): 240366.

Published online: September 20, 2024

DOI: https://doi.org/10.4491/eer.2024.366

A systematic review on the occurrence and removal of microplastics during municipal wastewater treatment plants

Anvar Asadi^1,^2,^†, Faranak Khodadoost², Nebile Daglioglu³, Setareh Eris⁴

¹Department of Environmental Health Engineering, School of Health, Kurdistan University of Medical Sciences, Sanandaj, Iran

²Students Research Committee, Department of Environmental Health Engineering, Kermanshah University of Medical Sciences, Kermanshah, Iran

³Institute of Forensic Sciences, Department of Forensic Toxicology, Ankara University, Ankara, Turkey

⁴Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

^†Corresponding author: E-mail: anvarasadi@sbmu.ac.ir, Tel: +98 918 877 2039 Fax: +98 87 3362 6969, ORCID: 0000-0002-5714-0878

Received June 19, 2024 Revised September 12, 2024 Accepted September 19, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This study uses a systematic review of Scopus, Web of Science, and ScienceDirect databases to investigate the removal rate of microplastics (MPs) at various stages of municipal wastewater treatment plants (WWTPs). In this context, 25 articles were included, which investigated 54 WWTPs. Results showed that the overall removal efficacy of MPs in studied WWTPs is about 88.1 (± 18.8) percent. The contribution of preliminary, primary, secondary, tertiary, and disinfection treatment in the removal of entry MPs are 37.4% (± 24), 62.7% (± 20.8), 63.7% (± 24.8), 62.8% (± 29.5), and 12.5% (± 15.7) (mean (± SD)), respectively. The critical aspects about MPs occurrence in WWTPs influent were summarized and on average 2267.2 (± 5376.4) MPs particles per liter (P/L) were present with a considerable wide range of 0.28 to 31400 P/L. In the next section, details about frequent shape (i.e., spherical (beads, pellets, granules), lines (filaments, fibers), films, fragments, and foams) of MPs in influent and effluent of included WWTPs and their removal efficacy during treatment steps are given. Finally, this study highlights future challenges in MP removal and suggests key areas for further research, such as improving treatment efficiency and understanding MP characteristics.

Keywords: Microplastics, Microplastics analysis, Occurrence, Removal efficiency, Treatment technologies, Wastewater treatment plant

Graphical Abstract

Keywords: Microplastics, Microplastics analysis, Occurrence, Removal efficiency, Treatment technologies, Wastewater treatment plant

Introduction

One of the most pressing challenges today is raising public awareness about environmental issues, particularly the impact of plastic pollution on human health [1]. Of particular concern is the infiltration of plastics into aquatic and terrestrial ecosystems, threatening biodiversity [2]. The global production of plastics escalates annually, and their persistence in the environment, due to their recalcitrant nature, exacerbates the problem [3]. Consequently, plastic debris accumulates in virtually all habitats [4]. In general, based on size classification, plastics can be megaplastics (> 1m), macroplastics (25 mm – 1 m), mesoplastics (5 – 25 mm), microplastics (5 mm – 1 μm), and nanoplastics (< 1 μm) [5]. MPs are further categorized as primary or secondary, with primary MPs being intentionally manufactured, while secondary MPs result from the fragmentation of larger plastic debris [5–7]. MPs are generally characterized as synthetic polymers smaller than 5 mm in size [8]. They are frequently detected in the effluent of WWTPs ranging between 29 and 447 P/L [9]. The most widely detected MP polymer types in the influent and effluent of WWTPs are polyethylene (PE), polypropylene (PP), polyamide (PA), polyester (PES), polystyrene (PS), and polyethylene terephthalate (PET) with abundances of 64.07%, 32.92%, 10.34%, 75.36%, 24.17%, and 28.90%, respectively [10]. In one study, the majority (≈ 90%) of MPs in WWTPs are PES which, originated from facial or body cleansers and water bottles [11]. MPs are insoluble in water, and their small size enables them to be ingested by aquatic organisms [12]. MP contaminants pose serious threats to a broad spectrum of terrestrial and aquatic animal species. They generate health effects including animals physical harm, contaminants adsorption, inflammatory responses, and behavioral modifications [13]. The ingestion and then accumulation of MPs in the animals can cause both physical and chemical damages such as clogging of the digestive system, feeding reduction, hindrance in mobility, and death [14]. However, the consequences of MPs on human health are poorly understood [15].

Primary MPs are often emitted from household products such as cosmetics (make-up, sunscreen), personal care products, toothpaste, facial cleansers, body scrubs, etc. [16]. Also, industrial products like abrasive plastic balls (used to clean ships) can be mentioned as another source of primary MPs [17, 18]. According to Osman et al. [19], cosmetics, personal care products, pharmaceuticals, detergents, and insecticides industries are the main sources of primary MPs. Plastic industries are also a source of primary MPs which mainly produce PP, PES, PS, and nylon polymers. Secondary MPs originate from the fragmentation of bigger plastic objects. In other words, secondary MPs are formed by the breakdown of the larger plastics such as scattered junk and plastic packaging, plastic particles derived from the erosion of tire wear, or plastic parts of vehicles, etc. [20, 21]. As well as household products, industrial products, and color flakiness can be referred to as the sources of secondary MPs [21]. Flaking paint and coating can be a significant source of MPs. As these materials break down, they contribute to the accumulation of MPs in the environment. Synthetic clothes can be classified into both categories (both primary and secondary MPs) [22]. Approximately 35% of primary MPs released into the oceans worldwide are from synthetic textile washing [22, 23]. The petrochemical industry is also contributing to the release of 1440,000 billion MPs into the environment and is a source of primary plastic producer [24].

Given the pervasive nature, MPs are found in WWTPs from various sources, including domestic wastewater, industrial effluents, stormwater, and landfill leachate. Therefore, an important route for MPs to enter aquatic ecosystems is WWTP effluents [25–28]. The discharge of WWTP effluents into the receiving waters leads to a significant increase in the MP concentration [29, 30]. Studies indicate that even with removal rates of 99.9%, 98.4%, and 88.1% of MPs in WWTPs in Sweden [31], France [32], and Scotland [33], respectively, significant numbers of MPs are still released into the aquatic environment. The amount of MPs released per day for the above-mentioned WWTPs were 4.25×10⁴, 8.40×10⁹, and 6.52×10⁷, respectively. Therefore, WWTPs are the main contributing to the MP load in the aquatic environment. During wastewater treatment, over 80% of MPs are transferred to sewage sludge [34, 35]. The sludge is used as fertilizer in agriculture, and this can be another possible entry path for MPs into the environment [36, 37].

Although most of MPs are retained in sewage sludge, the amount of MPs discharged from WWTPs is still high enough to pose a risk to aquatic environments [38]. The MPs abundance in surface water varies from 10⁻⁵ to 10⁵ pieces/m³ and 0.04 to 0.40 pieces/m³ in sediments [39]. However, controlling MPs pollution in low volume sludge is much easier than removal from high volume effluent discharges. Therefore, improving the efficiency of wastewater treatment processes in removing of MPs is an excellent option to reduce their entry into fresh water [35]. It should be noted that no special treatment facilities are designed to remove MPs from WWTPs [34, 38]. However, at each stage of wastewater treatment, a percentage of MPs is eliminated. So, it seems necessary to make a comparison between the various treatment processes and to identify which processes and which units are most effective in removing MPs. Furthermore, it is essential to investigate the parameters affecting the efficiency of various treatment units in removing MPs.

In recent years, several studies have been carried out to evaluate and investigate the capability of primary [40], secondary [32, 34, 41], and tertiary treatment processes [34, 37, 42] in removing MPs with different shapes and properties. However, such studies don’t provide a comprehensive insight into the removal of MPs in WWTP processes. In contrast, some studies evaluated the removal of MPs in the effluent of WWTPs [27, 37, 43], other studies conducted in laboratory and bench scale [44–46]. Furthermore, the differences between sampling methods (i.e., pumping, trawling, or filling bottles or buckets), extraction methods, and quantification and confirmation of MPs, as well as the heterogeneous nature of plastics entering WWTPs make it difficult to compare data from different studies [47, 48].

Accordingly, this review aimed to investigate the removal of MPs within WWTP systems. Initially, a concise overview of MP characteristics, extraction and quantification methods, and associated challenges was presented. Subsequently, data from 54 WWTPs across 25 studies were analyzed to examine the occurrence, transfer, and removal of MPs. The performance of various removal processes and operations within each WWTP unit was quantitatively compared. Additionally, the parameters influencing MP removal were elucidated in detail. The removal behavior of MPs with diverse shapes, morphologies, and particle sizes was also discussed. Based on these findings, recommendations were proposed to enhance the efficacy of WWTPs in mitigating MP pollution. Finally, suggestions for future research were outlined. This comprehensive analysis contributes to a deeper scientific understanding of the fate of MPs in WWTPs.

Materials and Methods

2.1. The Protocol of Study and Search Strategy

A focused literature search was conducted for articles on the removal efficiency of MPs in WWTPs using Web of Science, ScienceDirect, and Scopus databases. The search resulted in 461 articles, out of which 25 articles met the criteria for inclusion in the current review. Detailed information about the protocol of study and research strategy is available in the Supplementary Materials. Data extracted included: MPs concentration in WWTPs influent and effluent (P/L), the overall removal efficiency of WWTP and its units, MPs size range, extraction and determination methods, wastewater source, and operations and processes steps.

2.2. Inclusion/Exclusion Criteria and Data Extraction

The studies included in the systematic review met the following criteria: 1) a full-scale wastewater treatment system was evaluated, 2) the municipal wastewater treatment was investigated, 3) the total removal rate of MP in WWTPs or each unit treatment was reported, 4) MPs were reported based on particles per liter, and 5) the full text was accessible. The retention capacity of MPs during WWTP was calculated using: Retention Capacity (%) = ([Influent] − [Effluent]) / [Influent] × 100. If RC % was not directly reported, it was calculated using the Eq. (1) [35, 49].

(1)

R C = \frac{C_{I n f l u e n t} - C_{E f f l u e n t}}{C_{I n f l u e n t}} \times 100

where C_Influent and C_Effluent are concentrations of MPs (#P/L) in the influent and effluent of WWTP, respectively. Treatment steps in a typical WWTP include: preliminary (screening and/or grit removal), primary (sedimentation and/or chemical process), secondary (biological treatment and sedimentation), and tertiary (advanced treatment).

Results and Discussions

3.1. Challenges in Studying MPs Occurrences

3.1.1. Challenges in sampling and analysis

The absence of standardized analytical methods for MPs presents several challenges in research. Different methodologies used by the researcher result in a lack of reliable and comparable data on the concentration of MPs [50]. Indeed, several factors can affect the measured concentration of MPs in wastewater influent, such as the sampling method, the sampling duration, sampling season and time, the sampling location, and the identification and quantification methods. Other influential factors include wastewater type and source, MPs type and shape, selected size range, etc. [51, 52]. In this regard, it is common for samples, even samples collected from the exact locations (with similar flow conditions) on the same day but at different time points, to have different abundances. Moreover, it can be said that the long-term variations of the MP abundances are greater than the short-term ones. It is interesting to note that the MP concentration, the shape, and their size distribution are changed during rainy seasons.

In terms of the impact of when and how samples are taken on the presence of MPs in wastewater, it is observed that there is a fluctuation in the number of MPs throughout the day, and the concentration in the morning and afternoon is likely to differ. Nevertheless, certain studies [42] have shown that long-term variations in the abundance of MPs are more significant than short-term variations. Depending on the season, there is a lower concentration of MPs due to the dilution of domestic wastewater with street runoff, and there may also be changes in the predominant type and shape of MPs. In stormwater weather, MPs are produced from cigarette filter waste, car tires, paint on road markings, and many other sources. Ice melt and/or wind also contribute significantly to MPs entry to WWTPs. Therefore, MP varies between dry and wet seasons, with 70–80% of the annual MPs entering water bodies are in rainy season [53, 54]. According to Uogintė et al. [54], the total concentration of MP was 27% higher in spring than in other seasons.

The desired size range for studying MPs characteristics is another significant factor influencing concentration. When smaller MPs (less than 100 μm) are included in the measurement, more MPs can be detected per liter of wastewater samples compared to when the larger ones are measured [51]. Based on polymer type, MPs have different densities. Therefore, the number of MPs in various depths of the sampling site is not the same which consequently affects the number of measured particles per liter of wastewater. There is no specific standard procedure for sampling from different depths of wastewater. However, in some studies, sampling was performed based on the standard method of water quality analysis, which was constructed by the Environment Protection Authority (EPA), South Australia [55]. In such studies, samples were collected from 30 cm under the wastewater surface (sub-surface sampling) using hand or modified plastic containers. It should be mentioned that according to the EPA, sub-surface sampling is adequate for the times in which the sample source is shallow and well-mixed [56]. However, when the water/wastewater is too deep, the samples should be taken from various depths. Then, the samples were blended to give a mixture representative of whole water/wastewater. Furthermore, special equipment such as pumping systems, open tubes, and sealed floats is needed to collect samples from particular depths.

3.1.2. Influence of wastewater source

The wastewater source plays a crucial role, especially when it is industrial, as it is likely to contain various primary MPs. In the case of a combined wastewater collection network, plastic patches from tire wear are more likely to enter WWTPs through street runoff [25]. Domestic wastewater contains fibers from washing powders, micro-beads from personal care products, or fragments of broken bottles [55]. The abundance of micro-beads and fibers in sewage depends on factors such as culture, lifestyle, and study location [57]. Microfibers are typically found in washing machine effluent, increasing concentrations in densely populated areas [42].

3.1.3. Extraction and quantification methods

Extraction, separation, and identification and quantification methods are other important challenges that need to be taken into consideration. Various extraction methods, the use of different chemical reagents in the MPs extraction process, and the use of filters with different pore diameters resulted in the collection of various shapes and sizes of MPs [43]. Consequently, the measured concentration of MPs in a sample differs based on the extraction method. Wet peroxide oxidation is a widely used method for MP extraction in which a 30% hydrogen peroxide (H₂O₂) solution is commonly used as an oxidant. However, H₂O₂ can affect the physical properties of some MPs, such as the shape and size of particles. This has a significant impact on the number of detected MP particles per liter of sample. [42]. In samples with high concentrations of suspended inorganic particles, the MP separation is performed by density method [25, 38, 58]. Density separation is one of the most widely used methods for separation of MPs. In this method, using a high-density saturated salt solution (such as NaCl, ZnCl₂, and NaI), the light MP particles float on the surface where they can be skimmed away easily. However, heavier particles are precipitated to the bottom, so they cannot be measured and identified [43].

Despite efforts to distinguish MP particles from other substances, it has been found that a significant percentage (between 22% and 90%) of suspected MPs are non-plastic particles or natural fibers when using techniques such as wet oxidation, density-based MP separation, and staining of natural fibers [30, 38]. The non-plastic particles can include various materials, including: natural fibers (cellulosic fibers, animal fibers and synthetic fibers), glass and ceramic fragments, metal particles, biomass and organic matter, and paint particles. These materials pose a challenge in characterizing MPs in water, as they can easily be mistaken for organic or other types of matter [51]. To overcome this challenge, confirmation techniques and instrumentation such as Raman and Fourier-transform infrared spectroscopies, microscopies, thermal desorption gas chromatography, and imaging techniques are necessary [42]. Therefore, due to the difficulties in identification, measurement, sampling, and variations in wastewater types, accurately determining the concentration of MPs in wastewater is not possible.

The unit operations and processes used in the various WWTPs included in this study are summarized in Fig. 1. Each treatment step is named with a letter, and the variety of treatment processes/operations of one treatment step is indicated by the corresponding numbered letter. The removal efficiency of MP particles in each treatment step will be compared in subsequent sections. While WWTPs are not particularly engineered to eliminate plastic particles, they can effectively treat wastewater using various treatment stages, including preliminary, primary, secondary, and tertiary treatment processes, to purify wastewater before discharging into the environment. In preliminary treatment, MPs are removed via bar screening, skimming, settling, and grease and grit removal. MPs are trapped in sludge flocs in primary and secondary sedimentation. Tertiary treatment is optional and, if present, will vary between facilities. They usually remove plastic particles by filtration, flotation, and chemical oxidation.

3.2. MPs Occurrence in Influent and Effluent of WWTP

Table 1 presents the summarized results of various aspects of the WWTPs included in this study. These aspects include the number of MP per liter in the influent and effluent of the WWTPs, the total removal rate achieved during the treatment process, the source of the wastewater, the steps involved in the treatment process, extraction of MPs from wastewater and determination methods and the selected mesh size range used in the investigated WWTP. According to Table 1, the average (± SD) load of MPs at the influent of WWTPs was 2267.2 (± 5376.4) P/L, with a range varied between 0.28 to 31400 P/L. As mentioned above, the wide variations in the number of MP particles per liter of wastewater could be attributed to the utilization of diverse sampling techniques, sample pre-treatment methods, and analytical approaches [59]. The highest MP concentration in wastewater influent was reported in a WWTP in South Korea where the source of wastewater was municipal [55]. On the other hand, the lowest amount of MP was reported in a WWTP in China which receives both municipal and industrial wastewater [57]. The average (± SD) concentration of MPs in the effluent of the WWTPs of included studies was 36.5 (± 79.8) P/L. The highest and lowest concentrations of MPs in WWTP effluent were 447 and 0.05 P/L, located in the Danish sewage treatment system with an activated sludge process [51], and the Chinese sewage treatment system with an oxidation channel process [57], respectively. In Hidayaturrahman et al. [55] study, which reported the highest load of MPs per liter (31400 P/L) in wastewater influent, the lowest mesh size was 2.5 μm and samples were filtered with a 1.2 μm glass fiber (GF) filter (Whatman GF/C). However, the lack of utilization of density-based separation methods and the reliance solely on visual analysis of MP particles using a microscope without conducting verification tests could explain the wide variations in the number of particles per liter of wastewater. The sampling was conducted between 3 and 4 pm, during March and April, and it was observed that microbeads were the predominant type of MPs present at both the influent and effluent of the WWTP. Based on this study, the source of micro-beads was personal care products, which this type of plastics is widely used in pharmaceutical and personal care products (PPCPs). Manufacture of the microbead-containing PPCPs was prohibited in July 2017 in the United States and Europe [57].

The lowest load of MP in influent and effluent of WWTP was reported in a WWTP in China. This WWTP received industrial and domestic wastewater with a different portion. In their study, the lower limit of the studied mesh size was 25 μm and density-based separation was performed using NaCl salt solution with a density of 1.49 g/ml. Visual examination and pre-selection of MPs were carried out using a dissecting microscope and confirmed through FTIR–ATR analysis. The sampling occurred in February 2018, revealing that the dominant MP morphotypes were fragments (65%) and fibers (21%), primarily composed of PET. There were also limited amounts of films (12%) and foams (2%). These plastic materials likely originated from everyday consumer products such as plastic bottles, containers, toys, decorative items, and construction materials. Over time, these items have been broken down into small fragments, called secondary MPs [57]. The lower concentration of MP in the wastewater effluent of Lv et al. [57] study can be attributed to the high efficacy of treatment plants (99.5% in the MBR system versus 97% in the oxidation ditch channel) and also the lower load of MP in the influent.

In Denmark’s WWTP, which had the highest load of MPs in the effluent, the mesh size below 10 μm was considered which may be the main reason for high MP load in effluent. Sampling was carried out in the dry season. Fragment shapes were the dominant form of MPs, and fibers were presented in the lowest amount [51]. In this study, FTIR-ATR based on an FPA detector was used for the quantification and verification of MPs. This method yields more accurate estimations, particularly for the tiniest particles, than a single-point FTIR approach. By avoiding pre-selection using microscopy and incorporating density-based separation, the potential to measure MPs with different densities increases. Consequently, this approach may lead to a more precise detection of MPs.

3.4. Removal of MPs in Municipal WWTP

The average (± SD) overall retention capacity of MPs in the influent and effluent of 55 studied WWTPs was 88.1% (±18.9) (see Table 1). Despite not being specifically designed for MP removal, WWTPs can effectively remove the MPs in wastewater through primary, secondary, and tertiary treatments. More than 63% of the studied WWTPs achieved a removal efficiency of approximately 95% or higher for MPs. Further analysis revealed that among the treatment plants with known process types, 69% of WWTPs with secondary treatment and 70% with advanced (tertiary) treatment achieved a removal rate of 95% or higher for MPs. Among the WWTPs studied, only one had just primary treatment, which removed 60.4% of MPs. The effectiveness of different treatment processes, including preliminary, primary, secondary, and tertiary treatment, in removing MPs is discussed in the following. The efficiency of each treatment step in removing MPs may be influenced by factors such as the physicochemical properties of MPs (e.g., density, mesh size, type, and shape). Additionally, the methods used for detection and identification can affect the number of MPs detected in effluent across all treatment steps. Therefore, it is challenging to compare plastic removal efficiencies within one treatment step (e.g., grit removal) based on the current literature [59]. Comparison of results between similar and identical articles is more acceptable if the extraction and detection methods are the same and similar filtration/sieve pore size is used. This allows for a better evaluation of the efficiency of various wastewater treatment systems in removing MPs, as well as estimating the efficiency of different treatment units within a WWTP.

MPs are present in untreated wastewater of various sizes, and typically, larger particles (e.g., larger than 300 μm) are trapped in the initial treatment stages. Subsequently, their size range decreases in subsequent treatment units, and smaller MPs (e.g., 20 to 100 μm) dominate [34]. Therefore, using larger mesh sizes (e.g., 300 μm) to separate MPs results in more identification of MPs in preliminary/primary treatment and leads to underestimating measurement and identification of MPs in secondary and tertiary treatment. Hence, for accurate quantification of MPs a smaller mesh size is recommended. In this case, the efficacy of treatment steps will be more precise. In some WWTPs, the grit and grease removal unit is incorporated in the grit removal chamber, which exhibits high efficiency in removing MPs with a density lower than water, causing them to float on the surface [33, 66].

3.5. MPs Removal Rate During Screening and Grit Chamber

In this review, the MPs removal efficiency by screening and grit chamber in seven WWTPs was reported, and results are presented in Table 2. The average removal at this treatment step (preliminary treatment) was 37.4% (± 21.0), with the highest removal rate of 58.8% reported in an aerated grit chamber. According to Yang et al. [38], most MPs in preliminary treatment were fiber shape, and common types of MPs were PET> PS> PP with a size range between 862 to 1110 μm. Extraction, detection, and confirmation were performed by wet oxidation and density separation, microscope, and micro-FTIR, respectively. On the other hand, the rotary grit removal showed the lowest efficiency of −371% and increased the number of MPs in the preliminary treatment effluent. This may be due to mechanical stirring, causing the MPs to break into smaller fragments [57]. However, the efficiency of these treatment units was not included in calculating the overall retention of MP during preliminary treatment.

3.6. MPs Removal Rate During Primary Treatment in WWTP

The efficiency of primary treatment (primary settling tank) in MPs removal has been reported in 6 studies that investigated 8 WWTPs, and results are presented in Table 3. On average, the removal efficiency of MPs during primary treatment was 62.7%, with a standard deviation of 20.8. The highest removal rate (82.1%) was achieved in a primary settling tank coupled with a scum removal unit. Fibers were the dominant shape of MPs, which detected only by optical microscope and didn’t confirm with other techniques (i.e. FTIR, TGA-DSC, TED-GC-MS, etc.) [66]. In contrast, Bayo et al [25] reported the lowest removal rate (19.1%) of MPs during primary treatment using only a primary settling tank. The dominant sizes of the MPs in the influent and effluent of primary treatment were 0.82 mm and 0.74 mm, respectively. The most common type of MP was low-density PE film. The filter pore size used to extract MPs was 0.45 μm, but only MPs larger than 200 μm were reported. Extraction, detection and verification methods were filtration-density-based separation-vacuum supernatant filtration, digital microscopy and FTIR-ATR, respectively [25].

3.7. MP Removal During Secondary Treatment in WWTP

The MP removal efficiency during secondary treatment was reported in 16 articles included in this systematic review [25, 30, 32, 33, 37, 38, 40–43, 55, 57, 65, 66], and results are presented in Table 4. The overall retention of passed MP from preliminary and primary treatments by secondary treatment was 63.7% (± 24.8). During secondary treatment, sludge flocs or bacterial extracellular polymers in the aeration tank probably led to the accumulation of the remaining microlitter, which would then be settled in the secondary clarification tank [68]. The highest removal efficiency (97.1%) was reported in a WWTP that utilized the activated sludge process [65]. In this study, a grab sample was taken using a pumping system, sieved, and observed under an anatomical microscope equipped with a digital camera. The lowest pore size of the mesh used was 20 μm. The presorted samples were then analyzed and identified using the FTIR technique. Approximately 72% of MPs in the influent of the activated sludge state were found to be in the size range of 100–200 μm, with fiber being the dominant shape (80%) and polyester being the dominant type (70%) [65]. It should be noted that the activated sludge process accounted for 74% of the secondary treatment process in the investigated WWTPs. The lowest efficiency of MP removal (−66.6%.) was also reported in a WWTP with activated sludge process combined with clarification and disinfection. The reported reason for this negative efficiency at this stage of treatment was the higher turbulence in the effluent stream of the primary settling (sampling point 1) compared to the steady stream in the final effluent (sampling point 2). In addition, it was reported that the most MPs were PE with lower density, which tend to float on the water surface and accumulated on the surface of wastewater effluent [43]. This particular case was not included in calculating the average removal rate of MPs during secondary treatment.

3.8. MPs Removal Rate During Tertiary Treatment in WWTP

Among the selected studies, nine studies that considered 15 WWTPs reported the removal rate of MPs during tertiary treatment units, as shown in Table 5. Tertiary treatment can provide additional removal of remaining MPs that enter this treatment step, with an overall removal rate of 62.8% (± 29.5). The level of MP removal in tertiary treatment depends on the specific treatment processes applied, such as membrane filtration, biological filtration, sand filtration, dissolved air flotation, and so on. The highest removal rate was observed in rapid sand filtration composed of 1 m of gravel with a grain size of 3 – 5 mm and 0.5 m of quartz with a grain size of 0.1 – 0.5 mm [34]. The removal of MPs in rapid sand filtration could be attributed to trapping inside the grains and adhering to the sand grain’s surface, which the latter may be exacerbated by the addition of a coagulant. The dominant size, shape, and types of MPs that entered this stage of treatment were 20 – 100 μm, fibers, and polyacrylate, PE, PES, respectively. The removal rate of MPs with these properties using rapid sand filtration was 97.1% [34]. The lowest removal efficiency for tertiary treatment was reported for a trickling filter with a plastic bed, where the dominant forms of input MPs were fibers, fragments, and films [42]. In the output stream of this treatment unit, the fibers and fragments were reduced, but films remained nearly unchanged. However, it should be noted that the preliminary, primary, and secondary treatments already significantly decreased the plastic particles, and a further reduction in the number of plastic particles during tertiary treatment is challenging, with the remaining particles being less than 100 μm [34]. In general, applying tertiary treatment lead to a lower amount of MPs in the effluent than using only primary or secondary treatment.

3.9. MPs Removal Rate in the Disinfection Unit of WWTP

The removal efficacy of MPs by the disinfection unit was reported in three studies [40, 66, 67]. As can be seen in Table 6, the average removal of MPs in disinfection units of 4 investigated WWTPs was 12.5% (± 15.7). In some cases, chlorination had a positive effect on the removal of MP [66, 67], while reverse effects were observed in others [40]. According to Ruan et al., chlorination causes an 81.8% increase in MPs count in effluent of this unit, which may have been associated with the breakage of MPs to smaller particles due to severe oxidation redox [40]. In addition, UV disinfection increases MP number per liter of wastewater which may be due to the decomposing of MPs into smaller particles [57]. Fibers are susceptible to breakage during treatment and lead to higher concentrations in the effluent [69]. However, more studies are required for the reliable assessment of MP removal rate by the disinfection treatment units.

3.10. The Dominant Shapes of MP in the Influent and Effluent of WWTPs and their Removal Rate

The dominant shape of MPs in the influent and effluent of included WWTPs was studied. Out of the included WWTPs, 47 reported the shape of MPs in wastewater influent, while 51 reported it in effluent. Totally, the MP was categorized into fiber (significantly longer than wide) and particle (similar width and length), and their abundance is presented in Fig. 2a. According to Fig. 2a, fibers were the most common shape in 80.9% of wastewater influent and 76.5% of wastewater effluent among the reviewed WWTPs. The frequency of particles as dominant shape among reviewed WWTPs was 19.1% in wastewater influent and 23.5% in effluent. A significant effect was not observed between MP shape and removal efficiency during WWTPs. Similar results were reported by Azizi et al. [70], concluding that the MP shape is not a significant variable in MPs removal by conventional WWTPs. Other MP shapes such as irregular shapes, spherical beads, films, foam, flake, pellets, fragments, chip, sheet, granules, lines and microbeads were also categorized in the literature [59]. Fig. 2b presents the frequency of other particle shapes: granules, fragments, beads, films, flake, and sheet. Therefore, in terms of decreasing frequency, the order of MP shapes in influent was fibers, fragments, beads, films, and flakes; and in effluent, it was fibers, fragments, beads, flakes, and films. However, the large portion of fibers in WWTPs influent may be due to domestic washing machine discharges and clothes made of synthetic and natural fibers [7, 8, 71].

The average removal rate of MP with different shapes in the reviewed WWTPs was illustrated in Fig. 2c. Contrary to previous studies [72, 73], all MP shapes were highly removed (> 82.0%) by WWTP with different treatment steps. The lowest removal rates observed for fiber were 38.6% and 35.7%, in a WWTP with tertiary treatment (membrane bioreactors & conventional process) and secondary treatment (fixed bed filters), respectively [62]. This may relate to the decrease in the size of fibers during treatment relative to raw wastewater. Some researchers [30, 34, 65] observed an increase in the number of fiber particles following secondary treatment while a decrease in the number of fragment shapes. According to one study, primary settling and scum removal are effective in fiber removal [41], and their removal during secondary sedimentation was not significant. The breaking of MP in filters or MBR membranes and the production of fibers is one of the reasons for the increase in the number of fibers in the effluent of tertiary treatment that uses these treatment technologies [55]. A 2.5% increase in MP concentration in the effluent of MBR technology compared to conventional activated sludge was also reported [34]. In general, effluents from WWTPs using tertiary or advanced treatment have been reported to discharge more fibers [59].

Conclusions, Future Perspectives, and Suggestions

In this study, we systematically reviewed the efficiency of different treatment steps in municipal WWTPs for removing MPs smaller than 5 mm. Our findings indicate that conventional WWTPs can effectively remove MPs from the wastewater. WWTPs achieved removal rates exceeding 88%, even though they are not specifically designed for this purpose. However, due to the continuous discharge of WWTP effluent, significant quantities of MPs are still being released into aquatic ecosystems and freshwater systems. Secondary treatment (biological treatment unit and secondary clarifier) demonstrated the highest efficacy (63.7% (± 24.8)) in reducing MP counts in both influent and effluent. Contrariwise, the disinfection unit had the lowest impact on MP removal and, in some instances, even led to an increase in effluent MP concentration. This negative removal rate during disinfection was attributed to the intense redox reactions and subsequent fragmentation of MPs into smaller particles. Fibers, in particular, are prone to breakage during treatment, resulting in higher concentrations in the effluent. Consequently, further research is warranted to investigate the fragmentation of MPs during WWTP treatment processes.

Fibers generated from washing and cleaning processes represent the largest fraction of MP shapes in both the influent and effluent of WWTPs. As MP shape was not a significant factor in removal efficiency, future research should prioritize developing technologies specifically targeting fiber removal and exploring methodologies for analyzing MPs smaller than 100 μm. It is worth noting that the lack of standardized methods for quantifying MPs in wastewater made it challenging to compare the results of the included studies, leading to a wide variation in the reported occurrence of MPs (ranging from 0.28 to 31400 particles per liter). Therefore, establishing standardized methods for MP sampling, extraction, and analysis is crucial. Furthermore, future research should focus on small MP particles, particularly those smaller than 100 μm. Finally, while WWTPs remove a significant portion of incoming MPs, source reduction appears to be an important factor which needs future research.

Supplementary Information

eer-2024-366-supplementary.pdf

Acknowledgments

The authors also gratefully acknowledge the Research Council of Kermanshah University of Medical Sciences (Grant Number: 990276) for the financial assistance. This work was approved under the ethical code #IR.KUMS.REC.1399.351. All authors have read, understood, and complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors and are aware that with minor exceptions, no changes can be made to authorship once the paper is submitted.

Notes

Conflict-of-Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

A.A. (Associate Professor) searching, data curation, formal analysis, methodology, project administration, supervision, resources, writing-original draft. F.K. (MSc student) searching, data curation, formal analysis, writing. N.D. (Professor) writing - review & editing. S.E. (Assistance Professor) writing - review & editing.

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Fig. 1

Flow diagram of wastewater treatment plant with primary, secondary, and tertiary treatment processes.

Fig. 2

(a) The frequency of dominant shape of MPs in reviewed WWTPs (fiber (significantly longer than wide) and particle (similar width and length)), (b) Percentage of WWTPs reporting MP particles as the dominant shape in influent and effluent, and (c) The average removal of MPs based on shape in studied WWTPs.

Table S1. Table 1

Main characteristics of studies included in the systematic review.

Location	Influent (P/L)	Effluent (P/L)	Total MPs removal rate (%)	Wastewater source^a	WWTP process steps	Mesh range (μm)	Extraction (E) and determination (D) methods	Ref.
Canada	31.1	0.5	98.4	1, 2	A₂, C₁, D₇, D₆, E, G₁	1 for influent, 65 for effluent	E: sieve filtration (63 μm, digestion (30% H₂O₂), and vacuum filtration (1 μm). D: microscope with a digital camera, FTIR equipped with a microscope (ATR-FTIR, m-ATR-FTIR).	[41]
South Korea	29.8	0.43	98.5	1, 2	A₂, B₄, A₁, C, D₅, E, G₂	106–5000	E: sieve sorting, digestion (0.05 M Fe (II) solution, and 30% hydrogen peroxide (H₂O₂) solution), density separation (ZnCl₂). D: HT003 USB digital microscope, FTIR-ATR.	[60]
	16.4	0.14	99.1	–	A₂, B₄, A₁, D₄, D₃, G₂
	13.8	0.28	98.0	–	A₂, B₄, A₁, C, D₂, E, G₂
Spain	3.2	0.31	90.3	1, 3	B₁, C, D₁, E	200–5000	E: density separation (NaCl (2.05 M)), vacuum filter (0.45 μm). D: Trinocular Microscope coupled to HD digital camera, FTIR.	[25]
Finland	57.6	1	98.3	1	A₁, B, C, D₁, E, G	250–5000	E: sieve sorting, oxidation (0.05 M Fe (II) solution and 30% hydrogen peroxide (H₂O₂) solution), vacuum filtration (0.8 μm). D: optical microscope, FTIR and/or micro-Raman.	[43]
Denmark	10044	127	98.7	1 + 3	D₁	10–500	E: wet-sieve coupled surfactant adding, enzymatic degradation, oxidation (Fenton reaction). D: FPA-based FTIR imaging technique coupled with an IR spectroscope.	[51]
	8762	447	94.9	1 + 3	D₁
	6830	42	99.4	1, 3	D₁
	6021	29	99.5	1 (25%), 3 (75%)	D₁
	18285	214	98.8	1 + 3	D₁
	4994	182	96.4	1 + 3	D₁
	2223	35	98.4	1 + 3	D₁
	8149	19	99.8	1 + 3	D₁, F₇
	7601	43	99.4	1 + 3	D₁
	5362	65	98.8	1 + 3	D₁
China	2	0.2	90.0	1 + 3 (60%)	–	43–5000	E: sieve sorting, drying, wet peroxide oxidation (30% H₂O₀ and 0.5 M FeSO₄), density separation (NaCl). D: dissection microscope, micro-Raman spectroscopy.	[61]
	9.1	0.47	94.9	1 + 3	–
	3.3	0.35	89.4	1 + 3	–
	13.7	0.3	97.8	1 + 3	–
	9.7	0.8	91.7	1 + 3	–
	22.9	1.7	92.5	1 + 3	–
	1.6	0.32	79.3	1 + 3	–
Italy	2.5	0.4	84.0	1 + 2	B₁, C, D₁, E	63–5000	E: sieve sorting, density separation (hypersaline of NaCl), digestion by 15% H₂O₀. D: stereomicroscope, μFTIR-ATR.	[37]
Hong Kong	2.1	0.27	86.9	1	A₂, B₂, C, D₁, E, G₂	100–5000	E: sieve filtration (100 μm), centrifuge, filtration with glass fiber filter, digestion by 30% H₂O₂. D: microscopic examination, FTIR-ATR.	[40]
Hong Kong	1.01	0.4	60.4	1	A, C₂, G₁	100–5000		[40]
Netherlands	910	39	95.7	1	–	10–5000	E: density separation (NaCl), filtration over 0.7 μm glass filter. D: light microscopy, FTIR.	[62]
	73	65	11.0	–	–
	238	81	66.0	–	–
	91	56	38.5	–	D₁₀
	68	51	25.0	–	D₁, F₅
China	0.28	0.13	53.6	1 + 3	B₂, D₉, E, G₂	25–5000	E: sieve sorting, digestion (30% H₂O₂ and 20 mM Fe(II)), density separation (NaI), centrifugation. D: stereo microscope, FTIR-ATR.	[57]
China		0.05	82.1	–	B₃, D₅, F₅	25–5000		[57]
France	244	2.84	98.8	1	B₁, D, F	20–5000	E: sieve sorting, density separation (ZnCl₂). D: Stereomicroscope, micro-Raman spectroscopy	[58]
South Korea	31400	297	99.1	1	B, C, D₁, E, F₂, F₃	1.2–5000	E: filtration (1.2 μm GF filter, digestion (30% H₂O₂). D: microscope equipped with light projected.	[55]
South Korea	5840	66	98.9	–	B, C, D₁, E, F₂, F₇	1.2–5000		[55]
China	12.0	0.59	95.2	1	A, B₂, C, D₅, E, D₁₂, F₄, F₈, G₂	50–5000	E: sieve filtration (50 μm), filtration (10 μm), drying, digestion (FeSO₄ 0.05 M, H₂O₂ 30%), density separation (ZnCl₂), separatory funnel sedimentation, filtration. D: microscope, m-FTIR.	[38]
USA	147	3.7	97.6	1 + 3	A, C. D₁, E, G₁	60–5000	E: physical separation by filtration (43 μm stainless steel mesh filter). Drying, chemical digestion (H₂O₂, HCl), vacuum filtration. D: Stereomicroscope with a digital camera, m-FTIR-ATR.	[49]
	126	17.6	85.2	–	A, D₈, D₁, E, G₁
	146	17.2	85.5	–	A, D₈, D₁, E, G₁
Poland	552.5	0.138	99.0	–	A, B, C, D₁, D₁₁	109–5000	E: sieve filtration, digestion with Fenton's reagent (25 mL H₂O₂, 30% and 1 g of FeSO₄·7H₂O), density separation (ZnCl₂). D: optical microscope.	[63]
	150.5	0.08	99.0	–	D₁
	341.3	0.24	99.0	–	D₁
	21.2	0.96	95.0	–	D₁, D₁₁
	89.6	0.17	99.0	–	D₁, D₁₁
	19.4	0.44	98.0	–	D₁, D₁₁
UK	6.5	2	96.0	1 + 3	A₂, B, C, D₁, E, D₇	60–2800	E: wet peroxide oxidation protocols (H₂O₂; 1:1, v/v), filtration (1.2 μm GF). D: visual sorting with stereo microscope and FTIR-ATR.	[42]
France	293	35	88.1	1	A, B₁, C, D₇, E	100–5000	E: filtration (1.6 μm GF).. D: Stereomicroscope.	[32]
Turkey	26.6	7	73.0	1 + 3	A₃, B₅, C, F₁, E	60–2800	E: sieve filtration (55 μm, wet peroxide oxidation (30% H₂O₂, 0.05 M Fe₂SO₄), density separation (NaI), centrifugation and supernatant filtration. D: microscopic and μ-Raman.	[64]
Turkey	23.4	4.11	79.0	–	A₃, B₅, C, D₁, E	60–2800		[64]
Scotland	15.7	0.25	98.4	1	A₂, A₁, B₁, C, D₁, E	100–5000	E: sieve filtration (65 μm) and vacuum filtration (11 μm). D: microscopic and FTIR.	[33]
Finland	610	13.5	97.8	1 – 2 + 3	A₂, B, C, D₁, E, F₆	55–5000	E: sieves filtration, density separation (NaCl), centrifugation and supernatant filtration. D: Stereomicroscope.	[65]
USA	133	5.9	95.6	1 + 2	A, B, C₁, D₁, E	200–4750	E: sieve stack sorting. D: Stereomicroscope.	[66]
USA	92.9	2.60	97.2	–	A₂, B, C₁, D₁, E, D₇, E, F₁	200–4750	E: sieve stack sorting. D: Stereomicroscope.	[66]
China	79.9	28.4	64.4	1	A₁, A₂, B₂, D₁, E, G₁	47	E: sieve filtration (47 μm], digestion (30% H₂O₂) (some samples Fenton’s reagent), centrifugation and supernatant filtration (0.8 μm. D: fluorescence microscope, micro-Raman spectroscopy.	[67]
Mean (± SD)	2226.7 ± (5376.4)	36.5 ± (79.9)	88.1 ± (18.9)

1: Municipal, 2: Storm water, 3: Industrial, P/1: Particles/liter

Table 2

MPs removal efficiency during preliminary treatment.

Type of preliminary treatment	Removal efficiency (%)	Ref.
B₂	21.4	[57]
B₃	−371.4	[57]
A, B₂	58.8	[38]
A₂, B	6.0	[42]
A₂, A₁, B₁	44.5	[33]
A₂, B	58.6	[66]
A, B	35.1	[66]
Mean (± SD)	37.4 (± 21.0)

Table 3

MPs removal efficiency during primary treatment

Type of primary treatment	Removal efficiency (%)	Ref.
C	19.1	[25]
C₂	78.2	[40]
C	68.0	[32]
C	69.3	[42]
C	60.9	[33]
C₁	61.6	[66]
C₁	82.1	[66]
Mean (± SD)	62.7 (± 20.8)

Table 4

MPs removal efficiency during secondary treatment

Type of secondary treatment	Removal efficiency (%)	Ref.
D₇, D₆, E, G₁	80.8	[41]
D₁, E	88.0	[25]
D₁, E, G₂	66.0	[30]
D₁, E	97.1	[34]
D₁, E	− 66.7	[43]
D₁, E	64.0	[37]
D₁, E, G₂	77.5	[40]
D₉, E	40.9	[57]
D₁, E	54.7	[55]
D₁, E	37.5	[55]
D₁, E	79.2	[55]
D₅, E	54.5	[38]
D₇, E	61.1	[42]
D₁, E	92.0	[32]
D₁, E	92.6	[33]
D₁, E	73.0	[65]
D₁, E	60.0	[66]
D₁, E, D₇, E	11.9	[66]
D₁, E, G₁	16.6	[60]
Mean (± SD)	63.7 (± 24.8)

Table 5

MPs removal efficiency during tertiary treatment

Type of tertiary treatment	Removal efficiency (%)	Ref.
F₁₁	25.0	[30]
F₇	97.1	[34]
F₁₀	95.0	[34]
F_7, G	55.5	[37]
F₅	12.1	[62]
F₂	53.8	[55]
F₈	89.9
F₂	81.6
F_3,	79.4
F₂	47.1
F₇	73.8
D_12, F_4, F₈, G₂	71.7	[38]
F₉	4.0	[32]
F₆	83.6	[65]
F₁	72.7	[66]
Mean (± SD)	62.8 (± 29.5)

Table 6

MP removal rate during disinfection unit

Disinfection type	Removal efficiency (%)	Ref.
G₁	−81.8	[40]
G₁	30.2	[66]
G₁	0.21	[67]
G₁	7.1
Mean (± SD)	12.5 (± 15.7)