Microplastics in Wastewater Treatment Plants: Characteristics, Occurrence and Removal Technologies

Abstract

Pollution of the aquatic environment with microplastics has recently been recognised as a new environmental threat considering their negative impact on the ecosystem. Due to the low density and small particle size of microplastics, they are easily discharged into sewage systems and wastewater treatment plants. Thus, wastewater treatment plants are considered major sources of microplastic pollution in aquatic and terrestrial environments. Therefore, there is an urgent need for an in-depth understanding of the occurrence, behaviour, and fate of microplastics in wastewater treatment plants before they are discharged into natural water bodies. This paper comprehensively reviews the current state of knowledge on the characteristics and removal of microplastics in a series of wastewater treatment plants by comparing their removal efficiency in different unit processes, both during pretreatment, biological treatment, and tertiary treatment. The study found varying efficiencies in wastewater treatment technologies, with the first stage of treatment removing between 16.5 and 98.4% of microplastics, while during biological treatment the overall efficiency of microplastics removal ranges from 78.1 to 99.9% (membrane bioreactor). Nevertheless, given the large volumes of wastewater continuously discharged to receiving bodies, even tertiary treatment plants can be a significant source of microplastics in surface waters. The largest fraction of MPs removed in conventional wastewater treatment plants is trapped in the sludge. Among the critical treatment technologies, microplastic quantitative analysis showed that membrane bioreactors and filter-based treatment technologies have the highest microplastic removal efficiency. Based on a review of the existing literature, it was concluded that existing wastewater treatment plants are ineffective in removing microplastics completely, and there is a risk that they could be discharged into surrounding water sources.

1. Introduction

Since the mid-20th century, global production of plastics has increased rapidly to 400 million tons in 2022 [1]. Nowadays, they have become part of our lives, mainly due to their characteristics, i.e., plasticity, lightness, durability, and low cost [2]. After use, according to available statistics, plastic waste generates a mass of microplastics (MPs) into the environment. MPs are defined as small plastic fragments less than 5 mm in size [3]. In 2011, the concept of nanoplastics (NPs) was also introduced, which are defined as particles (nanospheres, nanofibers/nanotubes, and nanofilms) with smaller dimensions, i.e., from 1 to 100 nm [4,5]. This limitation is important because particles below this limit, unlike MPs, may be capable of breaching the cell membrane of living organisms [5]. Commonly known types of polymers from which MPs and NPs are formed include polypropylene (PP), polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC), polyamides (PA), polyester (PES), and polyethylene terephthalate (PET) [6,7]. These plastics are reversible and thermoplastic, are highly recyclable, and can be repeatedly heated, cooled, and shaped [8]. The recycling of plastics is far less than the amount of waste generated and accounts for only 9–16% of all waste, with the rest being incinerated, landfilled, or dispersed into the environment [9,10,11,12]. In addition to reversible polymers, thermosetting plastics are in use, mainly epoxy, phenolic, acrylic, and melamine resins, vinyl esters, and others [12,13], which are not recyclable. In addition, based on the origin of MPs, they can be categorised into two groups: primary and secondary [14]. Primary MPs come directly from products or materials containing microscale plastics (microbeads, microfibers, and granules) contained in cosmetics, such as facial scrubs, exfoliants, and toothpaste, as well as synthetic fabrics [15]. Secondary MPs, on the other hand, are formed from the breakdown of larger plastic objects through fragmentation, weathering, and degradation [16].

The low density, high persistence, and wide range and size, in addition to their non-biodegradable nature, are unique characteristics of MPs that distinguish them from most other pollutants and make them very difficult to remove, especially from aquatic ecosystems [17]. Plastic microparticles have a complex composition and contain a wide range of additives such as plasticizers, fillers, and stabilisers [18]. MPs can adsorb and transport toxic contaminants such as polycyclic aromatic hydrocarbons [19], heavy metals [20], polybrominated diphenyl ethers [21], pharmaceuticals, and personal care products [22] due to their small volume and large specific surface area, and hydrophobic nature [23]. As a result, MPs always cause chronic toxicity due to their accumulation in organisms [24]. Toxicity can arise directly from the polymeric materials used in plastic products [25], while MPs themselves can harm organisms and induce inflammation due to their small size and sharp ends [26]. Moreover, additives added to plastics to improve their properties can also pose toxicity risks to organisms. Phthalates and polybrominated diphenyl ethers, for example, are well-known endocrine-disrupting compounds (EDCs) [27].

MPs have been detected in many environmental matrices, especially in aquatic environments [28,29]. For example, the abundance of MPs in Taihu Lake and Qinghai Lake in China ranged from 0.01 to 6.8 × 10⁶ particles/km² and 0.05–7.5 × 10⁵ particles/km², respectively [30,31], and in the middle and lower reaches of the Yangtze River up to 1.95–9.00 × 10⁵ particles/km² [32]. In the surface waters of the central western Pacific Ocean and the subsurface waters of the Atlantic Ocean, the abundance of MPs was 34,039 ± 25,101 particles/km² and 1.15 ± 1.45 particles/m³, respectively [28,33]. In recent years, it has also been discovered that 95% of Arctic polar waters contain MPs of 0–1.31 particles/m³ [34]. It is estimated that 70–80% of plastic pollution originates from land-based sources and is transported by rivers to the seas and oceans [35,36]. They can enter surface waters through a number of routes, including stormwater runoff, wind advection, precipitation, and discharges of treated wastewater. Wastewater treatment plants (WWTPs) are considered and cited as major point sources of MPs and play an important role in their release into the environment, primarily aquatic [12,36,37,38,39].

At present, there are few works discussing both the fate of MPs in WWTPs and their potential impact on wastewater treatment processes. In addition, more research is needed to reveal the complex interactions between MPs and various wastewater treatment processes so as to minimise their negative impact on wastewater treatment performance. This review analysed the sources of MPs, the quantities and characteristics of MPs in WWTPs in different locations around the world, and the efficiency of their removal in different WWTP processes. In addition, the analysis included different types of MPs, which vary in shape, size, and colour, as well as their amounts in the aqueous phase. The discussion focused on summarising the possible impact of MPs on WWTPs, revealing research gaps in current research on MPs and wastewater treatment, and proposing research perspectives in the field.

2. Research Methodology

A detailed review of the scientific literature shows, that there are many publications on microplastics in wastewater treatment plants. In the literature research, we used the resources of the Web of Science Core Collection (WoS) and Scopus databases. Both databases include articles published since the beginning of the 20th century, with the WoS database being only slightly older.

Scientific papers were searched using the “topic” function, which allows the declaration of selected search terms (words or phrases). Two main key search terms were declared, namely (1) “microplastics” and (2) “wastewater treatment plants”. Subsequently, as the literature was examined in more detail, the search was conducted using more specific wording, such as:

-

physical methods of wastewater treatment (sedimentation, filtration, adsorption, etc.) + microplastics,

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chemical methods of wastewater treatment (coagulation, ozonation, Fenton process, etc.) + microplastics,

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biological methods of wastewater treatment (activated sludge, constructed wetlands, etc.) + microplastics

The databases were also searched for terms related to the characteristics of microplastics, such as the shape of MPs, the chemical composition of MPs, and the size of MPs.

The focus was mainly on literature not older than 10 years, with minor exceptions.

3. Characteristics of Microplastics in Wastewater Treatment Plants

WWTPs are usually considered as barriers to prevent pollutants from entering the environment; however, MPs are readily detected in both incoming and outgoing wastewater from WWTPs [12,36,40,41,42,43,44,45]. MPs enter the WWTPs from various sources. The main ones mentioned are municipal wastewater containing personal care products and wastewater from the manufacture and processing of plastic products [44]. Protective paints (e.g., marine paints, and furniture paints) contain many synthetic polymers, such as polyacrylate, PS, polyurethane, and epoxy resins, which can release MPs during paint application, removal, or abrasion [46]. In addition, abrasion of some household plastic products, car tyres, and synthetic textiles (e.g., shirts, PS fleece, blankets) are major sources of MPs in wastewater [43]. Fibre is produced in the manufacturing process of textiles, and the increasing consumption of textiles and the amount of wastewater from laundering, result in more frequent entry into the WWTPs [47]. Polymers are used in some industrial abrasives, drilling fluids (in gas and oil exploration), engine cleaners, and other industrial equipment. Improper disposal and handling can result in significant MP content in wastewater [48].

The concentration of MPs in the inlet to a series of WWTPs ranges from 0.28 × 10⁴ to 3.14 × 10⁴ particles/L in many countries (average value: 1.90 × 10³ particles/L) [49]. According to Sun et al. [39], the concentrations of MPs in the influent measured at dozens of WWTPs ranged from 1 to 10,044 particles/L, with much lower concentrations in the effluent leaving the WWTP, ranging from 0 to 447 particles/L. Large differences in MP concentrations can be related to a number of factors, such as the population served, wastewater sources (municipal or industrial), economy, and lifestyle, as well as the different sampling, pretreatment, and analysis methods used in the studies [39,49]. In most cases, MP concentrations in municipal wastewater are lower, ranging from 0.28 × 10² to 6.10 × 10² particles/L (average value: 1.27 × 10² particles/L). In municipal and industrial WWTPs, MP concentrations often range from 1.60 particles/L to 3.14 × 10⁴ particles/L (average value: 5.23 × 10³ particles/L) [49]. Mason et al. [38] conducted a statistical analysis at 17 WWTPs in the United States, and the results showed that the population served was positively correlated with the total particle content of MPs in wastewater. In contrast, Mintenig et al. [37] found that no significant correlation was observed between the number of MPs (<500 µm) and the equivalent population at the 12 WWTPs evaluated in Germany. Studies from various countries suggest that future research on MPs in wastewater should focus on regions with different economic levels and lifestyles to gain a comprehensive understanding of their occurrence.

The concentration of MPs is also related to the treatment technology used, with a gradual decrease from first to second (biological processes) and third stage treatment. The concentration of MPs after the pretreatment process generally ranges from 0.22 to 1.26 × 10⁴ particles/L (average value: 6.87 × 10² particles/L) [49]. In contrast, after the biological process, when anaerobic-anoxic-oxygen (A²O) treatment or biofilters were used, the concentration of MPs ranged from below the limit of quantification to 7.86 × 10³ particles/L (average value: 4.67 × 10² particles/L), resulting in a decrease in abundance of 20.45–95.45% (average 66.63%) [49]. Often, in order to further remove contaminants, tertiary treatment processes, such as advanced oxidation processes and membrane processes, are used in a large number of WWTPs. After the application of tertiary treatment processes, the concentration of MPs decreases from 0 to 2.97 × 10² particles/L (average value: 1.93 × 10¹ particles/L) in most of the studied WWTPs [49]. In general, WWTPs with a tertiary treatment process are characterised by lower concentrations of MPs in treated wastewater (0–51 particles/L) than WWTPs that used only a primary or secondary treatment process (9 × 10⁴–447 particles/L) (

Table 1. Inflow and outflow concentrations, daily discharges, and MP removal rates in WWTPs with different treatment processes in different countries.

Location	Capacity [m3/Year]	Purification Process	Inflow [MPs/L]	Outflow [MPs/L]	Discharge [MPs/d]	R [%]
Australia	1.12 × 108	First stage	-	1.5	4.60 × 108	-
Australia	4.75 × 106– 1.753 × 107	I and II stage/RO	-	0.21–0.28	3.60 × 106–1.00 × 107	-
Sweden	1.88 × 106	First and second stage	15.1	0.00825	4.25 × 104	99.9
France	8.76 × 107	I and II stage (biofilter)	293	35	8.40 × 109	88.1
Scotland	9.52 × 107	I and II stage	15.7	0.25	6.52 × 107	98.4
Netherlands	3.37 × 106–2.63 × 108	I and II stage	-	55–81	7.48 × 108	94
USA	7.89 × 107	I and II stage	-	0.023	4.97 × 106	-
Germany	1.9 × 105–1.40 × 105	I and II stage	-	0.08–7.52	4.19 × 104–1.24 × 107	-
Germany	1.3 × 107	I and II stage/final filtration,	-	0.01–0.38	2.79 × 105–2.62 × 106	-
Australia	6.21 × 106	I and II stage	-	0.4	8.16 × 105	-
Denmark	-	I and II stage	2223–10,044	29–447	-	-
Finland	3.65 × 106	I and II stage	57.6	1	1.00 × 107	98.3
Finland	8.0 × 105	I and II stage/BAF	610	13.5	3.65 × 109	-
Finland		I and II stage/(BAF, DF, MBR, RSF)	-	0.02–0.3	1.26 × 106–6.59 × 107	-
Finland	1.10 × 103	I stage/MBR(pilot)	57.6	0.4	-	99.3
Germany	1.13 × 104	I and II stage	-	80.4	2.47 × 106	-
The Netherlands	2.03 × 106	I degree/MBR	68	51	2.84 × 108	25
Netherlands	9240–720 × 103 PEQ	I and II stage	68–910	51–81	-	72
USA	8.58 × 105	I and II stage		0.004–0.195	5.28 × 104–1.49 × 107	-
USA	1.4 × 108–5.51 × 108	I and II stage	1	8.8 × 10−4	9.3 × 105	99.9
USA	-	I stage/AnMBR	91	0.5	-	99.4
USA	4.75 × 106–7.77 × 107	I and II stage/(GF, BAF)	-	0.009–0.127	1.01 × 105–9.63 × 106	-
USA	1.30 × 106–3.13 × 108	I and II stage/gravity filter.	-	0–2.43 × 10−4	0–2.08 × 102	97.2
USA	6.23 × 105	I and II stage/GF	91	2.6	4.43 × 106
USA	9.13 × 108	I and II stage	133	5.9	1.48 × 1010	95.6
Denmark	-	I and II stop./RSF	81.49	19	-	-
Israel	30 × 103	I and II degree/filter.sand + Cl2	64.78	1.97	5.91 × 107	97.0
Spain	210 × 103 PEQ	I and II degree	12.43	1.23	6.7 × 106	90.3
Spain	29,777 PEQ	I and II stage/(MBR, RSF)	4.40	092–1.08	12.96 × 106	75.5–79.0
South Korea	20.840–26.545	I and II stage/O3/RSF	4200–5840	33–66	8.8 × 108–1.37 × 109	98.9–99.2
South Korea	40 × 103	I stage/RSF, UV d	114–216	0.26–0.48	2.9 × 109	99.8
China	3.1 × 106	I and II stage/UV d	126	30.6	-	75
China	3.5 × 106	I and II stage	37.9	1.57–13.69	-	-
Italy	80,000 PEQ	I and II stage/UF	3.6	0.76	4.15 × 107	86
Thailand	130 × 103	I and II stage/UF	77	2.33	2.8 × 108	97
Turkey	87,500	I and II stage	135.3	8.5	5.25 × 108	93.7
Spain	70,417 PEQ	I and II stage/RSF/UV d	3.78	1.38	1.7 × 107	63.4

Notes: MPs: microplastics; PEQ: population equivalent; BAF: biologically active filter; MBR: membrane bioreactor; RO: reverse osmosis; RSF: rapid sand filter; UF: ultrafiltration; UV d: UV disinfection. AnMBR: anaerobic membrane bioreactor; GF: granular filter; DAF: dissolved air flotation; DF: disc filter; R: removal rate. Source: own elaboration based on [4,8,12,13,36,37,38,39,40,51,52,53].

). Despite the relatively low concentrations of MPs in wastewater from WWTPs, total discharges from WWTPs are still significantly high, as most of these facilities treat millions of litres of wastewater per day. The average value of the total daily discharge of MPs (estimated from annual effluent and concentration in wastewater) at the WWTPs studied was 2 × 10⁶ particles/day, which corresponds to an average annual discharge of 5 × 10⁷ m³/year [47]. In certain wastewater treatment plants in the Netherlands and the United States, the total daily discharge of microplastics can surpass 1 × 10¹⁰ particles per day [47]. The high discharge levels of MPs from WWTPs highlight the urgent need for effective treatment technologies to prevent their uncontrolled release into aquatic ecosystems. MPs concentrations are also influenced by the sampling method and how MPs are detected. Insufficient volumes of wastewater samples increase the uncertainty in the number of MPs and increase experimental errors. Therefore, standardisation or harmonisation of MPs sampling and analysis methods is urgently needed to better compare MPs concentrations in different studies [50].

Table 1 shows examples of MP concentrations in inflow and outflow and their daily discharges from WWTPs in different countries using different treatment processes.

Shape is another important indicator used to classify MPs, which can affect not only their removal efficiency in the WWTPs but also interactions with other contaminants or microorganisms in the wastewater [54]. In simple terms, MPs can be divided into fibres (with significant length relative to width) and particles (with similar width and length) [55]. MPs identified in the influent and effluent of WWTPs are generally classified into six main shapes, namely fibres, granules (small hard pieces), spherical shapes, films (very thin particles), foams (sponge-like mass), and fragments (small broken parts) (

Table 2. Distribution of inlet concentrations of MPs with different shapes in a wastewater treatment plant.

Location	Concentration in the Inflow [MPs/L]	Fibres [%]	Granules [%]	Spheres [%]	Films [%]	Foams [%]	Fragments [%]
Northern California, USA	0.195	90	-	-	-	1	9
	0.022	91	-	-	-	-	9
	0.064	58	-	0	4	4	35
	0.092	94	-	-	2	-	4
	0.072	59	-	-	-	-	41
	0.127	78	-	-	3	-	18
	0.047	100	-	-	-	-	-
Northern Ohio, USA	0.042	8	-	4	15	4	70
Central New York, USA	0.019	68	-	-	3	-	28
	0.08	58	-	1	8	2	30
Eastern Wisconsin, USA	0.007	39	-	2	5	1	53
	0.017	15	-	2	6	-	77
West New York, USA	0.009	68	-	5	2	5	21
	0.047	68	-	5	2	5	21
East New York, USA	0.004	13	-	6	13	3	65
New Jersey, USA	0.028	-	-	-	-	-	-
Sydney, Australia	0.280	80	20	-	-	-	-
	0.480	66	34	-	-	-	-
	1500	-	-	-	-	-	-
Hong Kong, China	2060	71	-	-	3	26	-
	1010	55	-	1	-	19	25
Scotland	15.7	18.5	-	3	9.9	1.3	67.3
Vancouver, Canada	31.1	65.6	0.45	5.4	0.2	0.22	28.1
Xiamem, China	6.55	17.7	49.8	2.5	-	-	30
Beijing, China	12.03	85.92	14.08	-	-	-	-

Note: Source: own elaboration based on [8,36,38,49,57,63,64,65].

). Depending on the sources of wastewater entering the WWTP, the percentage of solids in these shapes varies significantly. The most commonly detected MPs in wastewater are fibres, granules, fragments, and films, with the highest amounts at 91.32%, 70.38%, 65.43%, and 21.36%, respectively [3,41,49,56]. Many researchers have reported that the predominant morphology of MPs in WWTPs is primarily fibres, with an average percentage of 56.7–52.7%, respectively [8,57,58,59]. The presence of numerous fibres in wastewater is related to their release by discharges from domestic washing machines [47,59]. This result is consistent with the high content of polymers in wastewater from synthetic clothing production. In addition, the high fibre content in some samples may be due to the difficulty in distinguishing synthetic fibres from natural fibres, and studies have shown that natural fibres such as cotton and linen can account for more than half of the fibres in some wastewater samples [13]. The other most commonly observed shape of MPs in wastewater, originating from the erosion of everyday plastic products, are fragments, which average 28.8–34.4%, [39]. Spherical and granular shapes come from cosmetics and personal care products such as toothpaste, masks, and soaps [40], in addition to being formed from tyre abrasion and road surface wear [60]. Wastewater also contains film and foam MPs, which have an average abundance of about 10% or less. Film and foam MPs mainly come from the erosion of plastic bags and packaging [40,61,62]. Table 2 shows, as an example, the percentage of MPs of different shapes in the inflow to treatment plants in different countries.

Chemical composition is another important characteristic of MPs that affects particulate floating and falling, which significantly affects the rate of their removal. Currently, more than 30 types of polymers have been detected in MPs [39,49]. Polyethylene (PE), polypropylene (PP), polyamide (PA), polyester (PES), polystyrene (PS), and polyethylene terephthalate (PET) make up the top six most frequently detected MPs in wastewater, with the highest concentrations of 64.07%, 32.92%, 10.34%, 75.36%, 24.17%, and 28.90%, respectively [8,12,37,57]. A study on the presence of MPs in wastewater at a WWTP in Sydney, Australia, found that PE, PP, and PET had the highest concentrations in the wastewater influent of 11% to 42%, 3% to 32%, and 36%, respectively (

Table 3. MP concentration depending on the type of polymer in the WWTP.

Polymer	Abbreviation	Inflow [MPs/L]	Outflow [MPs/L]
Polyethylene	PE	0.03–1.05	0.00–0.67
Polypropylene	PP	0.02–1.42	0.00–0.22
Polyamide	PA	0.06–0.71	0.00–0.06
Polyester	PES	0.22–6.31	0.07–1.33
Polystyrene	PS	0.00–0.41	0.00–0.08
Polyethylene terephthalate	PET	0.01–0.63	0.00–0.16
Polyurethane	PUR/PU	0.07–1.40	0.00–0.02
Polyvinyl chloride	PVC	0.12–1.65	0.00
Polyvinyl acetate	PVA	0.26–0.50	0.00–0.01
Ethylene vinyl acetate	EVA	0.00–0.01	0.00
Polyacrylates	–	0.06–0.40	0.00–0.03
Polyvinyl ethylene	PVE	0.09	0.00
Polyvinyl fluoride	PVF	0.09	0.00
Styrene-butadiene-styrene	SBS	0.02	0.00
Styrene-ethylene-butadiene-styrene	SEBS	0.06	0.00
Styrene acrylonitrile	SAN	0.01	0.00
Polyvinyl alcohol	PVAL	0.03	0.00
Polyethylene and polypropylene	PE&PP	0.09	0.01
Acrylic polystyrene	PS acrylic	0.30	0.00
Polyvinyl acrylate	PV acrylate	0.09	0.00
Acrylonitrile-butadiene	–	0.80	0.01
Ethyl acrylate	–	0.14	0.01
Ethylene-propylene	–	0.28	0.00

Note: Source: own elaboration based on [4,8,12,37,39,56,66,67].

) [8,49]. It was also found that PP and PE are generally absent in wastewater from WWTPs using the air flotation process, due to their low density. In addition, MPs from PET (42.3%), PES (79.1%), and PA (61.2%), which are widely used in synthetic clothing and household products such as facial scrubs, water bottles, caps, nylon bags, and food packaging films, dominate the inlet of municipal and textile industry WWTPs [4,8,37,39,66,67]. These materials have a low density, leading to difficulties in their removal in current wastewater treatment facilities, especially when they are microfibers [12]. In addition, mechanical crushing of plastic products, the tyre and textile industries, and rubber particles in road dust have been identified as potentially important sources of MPs from PE, PP, PS, and PES [12,56]. The content of polymers other than those listed above accounted for only a small fraction of all MPs in wastewater, typically less than 5% or even <1%. Therefore, the most common polymers should be a research priority.

The size of MPs is one of the main factors influencing the transformation and removal efficiency of MPs in the WWTP [68]. Dimensions of 25 µm, 100 µm, and 500 µm are most commonly used for size classification [4,13,37,51,52]. In the inflow to the WWTPs, the number of MPs with dimensions above 500 µm can sometimes reach more than 70% [4,52]. Mintening et al. [37] detected MPs > 500 μm in wastewater from 12 German WWTPs in Lower Saxony, whereby eight synthetic polymers were identified, with PE (59%) and PP (16%) predominating. In contrast, no MPs of >500 μm were found in the wastewater after filtration. On average, however, MPs with dimensions smaller than 500 µm are present in more than 90%, and in some samples, about 60% of MPs have dimensions even below 100 µm [8,37,52]. In contrast, in a study by Magni et al. [69], the concentration of MPs < 1 mm was 65.0–86.9% in the inflow and 81.0–91.0% in the outflow. A more recent study showed that MPs < 25 µm were present in significant amounts in wastewater [52]. This result is consistent with observations from the Atlantic Ocean, where MPs less than 40 µm accounted for 64% of all MPs detected, among which more than half were less than 20 µm in size [70]. Smaller MP particles can be ingested by plankton and fish, which can cause a range of toxicological effects in these organisms [71]. Therefore, the study of MPs particle size, especially those of smaller size (less than 1 mm), can be of leading importance for the toxicity and environmental transformation of MPs.

Dyes and pigments are added during the production of plastic products to enhance their appeal and improve their performance [72]. Many researchers have documented the presence of transparent and coloured MPs (including white, black, blue, green, red, yellow, and other hues) in WWTPs and aquatic environments [73,74]. Although the effect of colour on the removal efficiency of MPs remains unclear, the dyes present in them are toxic to aquatic life. Studies also indicate that coloured microplastics may contain harmful substances, such as heavy metals and persistent organic pollutants [75]. Because coloured or transparent microplastics released from wastewater treatment plants resemble food, they are often ingested by organisms in aquatic environments. As a result, these MPs can accumulate in the organisms’ bodies and, eventually, enter the human food chain [76].

The high levels of MPs discharged from WWTPs highlight the urgent need for advanced treatment technologies to prevent their uncontrolled release into aquatic environments.

4. Removal Efficiency and Fate of Microplastics in Existing Wastewater Treatment Plants

The removal of MPs and NPs from aquatic environments poses a significant challenge for researchers and experts in water and wastewater treatment technologies. Technologies used in WWTPs include first-stage treatment processes (primary settling tank, flotation, sand filtration and others) and second-stage treatment processes (activated sludge, A²O process, biofilters and other bioreactors), and in some cases third-stage treatment processes (UV, O₃, chlorination, biologically active filters (BAF), disc filters (DF), rapid sand filters (RSF), membrane techniques and others) [49]. None of the current wastewater treatment processes are designed to remove MPs and NPs, as they were developed to remediate dissolved and suspended contaminants [38,77]. The MPs present in different unit processes in WWTPs vary in size, shape, type, and concentration, and the results obtained vary widely from one WWTP to another for the same unit process [12]. In order to effectively control the release of MPs into the environment by the WWTPs, it is necessary to study the characteristics, fate and removal of MPs in each of the technological processes used. The achieved efficiency of MP removal at various WWTPs around the world ranges from about 60% to 99.9%, depending on the technology used (Table 1). For example, in Wuhan (China) WWTP achieved MP removal of 64.4% [78], in Sydney (Australia) WWTP removal was 66% [8], and in Vancouver (Canada) WWTP—91.7% [63]. After first-, second- and third-stage treatment at a WWTP in Finland, MP removal was 99.9% [79], and in the UK, the total amount of MPs was reduced by 74%, 92% and 96%, respectively [80]. Aerated sand traps, A²O plants and advanced oxidation processes (UV and O₃) were adopted in the Beijing (China) WWTP as MPs treatment methods, and their MPs removal efficiencies were 58.84%, 54.47% and 71.67%, respectively [65]. In contrast, MP removal efficiencies for the same treatment processes at the Shanghai WWTP decreased to 49.56%, 26.01% and 0.78%, respectively [81]. Hidayaturrahman and Lee [56] studied the fate of MPs at different stages of treatment at three WWTPs in South Korea and found that the removal rate of MPs after the third stage of treatment could increase to more than 98%. Conley et al. [58] measured MPs loads and removal efficiencies at three WWTPs in South Carolina (USA) and estimated that these treatment plants could reduce 99.9% of MPs entering the environment.

MPs were found to have a negative impact on activated sludge (AS) because they disrupt microbial function and inhibit sludge functions such as hydrogen production and anaerobic digestion [82,83]. There is a growing of research that focuses on analysing the concentrations and composition of MPs and evaluating the effectiveness of their removal in WWTPs [48,84].

Figure 1 shows the estimated MP particle flow based on the ranges of values reported in the literature, indicating the efficiency of MP removal during the first, second, and third stage purification, respectively [50,85,86,87].

4.1. First-Stage (Pre-Treatment) in MPs Removal

Pretreatment begins with a screening process (sieve) to remove large floating elements, and the outflow is directed to a sand trap, which in most cases is a long narrow basin that slows the flow of water and allows solids such as sand and gravel to settle out [18,89]. Thanks to the aeration process in the back of the sand trap, solids are largely removed at this pretreatment stage through flotation and sedimentation. The wastewater then flows to the primary settling tanks, where the suspended solids are deposited by gravity, forming a sludge. At this stage, depending on the characteristics of the wastewater, a flotation process can also be used [90].

Sedimentation is the initial stage of wastewater treatment, where larger granular solids like sand and gravel are removed through sand traps. Additionally, finer suspended particles and colloidal matter are separated in settling tanks, also known as clarifiers. It not only brings significant results in the reduction in pollutants, but also provides optimal conditions for subsequent technologies, such as biological, filtration, and disinfection (second and third stage) processes due to its high suspended solids removal capacity. High MP removal efficiency in the first stage of wastewater treatment was obtained by Liu et al. [78], Hidayaturrahman and Lee [56], and Ziajahromi et al. [8], 41%, 57–64%, and 66%, respectively. Murphy et al. [36] also investigated the effectiveness of the pretreatment stage at a municipal wastewater treatment plant in Glasgow, Scotland. Their findings showed that the average amount of MPs decreased from 15.7 MPs/L to 3.4 MPs/L, resulting in a removal efficiency of approximately 78%. According to the results of Bayo et al. [41], over 70% of MPs were removed during the pretreatment stage at municipal WWTPs in Spain (

Table 4. MP removal efficiency at WWTPs by the method used for pretreatment, including pretreatment settling tank and flotation.

Type of Treatment Process	Removal Efficiency [%]
Primary settling tank	47.8
Aerated sand trap + primary settling tank	58.8
Coarse screen + fine screen + sand trap + primary settling tank	40.7
Dissolved air flotation (DAF)	95.0

Note: Source: own elaboration based on [65,93,94].

). Pretreatment is most effective in removing larger-sized microplastics. Michielssen et al. [88] observed that 84–88% of MPs with sizes in the 100–1000 μm range were removed in the WWTP during the sedimentation and clarification process. Dris et al. [51] found that the fraction of large MPs, about sizes 1000–5000 µm, significantly decreased from 45% to 7% after pre-treatment. Conley et al. [58] presented annual data on the concentration and removal efficiency of MPs in three U.S. WWTPs using different treatment methods. They observed a high MP removal efficiency of approximately 97.6% during the clarification process for microplastic fractions consisting of particles ranging from 418 to 60 μm. The effectiveness of MP removal through sedimentation is also significantly influenced by particle shape [58,91]. MPs in the form of fibres in the sedimentation process are the most difficult to remove from wastewater [57], while fragments and granules are the two shapes of MPs that are most easily eliminated (91% and 83%, respectively). Thus, fibres are the predominant shape of MPs in wastewater discharged from WWTPs [8,12,57]. This can be partially explained by the surface smoothness of each MP shape [92]. In contrast, fragments of other shapes are often angular, bifurcated, and twisted, which not only increases their capture capacity but also provides a greater opportunity for microbial colonisation. MP removal efficiency further depends on the type of polymer that is present in the raw wastewater. For example, at a WWTP in Wuhan, China, the dominant polymer in MPs is PA (61.2%), followed by PE (14.6%), and PP (10.7%). These polymers all have relatively low densities, with PA ranging from approximately 1.02 to 1.16 g/cm³, PE from 0.89 to 0.98 g/cm³, and PP from 0.83 to 0.92 g/cm³ [63]; while the main MPs materials in Beijing’s WWTP are PET and PES, with densities of 0.96–1.45 (42.26%) and 1.24–2.3 g/cm³ (19.1%), respectively [65]. This indicates that the removal rate of MPs in the sedimentation process is significantly dependent on the density of the pollutants, which plays a key role in the buoyancy and sinking of these fine plastic wastes in the wastewater. MPs with a higher density than the wastewater are easily eliminated from this matrix in the physical sedimentation process, unlike MPs with a lower density. Table 4 shows the efficiency of MP removal in WWTPs by methods used for primary treatment, including pre-sedimentation and flotation [93].

Often used in first-stage wastewater treatment is the coagulation/flocculation process, which, when combined with sedimentation, contributes to higher MP removal rates [10]. In wastewater treatment, both iron salts and aluminium salts are used as coagulants [95]. In the coagulation process, coagulation flocs interact with MPs via hydrogen bonds, van der Waals or electrostatic forces, leading to the aggregation of MPs with flocs [96]. Coagulants/flocculants having opposite charges to MPs effectively reduce the repulsion potential between MP particles. Flocculation of MPs with iron salts is caused by the adsorption of small Fe(OH)₃ aggregates with a high positive charge, neutralising MPs charges and eliminating the repulsive force between MPs. In neutral and alkaline environments, the size of floc aggregates increases, forming bridge bonds between MPs [97]. In the case of aluminium salts, MPs interact with coagulation flocs through hydrogen bonds.

Flotation is a separation technique used to isolate finely divided solids based on differences in their wettability. In this process, hydrophobic contaminants attach to air bubbles and rise to the surface, where they form a foam [98]. Dissolved air flotation (DAF) is a commonly used technique in the WWTPs for separating solids, oils, and low-density fibrous materials, including MPs [92]. Air under high pressure is dissolved in water, forming bubbles that attach solids (also MPs) to their surface, causing them to be removed [91]. For example, at the Hameenlinna WWTP (Finland), the DAF method achieved MPs removal of 95%, with a low MPs concentration (2 ± 0.07 MPs/L) [12]. Unlike sedimentation, air flotation technology results in contaminant removal by trapping low (e.g., PE, PP) and medium density (e.g., PS and PA) MP fragments, which can float alone or with air bubbles on the surface of the tank. Similarly to the sedimentation process, the morphology of MPs/NPs plays a significant role in determining their removal efficiency during the air flotation process. In the Xiamen WWTP in China, granules and other shapes were removed at 91% and 83%, respectively, while the removal rate of fibres was only about 79% [57]. However, in contrast to the sedimentation process, the flotation process has higher operating costs [12].

4.2. Second Stage Treatment (Biological) in the Removal of MPs/NPs

Plastics have traditionally been considered non-biodegradable, but it is now known that they are degraded and metabolised by a range of organisms, particularly microorganisms [99]. Microorganisms can convert complex polymers to simpler monomers, with aerobic degradation producing CO₂ and water, while anaerobic degradation produces CO₂, water, methane, and H₂S [9,100]. Considering the widespread presence of microorganisms in the environment and their capacity to degrade plastics, biodegradation stands out as one of the most promising solutions to the MPs/NPs problem.

Numerous studies have successfully tested a range of microorganisms for the degradation of MPs, including fungi, bacteria, and others [9]. Auta et al. [101] studied the removal of MPs in the form of PE, PS, PET and PP by two strains of Bacillus bacteria isolated from mangrove sludge, namely Bacillus cereus and Bacillus gottheilii. The fastest mass reduction and shortest degradation half-life (363 days) were found with B. cereus for MPs from PS, while B. gottheilii for PE had a half-life of 431 days. Studies have also shown several other types of functional bacteria that degrade MPs, such as Rhodococcus, which degrades 6.4 wt.% of PP polymer in 40 days [49,101], while the bacterium Ideonella sakaiensis is able to completely metabolise MPs from PET film in six weeks [102]. However, the hydraulic retention time (7–14 h) used in WWTPs does not allow for the effective degradation of MPs by microorganisms alone, such as those present in activated sludge. Along with microorganisms, other organisms also contribute to the degradation of MPs found in aquatic environments, namely the red clam (Tridacnamaxima) [103], Antarctic krill (Euphausia superba) [104], and some corals and microalgae [105,106].

4.2.1. Biological Methods of Wastewater Treatment

Second-stage wastewater treatment aims to remove residual organic matter and suspended solids from first-stage treatment. It includes biological treatment processes and clarification processes [39,40]. In practice, aerobic and anaerobic biological treatment methods are used to remove dissolved and colloidal biodegradable organic substances. An alternating aerobic-anaerobic system (anaerobic-anoxic-oxygen (A²O) treatment) is also used. Activated sludge (AS), biological filters (such as effluent filters and biofilters), membrane bioreactors (MBR), and hydrophytic treatment plants (constructed wetland—CW) are among the most commonly and widely used technologies for second-stage wastewater treatment, and at the same time the most effective methods of removing MPs from sewage [12,107].

Conventional activated sludge (AS) system is a popular technology used in WWTPs after primary treatment. MPs/NPs removal is mainly based on the degradation of the polymer by protozoa and microorganisms, and the formation of sludge aggregates containing MPs/NPs, which are then separated from the treated wastewater in a secondary settling tank (Figure 2) [12,64,91]. The AS process is widely used to remove soluble/colloidal organic pollutants and nutrients from various types of wastewater. Microorganisms secrete extracellular polymeric substances (EPS), adsorbing available pollutants, including MPs/NPs, and then decompose them to produce suitable products. Plastic residues can also be retained in sludge flocs as a result of “swallowing” by microorganisms [108]. However, it is still uncertain to what extent this process affects the removal of MPs/NPs from wastewater.

In biological wastewater treatment by the AS method, different removal efficiencies of MPs/NPs are obtained. Lares et al. [4], Murphy et al. [36], and Edo et al. [109] showed very high MPs/NPs removal efficiencies in a clastic AS system contributing 98%, 92.6% and 93.7%, respectively. Other studies conducted by Hidayaturrahman and Lee [56] and Bayo et al. [110] in an urban WWTP (Spain) showed lower efficiencies of 42–77% and about 62%, respectively. In contrast, in a municipal WWTP in Italy, approximately 64% of MPs were eliminated after employing a grid chamber and AS system [69]. The AS process, together with modified processes such as the A²O process, the sequential batch reactor process and others, show lower MPs removal from wastewater of 3.6–42.9% [4,40,111,112]. The use of coagulants in the form of iron(III) and aluminium(III) salts or organic flocculants during second-stage treatment may have a positive effect on MPs/NPs removal efficiency [36].

The removal rate of MPs in AS processes depends on the level of nutrients in the wastewater [113] and the retention time [40]. The longer the sludge retention time (SRT), the greater the chances of surface biofilm formation on MPs, which modifies the surface area, size, and concentration of contaminants [40,114]. Thus, changes in SRT can significantly affect the buoyancy of MPs, which increases the chances of their elimination in sedimentation processes, thereby improving removal efficiency [12]. However, further research is needed on the interaction between AS parameters and MP removal. The removal efficiency of MPs/NPs in the AS process can vary significantly, depending on factors such as the size and shape of the MPs [86]. For example, Liu et al. [115] observed that the majority of MPs removed by the AS process were smaller than 300 μm, whereas other studies reported the highest removal efficiencies for particles in the 1–5 mm size range [4]. Talvitie et al. [12] showed that the concentration of MPs with particle sizes of 100–300 μm decreased during second-stage treatment, and MPs with particle sizes of 20–100 μm accounted for about 80% of the total. Mintenig et al. [37] found that particulate matter larger than 500 μm was virtually undetected in wastewater after second-stage treatment, while Michielssen et al. [88] showed that particles >300 μm were still a major component of wastewater after treatment. The differences in the particle size distribution may be related to the different treatment methods and operational conditions adopted at the different WWTPs. Compared to primary treatment, second-stage treatment results in greater removal of MPs in the form of fragments than fibres, as confirmed by studies showing that the average fibre abundance increases markedly (up to several times) after second-stage treatment [8,40]. One possible reason for this is that easily settled or skimmed MPs are already largely removed during the first-stage treatment while floating fibres proceed to further cleaning stages.

Table 5. Efficiency of MP removal in biological processes and secondary settling tanks in WTPs.

Type of Treatment	Removal Efficiencies [%]
Activated sludge + secondary settling tank	60.0
Primary settling tank + subsequent biological treatment stages	68.3
Aerobic biological reactor + secondary settling tank	74.8
Aerated tank + secondary settling tank	84.0
Anaerobic + anoxic + aerobic processes	54.4
Anaerobic + anoxic + aerobic processes	16.0

Note: Source: own elaboration based on [65,78,93,94,111,112].

shows the results of MP removal efficiencies in different WWTPs using different technologies applied in second-stage treatment (i.e., biological processes and secondary settling tanks). It should be noted that even if the same biological treatment technology is used, the operating characteristics of the wastewater treatment plant and MPs can lead to differences in removal efficiency.

When analysing the removal efficiency of MPs in the AS process, it is important to pay attention to the possible effects of MPs on activated sludge, which is a living suspension of heterotrophic bacteria and protozoa [116]. First, MPs have been proven to affect the nitrification and denitrification of the AS process by promoting or inhibiting the activity of nitrifying and denitrifying bacteria [117]. For example, Li et al. [117] found that MPs have a negative effect on ammonia oxidation efficiency, but no clear effect on nitrite oxidation efficiency. Second, the presence of MPs affects sludge digestion. Analysis showed that PVC microplastics at >20 particles/g reduced both methane production potential, inhibiting hydrolysis and acidification. Mechanistic studies showed that leaching of bisphenol A (BPA) from PVC MPs was the main cause of reduced methane production [67,118].

4.2.2. Membrane Bioreactors

Membrane bioreactor (MBR) technology is a modern, recently frequently used, biological process for the removal of pollutants in WWTPs. MBRs are systems in which a process supported by biological catalysts (bacteria, enzymes) is combined with a membrane process (usually MF or UF modules) [119,120]. It uses two MBR design solutions: a membrane module outside the bioreactor (sMBR) and an immersed module (iMBR) (Figure 3) [120,121]. MBR is characterised by a dual removal mechanism, involving both biodegradation and membrane filtration. In this process, the UF/MF membrane directly separates solids from the liquid within the biological bioreactors. Additionally, other substances, such as biomass and macromolecules, are retained by the membrane and are subsequently removed along with the dead sludge [119,120].

Thanks to the use of membrane modules, the secondary settling tank used in classical AS can be eliminated in MBR, and all the biomass is retained by the membrane, making it possible to achieve high concentrations in the bioreactor (Figure 4) [119,120]. Currently, MBR is considered one of the most efficient technologies for the effective treatment of municipal and industrial wastewater [4,119,120]. This is attributed to its ability to produce high-quality treated wastewater, its compact footprint, the complete separation of hydraulic retention time (HRT) and sludge solids retention time (STR), and ease of scale-up.

Combining pressure membrane techniques with a biological process (MBR) can increase MP removal rates compared to other biological wastewater treatment methods. MP removal can reach values as high as 99.9% [12], due to high suspended solids concentrations (6000 mg/L to 10,000 mg/L) [121], with MPs being retained in the retentate. Removal efficiency is related to the size of MPs, as the membranes used in the MBR system typically have a pore size of about 0.1–2 μm for MF and 1 nm–0.05 μm for UF, which definitely affects the size of MPs retained and passing through the membrane [64,121]. Li et al. [122] studied the removal efficiency of PVC gel (particle size < 5 μm) by MBR with an immersed membrane of 0.1 μm pore size and 0.1 m² surface area. Under HRT of 2.5 h, a temperature of about 19.1 °C and a pH of 7.5, practically no MPs were detected in the filtrate of the MBR system. Studies at the WWTP in Kenkaveronniemi, Finland, have shown that MBR can eliminate MPs from wastewater ranging from 6.9 ± 1.0 particle/L to 0.005 ± 0.004 particle/L [12]. Similarly, Lares et al. [4] achieved a 99.4% removal of MPs, suggesting that the removal rate of MPs in the MBR system is both consistent and highly effective. Talvitie et al. [12] studied the removal of MPs from the final treatment stage in four WWTPs using MBR, as well as disc filters, dissolved air flotation and a high-speed sand filter. The study reported 99.9% removal of MPs (>300 µm to 20 µm) for MBR (second stage treatment), 40–98.5% removal by disc filter, 95% removal by dissolved air flotation and 97% removal by rapid sand filter (third stage treatment) (

Table 6. Average concentrations of MPs before and after treatment with different technologies.

Method	Type of Stream	Raw Wastewater [MPs/L]	Cleaned Wastewater [MPs/L]	Removal Rate [%]
10 µm disc filter	After the second-stage treatment	0.5	0.3	40.0
20 µm disc filter	After the second-stage treatment	2.0	0.03	98.5
Rapid sand filter	After the second-stage treatment	0.7	0.02	97.1
Flotation	After the second-stage treatment	2.0	0.1	95.0
Membrane bioreactor	After the first-stage treatment	6.9	0.005	99.9
Membrane bioreactor	After the first-stage treatment	57.6	0.4	99.4

Note: Source: own elaboration based on [4,13].

) [12]. Compared to other advanced treatment processes, the MBR demonstrated a significant improvement in MPs removal (99%), superior final effluent quality, and considerable potential for streamlining the treatment process by eliminating the need for conventional secondary settling tanks in the AS process.

Similarly to the AS process, MBR does not completely remove MPs of smaller sizes, especially fibres, due to its high length-to-width ratio [64,122]. Other key limitations of MBR technology include the management of biofilm thickness and membrane fouling, both of which significantly impact the overall effectiveness of the method [56,64,119]. Membrane fouling, which can be controlled by backwashing or chemical cleaning, negatively affects membrane life and potentially leads to increased maintenance costs [91]. However, compared to other technologies, the efficiency of MBR does not depend on the size, shape, and composition of MPs [91].

In conclusion, biological treatment methods are highly effective in removing MPs across a range of environmental conditions. The MBR process shows the highest efficiency among all treatment methods and can become the leading biological method for MP removal. The efficiency of MPs removal in standard second-stage treatment processes is summarised in

Table 7. MP removal using various second-stage technologies.

Treatment Type	Efficiency [%]	Type of Removed MPs
MBR, AS, and settling tank	83.1–91.9	Fragments
AS and clarification	92	Fragments, fibres
AS	93.8	Microgranules
AS	89.8	Microgranules
MBR	79.01	Fibres, PP, PS
A2O	71.67 ± 11.58	No data available
AS, sedimentation	64	Fibres
MBR	99	Fragments, PVC fibres
CW	97	Fragments, fibres
AS	52	PE < 100 µm
Biological aerated filter	99	PE 100–300 µm
A2O	54.4	-
A2O	28.1	PET, PE, PES, PAN, PAA
AS	66.7	PS
MBR	99.9	20–100 μm MPs
MBR	97.6	PES fibres and PE fragments
A2O	93.7	PE, PP, PE
MBR	99.4	PES, PE, PA and PP
AS	98.3	Different types of MPs
AS	75–91.9	Different types of MPs
Submerged MBR	100.0	-
Submerged anaerobic MBR	99.4	-
Submerged MBR (KUBOTA)	100	-

Note: Source: own elaboration based on [10,12,13,56,65,69,78,80,86,88,109,123,124,125].

.

4.3. Tertiary (Final) Treatment in the MPs Removal

Tertiary treatment is an optional process in WWTPs, as most wastewater after the second stage of treatment meets standards for discharge to the environment [39]. In many cases, tertiary treatment is an important additional treatment process to further improve wastewater quality prior to discharge to the environment and can meet the conditions required in water reuse [126]. A variety of techniques can be used in tertiary treatment, especially biofilters, coagulation, various filtration processes, including membrane processes, and various oxidation processes [12,16,87,126]. Studies have shown the positive effect of tertiary treatment on MP removal [12] i.e., that the concentration of MPs in wastewater after various advanced treatment processes is further reduced. Considering that the majority of MPs has already been removed from wastewater in the pretreatment and second-stage treatment processes, the efficiency of MPs removal in third-stage treatment processes is no longer so significant. It has been found that the concentration of MPs in the wastewater after the third stage only decreases by 0.2–2% compared to the inflow of wastewater to the third stage [50,64,107]. Therefore, the need for tertiary treatment to further remove MPs from wastewater depends on wastewater quality requirements, but more research is still needed to understand the impact of tertiary treatment methods on MPs removal.

4.3.1. Biological Methods of Tertiary Treatment

Tertiary treatment also includes biological processes, mainly biological beds (biofilters) and hydrophytic treatment.

Biofilter technology integrates physical and biological treatment processes, and the main mechanisms for MP removal are filtration and biofilm adsorption. The microbial layer developing on the surface of the inert filter material comes into direct contact with the MPs, while excess microorganisms and retained MPs can be effectively removed through backwashing [127]. Liu et al. [128] in their research used a biofilter in a pilot-scale WWTP that reduced MPs in wastewater by 79–89%. The filter was fed with wastewater from a conventional WWTP in Denmark, which contained 917 MPs/m³, corresponding to a mass concentration of 24.8 µg/m³. Most of the MPs were removed in the upper part of the biofilter, while the deeper layers additionally removed the remaining MPs. The biofilter preferentially retained MP particles of large size, and no particles larger than 100 μm were found in the final filter effluent. Biofilters, therefore, are able to significantly reduce the number of MPs in treated wastewater but do not ensure their complete elimination.

Hydrophytic treatment (constructed wetlands—CWs) plants are a well-known and natural wastewater treatment technology with relatively lower costs than other biological treatment methods. Recent studies have explored the feasibility of using constructed wetlands to remove MPs from wastewater [129,130]. Vegetated wetlands are a major site for the detachment, storage, transformation, and finally the release of MP particles [129,130]. Studies on the role and effectiveness of vegetated wetlands, including both natural and constructed types, in removing MPs from contaminated water have highlighted the significant contribution of macroinvertebrates (e.g., snails, beetles, and others) in the process. They found that macroinvertebrates living in wetlands ingest significant amounts of MPs [129,130]. They achieved over 90% MP removal efficiency in both horizontal and vertical flow CWs, which is comparable to other conventional tertiary treatment methods in WWTPs, such as biological filtration (84%), dissolved air flotation (95%), DF (40–98.5%), MBR (99.9%), and sand filters (97.1%) [130]. An MP removal efficiency of 98% was achieved across the entire WWTP when CWs were implemented as the third stage of wastewater treatment. Therefore, CWs can serve as an efficient, environmentally friendly, and cost-effective tertiary treatment option that substantially reduces MPs in wastewater.

4.3.2. Coagulation

Coagulation can also be used as a tertiary treatment to remove total phosphorus, which cannot be completely removed by first- and second-stage processes in WWTPs. The number of studies on the removal of MPs in WWTPs using the coagulation process is very limited. Hidayaturrahman and Lee [56] investigated MP removal using polyaluminum chloride (PAC) at varying initial MP concentrations of 4200 MPs/L, 5840 MPs/L, and 31,400 MPs/L. The results indicated that the MP removal efficiencies were 53.8%, 47.1%, and 81.6%, respectively, whereby the overall MP removal increased from 83.1 to 92.2%, from 75 to 95.4% and from 91.9 to 95.7% after the second-stage treatment at three different WWTPs [56]. Kwon et al. [131] studied the efficiency of MPs removal by coagulation also using PAC, as tertiary treatment at two different WWTPs that treat domestic and industrial wastewater. The removal efficiency of MPs was 42.26 and 15.79%, respectively, while the total efficiency in treated wastewater after coagulation was 96.33 and 98.1% [131]. Ma et al. [95] studied the removal of MPs (PE less than 0.5 mm in size) by an Al-based coagulant in combination with anionic and cationic polyacrylamide (PAM). The results indicated that the removal efficiency increased from 26 ± 3% (without anionic PAM) to 61 ± 4% (with anionic PAM at a concentration of 15 mg/L). In addition, the results showed that anionic PAM was more effective than cationic PAM in removing PE MPs.

Studies of MP removal by coagulation method have shown that the type and properties of MPs, coagulant dose, agitation speed, and water quality (pH, ionic strength, presence of contaminants in water) affect their elimination efficiency. It was found that the effective coagulation strictly depends on the concentration of MPs, with higher concentrations resulting in lower removal efficiency [56]. Removal of MPs also positively correlates with the coagulant dose, with the degree of MP removal decreasing with increasing coagulant dose [132,133]. This can be explained by the fact that the zeta potential of MPs decreases as the coagulant dose increases, making it more difficult for coagulation flocs to form. In addition, the efficiency of the coagulation process depends on the type of coagulant used, with aluminium-based coagulants showing better performance than iron-based coagulants due to the stronger affinity of Al³⁺ for MPs [93,97]. Wang et al. [134] evaluated the effect of MPs particle size on the efficiency of the coagulation process combined with sedimentation. The results showed that larger particles were removed from wastewater to a greater extent, i.e., MPs of >10 μm in 100%, while particles of 5–10 μm in 45–75%. The effectiveness of the MP coagulation process is also affected by the pH value of the aqueous solution. Ma et al. [95] studied the removal of PE by coagulation with AlCl₃·6H₂O at pH 6, 7, and 8. Higher removal efficiency (27.5%) was obtained at lower pH conditions, while when FeCl₃·6H₂O was used, the highest removal (17 ± 2%) for MPs was obtained at pH 8 [95]. The efficiency of MP removal using coagulation depends also on the shape of the MPs and the polymer type. Compared to the fibrous and granular forms, fibres are removed to the greatest extent (51–61%) because fibrous MPs particles are more easily bound to post-aggregation flocs. The results also show that PET was removed to the highest extent (59–69%) in contrast to PP, PS, and PAM. Similar findings were reported by Katrivesis et al. [135] and Lares et al. [4]. However, more research needs to be carried out on the efficiency of removing MPs of different types, shapes, and sizes of polymers from wastewater samples from different stages of wastewater treatment. It is important that future research focus on finding the best coagulants/flocculants and their optimal conditions for MP removal along with colloid elimination.

Electrocoagulation (EC) is an advanced chemical coagulation technology that is relatively cost-effective and environmentally friendly. In the EC process, metal ions (Al³⁺ and Fe³⁺) are released from electrodes (often made of aluminium and iron), which effectively act as coagulants [136]. Electrocoagulation has also been applied for the removal of MPs/NPs [136]. The EC process has been shown to be effective in the pH range of 3–10 [11], making it more attractive for removing MPs/NPs from wastewater and water without the addition of pH-correcting substances. MP removal efficiency is over 90%, and 99.2% in the presence of 0.2 g/L NaCl and pH 7.5 [136]. EC studies of laundry wastewater showed 98% removal of MPs using an Fe electrode in just 60 min [11]. Although the EC process is cost-effective and efficient for MPs/NPs removal, it has several operational limitations, such as the frequent replacement of sacrificial anodes, cathodic passivation, and significant energy costs. To overcome these limitations, it is necessary to develop more cost-effective anodes and to research modifications to the operating conditions to avoid cathodic passivation.

4.3.3. Classical Filtration Processes

Classical filtration is an effective and common method of removing MPs/NPs, most commonly used as the third stage of wastewater treatment [50]. Filters with various structures, pore sizes, and materials are used in the filtration process. Among these, the most common are metal filters (such as stainless steel), polymer filters (including polycarbonate, nitrocellulose, and nylon), and fibreglass filters [137]. Classical filters typically have a pore size ranging from 0.45 to 1 μm. Due to the sizes of MPs/NPs and other contaminants in the water and wastewater, filter media become clogged. To prevent this, backwashing, pre-filtration with a larger pore diameter filter, or coagulation/flocculation-assisted filtration are carried out [137].

The disc filter (DF) is a forward-looking technology for reducing the MPs/NPs concentration in WWTPs, primarily used as a tertiary treatment stage [13,56]. DF uses a physical barrier, e.g., a fabric or mesh with large pores (10–20 μm), on which a cake layer is formed. It can be used to remove substances of small size and low density. In a study in South Korea’s WWTP, a general MP removal efficiency of 99.1% was achieved, while the efficiency at the DF stage (pore size 10 μm) was 79.4% [48,56]. In contrast, the Helsinki (Finland) WWTP also found that DF with a pore size of 10 μm reduced the concentration of MPs from PP, polyester, and PA from 0.5 ± 0.1 MPs/L to 0.3 ± 0.1 MPs/L and from 2.0 ± 1.3 to 0.03 ± 0.01 MPs/L (98.5%) [12,13]. Simon et al. [138] used a polyester mesh DF (pore size of 18 μm), achieving an MPs (>10 μm) removal efficiency of 89.7%. Generally, DFs with smaller pore sizes demonstrate higher MP removal efficiencies and the efficiencies depend little on the shape of the MPs [13]. The study shows that DF offers relatively moderate MP removal efficiencies, which can be explained by the fact that the filter surface becomes fouled, and the pressurised backwash process causes the solids to pass through the membrane. The removal efficiency of MPs in the DF process can be improved by implementing coagulation prior to filtration.

Rapid sand filtration (RSF) is a widely used technology for removing contaminants, employed in both water and wastewater treatment plants, as a third treatment stage [64]. Filters used in RSF are typically constructed with a layer of gravel (1 m high and grain size 35 mm) and a layer of quartz sand (0.5 m high and grain size 0.1–0.5 mm); a third layer in the form of anthracite is also often used [137]. RSF is an effective method for MP particles of diameters above 20 μm and allows the removal of all types of MPs [9]. In a pilot study of wastewater treatment in Turku (Finland), the RSF process was used as a third treatment step, achieving a 97% removal of MPs (ranging from 0.7 to 0.02 MPs/L) across all shapes and sizes, including the smallest fractions (20–100 μm) [12,13]. In the study, conducted by Hidayaturrahman and Lee [56], with RSF having a sand filter with a depth of 6.8 m (sand grain size 0.8–1.2 mm), approx. 74% of the MPs in the effluent were retained at an MP concentration of 215 MPs/L. When combined with coagulation, the total MP removal efficiency can be increased to 98.9%, with a removal efficiency of 73.8% at the RSF stage [56]. The mechanism for the separation of suspended solids in RSF is by adsorption and/or mechanical straining. As a result of hydrophilic interactions, MP particles attach to the surface of sand grains or adsorb onto silica grains, which contributes to pore clogging and thus reduced efficiency over time [62]. The adsorption of MPs is essentially permanent because functional groups, like hydroxyl groups, on the surface of the MPs create stronger bonds, leading to a more secure attachment to the filter layers [98]. All the above studies have shown that the RSF process exhibits a lower MP removal efficiency than the MBR process, but higher than that of DAF, granular activated carbon (GAC) adsorption, and membrane filtration, highlighting its potential as an effective method for MP removal in WWTPs [9].

4.3.4. Membrane Filtration

Membrane filtration technology is widely used in water and wastewater treatment, with the most commonly used pressure-driven membrane processes [139]. Depending on the membrane pore size, the separation mechanism, and the size of the particles/molecules to be separated, they include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) [139,140]. The main disadvantage of membrane filtration is membrane fouling, which occurs as a result of the adsorption of molecules on the surface and inside the membrane pores. As a consequence of fouling, the performance of membrane processes is reduced, resulting in higher energy consumption, and increased operating time and costs [139,140]. Membrane technology is very effective in removing MPs with effectiveness influenced by the specific membrane technique used.

UF and MF are used in water and wastewater treatment to produce high-quality drinking water, as well as in water reuse, especially in industrial plants [19]. UF/MF processes are characterised by low energy consumption (transmembrane pressure (TMP) of only 1–10 bar) and small plant size. Asymmetric membranes are mainly used with pore sizes ranging from 0.05 to 10 μm for MF and 1 to 100 nm for UF, which can retain particles and macromolecules, including MPs [139,141]. The particle size of MPs is therefore larger than the pore size of the UF membrane, so they are completely retained by UF membranes [97], whereas the pore size of MF membranes means that they can retain MPs of more than 0.05–10 μm. Pizzichetti et al. [142] showed that a membrane made of three different polymers (polycarbonate, cellulose acetate, and polytetrafluoroethylene) with a pore size of 5 μm can retain MPs from PA in the range of 99.6–99.8% and MPs from PS in 94.3–96.8%. UF/MF membranes, while separating MPs from water, can also cause MPs to migrate into the water, through fragmentation or rupture, as they are themselves made of polymers [18]. MPs filtration leads to a 38% decrease in final water flux [143], as MPs are adsorbed inside the pores and on the membrane surface. To ensure the long-term stable operation of MF and UF membranes, fouling, which is a major operational problem, must be strictly controlled [143]. Effective and stable purification or pretreatment (coagulation-flocculation-sedimentation) is needed to reduce the effect of MPs on membrane fouling, which depends on the concentration and size of MPs/NPs [95,97,144]. Higher concentrations of them and particle sizes around 1 μm lead to a higher intensity of membrane fouling [144]. In a study by Yahyanezhad et al. [145] an MF membrane with a 0.1 μm pore size was employed to remove MPs from biologically treated wastewater. The MPs concentration was reduced from 10⁶ MPs/L to only 2 MPs/L, which practically means a removal of 98% of MPs. It was also found that the concentration of MPs in the wastewater influenced the efficiency of their removal, with higher concentrations reducing the efficiency of the process. A comparison of MP removal efficiency for two ceramic membranes (MF and UF) and their effect on fouling was carried out for synthetic and real wastewater from the first washing cycle in an industrial laundry [146]. In both cases, 100% fibre removal was achieved. Filtration of synthetic wastewater containing 80 µm nylon fibres showed significantly higher fouling of the MF membrane, compared to UF membranes. When real laundry wastewater was filtered, the decrease in permeability in the MF process was about 95%, while in UF it was only about 37%, demonstrating the greater suitability of UF membranes in removing MPs from laundry wastewater, with less need for membrane regeneration and longer operating times.

Reverse osmosis (RO) is a pressure-driven membrane process in which the TMP induces selective transport of solvent molecules in a direction opposite to that resulting from the osmotic pressure. The TMPs used in the RO process must exceed the osmotic pressure of the feed solution and are generally in the range of 1.0–8.0 MPa [39,141]. Low-molecular-weight substances are retained by RO membranes and the separation mechanism is described by the dissolution–diffusion model. RO is employed in municipal and industrial water treatment systems, for producing high-purity water in the energy and pharmaceutical industries, for the desalination of saline waters, and for water reuse from industrial and municipal wastewater [139]. RO is useful for the removal of MPs and NPs primarily as a third or even fourth stage of wastewater treatment. Ziajahromi et al. [8] investigated the removal efficiency of MPs and NPs by RO in a WWTP in Sydney, Australia, as a third stage of treatment. After the RO process, MP fibres were detected, with a removal efficiency of only 90.45% for MPs larger than 25 µm [8]. Following the first, second, and third stages of treatment and reverse osmosis, significant amounts of MPs/NPs were still present in treated water [8]. Studies have suggested that the release of MPs through RO membranes can occur through membrane defects and small holes in pipelines [8] or worn polymeric membranes [147]. Therefore, there is an urgent need for more research on the release of MPs from polymeric membranes used in RO in the WWTPs.

The paper [148] discusses the results of MP removal using an integrated membrane system (IMS) alongside traditional AS treatment, with RO as the tertiary step (Figure 5). The IMS was equipped with pretreatment including UF, MBR as the biological step, and RO as the tertiary step. The MBR was equipped with capillary PVDF (polyvinylidene fluoride) membranes with a pore size of 0.4 μm, while the RO system was designed with flat membranes having a pore size of 0.0001 μm. MP removal in the IMS was 93.2% after MBR treatment, reaching 98.0% after the RO step. The water recovered through the RO process is suitable for reuse as industrial water. The results demonstrated that IMS was more effective in removing MPs from wastewater. However, the possibility of fine MP fibres (<200 μm) bypassing the IMS, even with the inclusion of RO, should be taken into consideration.

Wang et al. [124] investigated the removal of phthalate esters (dimethyl phthalate, dibutyl phthalate, diisobutyl phthalate, and di(2-ethylhexyl) phthalate) and MPs from wastewater in four different WWTPs. Clarification, filtration, and reverse osmosis were applied, achieving phthalate ester removal efficiencies between 47.7% and 81.6%, and MPs removal efficiencies ranging from 63.5% to 95.4% across all treatment plants. The effluent contained MPs in the form of granules and fragments (<0.01 mm in size), with concentrations ranging from 276 to 1030 MPs/L and 103 to 4458 MPs/L, respectively.

4.3.5. Oxidation Processes

Chemical oxidation aims to mineralise polymers, converting them into CO₂, water, and other byproducts through mineralization. In order to increase the efficiency of the oxidation process, radiation from various sources (e.g., UV-VIS radiation, solar energy) as well as electricity and ultrasound are used [149]. Advanced oxidation processes (AOPs) are promising technologies that use radical reactions to degrade environmental pollutants [150]. Recently, studies describing the removal of MPs by oxidising agents are emerging, with ozonation, Fenton methods, chemical disinfection methods and photocatalysis being the most widely used and effective [16].

Ozonation can cause the oxidation of various polymeric substances containing unsaturated bonds and aromatic rings by increasing surface tension, improving surface adhesion properties, increasing solubility, lowering the melting point, and modifying mechanical properties [151]. This process is used to increase the effectiveness of some conventional biological methods. Laboratory studies show that the mineralisation of PS films by Penicillium variabile was significantly increased from 0.01% to 0.15% when ozone treatment was used [152]. Studies confirmed the high degree of PAM polymer degradation (>90%) at 35–45 °C by direct ozone treatment, compared to other MP removal methods such as DF (79.4%), RSF, and GAC filtration (73.8%) [56,153]. By combining the GAC filtration method with ozonation, an increase in removal efficiency of 17.2–22.2% was achieved [153]. These examples indicate that ozonation is useful as a tertiary stage of wastewater treatment. One of the factors limiting the use of ozonation for MP removal is the high cost of ozone production and environmental concerns [150], it means if the oxidation is not complete, intermediates may be formed that have negative impacts on human health and the ecosystem [91].

The Fenton process is among the most commonly used advanced oxidation processes for wastewater treatment. In the process, the reaction of hydrogen peroxide (H₂O₂) and heterogeneous Fe²⁺-containing catalysts produces highly reactive hydroxyl radicals, which then oxidise the target organic and other pollutants to CO₂, water, and mineral products [154]. Tagg et al. [155] studied the effect of Fenton’s reagent on MPs made of PE, PP, and PVC and found no significant changes in any of the polymers, despite applying three different doses of H₂O₂ and FeSO₄·H₂O. To overcome the drawbacks of the conventional Fenton process, innovative approaches have been developed in which the main oxidant (H₂O₂) is produced electrochemically or activated using UV light to generate hydroxyl radicals [150]. Miao et al. [149] used the electro-Fenton method to degrade MPs from PVC, achieving 75% dichlorination efficiency and 56% mass loss efficiency after 6 h at 100 °C. Given its effectiveness for PVC MPs, this method may also be a viable solution for degrading other chlorinated MP species [149]. Feng et al. [156] reported over 99% mineralisation of cross-linked sulphonated PS foams in only 250 min using a photochemically enhanced Fenton process. The conventional Fenton process is not sufficient to cause significant changes in MPs through the *OH radicals alone; however, the introduction of UV irradiation boosts the rate of oxidative degradation [156]. Overall, further research is needed to effectively apply the Fenton process in the removal of MPs.

Photocatalysis is a chemical process in which light (usually UV or visible radiation) activates a catalyst, leading to the acceleration of chemical reactions [157]. The photocatalysis process begins with the excitation of the photocatalyst by absorbing an appropriate amount of energy from a specific light source. Photo-excitation leads to the formation of photogenerated pairs of electrons and eCB⁻/hVB⁺ holes (CB-conductivity band, VB-valence band). The valence band holes (hVB⁺) of the photocatalyst react with water molecules or surface-bound hydroxyl groups to generate hydroxyl radicals (HO*). At the same time, the conduction band electrons (eCB⁻) reduce oxygen (O₂) present in the solution, producing superoxide anions (O₂⁻), which then interact with water to form hydroperoxide radicals (HOO*) [157,158]. These highly reactive radicals then oxidise various organic pollutants, including polymers [11,159,160,161]. Various mechanisms have been proposed for the photocatalysis of MPs, among which the promotion of the degradation process by hydroxyl radicals is predominant [160]. Metal oxide nanomaterials with semiconducting properties (ZnO, TiO₂) are used to generate the desired reactive radicals [157]. ZnO nanoparticles are considered to be one of the most effective photocatalysts because of their ideal bandgap (3.37 eV) for efficient catalysis, strong redox potential, non-toxic nature, superior electron mobility, and the flexibility to form different sizes and shapes [159,160]. The photocatalytic degradation of low-density polyethylene-based MPs was studied using heterogeneous zinc oxide nanocatalysts in the form of rods [162]. Based on optical images, morphological changes in MPs were observed, including the formation of wrinkles, brittleness, cracks, and stains on the surfaces exposed to the photocatalytic process. The results also showed changes in the elastic properties of MPs in samples exposed to photocatalytic conditions compared to untreated samples. This is directly correlated with the modifications in the strength of chemical bonds in polymer molecules. Liang et al. [161] reported FTIR spectra that identified the formation of new functional groups, such as carbonyl and vinyl groups, during the photocatalytic treatment.

Chlorine is widely used as a disinfectant for WWTPs. Studies have shown that the common polymers that form MPs are not completely resistant to chlorine [163]. The chlorination process increases the abundance of MPs due to particle fragmentation [10]. For example, chlorine during the disinfection process potentially breaks existing bonds in polystyrene and poly(styrene-co-butadiene), forming new structures, i.e., asymmetric chains and symmetric C-C-C chains, and non-linear structures containing -CH₂ groups [163]. During chlorination, new carbon-chlorine bonds (Cl-CH₂-C-H) are also formed, which increases toxicity and hydrophobicity and results in easier adsorption of MPs and accumulation of harmful contaminants [54]. Chlorination changes the physical and chemical properties of microplastics because of its strong oxidising properties.

UV oxidation occurs on the surface of MPs, causing a change in their topography and chemical properties [164]. Primary MPs have a relatively homogeneous and compact structure, whereas after UV oxidation, the surface of MPs becomes rough and, in the case of PE, PP and PS, oxidation with the formation of structures in the form of cracks, grains and flakes is a common degradation phenomenon. The resulting MP structures then readily decompose further, leading to the formation of smaller structures and even NPs [164]. As a result of the UV-induced cleavage of C-C and C-H bonds, superoxide free radicals are formed [164]. In addition, hydroxyl (OH), chromophore, carbonyl (C=O), and hydroperoxide (ROOH) groups are formed on the surfaces of MPs, forming oxygen-containing free radicals and initiating chain reactions [164,165]. The superoxide free radicals formed by UV irradiation undergo secondary reactions to form cross-linking compounds, and the molecular chain with the carbonyl group is broken to reduce the relative molecular weight [165]. However, the intermediate products and toxicity of UV-oxidised MPs are less well known. The effects of UV irradiation time and environmental differences on the degradation of MPs require further study.

The efficiency of MP removal in the third treatment stage in WWTPs depends on the unit processes used, with the best results shown by technologies based on the filtration mechanism, mainly membranes [39,87]. However, MPs have irregular shapes that can cause filter wear [85] and pose a high risk of membrane fouling due to their surface properties [97]. It is worth noting that the concentration of MPs in both the influent and effluent of WWTPs with tertiary treatment units can be very low (<1 molecule/L in most cases) and therefore a limited sample volume (a few tens of litres) may give false zero results. Therefore, larger sample volumes are required to reliably assess the removal efficiency of MPs in tertiary treatment processes than for the assessment of primary and secondary treatment processes. Regarding the particle size of MPs, it is reported that MPs with particle size < 20 μm predominate in the final effluent [40]. Regarding the shapes and types of particulate polymers in the effluent after tertiary treatment, in most studies, fibres dominate the final effluent [8,12,51].

5. Summary

The table (

Table 8. Advantages, disadvantages, and effectiveness of various purification methods for removing microplastics.

Type of Process		Method Advantages	Method Disadvantages	Effectiveness in MPs Removal
Physical treatment technologies for MPs removal	Adsorption	Adsorbents with high surface area and porosity effectively retain even large MPs.	Spherical MP particles with a diameter of 10 μm are adsorbed to a lesser extent.	Low to moderate effectiveness
	Density separation	Enables the removal of low-density solid particles.	Heavy salts are very expensive and some of them are hazardous. Not useful for large-scale particulate removal.	Moderate to high effectiveness
	Disc filters	Formation of slime cakes; floating MPs are especially removed.	Need to backwash.	Moderate to high effectiveness
	GAC Filtration	Removes small-sized MP particles.	GAC Filters Clogging.	Moderate to high effectiveness
	Gravel/Sedimentation	Low-cost process; effective for large MP particles	Multiple stages of purification are needed to remove small solid particles.	Low to moderate effectiveness
	Magnetic separation	Effective removal of smaller MPs; better for drinking water treatment.	MPs recovery from sludge is lower.	Moderate effectiveness
	Membrane filtration	High MP removal efficiency; suitable for various water sources; compact and modular design.	Prone to fouling and clogging; high operational costs; requires regular maintenance and replacement.	High effective
	Rapid sand filter	Low operating and maintenance costs.	Clogging reduces efficiency. Regular backwashing is necessary.	Moderate to high effectiveness
Chemical treatment technologies for MPs removal	Classical Fenton process	Cost-effective and uses optimal reagents.	Limited to specific MP types; low efficiency.	Low effectiveness
	Coagulation	Flexible operating conditions; simple to use; suitable for small MPs.	Not effective for large MPs; uses significant amounts of chemicals.	Moderate to high effectiveness
	Electrocoagulation	Effective sediment reduction; cost-effective; no secondary pollution.	Sacrificial anodes must be replaced many times; passivation of the cathode occurs; electrical energy is required.	Moderate to high effectiveness
	Electro-Fenton process	Environmentally friendly process; low reagent costs; less sludge production.	Further modifications are necessary to improve effectiveness.	Moderate effectiveness
	Ozonation	Effective tertiary treatment; removes MPs by changing properties/morphology.	Complex ozone production; high costs; potential environmental pollution.	Moderate to high effectiveness
	Photocatalytic degradation	Environmentally friendly; avoids excessive chemicals.	Generates secondary pollutants; energy-intensive.	Low to moderate effectiveness
	Photo-Fenton process	Highly efficient; no excessive catalyst/chemical use required.	Requires optimal pH maintenance; further research is needed.	Further research is needed
Biological treatment technologies for MPs removal	Activated sludge (AS)	Well-established biological process.	Removal efficiency depends on microorganism activity and MP properties.	Varied removal
	Anaerobic-anoxic-aerobic process (A2O)	Integrated treatment approach.	Highly variable removal efficiency based on conditions.	Varied removal
	Constructed wetlands (CWs)	Environmentally friendly and cost-effective; provides simultaneous removal of nutrients, organic matter, and MPs; minimal energy consumption.	Slower treatment process compared to mechanical or chemical methods; removal efficiency depends on many factors; requires large land areas.	Moderate effectiveness
	Membrane bioreactors (MBRs)	Combines biological treatment with membrane filtration. Highly efficient at removing MPs; produces high-quality effluent; capable of removing MPs of varying sizes due to the membrane’s fine pore size.	High operational and maintenance costs; membrane fouling can reduce efficiency and requires regular cleaning.	Very high effectiveness

) summarises the advantages, limitations, and integrated MPs removal efficiency of different technologies in wastewater treatment processes.

6. Conclusions and Future Prospects

MPs are typically detected in both influent and treated effluent, with concentrations ranging from 1 to 10,044 particles/L and from 0 to 447 particles/L, respectively. Despite the relatively low concentration of MPs in purified wastewater, the total discharge of MPs is still high, averaging 2 × 10⁶ particles/day, corresponding to an annual discharge of 5 × 10⁷ m³/year. In addition, large quantities of MPs from the WWTP are often returned to the environment through sludge, utilised membranes, and streams containing filter concentrate. If the sludge containing MPs is then used as a fertiliser or landfilled, contamination of natural water bodies will occur due to transport by storm runoff or due to MPs returning to the WWTP via leachate. The most common polymers detected in WWTPs are polyester, polyethylene, polyethylene terephthalate, and polyamide, with fibres accounting for the largest proportion of observed MPs in the classification of different shapes. It has been found that primary- and second-stage treatment in WWTPs remove the majority of MPs, while the introduction of tertiary treatment, further increases the efficiency of MPs removal.

It is not possible to state unequivocally which technology is likely to produce a significant removal effect for MPs compared to others, due to insufficient studies that thoroughly analyse the removal efficiency of each method. In addition, although the same technology is used, there are many differences in the design and operating parameters of the plant in each WWTP, which can affect the removal efficiency. Filtration-based methods are regarded as one of the most effective physical treatment methods; however, additional research is still needed for their large-scale application, especially in municipal wastewater treatment. In membrane filtration technology, MPs tend to adsorb onto the membrane surface due to interactions with the pores and membrane, which complicates the process and reduces its efficiency. Membrane bioreactor technology is the most effective biological treatment methods, capable of removing up to 99% of MPs. In chemical treatment, coagulation, Fenton processes, and other advanced oxidation processes show promising results in removing MPs. Therefore, complete treatment technologies or methods are needed that not only capture MPs in the treatment process but will result in their degradation or transfer to another component with low environmental impact.

Future research on MPs in the WWTPs should focus on the following issues:

Development of standardised sampling methods and their analysis. At the same time, further research should focus on the development of disposal methods for specific MPs, especially for different industries. More research is needed to determine the toxicological effects of MPs.

Most of the current research on the potential effects of MPs on wastewater treatment processes is based on laboratory studies, it is important to consider the actual environment in the WWTP when designing experiments.

Factors influencing treatment processes in the removal of MPs in WWTPs, such as hydraulic retention time, salinity, presence of dissolved organic matter, as well as temperature, pH, and dissolved oxygen concentration in the wastewater, also require in-depth research.

The mechanisms revealing the influence of MPs on various wastewater treatment processes, including activated sludge structure and function, are still unexplained.

Future research should take into account that MPs can easily adsorb other contaminants (e.g., heavy metals and organic compounds) on their surface and also contain many chemical additives, which is a serious threat to aquatic organisms.

To date, studies on MPs in the WWTPs have mainly considered MPs > 20 µm in size. However, it has been reported that smaller MPs are present in large quantities in the aquatic environment and may exhibit serious biotoxicity as they can enter the circulatory system of aquatic species. Therefore, it is worth considering fine MPs (<20 µm) in future studies.

Source control may provide an alternative solution to prevent contamination of MPs. Future efforts could be devoted to the separation of MPs from wastewater at the household scale, as well as to improving the regulation of plastics.

New challenges for water and wastewater management resulting from advances in knowledge, technology, and analytical tools, lead to changes in regulations governing wastewater quality, including the inclusion of new parameters. Microplastic contamination of the aquatic environment has recently been recognised as a serious ecological threat, which creates an urgent need for a thorough understanding of the occurrence, behaviour, and fate of microplastics in wastewater treatment processes before they are discharged into natural water bodies. In light of the new regulations on microplastics that wastewater treatment plants will soon have to face, it is necessary to collect and understand the results of research on the removal of these pollutants using different technologies. This is the only way to ensure their effective elimination in WWTPs and reduce emissions to the environment.