Fate and Removal of Microplastics from Industrial Wastewaters

Abstract

Industrial sites are typically located in close proximity to bodies of water, making industrial wastewater a prevalent source of pollution. Microplastics, which are plastic fragments generated from everyday activities or industrial operations and are smaller than 5 mm in size, can readily find their way into wastewater treatment plants (WWTPs). The objective of this research was to offer extensive insight into the fate of microplastics in industrial WWTPs worldwide, as well as to explore the effectiveness of diverse advanced treatment technologies in eliminating microplastics. The prevalence of microplastics and their negative impact on aquatic environments has been acknowledged in recent years. The progressive discharge of plastic waste, insufficient detection processes with specialized elimination methods and a sluggish disposal rate have led to the continuous presence of microplastics in various ecosystems worldwide, such as domestic wastewater and industrial wastewater. Research outcomes have revealed that they can adsorb a variety of pathogens, heavy metals and chemical substances that are commonly used in production processes. Microplastics can be consumed by aquatic life, which might lead them up the food chain to human bodies, resulting in potential digestion tract blockage, digestion disturbance and diminished reproductive growth. Microplastics have thus become a growing threat and cause for concern, demanding the containment of their dispersion. This work offers a critical evaluation of current and developing techniques for microplastic detection and separation from industrial wastewater, which are the most challenging endeavors when treating systems containing microplastics. A review of the effect of microplastics on aquatic environments and human health is also conducted. This analysis offers a comprehensive view of the full microplastic detection and removal strategies and their related concerns in order to establish a waste disposal standard that minimizes the potential hazardous effects of microplastics in aquatic systems.

1. Introduction

The pollution of aquatic ecosystems with plastic has been a pressing and enduring concern [1]. The lightweight nature of plastic enables it to be carried over vast distances by wind and ocean currents [2]. ISO technical report 21,960 provides a definition of microplastics, referring to particles ranging from 1 µm to 1 mm in size [3] which may be categorized into primary and secondary types [4,5]. Among microplastics, particles with dimensions from 1 mm to 5 mm are known as large microplastics. On the other hand, particles with dimensions in the range of 1 nm to 1 µm are defined as nanoplastics [3]. The former are deliberately constructed to be microscopic and can be found in various cosmetic products, while the latter result from the degradation of larger debris.

Industrial and residential activity are responsible for microplastic (MP) pollution [6,7]. The correlation of the level of industrialization to MP is widely accepted [8]. According to research [9,10] and model studies [11], industrial microplastic emissions are higher than from other areas. Industrial waste discharge is one of the leading causes of MP pollution in rivers [12] and oceans [13]. Microplastics are found in various environments [14], including the atmosphere [15], ground [16], ocean [17] and freshwater [18]. due to their small dimensions and high specific surface area [19], MPs are capable of adsorbing various pollutants, including heavy metals [20], polycyclic hydrocarbons [21], polybrominated diphenyl ethers [22], drugs and personal care products, from the environment [20,22,23]. Moreover, they offer a large surface area for the adsorption of various chemicals [24,25]. These particles, in addition to nanoplastics, which can potentially enter cells and disrupt cellular processes, are of great concern for environmental removal efforts [26]. There is a lack of literature regarding the impact of industrial sources on the microplastic contamination of urban wastewater, as indicated by prior studies [27]. As a result, researchers have shifted their attention toward identifying and regulating the sources of MPs, particularly in industrial zones [28,29]. In order to effectively manage MP pollution, it is crucial to obtain a deeper understanding of the significance of their sources and means of reaching sewer systems [11].

The effluents of urban WWTPs or facilities treating sewage are known to be significant recipients of microplastic (MP) discharges, which can consist of primary and secondary microplastics [30]. The appearance of microplastics has been observed in both the influent (ranging from approximately 645 to 1567 MPs/L) and effluent (ranging from approximately 16 to 131 MPs/L) of WWTPs, as reported in a study cited in Franco et al. [31]. Additionally, recent research has revealed the existence of MPs in sludge (ranging from 2742 to 15385 MPs/kg) [32]. Although numerous factors [33] can influence the quantity, composition and morphology of microplastics, the main factor is the source of the MPs [34]. Typically, MPs found in urban wastewater arise from daily activities, such as the use of products such as toothpaste, cleansers and shower gels [35]. Additionally, plastics in waste can be broken down by microorganisms in the leachate and ultimately end up in WWTPs [36].

Information about the variety of microplastics (MPs) from different sources is scarce. Specifically, there is limited knowledge about the traits of microplastics in industrial wastewater [37]. Although the concentrations of microplastics (MPs) in treated wastewater are lower than in non-treated wastewater, they still contribute significantly to environmental contamination [13]. Research by Conley et al. [38] estimated that three wastewater treatment plants (WWTPs) in the USA release 500–1000 million MPs daily. Similarly, Wolff et al. [39] reported the release of approximately 3000 to 5900 MPs/m³ of treated effluent from a German plant. Various techniques have been studied for removing MPs from water, including coagulation/flocculation, sedimentation, magnetic extraction and membrane filtration, which can reduce the quantity of MPs by approximately 80% [40]. Nanofiltration (NF) and reverse osmosis (RO) were previously also utilized in potable water treatment; however, they are prone to fouling caused by MPs [41]. Despite the availability of advanced treatment technologies, further optimization is needed before they are mandated by wastewater treatment legislation [42]. There are still gaps in understanding the effectiveness of wastewater treatment processes in eliminating MPs and the effect of MPs on these processes.

Moreover, there are inadequate data on the heterogeneity of microplastics (MPs) originating from diverse sources, particularly with respect to the attributes of microplastics found in industrial wastewater [43]. As a result, the exact share of contamination owed to industrial sources remains imprecise [44]. One possible explanation for this is that only a limited number of countries have established policies on these issues, resulting in MP pollution becoming a worldwide issue, as stated in [45]. The presence and distribution of microplastics (MPs) in various settings have been thoroughly studied, but their causes and fates in certain cases are still unclear [32]. Wastewater treatment plants (WWTPs) are some of the significant contributors of MPs [46]. However, while removal methods for other pollutants have been thoroughly investigated, there are still gaps in the knowledge regarding emerging pollutants such as MPs [32]. The detection and characterization of microplastics in the environment poses a multifaceted problem that has yet to be fully resolved. Early efforts in this area delved into characterizing the extent of microplastic pollution, but the research since then has broadened to include investigations into the processes and fate of microplastics [47]. The high quantities detected in the environment underscore the importance of conducting studies on their global occurrence and distribution [27]. Furthermore, the development of effective removal technologies remains a significant challenge in preventing the release of large quantities of MPs into the environment [48].

Acquiring a comprehensive understanding of the behavior of microplastics in wastewater treatment plants (WWTPs) is of the utmost urgency, as WWTPs are widely recognized as the primary contributors of microplastic contamination in aquatic ecosystems. [29]. The main objective of this work was to comprehend the fate of microplastics in industrial treatment plants globally, in addition to ascertaining the microplastics removal efficiency of different advanced treatments. This review aims to address this gap by examining literature from various WWTPs in seven countries to provide reliable and comprehensive information on the characteristics of microplastics and their removal technologies. The selection of different types of WWTPs serves a threefold purpose: (i) to gain an in-depth understanding of MPs in different WWTPs, (ii) to analyze the elimination patterns of different microplastic shapes, polymer categories and particle sizes and (iii) to evaluate treatment technologies worldwide. This work is unique in its objective to review current works and present up-to-date information on the efficiency of microplastics removal, highlighting the factors that influence the process and providing informative results to better comprehend the fate of microplastics in WWTPs.

2. Fate of Microplastics

The fate of microplastics (MPs) is determined by a variety of factors, such as biodegradation, physical abrasion and chemical oxidation [49]. The interplay between these processes and the transport of MPs determines their fate [50]. To evaluate the environmental hazards of MPs, regulators and researchers rely on environmental fate modeling [51]. There are three primary fates for MPs: aquatic environments [52,53], air environments (e.g., humans and sea birds) [54] and terrestrial environments (such as soil and landfills) [55]. The fate of MPs in different environments is affected by a range of natural factors, such as temperature and sunlight, and their physicochemical properties, including size and density [56]. The characteristics of microplastic polymers play a crucial role in their persistence, fate, degradation and ability to adsorb or release organic contaminants [9]. Understanding the fate of MPs is complex and challenging due to the tedious and cost-intensive analysis involved [57]. Several studies have explored the partitioning of microplastics during wastewater treatment [35,38,58,59]. Specifically, the fate of microplastics is largely dependent on their physicochemical properties and the treatment they undergo. Adsorption, for instance, refers to the transformation of pollutants from a liquid to a solid phase; thus, the fate of adsorbed substances is linked to the fate of solids (e.g., agriculture) [60].

There are numerous aspects that have an effect on the transportation of microplastics in water systems, including their density, shape, size and interactions with biota. Additionally, various characteristics [61] of the host environment, such as how fast the flow is, how deep it is, the topography, weather conditions and human activities in the vicinity [41], can also play a role. Different studies have reported varying levels of efficiency in removing MPs from water, with removal rates ranging from about 60% to almost 100%. The efficiency of removing MPs is affected by a range of factors, such as the plant layout and technology, influent attributes, pre- and primary treatments and secondary treatments.

Ngo et al. [62] noticed that fibers are usually not easy to remove. Additionally, polyethylene (PET) and polypropylene (PP) are commonly found in the influent but are often encountered in the effluent because of their low density, making them easier to remove by skimming. Fahrenfeld et al. [63] argued that films and fibers are the most successfully eliminated items. This discrepancy is likely due to differences in plant topography and research protocols followed during the studies.

In pretreatments and primary treatments, the elimination efficiency can vary from 40.7% to 91.7% [62], depending on whether an aerated grit chamber is present or not and on the polymer categories detected in the influent. Fibers are often the least-removed items at these steps. Pre-treatments have the highest impact on the relative size distribution of MPs, as they are able to remove particles of a larger size. Secondary treatments, such as activated sludge and sedimentation, can lead to removal efficiencies in the range of 28.1% to 66.7% [62], depending on their diverse configurations and retention time. Longer retention times can boost the development of biofilms on microplastics, leading to a higher weight and thus better results. Some studies have reported a total removal rate of additional treatments reaching higher than 90% [64,65]. Nevertheless, further research is required on this subject.

Fragment removal appears to be higher than fiber removal in secondary treatments, with some studies reporting higher amounts of fibers after secondary treatments. This discrepancy may be attributed to undiscovered interactions among microplastics, microbes and flocculants. Additionally, recent studies have confirmed that secondary treatments have lower elimination rates for fibers. In some studies [66,67], the relative abundance of fibers in the effluent was found to be at higher levels than in the influent [68], which may due to the diverse sampling mechanisms applied to each stream.

In conclusion, the efficiency of MPs removal from water is influenced by multiple factors, and different types of MPs may be more or less effectively removed depending on the treatment process and operational conditions. Additional research is required to comprehend the movements of MPs in water systems and to develop more effective treatment technologies for their elimination.

Ngo et al. [62] have stated that using a membrane biological reactor (MBR) is the most efficient option for the extraction of MPs, steadily achieving an elimination rate higher than 99.5%. However, the researchers have emphasized the importance of studying fouling after a certain time of operation [62]. Bayo et al. [69] observed that during an 18-month period in a WWTP in Spain, the MBR method demonstrated better removal rates for MPs than the typical activated sludge (CAS) and rapid sand filtration (RSF) process (79% vs. 75%), although there was no statistical significance in the difference. This work found that the elimination of MPs was not as high as expected, but it was comparable to findings by Raju et al. [66]. Bayo et al. [69] observed considerable differences in elimination rates when they used activated sludge and rapid sand filtration in combination with an MBR. Sun et al. [13] have confirmed that membrane filtration is considered the most effective for MPs removal. Membrane fouling is a major challenge associated with membrane processes. Microplastics (MPs) can contribute to this issue because of their small dimensions and surface attributes. Inadequate control of fouling can lead to significant operational and financial concerns. As a result, many studies have been conducted to address membrane fouling [70]. Ngo et al. [62] found major inconsistencies in MP removal using RSF, and Sun et al. [13] discovered that biological aerated filters did not cause a significant change in MPs in the effluent. Park et al. found that wastewater treatment plants (WWTPs) utilizing advanced phosphorus removal techniques demonstrated a greater removal efficiency compared to those without such processes [68].

The research outcomes from Freeman et al. [45] and Sun et al. [13] suggest that polyester polyethylene MPs are the most common pollutants detected in effluents from WWTPs. Moreover, some authors have found that the smallest size classes of these microparticles have the highest frequency [64], while others have not confirmed this finding [67]. Bayo et al. [67] identified no significant change in effluent concentrations among seasons, but Mintenig et al. [71] noted a heightened concentration during a strong rain event. However, the impact on the size variations and polymeric variety have yet to be fully understood. The removal of MPs can be improved through pretreatment and primary treatments and secondary and tertiary treatments can also be effective, depending on the technology employed. Temporal patterns exist, but further research is needed to confirm them.

3. Water Treatment Technologies for the Removal of Microplastics

According to the literature [72], WWTPs are not created for the purpose of degrading plastic materials. However, there are various technologies available for removing microplastics (MPs) from wastewater, including pretreatments such as screening and grit chambers, initial sedimentation, activated sludge processes, coagulation, constructed wetlands and filtration. Based on their installation and maintenance costs, these technologies can be categorized from low-cost to high cost categories [73].

Figure 1 depicts a typical wastewater treatment plant (WWTP) system and the MPs/NPs elimination rates [7].

3.1. Primary Treatment

Numerous studies have shown that primary treatment processes such as skimming and settling can significantly reduce the amount of microplastics found in WWTPs [65,74]. The elimination efficiency during this stage relies on the physical properties of the MPs, such as their density, dimensions and shape, and the characteristics of the water sample, such as the suspended solid concentration. Temperature plays a critical role in the implementation of a microplastics (MPs) procedure, as higher temperatures can enhance the efficiency of the oxidation process. However, there is a risk of modifying the characteristics of the plastic particles, potentially resulting in morphological alterations and causing them to break down [75]. Floatation and gravity settling affect the elimination of the MPs at this phase. Gravity can cause MPs with higher densities to settle quickly, while those with lower densities may float and be separated [66]. In the presence of elevated levels of suspended solids, microplastics (MPs) have the potential to aggregate, leading to increased sedimentation rates and the improved extraction of MPs from wastewater. As a result, this may help alleviate the burden of MPs on other treatment processes by reducing their overall load [67]. However, MPs connected to unstable flocs could potentially not settle as a solid bulk and may consequently escape, resulting in a lower removal of MPs [76]. In addition, the retention of MPs is influenced by the hydraulic retention time and turbulence. As primary treatment is highly effective at removing large and fragmented MPs [77], it should be the primary area of focus for improving the removal of MPs [78]. Retrofitting secondary plants with primary clarifiers can likewise enhance the efficiency of microplastics elimination [38], but this strategy might be affected by the locale, availability of land, current facility architecture, influent load on WWTPs and cost [38].

3.2. Secondary Treatment

Studies have shown that the activated sludge method is successful at eliminating MPs, suspended solids, nutrients and dissolved organic matter [59,79]. It has been observed that sludge contains more microfibers and foams (which are round or irregular in shape) than the secondary effluent, indicating a greater tendency for fibers to bio-foul in sludge. In contrast, small openings in foam surfaces attract particulates, thus augmenting their weight and allowing them to settle in the sludge quickly [66,80]. For granular MPs, settling is determined by Stoke’s law [81].

3.3. Tertiary Treatment

The existing treatment systems primarily aim to eliminate solids, organic substances, nutrients, pathogens and emerging pollutants, and they are not designed to specifically metabolize plastic materials [72]. Although the conventional treatment processes exhibit a reasonable level of MPs elimination, there are still some small-sized MPs in the effluent which can be extracted before discharge [72]. The available options were devised with the intention to eliminate the remaining solids and organics in the wastewater, including—among others—coagulation, membrane filtration and bioreactors, constructed wetlands and advanced oxidation processes [72]. Incorporating advanced treatment methods such as tertiary treatment is necessary to improve the elimination efficiency of MPs/NPs, resulting in low concentrations of MPs in the effluent from tertiary treatment systems [82].

Coagulation involves the destabilization of colloidal particles and their subsequent agglomeration in floccules through the addition of chemical reagents, succeeded by a physical separation. Coagulation neutralizes the charge of colloidal particles and facilitates sweep flocculation, resulting in stable flocs that settle quickly. The efficiency rate of coagulation is impacted by the physicochemical attributes of the material, such as its dimensions, shape, pH and organic content. The category and quantity of coagulant employed and the concentration of MPs in the water also impact the results of the method [7]. The existing literature provides information on the effectiveness of coagulation/flocculation for the removal of various microplastic particle types (including polyethylene, polyester etc.), each with diverse dimensions and shapes, either untreated or aged. The experiments employed frequently utilized coagulants such as iron and aluminum salts. Synthetic matrices with well-defined properties are frequently used to study the coagulation/flocculation mechanism under laboratory conditions [83].

The results obtained, however, differ based on the conditions of the coagulation process, the polymers and their properties, as well as the characteristics of the matrix in which microplastics are found [84]. In their research, Vasiljević et al. examined the use of FeCl3 for coagulation and flocculation in a synthetic matrix and reached an elimination efficiency of 67–99% for textile fibers as opposed to the starting concentration. The maximum rate was observed at a FeCl₃ concentration of approximately 3.9 mM, whereas the minimum was at a concentration of 0.7 mM. With the exception of the highest dose, all applied doses exhibited similar removal efficiency values, indicating complete elimination. Using PACl as a coagulant resulted in marginally lower values ranging from approximately 57% to 91%. For the combination of coagulants, elimination efficiencies of 35% were reached for Fe/Al, whereas an elimination efficiency of 66% was achieved for Al/Fe and 78% for Fe + Al. These results suggest that combining coagulants does not improve the elimination rate of textile fibers as opposed to single materials [85].

Jachimowicz et al. studied the removal of microplastics (MPs) from wastewater in a wastewater treatment plant (WWTP), using different coagulants and flocculants at varying dosages prior to the initial primary. The study found that the best elimination rate (90%) was reached when using a dose of 2.5 mL PAX/m³, and the initial sludge produced had a low MP concentration that was easy to manage in ensuing treatment phases. At the higher dosage, PIX was able to substantially improve the elimination of P-PO4 (reaching 94%) and COD (approximately 73%) [86]. Mathieu Lapointe et al. have made a valiant effort to explain the coagulation and flocculation processes involved in microplastics (MPs) removal by studying both pristine and worn-down MPs. They found that worn-down MPs exhibited an altered surface chemistry and roughness, which in turn affected their affinity for coagulants and flocculants. The results showed that coagulant efficiency increased in every case of worn-down MPs. For instance, elimination rates of approximately 97% and 99% were noted for PEST and worn-down PE. However, larger, pristine PE microplastics posed the highest resistance to coagulation and flocculation, reaching an elimination of only 82%, even under optimal conditions [87]. In conclusion, the coagulation process appears to hold relatively new, promising and still-evolving ground, given its numerous benefits. Scientific research teams have been making strides toward advancing this method.

According to one study [88], the data reported on the removal of microplastics (MPs) through disc filters yielded inconsistent outcomes because of variations in filter sizes, types of MPs studied and the methods of characterization employed. Disc filters typically consist of fiber materials with varied mesh pore dimensions and are assumed to remove particles larger than the mesh size. These filters have been found to effectively eliminate MPs at rates ranging from 40 to 98.5%, in addition to fine particulate matter [88]. Larger MPs are retained on the filter, while small-sized ones may go through [7].

The use of sand filtration in tertiary wastewater treatment may require physicochemical and biological interactions to eliminate particulates, nutrients and microorganisms, as well as microplastics (MPs) and nanoplastics (NPs) [59]. Sand filters, including rapid and gravity sand filters, act as a fence, preventing the percolation of MPs and NPs. While very small pores between the sand grains trap larger MPs on the surface, smaller MPs may pass through [59]. The confinement of MPs on the filter surface depends on their size and the employment of coagulation as a first treatment stage to form flocs that can be retained by the sand layer. Studies have reported an MPs removal efficiency of up to 73.8% using coagulation and sand filtration [59,72].

Figure 2 depicts the effect of MPs on various WWTP treatments, ultimately leading to reduced treatment efficiency [7].

Ozone disinfection, another wastewater treatment mechanism, effectively destroys microorganisms and oxidizing compounds. Additionally, ozone has proven to be highly effective in removing micropollutants, including MPs, from wastewater, achieving a removal efficiency of almost 90% in some studies [59]. Constructed wetlands are considered a sustainable option for secondary or tertiary treatment in WWTPs, according to surface, horizontal subsurface or vertical flow. They have been found to enhance the elimination of MPs in WWTPs. Long et al. [89] verified removal efficiencies of approximately 26% and 10% for MPs in rural WWTPs using a horizontal subsurface flow, while Zhou et al. [90] verified a removal of MPs of more than 89%. The efficiency of constructed wetlands in removing MPs is affected by the plant’s architecture and environmental conditions. Membrane bioreactors (MBRs) combine bioreactors with filtration to eliminate suspended or dissolved inorganic and organic materials as well as MPs and NPs [91]. The mechanisms of MPs removal in MBRs include trapping the MPs and stopping them from infiltrating the effluent. Sedimentation is the main mechanism for removing larger particles in anaerobic conditions, while the removal mechanism for small-sized particles [92] in aerobic conditions is sludge adsorption interception.

Dissolved air flotation (DAF) can take place before or after the initial/secondary/final steps to remove lightweight particles, oils and greases. Although its main purpose is not to eliminate MPs and NPs, it has been found to have an efficiency of 95% in removing MPs [72]. Advanced oxidation processes are promising options for the tertiary treatment of WWTPs to mitigate the newly found contaminants, including MPs and NPs. Kang et al. [93] studied the decomposition of microplastics using magnetic carbon nanotubes (CNTs) under hydrothermal conditions; these release products that microorganisms can use as a source of carbon in the water. Membrane technology is effective in removing MPs from wastewater and has several advantages in treating various pollutants [32]. Microfiltration, ultrafiltration, nanofiltration and reverse osmosis can be utilized either as a secondary step or in combination with biological processes for the treatment of the primary effluent [94,95]. Considering the aforementioned details, it is evident that each treatment method has both advantages and limitations (

Table 1. Advantages and drawbacks of treatment options used for the elimination of MPs from aquatic environments.

Removal Method	Advantages	Drawbacks	References
Sand filtration	Quick and effective MP elimination. Low cost of operation and maintenance.	The filter material is highly porous.	[62]
Dissolved air flotation	Compact design, short retention period, high hydraulic capacity and small footprint of flocculation and flotation units, enabling low capital costs.	Apart from the adhesion via hydrophobic forces, there are concerns regarding the mechanism of interactions between bubbles and particles.	[96,97]
MBR	High elimination efficiency of MPs. Effective removal of MPs because of the smallest pore size. A combination of biodegradation and membrane filtration.	It is challenging to regulate different factors that impact the treatment efficacy, such as the type, dimensions and concentration of MPs. More expensive and with greater energy requirements than sedimentation.	[79]
Constructed wetlands	Requires less frequent maintenance. Limited operating cost.	Limited information about the related mechanisms. The impact of external factors is not fully known.	[98]
Coagulation–Flocculation–Sedimentation	Simple and easy to operate. Ability to capture and eliminate small MPs.	High requirement for chemicals. Mostly studied only in laboratories. Not widely studied at the commercial level.	[87]
RO	Elimination through semi-permeable membrane. Ease of treatment.	Lower elimination rate of MPs than that of the MBR process. Even after RO, plastic debris still remains in WWTPs.	[62]
AOP Fenton	Simple process with high mineralization percentages for recalcitrant organic pollutants.	Low pH requirement, needs quite large amounts of ferrous ions and the formulation of iron sludge, i.e., a secondary contamination source.	[99]
AOP Ozonation	High elimination rate.	No effect on salinity (ozone)	[100,101]
AOP Photocatalysis	High degradation percentage.	Exposure to carcinogenic UV light.	[102]

) that are unique to the individual technique.

Overall, the removal process is affected by various factors, including the treatment technology, environmental conditions and the type of MP.

Table 2. Information on the removal efficiencies of microplastics.

Technology	Types of Polymers	WWTPs Location	Concentration of MPs in Influent (Particles L−1)	Concentration of MPs in Effluent (Particles L−1)	WWTP Treatment	Concentration of MPs % (Influent)	MPs Total Removal Efficiency	Reference
Grit and grease removal; primary clarifier; activated sludge reactor; secondary clarifier	Seventeen polymer families were detected, with low-density polyethylene (LPDE) being the most frequently encountered one (52.4%).	Spain/Cabezo Beaza WWTP	3.20 (±0.67)	0.31 (±0.06)	Primary Treatment; secondary Treatment	52.40%	90.30%	[67]
Grit chamber; primary settling tank; anaerobic–anoxic–oxic	The main polymer compositions of MPs included polyethylene. Terephthalate (PET); polyethylene (PE); nylon; polyvinyl chloride (PVC).	Vietnam/HoaCam WWTP	183–443 particles/L	138–340 particles/L	Primary Treatment; secondary Treatment; tertiary treatment	PET was the most common MP at 22–29.9% of the MPs	25.50%	[103]
Grit chamber; primary settling tank; activated sludge	The main polymer compositions of MPs included polyethylene terephthalate, polyethylene, nylon and polyvinyl chloride	Vietnam/DaNang WWTP	183–443 particles/L	138–340 particles/L	Primary treatment; secondary treatment; tertiary treatment	PET was the most common MP at 22–29.9% of the MPs	21.80%	[103]
Grit chamber; primary settling tank; sequencing batch reactor	Polyethylene terephthalate (PET); polyethylene (PE); nylon; polyvinyl chloride (PVC)	Vietnam/HoaKhanh WWTP	183–443 particles/L	138–340 particles/L	Primary treatment; secondary treatment; tertiary treatment	PET was the most common MP at 22–29.9% of the MPs	25.30%	[103]
Biofilter	N/A	France/Seine-Centre WWTP	260–320 × 1000 particles m−3	14–50 × 1000 particles m−3	Primary treatment; secondary treatment	N/A	83 to 95%	[104]
Filtered via stainless steel sieves of various mesh sizes	PVC, HDPE, PEMA, PP, PS and PE were the most common MPs detected	Spain/5 municipal and 2 industrial WWTPs in Cadiz	264–1567 partickes/L	39–131 partickes/L	Primary treatment	N/A	78–97%	[27]
Ultrafiltration composite membrane made of polyacrylonitrile with added rGO	polyethylene terephthalate	Poland/Silesian Province	not mentioned	not mentioned		N/A	>80%	[105]
Activated sludge; secondary settling tank	Not specified (they were categorized by type/shape)	South Korea/Three WWTP in Daegu	4200 MP/L (total) in WWTP A31400 in WWTP B5840 in WWTP C	33 MP/L in WWTP A 297 in WWTP B 66 in WWTP C	Primary treatment; secondary treatment; tertiary treatment	31.4–53.4% for WWTP-A and C70.4% for WWTP-B	98.9–99.2% after tertiary treatment	[59]
Biofilter	Polypropylene (31.6%); polyethylene (21.9%); polystyrene (10.1%); propylene/ethylene copolymer (9.2%); polyethylene terephthalate (7.5%).	China/7 WWTPs in Xiamen	1.57 to 13.69 items/L	0.20–1.73 items/L	Primary treatment; secondary treatment	not mentioned (we have percentages of distribution for MP types but not total mp concentration)	79.3 to 97.8%	[106]
Gravity filters/membrane reactor	Polyethylene was the most frequent polymer type in both size classes. PP, PE, PA, styrene295 acrylonitrile (SAN) and PEST were found.	Germany/12 WWTPs in Lower Saxony	0 to 5 × 10 m−3 MP (size >500 nm)1 × 10 to 9 × 10 + 3 m−3 MP (size <500 nm)	Not mentioned	Tertiary treatment	Not mentioned	Up yo 97% after tertiary treatment	[71]
Sand filtration	Polyethylene (PE); polypropylene (PP); polystyrene (PS); polyvinyl chloride (PVC); polyethylene terephthalate (PET); polyamide (PA). Polyethylene (PE) polypropylene (PP) and polystyrene (PS) constituted almost 83% of all the MPs.	China/9 domestic wastewater treatment plants (WWTPs), 5 industrial WWTPs, wastewater from ten industrial facilities, 4livestock farms and another 4 fish ponds	18–890 n L−1	6–26 n L−1,	Primary treatment; secondary treatment	PE, PP and PS comprised 83% of total MPs	35% to 98%, depending on the WWTP	[107]

summarizes the location, MPs concentration in influent and effluent, MPs removal and primary treatment options of the WWTPs in the studies discussed above.

Bayo et al. [67] investigated the abundance, concentration and removal efficiency of MPs in a classical WWTP with an activated sludge process. The WWTP exhibited an efficiency of practically 90% in the final effluent, identifying more than 540 MPs, including fragments (~47%), films (~35%), beads, foam and fibers. The highest concentration was achieved by low- and high-density polyethylene (LDPE and HDPE).

Mintenig et al. [71] conducted a study to examine the appearance of microplastics (MPs) in the effluent of 12 plants in Germany. The WWTPs treated both municipal and industrial wastewater. The findings revealed that all the effluents contained MPs with sizes ranging from >500 μm to <500 μm. The concentration of microplastics larger than 500 μm varied between 0 and 5 × 10¹ m⁻³, while the concentration of microplastics that were smaller than that ranged from 1 × 10¹ to 9 × 10³ m⁻³. There was no significant correlation between the number, size, or polymer type of MPs found in the different WWTPs. However, MPs larger than 500 μm were absent in the effluents of only two WWTPs, which were located in Schillig and Oldenburg (ap). Polyethylene (PE) was the most common plastic particle found in MPs > 500 μm, accounting for 59% of the total, followed by polypropylene (PP), with an average of 16%. In MPs < 500 μm, PE was also the prevailing polymer, accounting for 40% of the total [71].

Wang et al. [107] determined that the majority of the microplastics in the influent and effluent of various wastewater treatment plants in Changzhou, China, were polyethylene (PE), polypropylene (PP) and polystyrene (PS), comprising nearly 83% of all detected MPs. Most of these MPs were fragments and films less than 500 μm in size. Additionally, the study identified the presence of plastic particles in wastewater from four livestock farms; these particles were composed mainly of PE (38%) and polyamide (PAM) (32%). All the samples from the fish ponds contained MPs as well. There were no marked differences in the concentration of MPs from the various sources, implying that they could all potentially contribute to MPs pollution. However, there were variations in the size and color of the MPs detected in them, although there were no significant differences in the type and morphology of the polymer present [107].

In their study, Van Do et al. [103] examined the occurrence and dissemination of microplastics (MPs) in the influent and effluent of three typical industrial treatment facilities located in Danang city, Vietnam. The study showed that the quantity of MPs in the influent and effluent varied between approximately 180–440 and 140–340 particles/L, respectively. The average elimination rates of the three plants were found to be approximately 25%, 22% and 25% respectively [103]. The main types of polymers detected in the MPs were polyethylene terephthalate (PET), polyethylene (PE), nylon and polyvinyl chloride (PVC).

Dris et al. [104] observed the presence of MPs, mainly fibers, in total atmospheric fallout. High levels of fibers were also found in wastewater (260–320 × 10³ particles/m³); however, after treatment, the contamination was reduced to approximately 15–50 × 10³ particles m³. The study also investigated the river Seine, using two sampling devices. The findings indicated that the plankton net (0.8 cm mesh) mainly caught fibers.

Similarly, Franco et al. [27] examined influents and effluents from five municipal wastewater treatment plants (MWWTPs) and two industrial plants in Cadiz. The study showed that quantities of MPs were higher in industrial than in municipal plants, with elimination rates varying between 78% and 97%. The dimensions and shapes of the microplastics in the effluents were similar to those in the influents, implying that no shapes and sizes are preferred during the elimination process. The most frequently identified MPs in the influents were Polyvinyl chloride (PVC), high-density polyethylene (HDPE), Poly (ethyl methacrylate) (PEMA), Polypropylene (PP), Polystyrene (PS) and Polyethylene (PE). The study claimed that approximately 8.5 × 10¹¹ microplastics may be released annually by the treated wastewaters [27].

Hidayaturrahman et al. [59] conducted a study to investigate the elimination efficiency of microplastics at various phases of treatment in three facilities and to evaluate the effectiveness of tertiary treatment using coagulation, ozone, membrane and rapid sand filtration. The study found that the first two treatments were successful in extracting MPs, with elimination rates varying between 75% and 92%. The percentage reached over 98% after the third treatment. The most frequently encountered MPs in all the samples were microbeads from personal care products and larger fragments of plastics. Microplastics were, however, still detected in significant quantities in the final effluent [59].

The results obtained by Long et al. [106] are consistent with those of this study. Long et al. used a better sampling process, which involved an electromagnetic flowmeter and a fast digital camera, to analyze sixty samples. The study found that the concentration of MPs in the influent ranged from 1.57 to 13.69 items/L, while in the effluent, approximately 79% to 98% of MPs were removed. The study estimated that approximately 6.5 × 10⁸ MPs were released from the seven plants daily. The micro-Raman spectroscopic and light microscopic analyses revealed that plastic polymers accounted for approximately 62% of the particles, with polypropylene (~31%) and polyethylene (~22%) being the most commonly detected polymers. Additionally, the majority of the MPs were white (27%) or clear (about 26%) in color [106].

3.4. Color and Range of Size and Shape of MPs

The various dimensions, shapes and colors that were found in the collected samples from the WWTPs are described in

Table 3. Characteristics of MPs in industrial wastewater influent.

WWTPs Location	Shape	Range Size Particles	Color	Reference
Spain/Cabezo Beaza WWTP	The shapes of the detected MPs were fragments (46.9%), films (34.0%), beads (11.5%), fibers (7.4%) and foam (0.2%). LPDE’s most common shape was film (27.7%)	400–600 nm	Beige (37%), white (23%), black (8%), blue (7%) and green (4%)	[67]
Vietnam/HoaCam WWTP	Fibers and fragments	1.6 to 5000 μm	Yellow, white, blue and black Black 50.48–58.55% Yellow 21.53 to 32.99%. White 8.81 to 22.34% Blue < 2%	[103]
Vietnam/DaNang WWTP	Fibers and fragments	1.6 to 5000 μm	yellow, white, blue and black Black 50.48–58.55% Yellow 21.53 to 32.99%. White 8.81 to 22.34% Blue < 2%	[103]
Vietnam/HoaKhanh WWTP	Fibers and fragments	1.6 to 5000 μm	yellow, white, blue and black Black 50.48–58.55% Yellow 21.53 to 32.99% White 8.81 to 22.34% Blue <2%	[103]
France/Seine-Centre WWTP	Fibers	100–5000-mm	Transparent, green, blue, and red	[104]
Spain/5 municipal and 2 industrial WWTPs in Cadiz	(A) Flake, (B) sphere, (C) fiber, (D) filament and (E) fragment.	<355 μm	N/A	[27]
Poland/Silesian Province	Not mentioned	<150 nm	N/A	[105]
South Korea/Three WWTP in Daegu	Microbeads, fragments, fibers and sheets	<65 μm	N/A	[59]
China/7 WWTPs in Xiamen	Granules (~41%) were the most frequently found shape of the MPs, while fragments follow behind at about 31% and fibers are last at approximately 24%.	43 μm–355 μm	black, yellow, red, blue, green, white and clear Mostly white (27.3%) and clear (25.8%)	[106]
Germany/12 WWTPs in Lower Saxony	Fibers	20 μm.	N/A	[71]
China/9 domestic and 5 industrial WWTPs, wastewater from 10 industrial facilities, 4 livestock farms and 4 fish ponds	Fragments and films were the most abundant shapes	<500 μm	Transparent, yellow, gray, green white, blue and pink Transparent (72%), white (20%) and colored (8%)	[107]

.

The final effluent, which is discharged from the WWTPs after undergoing various treatment processes, can exhibit different characteristics and concentrations of microplastics (MPs), which are determined by the treatment used. Therefore, it is essential to assess the behavior of MPs in treatment plants and the associated exposure risks in the environment through the disposed effluent or sludge. In WWTPs, the secondary treatment process, namely, biological treatment, is considered the most critical technology, with advanced oxidation being the most widely used method. Most WWTPs also employ additional treatments, such as advanced oxidation and membrane filtration, to further remove contaminants.

Microplastics are a diverse group of polymer mixtures with diverse dimensions and structure, and each shape has its unique physicochemical and toxicity properties [108]. The shape of microplastics is a significant classification factor and impacts their elimination efficiency in WWTPs [109]. Fibers, fragments and films are the most widely detected microplastics in wastewater [59,67]. Among the various shapes, fibers, which are filamentary microstructures, are the most frequently encountered types. Additionally, different types of polymers, including polyethene, polypropylene and polyester, have been identified as some of the most frequently identified microplastics in the wastewater [59,67].

The abundance of microplastics gradually decreases with additional treatments. Primary treatments, which rely on physical mechanisms, are the initial barrier to remove MPs in WWTPs and are more effective in extracting fiber microplastics because fibers are easily trapped through flocculation and settling [109]. The microplastic composition in the effluent of WWTPs is diverse [67,69], and even post-treatment, MPs are still detected in the resulting effluent and are continuously disposed of in bodies of water, thereby causing environmental pollution [59,72]. Therefore, these findings emphasize the importance of considering the environmental pollution caused by MPs/NPs released from WWTPs [59,71].

4. Challenges and Future Perspectives

The application of membrane technology has demonstrated impressive results in the elimination of MPs. However, the issue of membrane blocking and fouling must be addressed for large-scale implementation. The presence of other pollutants in wastewater may adsorb and interact with MPs through various mechanisms, such as electrostatic or hydrophobic interactions and ion exchanges. Further research is needed to explore surface modifications or coatings to enhance the sorption of MPs. Pilot studies and cost analyses are necessary to assess the feasibility of diverse treatment methods for mitigating MPs. The scientific community has reported concentrations of MPs from various WWTPs worldwide, but the standardization of sampling, purification and characterization processes is essential for producing reliable research results. Further analysis is needed to comprehend the concentration and characteristic profiles of MPs across different WWTPs. Geographical variations and human behaviors and developmental activities can be significant factors contributing to MPs pollution; thus, data from various regions can inform effective strategies for controlling MPs at their source.

5. Conclusions

Plastic pollution is a global problem that poses a significant threat to aquatic health. The presence of microplastics (MPs) is indicative of significant plastic pollution. As the global production and consumption of plastics continue to increase, the contamination of the environment with microplastics is expected to rise, leading to severe damage to ecosystems. Therefore, it is essential to control the emission of plastic at the source to prevent plastic pollution. Once plastics enter marine and freshwater environments, they break down into smaller pieces that can travel thousands of kilometers, causing harm to ecosystems far from the emission source. To address plastic pollution, an integrated and interdisciplinary approach that considers various disciplines is required. Recent research has shed light on the fate and effect of MPs on the environment.

There are two types of MPs: primary and secondary. Primary MPs are intentionally manufactured as micro-sized plastics, while secondary MPs are the byproduct of the fragmentation and degradation of larger plastics in the aquatic environment. Secondary MPs make up the majority of MPs found in aquatic environments. The issue of MPs generated in WWTPs of various sizes requires further investigation. Some microplastics from WWTPs can be transferred and disseminated in the aquatic environment, necessitating collaborations at local and global levels to create management plans that define the highest allowed levels of MPs in effluent. The efficiency of microplastic (MP) removal is influenced by their physical characteristics such as size, shape and density. Previous studies have shown that the most common types of MPs found in water samples are secondary microplastics, consisting of fibers and fragments, with the primary polymers being polyethylene terephthalate, polystyrene, polypropylene and polyethylene. The range of removal efficiency reported in the literature is 21.8–97.8%. In order to better understand the fate of microplastics in WWTPs and other environmental matrices, it is important to develop standardized methods for their sampling and analysis. Furthermore, research should focus on specific types of microplastics, especially in industrial areas, and their effects on plant roots. This study contributes to the critical understanding of treatment technologies for the removal of microplastics. Although treatment plants have demonstrated substantial removal efficiency, the complete removal of MPs has not been achieved. MP abundance has been observed to accumulate or be stored in WWTP sludge without proper treatment. Among the various shapes of MPs, fibers are the most commonly found MPs, while black, blue and red are the most frequently observed colors. To mitigate the risks posed, it is crucial to determine the sources of the polymers detected and their transportation pathways. However, additional research and established techniques are necessary to accurately identify these sources.