The Occurrence and Removal of Microplastics from Stormwater Using Green Infrastructure

Abstract

Microplastics (MPs) are becoming an increasingly common pollutant in the aquatic environment, including stormwater. This is a serious problem, as stormwater is becoming an essential transport route for MPs from urban areas to surface waters. Rainwater flowing from roofs, roads, and other impermeable surfaces contains a variety of plastic particles originating from tire abrasion or waste disposal. This article presents an overview of current research on the occurrence of MPs in stormwater. The potential of selected green infrastructure solutions—particularly bioretention systems, constructed wetlands, and permeable pavements—for their reduction is assessed. Individual solutions present how the change in filter material, selection of vegetation, or the method of conducting the process (e.g., direction of stormwater flow in constructed wetlands) affects their effectiveness. The potential of green infrastructure is also compared with the traditional gray solution of sewage management in cities. This article emphasizes the importance of integrating such solutions in spatial planning as an effective tool to combat climate change and limit the spread of microplastics in the environment.

1. Introduction

According to the State of the Global Climate Report, 2024 saw an increase in global temperature by over 1.5 °C compared with the so-called pre-industrial era (1850–1900). The year 2024 was the warmest in the 175-year observation period. The comparison of the record temperatures recorded in Europe in 2020 (Spain—47.1 °C) to those in 2024 (Italy—48.2 °C) is also a big surprise [1]. The observed climate changes are a change in global temperature and a disruption of the hydrological cycle. A significantly increased rate of water evaporation, even with slightly higher rainfall, can lead to more frequent cases of drought, resulting in a decrease in groundwater and underground water levels, which also affects the deterioration of their quality [2,3]. In turn, heavy, sudden rainfall, especially in cities, causes excess water not to drain away promptly, which can lead to flash floods [4]. Just one minute of heavy rainfall (level V) can bring up to 8 L of water to a surface of up to 1 m². Changes in precipitation patterns are observed worldwide, leading to their irregular distribution. Regions previously characterized by stable precipitation amounts are now struggling with deficits or excesses, resulting in droughts, floods, and floods [2,5,6]. Floods became the deadliest type of disaster in 2024, accounting for over 50% of all natural disasters (an increase of 15% compared with 2020) [1]. We cannot protect ourselves from climate change. We must reckon that extreme weather phenomena will occur more often and more intensively, significantly affecting the resources and quality of drinking water [7,8]. According to the United Nations (UN) estimates, the availability of clean and safe drinking water may decrease by as much as 40% in the next decade [9]. By 2050, the water demand will double, and more than half of the world’s population will be at risk of water deficit [10]. Since groundwater abstraction is increasing annually by 1% to 2%, it contributes to water shortages in some areas of the world [11]. Among the European Union (EU) member states, the highest freshwater resources are recorded in Croatia (approximately 28,800 m³ per capita). Finland and Sweden also have large freshwater resources per capita, amounting to approximately 20,000 m³. In contrast, relatively low levels per capita (below 3000 m³) are recorded in the six most populated EU Member States (France, Denmark, Spain, Germany, Italy, and Belgium) [12]. According to the UN report, a country experiences “water stress” when its annual water resources fall below 1700 m³ per capita [13]. This occurs among the EU Member States in Poland, the Czech Republic, Cyprus, and Malta [12]. As seen in this new reality, stormwater, previously treated mainly as waste to be quickly drained from cities, can now become a key resource in the fight against the growing climate crisis [14,15,16,17]. However, it is impossible to reuse it directly due to various pollutants. Therefore, appropriate stormwater treatment is an important scientific and technological challenge to reduce the demand for conventional water resources. Well-designed urban green infrastructure is a promising approach to reducing stormwater pollution and managing it sustainably. The article shows that despite the increasingly frequent research on the occurrence of microplastics in stormwater, the topic of their removal, especially in the recently promoted green technologies, still leaves many questions and areas to be researched.

The aim of this narrative–thematic review, which incorporates elements of a systematic structure, is to assess the effectiveness of green infrastructure (GI) in removing microplastics from stormwater. The following research questions were posed: (1) What are the dominant types of microplastics found in stormwater? (2) Which green infrastructure technologies, bioretention cells, constructed wetlands, and permeable pavement are most effective in removing them? (3) What are the limitations, costs, durability, and reliability of the analyzed GI solutions? and (4) What are the main research gaps and needs for further analysis?

2. Methodology

Although the review is narrative and thematic, a rigorous methodological approach was employed to enhance its clarity and scientific validity. Literature searches were conducted in Scopus, Web of Science, ScienceDirect, PubMed, Google Scholar, and reports from institutions such as the EPA, EEA, and WHO. The time frame covered the period 2005–2025. A broad set of keywords and phrases were used, including “microplastic”, “stormwater”, “green infrastructure”, “gray infrastructure”, “bioretention cell”, “permeable pavement”, “constructed wetlands”, “nature-based solutions”, and “removal of microplastic”. Peer-reviewed articles and technical reports, presented in both English and Polish, were included in the analysis. Publications focusing exclusively on other types of green infrastructure (e.g., green roofs, green walls), articles duplicating previously identified data or not meeting quality criteria, as well as articles not available in full text, were excluded (open access). Data were extracted from the selected publications regarding the type of infrastructure used (bioretention cell, constructed wetlands, PICP), microplastic removal efficiency (%), particle size ranges, sampling locations, investment and operating costs, system durability and reliability, as well as mechanisms of microplastic retention and migration within GI. The results were summarized in a tabular form and subjected to a critical thematic analysis, which identified dominant trends, limitations, and future research directions. Ultimately, after screening titles, abstracts, and full texts, 169 publications were identified for inclusion in the review. This review was not formally registered (e.g., in PROSPERO), but its search strategy was developed before the selection process began, documented internally, and consistently applied across the databases analyzed.

3. Results and Discussion

3.1. Occurrence of Microplastics in Stormwater

With urbanization, industrialization, and climate change, the composition of stormwater has deteriorated significantly. Not only classic pollutants such as sand [18,19], biogenic compounds [20], petroleum substances [21], suspensions, and heavy metals [22,23] are now recorded, but also a new class of pollutants, the so-called emerging pollutants (EP) [24,25,26]. These are pollutants that are difficult to remove and seriously threaten the environment and human health. In most cases, there are no regulatory standards for this type of pollutant. These include pharmaceuticals, personal hygiene products, hormones, and microplastics [24,25,26]. Also, stormwater contains pathogenic microorganisms, including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Aeromonas hydrophila [27,28].

In the case of the migration of pollutants to stormwater, we can distinguish two phases [18]. In the first phase—atmospheric deposition (AD), pollutants are captured from the air, including hydrocarbon compounds, carbon monoxide, sulfur dioxide, dust, suspensions, and solid particles [24,29,30]. The pollutants “acquired” in this phase constitute about 20–25% of all contaminants present in stormwater [31]. In the next phase, pollutants are washed out from the surface of the catchment area (including aerosols deposited on surfaces or runoff from car washing) [32]. Figure 1 presents the sources of pollution and the fate of stormwater in the natural environment.

Due to the fate of rainwater and the emerging problem of several new anthropogenic pollutants in it, including microplastics, an important step seems to be aimed at, among other things, their detection and removal.

3.1.1. Microplastics in Stormwater

The ubiquity of plastics in almost every aspect of modern life has led to an intensive increase in their production on a global scale. According to Global Statistic data, in 2023, global plastic production reached 413.8 million tons. Along with this phenomenon, the problem of generating vast amounts of plastic waste is growing, which increasingly ends up in the environment after the end of its life cycle. Low recycling rates further deepen this problem’s scale, promoting plastic accumulation in ecosystems. As a result, microplastics are becoming a persistent, difficult-to-remove pollutant, occurring in both the aquatic and terrestrial environments. Its presence raises growing concerns about the impact on the health of organisms and the functioning of entire ecosystems. Studies have shown that MP contamination in stormwater is highly diverse and variable over time—its concentration can range from 0.9 to 200 MP particles/L [33,34,35]. In comparison, values ranging from 2 to 54 MP/L have been reported in sewage treatment plant outflows [36,37]. These data may suggest that stormwater from urban areas is the main route of microplastic entry into aquatic ecosystems. Studies conducted in urban areas detected the presence of microplastics in all analyzed stormwater samples. High concentrations of these particles could result from their accumulation on impermeable surfaces during droughts and their resuspension in storm drains during heavy rainfall.

Studies show that each storm runoff can release significant amounts of microplastics into the aquatic environment [38,39]. It has been estimated that a single storm runoff during rainfall can release from 1.9 million to even 9.6 billion microplastic particles, which confirms stormwater’s important role as a pollution vector [37]. The scale and toxicity of this pollution are influenced not only by the quantity but also by the quality of emitted particles. Microplastics contain various chemical additives used at the stage of their production, such as acidifiers, lubricants, light and temperature stabilizers, dyes, antistatic agents, antioxidants, and plasticizers [40]. Most of these compounds do not form permanent chemical bonds with the polymer matrix, so they can be easily released into the environment [41,42,43]. The process of plastic fragmentation, combined with the migration of these substances, can lead to an exponential increase in the presence of toxic compounds in the environment, generating the so-called “toxic debt” [44].

Additionally, microplastic particles carried by stormwater can transport various chemical pollutants, including polycyclic aromatic hydrocarbons and additives used during the production of plastics [45]. In this context, microplastics act as a vector of harmful substances, accumulating toxins and increasing their bioavailability for aquatic organisms, which can lead to disorders of their health and functioning [46,47]. When marine organisms ingest microplastic particles, they can cause several adverse biological effects, including immunotoxicity and disruption of metabolic processes [48,49]. Microplastics in water and food chains also raise serious concerns from a human health perspective, especially in their increasing concentration in drinking water and food products. Microplastics can move through food chains, causing ecological and health effects [50,51,52,53]. Their ability to release toxic substances and transfer pollutants, as well as their high resistance to degradation and tendency to bioaccumulate in the food chain, make them a serious threat to aquatic ecosystems [54]. Therefore, identifying the sources and mechanisms of plastic transport in the environment is essential to develop effective strategies to reduce their negative impact.

3.1.2. Sources of Microplastics in Stormwater

Microplastics present in the environment come from two main types of sources: primary and secondary [55], which translates into the diversity of their shapes and sizes. Primary microplastics are intentionally produced microscopic particles, while secondary microplastics are created due to the degradation of larger plastic waste. However, it should be emphasized that in practice, identifying the exact origin of microplastics present in the environment is very difficult and often even impossible [56]. Figure 2 illustrates how microplastics are introduced to the urban environment through stormwater [56,57,58,59,60,61,62,63,64]

Sources of primary MPs include, among others, plastic pellets used in industry, micro granules contained in body care products (e.g., in scrubs, toothpaste, shower gels, and make-up) [56,65,66] as well as dust from paints or cleaning agents [67]. In addition, primary MPs can also be released during production—released fibers end up in sewage [68,69]. Additional sources include artificial surfaces, such as plastic running tracks in schools, synthetic turf, rubber roads in cities, and tire abrasion [62,70,71,72]. Secondary microplastics are formed due to the degradation of larger plastic waste. This process occurs under the influence of various factors, such as mechanical, chemical, thermal processes, photooxidation, and biological processes. As a result of these interactions, larger plastic fragments disintegrate into smaller particles [73,74]. Secondary MPs occur in both terrestrial and aquatic environments, and their primary sources are plastic waste lying on land and along coastlines or pollutants transported by rivers and sewage discharged through sewage systems [73]. Studies have shown that microplastics (particles less than 5 mm in diameter) are widely present on all continents—from the deepest oceans, through mountain peaks, to the most remote and inaccessible regions of the world, including polar regions [42].

Climatic factors such as precipitation and wind play a key role in transporting microplastics from terrestrial to aquatic environments. Plastics disposed of in the environment can fragment into smaller particles under the influence of solar radiation, wind, water, waves, and microbiological activity [75], facilitating further transport. Rainfall washes the resulting microplastics from urban surfaces such as roads, roofs, and pavements into storm sewers, directing them to rivers, lakes, and ultimately to the oceans. Strong winds can transport microplastics over considerable distances, leading to their presence even in remote and seemingly untouched ecosystems. Stormwater from urban areas is a source of microplastics (MPs) entering aquatic ecosystems [76]. Microplastics enter them from numerous sources; the most common sources of MPs in stormwater are surface runoff from roofs and roads, tire abrasion, urban waste, artificial turf, outdoor paints, industrial sewage, garbage, and construction waste [71,77,78,79,80].

Along with stormwater, microplastics are transported to water reservoirs such as retention ponds, rivers, or lakes, from where they can be transported to marine ecosystems [81,82]. Urban areas, characterized by a large share of hard and impermeable surfaces, such as streets, parking lots, roofs, or compacted soil, generate intensive surface runoff, which favors the transport of microplastics to water reservoirs [39,71,82]. It should be emphasized that microplastics can accumulate on these surfaces and then be washed away during rainfall [71]. Transport-related microplastics come primarily from two essential sources: road dust and vehicle wear, such as tires and brakes. The first is road dust, which is considered a significant source of diffuse pollution, especially in urban environments. Studies by Su et al. showed that the concentration of microplastics in street dust samples from Phillip Bay (Australia), collected under different precipitation conditions, ranged from 20.6 to 529.3 particles/kg [83]. The most common were fibers (70.8%) and fine particles below 1 mm (41.9%). In terms of composition, polymers such as polyester and polypropylene dominated (totaling 26.3%) [84]. The second primary sources of microplastics in urban areas are tire wear particles (TWP) and brake wear particles (BWP). TWP particles are generated by mechanical abrasion of tires, which consist of complex rubber compounds, often containing polymer additives such as polyurethanes and acrylics [85,86]. According to observations [85], the mechanical wear process of tires leads to their degradation and TWP emission, which is associated with a significant environmental risk. Similar conclusions were presented in another study, indicating that TWP is one of the primary sources of microplastics in both air and surface water. In turn, Ashiq et al. classified tires as the primary source of microplastics in urban stormwater [57]. As a result of intensive vehicle traffic, microplastic particles are generated, which can be easily transported by stormwater [87]. Global modeling conducted by Evangeliou et al. showed that TWP and BWP can be effectively transported through the atmosphere over considerable distances, affecting the environment outside urban areas [88]. Griffith University studies have confirmed the presence of TWPs in urban storm runoff, indicating a wide range of particle sizes, from visible rubber fragments to invisible microparticles. It is estimated that as much as 6.6 million tons of TWPs are released globally each year, making them a significant source of transport-related microplastics [89]. Another vital pollutant is municipal waste—such as plastic bags or bottles—which fragment and are flushed into storm sewers. Werbowski et al. indicated improper waste management as a significant factor in increasing the load of microplastics in urban stormwater [35]. The degradation of four common municipal plastic wastes (coffee cup lids, chocolate wrappers, and plastic bottles) made of high-volume polymers—PP (polypropylene), PS (polystyrene), PE (polyethylene), and PE-LD—under the influence of UV radiation was studied. The results show that PP, PS, and PET showed a clear degradation, and longer exposure led to a greater release of microplastics [90]. PP showed the highest susceptibility to UV, which is significant because this polymer is the most frequently identified component of microplastics in stormwater and urban sediments [91,92].

Another source of microplastics is artificial surfaces, such as synthetic turf and sports fields, which contain granulated rubber. During their use, microplastic particles, including contaminants present in rubber materials, can evaporate or be washed away by rain, constituting a persistent and difficult-to-remove environmental pollutant [71,93]. Exposure of these objects to atmospheric factors—oxidants, light, high temperatures, and precipitation—promotes material degradation and increases toxic emissions. Rubber particles from artificial surfaces can accumulate in soil and surface waters, contributing to the contamination of urban freshwater systems [67]. A study was conducted on five types of playing fields on a campus in the Daxing District of Beijing—including artificial turf, a runway, a basketball court, a badminton court, and a tennis court—and found high concentrations of microplastics. The identified materials included EPDM (ethylene propylene diene monomer rubber), SBR (styrene butadiene rubber), PE, PP, PO, viscose, and nylon. The highest levels of contamination were recorded on artificial turf and the runway. It was found that the frequency of use and the type of sports activity can affect microplastic emissions [62]. The influence of external building coatings cannot be ignored either. Paints, varnishes, and other protective coatings can contain polymers that erode under the influence of weathering. Hale et al. described the degradation mechanisms of these materials, indicating their contribution to stormwater pollution [61]. Moreover, it has been pointed out that industrial emissions, including those from the degradation of polymer coatings in everyday materials, may be a source of microplastics in storm sewer systems [94]. Stormwater retention and transport systems should also be considered, as they are increasingly made of plastics such as PVC, HDPE, or PP. This applies to both compact filters used in households and large installations, including inspection wells, tanks, or pipelines [95]. Studies by Zhang et al. have shown that because of long-term exposure to UV radiation (45 days) and intensive hydraulic cleaning (72 h), there is a significant release of microplastics—from 160 to 1905 particles per gram of material, regardless of the type of polymer. These results confirm that plastic installations used in stormwater retention systems can be a significant source of environmental microplastics [62]. Finally, construction waste, including plastic pipes and insulation fragments, can enter stormwater during construction work. Dris et al. emphasize that construction sites often generate microplastics, and the contaminants are washed into nearby stormwater systems, especially in urban areas where construction work is carried out daily [86].

Additionally, retention ponds and reservoirs should be remembered, which play a key role in managing urban runoff, enabling its capture and storage. The primary purpose of these systems is to reduce the risk of flooding and to support the sedimentation process of suspensions before the water is discharged into the natural environment or storm sewer system [96,97,98]. The conducted studies have shown [97] that among the retained MP particles, the most numerous groups were fragments (57%), followed by pellets (27%), foils (6%), and fibers (9%). The obtained results show that storm ponds play an important role as environmental absorbers of microplastics, retaining their various forms and limiting their further transport in the urban environment. In summary, the multiple sources of microplastics in stormwater—including built-up runoff, tire abrasion, litter, artificial turf, building coatings, industrial effluents, and construction waste—are critical to understanding and addressing the broad impacts of microplastic pollution on aquatic ecosystems. To better illustrate the scale and diversity of this phenomenon,

Table 1. Characteristics of microplastics detected in stormwater in different locations.

Place	Microplastics Concentration	Shape	Type of Material	References
Recycled Tire Parking	186 ± 173 p/L	79% fibers 17% rubber 1% fragments	PES > PU > PE	[33]
Three Ponds in Urban Area	0.5–22.9 p/L	-	PP > PE > PVC > PS	[99]
Urban Area, High Traffic	0.4–3.2 p/L	58% fibers 15% fragments 21% microbeads 4% rubber	-	[35]
Urban Area, Residential and Offices	8.28 ± 6.90 p/L	20–97% fibers 10–64% rubber 5–40% fragments	WF > WF/PET > PP	[35]
Urban Redevelopment Area	9–280 p/L	47% asphalt 39% TWP	-	[100]
Densely Populated Urban Area	0.7–200.4 p/L	47.7% fibers 42.5% fragments	-	[37]
Car Park in City Center	3.8–59 p/L w tym 2.5–58 p/L TWP	80% fibers 20% fragments	PET, PP, PVC	[100]
Metropolitan City	0.4–36.48 p/L	films, fragments, fibers, foams, pellets	PE, PP, PET, PS	[101]
Stormwater Ponds	270 p/L	fibers, fragments	PP > poliester > akryl > inne > PA > PE > PS	[102]
Stormwater Drains	9.22–20.21 p/L	86–99% fibers, 1–13% fragments	PP, PE, PTFE, PVDF	[103]
Urban Area	0.7–200.4 p/L	47.7% fibers 42.5% fragments	PP, PE, PS	[37,104]
City Roads	0.68–5.9 mg/g	-	PP, PS, PET, PVC, PMMA, PE	[104]
Metropolitan City	57.5 MP units/kg	57% fragnents, 27% pellet, 9% fibres, 6% film,	PE, PES, PS, PA, PP	[97]
Urban Area—Streams	17–303 p/m3	39% fragments, 34% fibers	PE, PET, PVC, EVA	[105]
Urban Area	118–8894 items/kg	Fragments, fibers, pellets	PP, PS, poliester, PE, PU, PVC, PA	[80]

PP: polypropylene, PS: polystyrene, PET: polyethylene terephthalate, PVC: polyvinyl chloride, PMMA: poly (methyl methacrylate), PE: polyethylene, PA: polyamide, PU: polyurethane, EVA: ethylene vinyl acetate, PTFE: polytetrafluoroethylene.

presents the characteristics of microplastics detected in stormwater at different locations.

3.1.3. Shapes of Microplastics in Stormwater

Microplastics in stormwater pose a serious environmental threat, mainly due to their diverse chemical composition, physical properties, and morphology, which directly affect their mobility in the environment and potential ecological effects. In the context of these threats, it is essential to understand the forms of microplastics found in stormwater, which is illustrated in the diagram (Figure 3) [106,107,108].

According to the current state of knowledge, MPs detected in stormwater take various forms, such as fibers, fragments, granules, beads, films, foams, and particles, which may have different origins and affect ecosystems in other ways [33,86]. Hartmann et al. drew attention in their studies to the large morphological diversity of microplastics present in the environment and the need to unify the classification of these contaminants [106]. Studies conducted by Utami et al. [39] (have shown a clear dominance of fibers as a form of microplastic in stormwater samples. These fibers most often originate from textile materials made of plastics (e.g., polyester or polyamide) and are introduced into the atmosphere and water sediments through air transport and deposition with atmospheric precipitation [39]. The presence of microplastics originating from fibers carried by atmospheric rainfall has also been confirmed by other studies, suggesting their dominant role in the urban precipitation cycle [43,86]. Utami et al. [39], also emphasized the importance of particle size, indicating that the highest concentrations of microplastics in stormwater concern fractions with sizes from 101 µm to 500 µm [39]. This site has significant ecological significance—smaller particles are more easily transported in the ecosystem, have a greater potential to penetrate biological barriers, and can accumulate in aquatic organisms, initiating toxicological processes [22,82]. The morphology of microplastics (MPs) plays a key role in shaping their environmental impacts. According to the findings of An et al., the physicochemical properties of MPs—such as shape, density, specific surface area, and electrostatic charge—modified by environmental conditions have a significant impact on the way they are distributed and potential migration routes. The authors emphasize that different morphologies of microplastics interact differently with ecological factors, such as rainfall intensity and urban runoff characteristics, which results in a direct correlation between the shape of particles and their ecological impact [56]. Another significant fraction of microplastics in stormwater is synthetic fibers, released mainly during the washing of textiles made of polyester (PES), polyamide (PA), and acrylic. The intensification of synthetic material production is one of the main factors in the growth of global waste, and this trend is intensifying with demographic development and consumption growth. It is estimated that by 2030, global fiber production will reach 147 million tons [109]. Given the growing contribution of synthetic textiles to environmental pollution with microplastics, increasing attention is being paid to the emission of microfibers not only during washing but also during the drying process of clothes. Studies show that during a 15 min drying cycle, a domestic tumble dryer can release from 433.128 ± 70.878 to 561.810 ± 102.156 microfibers, depending on the type of fiber and the load size [110]. Fragments and films, in turn, are often derived from the decomposition of larger objects made of such materials as polyvinyl chloride (PVC), polystyrene (PS), polyethylene (PE), and polypropylene (PP) [108,111]. Once in the environment, plastic fragments and granules undergo aging, a process that involves changes in the properties of polymers that can affect the composition, physical structure, and surface characteristics [112]. In addition to these categories, industrial granules and pellets, mainly composed of PE, PS, and PP, are also used as raw materials in the production of plastics. Emissions of these microplastics can occur during transport, storage, and processing, leading to their dispersion in the atmospheric and aquatic environment [113]. In the case of PE, PP, and PS granules, different levels of degradation were observed depending on the medium, whether it was air, ultrapure water, or synthetic seawater, as well as differences in terms of surface functional groups and textures, which were examined using FTIR, Raman, and SEM methods [114]. Other studies have highlighted that stormwater is a key medium for assessing microplastic abundance and distribution, indicating the need for research in this direction. It has been noted that stormwater can effectively transport various forms of microplastics, which supports the view that rainfall can serve as a vector for the dispersion of these particles in the environment [115]. It has been shown that persistent rainfall is responsible for the increase in microplastic amount and diversity in surface waters. A correlation has been observed between the intensity of stormwater and the increased amount of MPs in urban water bodies [89]. Additionally, it has been observed that the migration of microplastics in water and their abundance are greater in the early phase of rainfall. In contrast, the concentration of MPs in water decreases with the extension of the rainfall duration. These observations are confirmed by the study’s results on the impact of precipitation on the concentration and migration of microplastics in surface waters by Gunther et al. (2023), who showed that stormwater runoff is the main entry route of microplastics into freshwater environments, especially in the context of highly urbanized areas [116]. The morphological diversity of microplastics in stormwater reflects their diverse origin, which emphasizes the importance of identifying the main sources of these pollutants in the urban environment. One of the main sources of microplastics in stormwater is runoff from roofs and paved surfaces. Roofing materials, paints, and waterproofing agents often contain plastics, which gradually degrade and are washed away by rain into storm sewer systems [117,118,119]. Studies have shown significant correlations between the characteristics of urban catchments and the concentrations of microplastics in surface runoff, confirming the influence of built-up areas on microplastics in stormwater [120]. It has been noted that the degradation of large plastic items contributes to generating secondary MPs that enter stormwater systems [121].

3.2. Removing Microplastics from Stormwater Using Green Urban Infrastructure

The approach to water in the urban environment has evolved significantly over its historical and functional transformations. Initially, the priority was the so-called “water-supplied city”, then the “severed city”, then the “drained city”, “water cycle city”, and finally, the “water-sensitive city”. This last approach seems to be the best way to achieve sustainable development. In many countries, emphasis is placed on developing the so-called sustainable urban drainage system (SUDS), which aims to imitate and restore hydrological processes existing before the development of cities [122]. This involves, among other things, replacing or supplementing the traditional “gray urban infrastructure” intended only to drain stormwater with solutions based on nature, the so-called green infrastructure [123]. Implementing these systems in urban infrastructure will allow, among other things, for long-term solutions to various environmental, social, and economic challenges. This is particularly important in mitigating and adapting to climate change in cities. Green infrastructure’s main task/goal is to retain and purify stormwater at the place of precipitation. Importantly, these solutions simultaneously allow for the absorption of carbon dioxide, reduction in air pollution, and mitigation of the so-called urban heat island (UHI) effect.

Table 2. Comparison of the conventional approach to stormwater management to solutions based on urban green infrastructure [123,124,125,126,127,128,129,130].

Conventional Stormwater Management System—Gray Infrastructure (Stormwater Drainage)	Green Infrastructure in Stormwater Treatment (Bioretention System, Constructed Wetlands, and Permeable Pavements)
rapid removal of stormwater using stormwater drainage	retention, infiltration, local stormwater treatment
rainwater runoff velocity—high	rainwater runoff velocity—low
removal of classic pollutants: - suspension (20–40%) - nutrients (10–30%) - heavy metals (10–50%) - pathogenic microorganisms—low - microplastics < 10%	removal of classic pollutants: - suspension (70–95%) - nutrients (30–80%) - heavy metals (50–90%) - pathogenic microorganisms > 70% - microplastics > 80%
investment costs—higher	investment costs—lower
operating costs—related to modernization, inspection, and repairs	operating costs—related to regular cleaning and plant care
system operating time—from 50 to 100 years depending on the material	system operating time—from 20 to 30 years
aesthetics—none/negative	aesthetics—improving the landscape, green areas
dd	flexibility of expansion—high due to the possibility of using modular solutions
	environmental aspects: - increasing permeable surfaces in the city - reducing the occurrence of the urban heat island - reducing noise - increasing biodiversity - improving air quality and microclimate - mitigating maximum intensity and delay of outflow to the receiver - reduction in stormwater treatment costs - protection of the natural water cycle in the environment and reconstruction of groundwater resources

compares the approach to stormwater management in gray and green infrastructure.

In many cases, green infrastructure is designed primarily to prevent flooding by reducing the volume of stormwater runoff and peak flows. Purification in these systems occurs through processes such as deposition, filtration, adsorption, and biotransformation of pollutants contained in stormwater [124]. Among the solutions used in cities, bioretention areas, constructed wetlands, and permeable pavements are very popular [27,123,124,125,126,127,128,129,130,131].

3.2.1. Bioretention Systems

Bioretention systems, often called rain gardens or bioretention cells, are closed systems (separated underground depressions) used to collect, purify, and infiltrate stormwater [132,133]. The benefits of using bioretention systems in stormwater purification include improving runoff quality, reducing its volume with peak flow, increasing biodiversity, and integrating it with the urban landscape [132]. A typical bioretention system consists of three layers: (1) surface—covered with various species of trees or shrubs, (2) growth-containing soil, sand, gravel, or other organic substrate, and (3) drainage [63]. The basic mechanisms of microplastic retention in bioretention cells include physical filtration and adsorption on the substrate. Therefore, the efficiency of the MP retention process is significantly influenced not only by the size and type of microplastics but also by the properties of the filter medium. Ahmad et al. [134] conducted column studies, showing that the addition of biochar and kaolin improves the performance of bioretention cells, among other benefits, by increasing the degree of MP removal, organic matter, microbial community structure, and plant condition. Biochar, kaolin, and their combined composite eliminated MPs by 90%, 94%, and 97%, respectively [134]. Therefore, the efficiency of the MP retention process is significantly influenced not only by the size and type of microplastics but also by the characteristics of the filter medium. Bioretention systems with various types of media, such as sand, compost, or proprietary blends, can remove MPs with particle sizes ranging from 106 µm to 5000 µm by up to 99% [135]. Recent studies have shown that bioretention cells effectively remove microplastics from urban stormwater runoff, including those as small as 25 µm. A two-year field study demonstrated a 71% reduction in microplastic concentration. The inlet MP concentration decreased on average from 227 MP/L to 66.5 MP/L [87]. Gilbereath et al.’s [34] research shows that a medium consisting mainly of sandy clay (70%) allows for 95% MP removal. Interestingly, it was also found that reducing the amount of sand content by only 8% in the filtration medium can reduce MP removal by up to 85%. As studies show, sand filter media contribute to increased hydraulic conductivity, which results in improved efficiency in removing pollutants contained in stormwater, including MP particles [87]. Other mechanisms, such as electrostatic interactions and hydrophobicity, also affect MP retention by sand particles [135]. Therefore, among others, in Canada, it is recommended that the media used in bioretention systems consist of 75–95% sand (0.05–2 mm in size), 7–22% silt (0.002–0.05 mm in size), and 3–12% clay (<0.002 mm) [87]. Although bioretention systems are highly effective in retaining microplastic particles, the mechanisms of their transport, retention, and possible further migration within the filter medium are not yet well understood. The migration of microplastics in a porous substrate, typically the layer that fills the bioretention cell, occurs primarily in the vertical direction. The few field studies conducted consistently indicate their most significant accumulation in the growth layer of the bioretention cell (from 2 cm to 10 cm) [63,136]. In addition, it was found that the microplastic particles contained are of low density (including LDPE and PE) [75]. It is assumed that 90% of MP particles are “trapped” in the filter for a long time in such a situation because their degradation in the soil is considered low compared with other environments [87]. It was also observed that fibrous MPs pass more easily through the pores of fillers than granular and thin-film MPs [137]. According to the 2020 review by Spraakman and Rodgers [138], about 35 numerical models are used to analyze the behavior of stormwater and pollutants contained in it in bioretention systems and their catchments. Only 12 of them consider the transport of suspended solids. However, none of them has been developed for microplastic particles. Additionally, since there is still no clear definition that allows for the classification of microplastics as colloids, and their physicochemical properties (including shape, size, density, surface charge, and aggregation state) depend on the type of polymer, adapting the appropriate model becomes even more problematic. It has been found that only microplastic particles with sizes ranging from 1 µm to 10 µm often exhibit colloidal behavior. Therefore, for this type of particle, the DLVO (Derjaguin–Landau–Verwey–Overbeek) theories and the clean bed filtration (CFT) model can be used to simulate their behavior in porous media. It is assumed that this type of microplastic particle undergoes physical mechanical retention, as is the case for colloids (depth filtration, interception, sedimentation) [136]. There are few studies on the potential effect of plants in bioretention systems on removing MPs. Comparing the removal of MPs with particle size from 100 µm to 300 µm, no differences were found between the sand filter and the bioretention cell with vegetation. However, differences were noted in removing smaller MP particles (from 20 to 100 µm) compared with the sand filter without vegetation [138]. The use of plants in purification processes is associated with the occurrence of mechanisms such as phytodegradation, phytoextraction, and rhizosphere interactions (reduction/oxidation, sorption/adsorption, and microbial remediation) [23]. Suitable plants for bioretention should grow quickly, be easy to plant and maintain, and have a good capacity to accumulate pollutants in the aboveground biomass. A study conducted by Kuoppamaki et al. in 2021 [139] visually examined the vertical transport of colored PE beads in vegetated and unvegetated columns. They found, among other things, that the MP beads moved deeper in the columns with vegetation (to a depth of 8–12 cm) compared with 4–5 cm in unvegetated columns [136,140].

The type of root system (fibrous or rhizomatic) may also be of major importance because MP particles accumulate around it [69,91,141,142]. The presence of numerous root hairs results in an increased sorption surface, contributing to the adsorption of smaller MP particles. However, there are still no studies indicating the most beneficial root system for MP removal [143]. Despite the significant phytoremediation potential, the mechanisms of microplastic biodegradation by plants remain poorly understood. They require further research, especially in the field of enzymatic activity and cooperation with soil microorganisms. As already indicated, the effective use of plants in MP removal can be enhanced by integrating them with other strategies, such as modifying the filter medium, e.g., by adding biochar. There are also numerous other research gaps: It is not known whether microplastics are permanently retained or whether they move to groundwater. There is also a lack of data on the effect of microplastics on the clogging of bioretention media and the rate of degradation of their hydraulic and filtration efficiency.

3.2.2. Constructed Wetland

Constructed wetland (CW) is a system that mimics natural wetlands, using aquatic vegetation (macrophytes), microorganisms, and filter substrate to remove low concentrations of pollutants from large volumes of stormwater runoff [144]. Among the various natural treatment systems, the simple design, low investment costs, and operating costs offered by CW make them a suitable alternative for both developed and developing countries. According to the life form of the dominant macrophytes, CW for stormwater treatment can be divided into three systems: (1) free-floating, (2) submerged, and (3) rooted emergent [144]. Another classification based on the water flow regime distinguishes two basic types of constructed wetlands: surface flow (1) and subsurface flow (2). In the underground system, we can also distinguish horizontal flow (HF) and vertical flow (VF) systems [129]. Surface flow CWs are often densely overgrown with various plant species, and the water depth is less than 0.4 m. In the underground flow CW system, a layer of soil or gravel is used as a substrate for the growth of rooted marsh plants. The operation of constructed wetlands is based on a combination of biological, chemical, and physical processes. When stormwater reaches the CW, several processes take place, such as sedimentation, precipitation, adsorption on the surface of the biofilm, degradation by microorganisms, and absorption by plant roots. As a result of the synergy of these processes, pollutants are removed from stormwater. Initially, CWs were designed to remove organic contaminants and suspensions from sewage or stormwater. Their more modern solutions also allow for the removal of nutrients, pharmaceuticals, and pesticides [145]. Recently, CWs have also been recognized as a promising solution in reducing MP pollution. However, the number of available studies is still limited, especially for large-scale systems [146]. It has been shown that constructed wetlands act as a barrier preventing the spread of microplastic particles in the environment. It has been found, among others, that their concentration in the water entering the CW can be up to four times higher than in the water leaving it [147,148]. The available literature focuses mainly on subsurface flow systems for microplastic removal [145]. The degree of MP removal in these systems can range from 20% to 100% depending on, among others, the amount, shape, and size of MPs and the filter material used [63,134]. The primary mechanism for microplastic (MP) removal in constructed wetlands (CWs) is substrate retention. Factors such as substrate particle size and CW structure significantly influence this retention process. In the case of sand matrices, almost 100% capture of MP particles larger than 45 μm has been reported. For a gravel filter matrix (d₅₀-2.1 mm), MP removal at 90% is possible [147]. As studies show, the filter matrix used has a significant impact on these systems. Therefore, using other materials, such as biochar or bark, in CW systems seems to be an interesting solution. Rullander et al. [148] showed that the use of biochar allows for over 97% removal of MP from rainwater. They also found, among other things, that larger particles are more easily retained, while those below 50 µm are more often recorded in water discharged from treatment plants [148,149]. The literature review also indicates that vertical flow (VF) systems, particularly, are highly effective in removing microplastics [146]. Interesting results were obtained by the team of Bydalek et al. [145], who showed for the first time that CWs with surface flow can effectively retain MPs. The degree of MP removal was 95%, and its concentration in the discharge was about 0.3 MP/L [145]. Plants and invertebrates are important links/elements influencing the efficiency of wastewater treatment in CWs. Studies show that there are significant numbers of snails, polychaetes, and beetles in wetlands, which also contribute to the removal of microplastic particles by swallowing them. It has been shown that earthworms can significantly increase the removal of microplastics in CWs—up to 98.8% [146,150]. The efficiency of microplastic removal can also be increased by the appropriate selection of plants or their density. It has been found that reeds can effectively capture microplastics (PE, PA, or PET) by reducing, among other things, the velocity of wastewater. The stems and roots of plants generate turbulence in the water, causing convection currents that stop the flow of microplastics and encourage their deposition [63,69]. Therefore, among the plants that can effectively remove PVC microplastics due to their extensive root system or rhizome part are Vallisneria natans or Water hyacinth. Their shape has a significant impact on MP reduction using plants. Even 99.7% reductions have been recorded for spherical microplastics and 99.99% for fibrous ones [150,151].

3.2.3. Permeable Pavements

In recent years, many cities have undergone unfavorable changes known as “concreteness”. Surfaces tightly covered with concrete or asphalt absorb solar heat, which in summer increases temperature and the formation of UHI [152]. The WHO emphasizes that for a city to be friendly to its inhabitants, the size of impermeable surface areas (ISA) should not exceed 50 m² per person. The global average is 93 m² per person. In the case of Poland, the ISA index is about 110 m² per person. This value is comparable to other EU countries, including Germany, Italy, the Czech Republic, and Austria. However, there are many countries where this index is lower, including Brazil and the United Kingdom (35–100 m²/person). Of course, there are places where the ISA index reaches values from 150 to 220 m²/person (e.g., France, Portugal, Belgium, Sweden, Spain) or even 300 m²/person (e.g., the USA) [153]. Urbanized areas with many impermeable surfaces are not only an aesthetic problem for the city that affects the quality of life of its inhabitants but also an environmental problem. Stormwater that could supply groundwater is directed directly from paved areas to storm sewers. In some situations, it is not possible to give up paved surfaces in cities, but one can consider replacing them with permeable pavements. This solution is well suited for urban areas with high density and limited space (i.e., highly urban and ultra-urban environments). Porous pavements can additionally purify stormwater by removing suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), heavy metals, and process oils compared with typical impermeable pavements [79,154,155]. These systems can also reduce runoff temperatures by 2 °C to 4 °C compared with traditional pavements [154].

Table 3. Comparison of various parameters for conventional and permeable pavements [154].

	Permeable Pavements	Conventional Surface
Water Retention	High	Very low
Structural Capacity	Low	High
Vertical Permeability	High	Very low
Pavement Pollution Level	Low	High
Pavement Surface Overflow	Low	High
Groundwater Resource	Yes	No
Noise Level	Low	High
Urban Heat Island	Medium	High
Vegetation Growth	Supports	Does not support
Reduction in Pollution from Stormwater	Yes	No
Microplastic Removal	Yes > 90%	No
Investment Cost	Medium/high	Low

compares various parameters for conventional and permeable pavements.

Permeable pavements can be divided into three groups: porous asphalt (1), permeable concrete (2), and permeable interlocking concrete pavers (PICP) (3) [79]. Porous asphalt and permeable concrete are traditional versions, but they only have a reduced amount of sand and fine particles to provide greater porosity and infiltration. PICP consists of paving stones with small holes between permeable joints containing highly permeable, fine-grained aggregates [156]. Due to their construction, permeable surfaces can be divided into drainage (DP), semi-permeable (SP), and fully permeable (FB). In the case of drainage surfaces, only the upper layer, the so-called pavement surface, is permeable, under which there is an impermeable sub-base. This solution is used on elevated roads or motorways. SP solutions are used to construct low-load highways or parking lots in which the upper layer and the sub-base are permeable. In the case of fully permeable surfaces (FB), stormwater permeates the entire structure. These systems are most often used to build sidewalks or bicycle paths [79]. It has been shown that permeable surfaces significantly improve the quality of stormwater; however, in the case of removing microplastics from them, there are still many research gaps resulting from the small number of studies conducted in this area. The fate of MP particles on the road surface is still unknown and has not been described. As stated by Rasmussen et al. [157], we still do not know whether MP mainly moves into the pores of the asphalt or is thrown out of the road surface by car traffic. We also do not know the answer to the question of how deeply MPs migrate into the individual layers of permeable surfaces [125,126,127,157,158]. Garcia-Haba et al. (2024) [127] showed that MP particles tend to accumulate mainly in the upper layer of the permeable surface (MP-fragments) and in the geotextile layers (MP-fibers). In their studies, they emphasized that geotextiles play a significant role in capturing microplastics, which prevents the penetration of stormwater into natural water bodies. They showed that the permeable surfaces of two geotextiles exhibited higher microplastic retention efficiency than those with one layer of geotextile. They found that the efficiency of capturing microplastic particles ranged from 89% to 99.6%. A similar degree of removal (>90%) of polyethylene (PE) and polyethylene terephthalate (PET) particles from stormwater was demonstrated in the studies of Kong et al. in 2025 [126]. They also showed that the density of MP particles had a significant impact on the migration/transport of their permeable surfaces. Low-density PE particles were retained more effectively than high-density PET particles, which showed greater mobility. They also observed that the efficiency of both PE and PET removal decreased with increasing rainfall intensity.

3.2.4. Influence of the Shape of MPs on Their Removal Using Green Infrastructure

As already mentioned, the shape of microplastics has a key impact on their transport and how they are removed from stormwater. Therefore, understanding the dominant shape of microplastics is essential for designing effective strategies for their removal [90,125]. Solutions such as bioretention systems or CWs can be optimized for specific shapes or sizes of MPs, increasing their effectiveness [87,131]. According to reports, the dominant types of MPs in stormwater are those in the shape of fragments, pellets, and fibers [90,144]. However, because fibers tend to have low deposition rates, their poor retention is noted in bioretention systems [158]. Their deposition rate is even 76% lower than that of spherical particles, which is why their transport over longer distances is observed. In bioretention cells, spherical MPs and fragments are generally more easily retained. This is associated with high deposition rates. Their deposition in the first 2 cm of the soil layer is also observed [144].

In CWs, their shape plays a key role in MP retention. For example, microfibers have a greater tendency to adhere to plants, while fragments are more likely to settle in the filter medium. Fibers are also lighter and more buoyant than fragments, so they can be suspended longer, reducing their deposition efficiency [141,159]. Their elongated shape is also potentially more challenging to capture compared with more compact shapes-fragments [145]. It has been observed that small MPs < 0.1 mm in the shape of a fragment are highly retained (from 89% to even 99.6%) on permeable surfaces. Due to their irregular shape, they tend to accumulate/remain on the surface and in the inner layer of the geotextile of permeable pavements [121].

3.2.5. A “Bottleneck” in the Use of Green Infrastructure

The presence of microplastics in bioretention systems and artificial wetlands may, however, contribute to several problems that limit their long-term effectiveness. Although these systems effectively retain microplastics (especially polypropylene and polyethylene), these particles can migrate to a depth of 20–30 cm, accumulating in the filter layer. Consequently, this can lead to several unfavorable phenomena. Firstly, it may reduce hydraulic permeability (clogging of the pores of the filter medium by MPs), decrease infiltration, increase surface runoff, and necessitate more frequent system maintenance. MP-PVC can also increase the pH of the filter layer. In artificial wetlands, where sediments are not regularly removed, this effect can also accumulate, leading to significant deterioration in the system’s performance [159,160,161]. Moreover, it has been demonstrated that the accumulation of MPs in CWs also contributes to the disturbance of nitrogen metabolism by reducing the activity of enzymes and the number of microorganisms responsible for the biological removal processes. A decrease in the number of nitrifying bacteria (Nitrospira) and an increase in the share of denitrifying bacteria (Dechloromonas, Hydrogenophaga) are observed, which disrupts the nitrogen cycle and may limit the removal of NH₄⁺-N and TN. On the other hand, an increase in the efficiency of NO₃⁻-N removal is also observed, which may be related to the intensification of denitrification processes [161]. Vegetation and macroinvertebrates play an essential role in the retention of MPs, but they are also exposed to the adverse effects of their presence. Due to the small size of the particles, they can be adsorbed or assimilated by plants and accumulate in their cells, which may consequently lead to difficulties in their uptake of water and nutrients. This may, therefore, affect plant growth disturbance (biomass decrease by up to 50%) and photosynthesis [159,160,162]. MPs also stimulate the production of reactive oxygen species in plants in amounts exceeding their scavenging capacity, which can lead to oxidative stress in plant cells [100,159]. MPs can also bind heavy metals, modifying their mobility and bioavailability, especially in the surface layer of CW systems, where organic matter accumulates [152,162]. In addition, harmful volatile organic compounds can be released during the degradation of microplastics in CWs. Therefore, although CWs are a green and effective strategy for microplastic removal and capture, there are still challenges in optimizing their processes and monitoring their effectiveness [163].

The effectiveness, cost-effectiveness, and investment and operating costs of each technology are key indicators that have a decisive influence on the selection of a specific stormwater management solution. Whether a given technology is used in each location may, in practice, mean that even ecologically beneficial solutions, such as green infrastructure systems, will not be implemented on a large scale if they prove unprofitable or difficult to maintain at a given level of effectiveness. Conducting a comprehensive assessment of the cost-effectiveness of green infrastructure is generally difficult due to the lack of consistent cost data. When it comes to the cost-effectiveness of green solutions, it is essential to remember that, compared with traditional solutions (gray infrastructure), they provide numerous benefits simultaneously. However, trying to capture these multi-faceted benefits using a traditional cost-effectiveness key may risk underestimating their total, not always tangible, value [164]. It is estimated that the cost of installing permeable surfaces varies depending on the material used and complexity, from USD 8 to USD 40 (per square foot). However, it is essential to note that the initial investment is typically 20–30% higher than for conventional solutions. However, the annual maintenance costs are surprisingly affordable (from USD 0.05 to USD 0.37 per square foot). With proper maintenance (on average annual desilting), these solutions can last from 20 to even 60 years, depending on the materials used [165]. According to studies, the costs of permeable surfaces can be significantly reduced by using recycled materials such as rice husk ash (RHA) and recycled asphalt paving (RAP). It has been estimated that the use of 100% RAP and 20% RHA can reduce costs compared with conventional methods by about 40% [166]. When comparing the two solutions, which are construction wetland and bio-retention cells, according to the US EPA, the cost of building a CW is about 25% more expensive. The cost of building a standard bio-retention bioremediation system ranges from USD 15 to USD 400 per m². In the case of these solutions, it is essential to consider the costs associated with routine work, such as weeding, inspections, replanting, or replacing mulch (depending on the activity, which occurs times a year). It is estimated that well-maintained chambers remain effective for 20–25 years, while neglected ones lose their efficiency after 5–10 years [167,168]. When analyzing the costs of these solutions, the profitability of their use increases significantly when non-economic benefits, such as biodiversity, improved air quality, aesthetic considerations, and stormwater management, are considered.

4. Conclusions

As presented in the article, a change in the approach to stormwater management in cities’—from the concept of a “water-supplied city” to the idea of a “water-sensitive city”—green infrastructure can have a beneficial effect; in addition to sustainable stormwater management, it also leads to the effective removal of microplastics from it.

As shown by numerous publications, in the case of green infrastructure, we observe the phenomenon of retention of microplastic particles in the filter medium (including soil, vegetation, or even living organisms) rather than their removal from the system. The effectiveness of MP retention is influenced by many factors, such as the design of the system, the filter materials used, and the presence of vegetation. The specific mechanisms by which MPs are transported and filtered in bioretention systems or CWs remain unclear. Further research is needed to understand them and, to a greater extent, influence the possibility of capturing MPs of a specific shape or size. Today, there are still many gaps and research areas regarding, among others, the behavior of MP particles in individual layers of bioretention systems and the selection of the most appropriate vegetation (including an indication of the root system or analysis of particle behavior in the rhizosphere area). According to the literature review, it was found that there is also another research gap—a small number of studies on the possibility of removing MP particles below 25 µm in size. Most studies concern particles with sizes from 100 µm to 5000 µm [89,133].

Of course, MP particles undergo fragmentation and migrate deeper into the filling; they undergo partial degradation. This is possible thanks to the activity of soil microorganisms, especially those located near plant roots (MP particles become their source of carbon) [69]. When thinking about designing green systems not only as an element of rainwater management but also as a method of purifying them from MP microparticles, it is important to use such supporting solutions (including additives in the form of biochar or other sorbents) to prevent their leaching, e.g., during wet periods.