An overview of the occurrence and distribution of plastics in wastewater treatment plants and the necessity of developing up-to-date management strategies

Abstract

Although studies suggest that wastewater treatment plants (WWTPs) are the main pathway for plastics into receiving waters, studies on the origin and fate of plastics entering WWTPs are imprecise and largely unexplored. The analysis of plastics in samples from WWTPs is also a relatively young and growing field compared with the marine environment. Furthermore, recent studies have shown that plastics are not uniformly distributed in WWTPs due to environmental factors and the inherent properties of plastics. Accordingly, this review article attempts to describe the current state of knowledge on plastic pollution in WWTPs and identify future research areas. In particular, this study describes the sources of plastics entering WWTPs and the analytical techniques used for the occurrence and properties of plastics in WWTPs. It also defines the role of these plastics as a possible source of microplastics and discusses the problems they can cause in WWTPs. The factors that can influence the variations in the number of plastics are defined. Furthermore, the policy needs for managing plastic pollution as a contribution to achieving the United Nations Sustainable Development Goals are assessed.

Impact statement

Plastic pollution is a global environmental problem that is becoming increasingly important. Wastewater treatment plants (WWTPs) are a significant source of plastic pollution as they receive a large amount of plastic waste from households and businesses. However, the literature on plastics entering WWTPs and their fate is insufficient and largely unexplored. This review article attempts to describe the current state of knowledge on plastic pollution in WWTPs and identify future research areas.

Introduction

Environmental pollution through chemicals is a global concern, specifically for aquatic ecosystems, threatening the water supplies and food production as they pose significant environmental risks due to their novelty or lack of information about their fate (Bashir et al., 2020). Even those chemicals that have been known for some time may only recently be identified as potentially hazardous to the environment (Bhat et al., 2022). Microplastics (MPs) classify under this category, with recorded data showing increasing concern, particularly in terms of pollution levels in aquatic systems (Andrady, 2011; Twiss, 2016; Blettler et al., 2018). These pollutants are chemicals with no legal status, and their effects on human health or the environment are unknown, according to government-related organizations (Deblonde et al., 2011; Yaşar et al., 2013). Plastic debris poses a particular threat because it is resistant to degradation processes, leading to ubiquitous accumulation in the environment, and because it is resistant to corrosion and damage from various factors (Hu et al., 2019). Furthermore, earlier studies also show that as the volume of this waste in water bodies increases, the amount of solar heat energy trapped in the water per unit volume decreases, resulting in additional energy escaping to the nearby environment and affecting global warming (Ford et al., 2022).

Plastic particles accumulate in wastewater treatment plants (WWTPs) from various sources such as domestic sewage, industrial effluents, rainwater and landfills (Okoffo et al., 2019). Therefore, they have been identified as major sources of plastic release into the environment. Due to primary MPs (PMPs) added to cosmetics and personal care products, and secondary MPs (SMPs) formed during the degradation of synthetic plastics after washing, MPs can be detected in wastewater (Lares et al., 2018). During rainy weather, urban storm water, which may contain MPs from traffic, airborne and synthetic degradation, enters the sewer system either independently or together with street runoff (Sugiura et al., 2021).

MPs pose the problems mentioned previously. Therefore, it is crucial to understand their likely sources and pathways in order to reduce their negative impacts on biotic and abiotic habitats. In addition to MPs, the presence of nanoplastics (NPs) resulting from fragmentation of MP in WWTPs also raises serious questions due to their physicochemical properties and potential damage to ecosystems (Twiss, 2016; Ali et al., 2021). Despite the growing awareness of the scale, nature and impact of MP pollution, there is still not enough information on how plastics enter WWTPs and what happens to them (Rasmussen et al., 2021). Therefore, the purpose of this study is to identify research gaps and provide a thorough overview of the current state of knowledge on plastic pollution in WWTPs. The study draws attention to the lack of knowledge about the sources of plastic inputs into WWTPs and emphasizes the need for further thorough research in this area. Additionally, the study investigates the function of plastics as a potential source of MPs and the difficulties they pose in wastewater treatment, as well as the analytical methods used to assess the presence and properties of plastics in WWTPs. The variables that affect the fluctuation of plastic content in WWTPs are being studied in order to gain a better understanding of the complicated dynamics of plastic pollution in these systems. In addition, the need for policies to effectively manage plastic pollution in line with the United Nations Sustainable Development Goals (SDGs) was assessed.

WWTPs as a potential source of plastic pollution in the environment

WWTPs are designed to effectively remove essential contaminants and purify water by removing debris, organic compounds and inorganic pollutants. Maintaining quality standards for wastewater from urban areas has been a critical aspect of wastewater management. However, the current treatment plants may not provide an adequate solution for the complete removal of plastics from domestic wastewater.

WWTPs usually involves several treatment steps, including screening, sedimentation, flocculation and aeration, which together contribute to the removal of various pollutants, including macroplastics (MaPs) and MPs (Ziajahromi et al., 2021; Keerthana Devi et al., 2022). The initial step of the treatment process usually involves screening of the influent wastewater to remove larger plastic debris, primarily MaPs. Subsequently, in the secondary treatment stage, advanced oxidation tanks ensure that dissolved organic wastes, including MPs, are eliminated through various mechanisms. To ensure the highest possible water quality, tertiary treatment protocols are often used in WWTPs following secondary treatment. Advanced treatment methods, such as activated sand filters, biofilm reactors and membrane bioreactors, are often used as part of tertiary treatment to provide additional purification and removal of MPs before the treated wastewater is discharged into nearby rivers or water bodies (Mahon et al., 2017). Typical processes of WWTP and indicated sources of plastics are schematically given in Figure 1.

Figure 1. Typical processes of a tertiary WWTP (indicated sources of plastics in WWTP, and primary, secondary and tertiary processes).

Nevertheless, it has been found that the efficacy of treatment varies depending on the specific design and operating parameters of each WWTP. In particular, the accumulation of plastics in treatment units such as sedimentation tanks and pumps poses operational challenges, leading to increased maintenance and reduced efficiency. In addition, the accumulation of plastics in aeration tanks hinders oxygen transport and thus affects the performance of treatment processes (Rasmussen et al., 2021).

According to Karapanagioti (2017), there are two main pathways by which plastics can enter a WWTP. The first is direct exposure, which occurs when people intentionally or accidentally flush solid waste down toilets or sinks (Mattsson et al., 2015). The second is indirect introduction into combined sewer systems, where the sewer system carries both stormwater and wastewater (Di Nunno et al., 2021). In addition, WWTPs serving smaller towns and suburban areas are more likely to contain certain types of MaPs such as cotton buds, as well as other wastes such as condoms, wet wipes, sanitary pads and baby wipes (Alda-Vidal et al., 2020; Besley and Cassidy, 2022; Köklü et al., 2023). This suggests that people in urban areas are more environmentally aware and active than people in suburban areas. This difference could be related to a greater awareness of the functions of urban WWTPs and a greater commitment, sensitivity and responsibility for environmental protection (Mourgkogiannis et al., 2018). Items such as cotton buds, plastic caps and non-plastics such as condoms and baby wipes are common among the plastics found in various WWTPs (Besley and Cassidy, 2022). Due to the difficulty in identifying tiny plastic waste, these smaller plastics can pollute the environment in overflow situations when wastewater is discharged into water bodies (Akarsu et al., 2023).

Both MPs and MaPs can also enter wastewater directly, for example, when products containing plastics are washed into wastewater (e.g., textile fibers released during laundry, microbeads in consumer goods, sanitary products or cotton buds), or indirectly via the combined sewer system from street debris and litter (Nunno et al., 2021). Although an average of 70% of wastewater is treated in high-income countries, only 20% of wastewater generated is treated worldwide (Duis and Coors, 2016). Furthermore, during periods of heavy rainfall or snowfall, overflows in combined sewers, together with the lack of functioning treatment plants, often result in MPs into the environment through the inefficient treatment of wastewater (Winton et al., 2020).

Some WWTPs should be known for the fact that their effluents flow directly into water bodies (Sun et al., 2023). The significant contribution of WWTP effluents to plastic pollution has been underlined by studies that estimate the daily release of millions or perhaps billions of plastic particles into receiving waters from WWTPs (Carr et al., 2016; Kalčíková et al., 2017; Leslie et al., 2017; Gündoğdu et al., 2018). For instance, one study found that a WWTP with a capacity of about 150,000 m³.d⁻¹ discharges about 87.6 million MPs into the Mersin Bay (Akarsu et al., 2020). According to another study, more than 80% of the 210 trillion microbeads that enter the waters of mainland China each year originate from WWTP effluent (Cheung and Fok, 2017). Similarly, it is estimated that between 50,000 and 15 million plastic particles per day enter freshwater from some sewage treatment plants in the United States (Mason et al., 2016).

The transfer of plastic pollutants from aquatic to terrestrial ecosystems is primarily attributed to the use and disposal of sewage sludge from wastewater treatment. Sewage sludge, which is known to contain high levels of organic matter, nutrients and contaminants such as plastics, has been found to carry an average of 14,750 particles per kilogram (Ragoobur et al., 2021). Initial studies of plastic pollution from sewage sludge application have shown elevated concentration of MP in topsoil compared to non-fertilized soils (Corradini et al., 2019; Tagg et al., 2022; Hassan et al., 2023).

Restrictions such as the German regulation limiting the application of sewage sludge to 5 tons per hectare every 3 years have led to a decline in agricultural use, a trend that is expected to continue if regulations are tightened. Nonetheless, significant amounts of sewage sludge and associated plastics have been intentionally applied to soils in the past. Notably, MPs and NPs pose a significant threat to soil biota by affecting plant growth, organism reproduction and soil biodiversity (Hale et al., 2020). As soil is an important habitat for terrestrial organisms, its fauna is increasingly affected by the ecotoxicological impacts of MPs and NPs, which are ingested by soil invertebrates and poultry and potentially serve as entry points for humans and other animals (Cox et al., 2019). The leaching of additives such as bisphenol A and phthalates from MPs and NPs disrupts the endocrine system of vertebrates and has estrogenic effects (Zhang and Chen, 2020). Several factors, including sunlight, oxygen content, temperature, soil microorganisms and terrestrial biota, contribute to the rate of degradation of plastic waste in the upper soil layer (Wong et al., 2020).

Sources and pathways of plastic waste in wastewater treatment plants

MPs can be divided into two categories depending on their origin: PMPs and SMPs. PMPs, which account for an estimated 19–31% of MPs in the oceans, are plastics that enter the environment directly in the form of small particles. Examples of PMPs are plastics from the manufacture, use or maintenance of MaPs objects such as personal care products (facial cleansers, toothpaste, etc.), tire wear from driving and synthetic textile products from washing (Boucher and Friot, 2017).

On the other hand, SMPs, which is estimated to account for 69–81% of MPs in the oceans (Evangeliou et al., 2022), is formed by the degradation of MaP- and mesoplastic into smaller dimensions. SMPs can enter the marine environment from both terrestrial and marine sources. A major terrestrial source of SMPs is WWTPs, which receive a variety of primary plastic wastes, such as wet wipes, plastic gloves, bandages, diapers, feminine hygiene products and plastic medical consumables (Figure 2). These items are often improperly disposed of in toilets, where they can cause blockages, clogs and overflows and disrupt biological treatment processes in WWTPs. In addition, these items can cause unpleasant odors in sewage systems, pumping stations and treatment plant pipes. Approximately, 75–90% of plastic in the marine environment comes from land-based sources transported via rivers (1.4 Mt/year) and coastal areas (5.1 Mt/year), whereas 10–25% comes from activities such as commercial fishing, maritime transport and sea travel (Belzagui Elder, 2017). Improper disposal of plastic waste is a major contributor to the accumulation of SMPs in the marine environment (Pandey et al., 2022).

Figure 2. Visual data depicting the MaPs wastes found within a WWTP (with Prof. Dr. Mustafa Öztürk’s permission and consent).

The presence of MPs in the influent and effluent of WWTP has been reported in several countries, including the United States (Mason et al., 2016; Ridall et al., 2023), the Netherlands (Leslie et al., 2017), Germany (Mintenig et al., 2017; Barkmann-Metaj et al., 2023), England (Murphy et al., 2016), Sweden (Magnusson and Noren, 2014), Australia (Ziajahromi et al., 2017) and Turkey (Akarsu et al., 2020; Vardar et al., 2021; Koyuncuoğlu and Erden, 2023). Despite the high efficiency of WWTPs in removing of MPs, significant amounts of MPs can still enter into receiving waters. For example, one WWTP serving 650,000 residents was found to have an MP removal efficiency of 98.41%, yet an estimated 65 million MPs were discharged daily (Murphy et al., 2016). Similarly, a study conducted at 17 WWTPs in the United States reported releases of over 4 million MPs per plant per day (Mason et al., 2016). Another example is a WWTP in northern Italy with a population of about 1,200,000 that had a MP removal efficiency of 84%, but still released an estimated 160,000,000 MPs per day (Magni et al., 2019). In a study by Vardar et al., (2021), it was predicted that Ambarlı Advanced Biological WWTP contributes approximately 2,934 × 10⁶ MPs per day into the Sea of Marmara. These results highlight that despite their treatment efforts, WWTPs can be a significant source of MP pollution, as they continuously discharge large amounts of effluent into the aquatic environment (Bozdaş et al., 2020).

Sampling, sample pre-treatment and analytical methodologies

Sampling and description of WWTP

MPs in WWTPs show heterogeneous distribution in both effluent and sludge (Gao et al., 2023). Although there are no standardized methods for sampling MPs in WWTPs, several successful approaches have been used. These include non-discrete techniques such as continuous pumping combined with in situ filtration and discrete sampling methods such as manual sampling or the use of an auto-sampler (Üstün et al., 2022). These different sampling methods allow researchers to effectively detect MP and investigate its presence in different parts of the WWTP (Figure 3).

Figure 3. Classification, measurement method and some typical apparatuses (adapted from Ye et al., 2022).

Among them, grab sampling is the most common method for collecting water, sediment or other environmental matrices to quantify the presence and concentration of MPs (Sönmez et al., 2023). There are a number of different methods for conducting grab sampling, but the most common method is to use a container to collect a set amount of samples (Green et al., 2018). Various sample volumes have been reported for collecting samples from organically contaminated wastewater, ranging from 0.1 L to 50 L (Michielssen et al., 2016; Leslie et al., 2017; Mintenig et al., 2017). These samples are usually taken from both the influent and the biological unit of the WWTP (Table 1).

Table 1. A summary of commonly applied sampling approaches (adapted from Gao et al., 2023)

Sample preparation

MPs found in the environmental matrix have the ability to absorb various pollutants and may also have altered density due to biofilm layers covering its surface (Tu et al., 2020). Therefore, it is important to wash the collected particles several times with distilled water before determining the morphological characteristics and performing the chemical structure analysis. This washing procedure effectively removes potential contaminants and impurities that could interfere with the subsequent analysis. In some cases, the MaPs and MPs are analyzed directly without undergoing additional processes to detect contaminants or organisms that may be adsorbed on their surface. In the case of water matrices, the separation of MPs is usually done with a series of screens or filters with different mesh or pore sizes through which the collected wastewater is passed (Costa et al., 2021). For example, Ziajahromi et al. (2017) developed a method using a large-volume sampling device with multiple mesh screens to effectively separate various size of MPs from wastewater. This method showed high efficiency with a retention efficiency of 92% for the 25 μm mesh screen and 99% for the 500 μm mesh screen. As an alternative, Besley et al. (2017) proposed a slightly different approach using a fully saturated NaCl solution in combination with filtration for MP extraction. This method allows the quantification of MPs in the size range of 0.3–5 mm.

Filtration is also used to separate them from the aqueous environment to determine the abundance and characterize the morphological properties of MPs. Various filter paper pore sizes have been used in different studies, including 0.2 μm (alumina oxide), 0.45 μm (GF/C), 1.2 μm (GF/C) and 5 μm (silicone, silver) (Robertson, 2018). Among the filter options, glass fiber filters have been widely used in MP research (Hanvey et al., 2017). These filters are commonly selected due to their suitability for capturing MPs and their compatibility with subsequent analysis techniques.

On the other hand, density fractionation methods are widely used to extract MPs from a complex soil matrix. The methods are based on the principle of combining the sample with a saturated salt solution of known density, followed by separation of the MP from its environment after a certain retention time (Sönmez et al., 2022). Retention times for MPs can vary considerably, ranging from 5 min to 48 h (Fries et al., 2013). After the specified retention time, the MPs are separated from the supernatant of the separating funnel so that they are available for subsequent analysis. Sodium chloride (NaCl), sodium iodide (NaI) and zinc chloride (ZnCl₂) are commonly used salts in the density-based separation process (Nabi et al., 2022). Among them, NaCl with a density of 1.2 g.L⁻¹ is preferred because of its affordability and nontoxic properties (Li et al., 2020). The saturated salt solution with NaCl creates a density gradient that floats the MP particles, which makes the separation process. Using the density separation method with NaCl provides a cost-effective and safe approach to isolate MP based on their density properties (Parashar and Hait, 2023).

Following the separation process, a clean-up procedure to remove microbes and various organic deposits is frequently used (Li et al., 2020). In the literature, various approaches including peroxide digestion (H₂O₂), alkaline digestion (NaOH) and acid digestion (HNO₃ and H₂SO₄) have been used to degrade organic matter in wastewater samples and sewage sludge (Sönmez et al., 2022). Sewage sludge is a more challenging sample than other environmental matrices, especially sediment, as it contains a mixture of organic material from human waste, inorganic solids, food waste, trace chemicals, heavy metals, microbes, pharmaceuticals and other micropollutants (Zhang and Chen, 2020; Gao et al., 2023). Several extraction techniques including oxidative digestion (Bretas Alvim et al., 2020; Cunsolo et al., 2021), alkaline treatment (Mintenig et al., 2017) and acid-based digestion (Hernández-Arenas et al., 2021) have been proposed to remove organics efficiently with minimal impact on MPs. The applicability of Fenton’s reagent in the extraction of plastics from environmental matrices such as sludge and soil in combination with density separation has been confirmed (Hurley et al., 2018). Oxidation with hydrogen peroxide coupled with density separation has been shown to reduce chemical consumption, sample pretreatment time and cost by adjusting the reaction temperature (Sujathan et al., 2017; Zhang and Chen, 2020). For samples with high organic content, a mixture of 30% H₂O₂ and 0.05 M Fe(II), known as Fenton reagent, is commonly used. The temperature is usually increased to 50°C to promote decomposition of the organic compounds during the oxidation process (Hurley et al., 2018).

Enzymatic treatments with enzymes such as amylase, lipase, chitinase, proteinase and fibers have also been used to remove organic matter from MPs (Cole et al., 2014; Parashar and Hait, 2023). However, it should be noted that enzymatic treatments can be costly and may have limitations in terms of MP processing efficiency (Zhu and Wang, 2020). In addition, acid and alkali treatments can be used to remove organic matter. However, care should be taken as they can potentially damage certain MP species that are sensitive to pH changes (Sönmez et al., 2022). Among the various isolation methods, the peroxide oxidation is considered effective for MP extraction and offers advantages over other methods.

Analytical methods

Previous studies on MaPs and MPs have performed abundance, enumeration and identification using stereomicroscopy and visual identification based on their physical characterization of type, morphology and color (Derraik, 2002; Andrady, 2011) (Figure 3). However, with increasing number of studies, this method has been found to have more drawbacks. There is a risk of up to 70% in the visual identification of possible plastic particles under the stereomicroscope (Hidalgo-Ruz et al., 2012). To overcome this problem, the use of selected dyes to determine the abundance and types of MPs has become popular (Hurley et al., 2018). However, it is not feasible to find a single dye that is suitable for all types of polymers. While staining remains an effective technique for quantifying and distinguishing different types of MPs, it is not sufficient to determine the colors of MPs, as shown by the research of Tu et al. (2020). When a thorough knowledge of the surface structure of MPs is required, for example, in a study of weathering mechanisms or confirmation of plastisphere potential, the limitations of optical microscopy become apparent. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) is a useful technique in such situations (Huang et al., 2023). SEM is a very useful tool for the morphological analysis of MPs, as shown by Wagner et al. (2017).

By comparing the characteristic chemical fingerprints of novel particles with a database of spectra of known materials, vibrational spectroscopy is a useful method for detecting unknown particles (Vianello et al., 2013). To identify possible MPs, this method first examines the particles visually. If MP are found, this is then confirmed using methods such as Raman spectroscopy or attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) (Yang et al., 2021; De Frond et al., 2023). According to Chalmers (2000), the spectrum of a polymer in ATR-FTIR represents the link between the observed infrared intensity and the wavelength of light. However, it is important to note that FT-IR can identify polar groups more accurately (Silva et al., 2018). While manual processing of particles larger than 500 μm is relatively easy, this method becomes increasingly impractical as particle size decreases (Sönmez et al., 2022).

On the other hand, Raman spectroscopy offers several advantages over FTIR, such as better resolution and response to nonpolar symmetric bonds, by exposing the sample to a monochromatic light source, typically a laser (Imhof et al., 2016; Ivleva et al., 2017; Yang et al., 2021). By exciting the molecules with a laser of a single wavelength, the interaction between the radiation and the sample is detected (Li et al., 2018a). The spatial resolution of the Raman microscope improves as the excitation wavelength of the laser decreases (Anger et al., 2018). As another vibrational spectroscopic technique in the analysis of MPs, Raman spectroscopy decodes the molecular vibrations of MPs, provides their vibrational spectrum and gives information about the different components present in the sample (Ribeiro-Claro et al., 2017). Raman spectroscopy allows the characterization of MPs ranging in size from 1 to 20 μm, with no limitations on sample size and thickness (Li et al., 2018a). However, interference from the presence of microorganisms or organic or inorganic contaminants can cause interference with the fluorescence signal (Li et al., 2018a). Despite its advantages, Raman spectroscopy has some known disadvantages, including a long processing time, potential polymer degradation and interference from fluorescence (Parashar and Hait, 2023).

Gas chromatography is widely recognized as one of the most popular and efficient chromatographic methods for the characterization of MPs, despite its destructive nature (Gniadek and Dąbrowska, 2019). Gas chromatography allows the separation and identification of volatile organic compounds (VOCs) released from MPs. Pyrolysis gas chromatography–mass spectrometry (Pyr-GC–MS) and thermal extraction–desorption gas chromatography–mass spectrometry (TED-GC–MS) are two promising approaches to obtain accurate information on polymers, additives and contaminants (Pipkin et al., 2021). Pyr-GC–MS can analyze the chemical composition and structural properties of high molecular weight polymers. This method provides a detailed description of the sample as well as an accurate assessment of its chemical properties (Tianniam et al., 2010). TED-GC–MS, on the other hand, is a more advanced thermal analysis method that combines thermogravimetric analytical solid-phase extraction (TGA-SPE) with thermal desorption gas chromatography–mass spectrometry (TDS-GC–MS) (Dümichen et al., 2015).

Occurrence and characteristics of plastics in WWTPs influents

In numerous investigations, plastics have been found in both influent and effluent samples from WWTPs. Reported concentrations of plastic particles in influent samples ranged from 0.0009 to 15,000 particles/L, while in effluent samples they ranged from 0.004 to 1,800 particles/L (Table 2). The use of different sampling methods, sample pretreatment techniques and analytical methods could be responsible for these differences in particle counts (Sönmez et al., 2022). Most of the plastic particles found were larger than 500 μm in the influent samples, while most of them were smaller than 500 μm in the effluent samples (Bayo et al., 2020; Üstün et al., 2022). However, in some studies, plastic particles smaller than 100 μm were also found in effluent samples (Jiang et al., 2022). In particular, with regard to samples with a mesh size of 10 μm, which mainly contained millimeter-sized debris, there are not many studies specifically address the presence of nano-sized plastics in wastewater samples (Okoffo et al., 2019). This discrepancy can be explained by a lack of information or evidence on the presence of nanoscale plastics in wastewater, as well as inadequate sampling procedures at WWTPs (Lehner et al., 2019). However, due to the environmental impacts associated with nano-sized plastics, it is essential to consider them in future studies focusing on WWTPs.

Table 2. MPs sampling, sample processing, occurrence and identification methods for wastewater and sludge in various WWTPs

It has been shown that when a storm sewer is connected to a WWTP, the amount of plastics in the influent of the treatment plant generally increases (Sun et al., 2019). This leads to an increase in the amount of plastics associated with the wastewater system due to the release of plastics from brake and tire wear, which eventually enter the sewer system via road runoff (Mason et al., 2016; Michielssen et al., 2016; Wagner et al., 2018). Plastics in the influent not only affect the technologies and processes used in the WWTP, but also the amount of plastics that end up in the effluent (Sun et al., 2019). It should be mentioned that some wastewater pipes are made of PVC polymers, the abrasion of which can increase the total amount of plastics in the WWTP (Xu et al., 2019a).

Plastic particles have been identified in the influent and effluent of WWTP as spherical beads, microbeads, pellets, fibers, particles, flakes, films, fragments, foams, paint chips, nurdles, foils, spheres, sheets, granules, lines and irregular shapes (Hamidian et al., 2021). Fibers have been found to be the most common form with an average of 65.4% of the wastewater samples. Irregular pieces came second with an average of 42.6% (Lv et al., 2019). According to the study by Mason et al. (2016), the most common plastic particles in 17 wastewater samples from effluent treatment plants were microfibers (59%), followed by fragments (33%), films (5%), foams (2%) and pellets (1%). These results indicate that the main sources of plastics entering WWTPs are secondary plastics that have been degraded and synthetic fibers (Kang et al., 2018). There is a possibility that natural fibers such as cotton have been misrecorded or classified as synthetic fibers during identification and quantification, and it is important to consider that current sampling and analytical techniques may not be sufficient to fully capture and identify plastics in effluents (Talvitie et al., 2017b; Sun et al., 2019).

The predominant polymer types in the influent and effluent of the WWTP were polyethersulfone (PES, ca. 30–90%), polyethylene (PE, ca. 6–60%), polyethylene terephthalate (PET, ca. 5–40%), polyamide (PA, ca. 5–35%), acrylate (ca. 4–31%), polypropylene (PP, ca. 4–27%), alkyds (ca. 4–25%), polystyrene (PS, ca. 4–25%), polyurethane (PU, ca. 3–25%), polyvinyl acetate (PVA, ca. 3–20%), polylactic acid (PLA, ca. 2–18%) and polytetrafluoroethylene (PTFE, ca. 2–8%) (Hurley et al., 2018; Long et al., 2019; Wolff et al., 2019). Since PES, PET and PA are commonly used in synthetic garments, laundry effluents could be the reason for their presence in wastewater. PE, on the other hand, the most commonly produced plastic in the world, is used in personal care products such as toothpaste, body and facial cleansers, water bottles and food packaging films (Alavian Petroody et al., 2020). A recent study in a Finnish WWTP found that MP dominate in the following order: PET > PE > PAR > PVC > PS > PP (Talvitie et al., 2017a). Another study conducted in three Australian WWTPs found that the concentrations of different types of polymers (PET, PE, PVC, PP, PS and nylon) varied, with PET and PE being the predominant polymers (Ziajahromi et al., 2017). It is crucial to focus research efforts on the identification and quantification of these polymers in influent and effluent samples, as they are known to be generated by routine human activities, even though the exact sources and pathways by which plastics enter WWTPs are not yet fully known.

Fate of plastics in WWTPs

Plastics not only accumulate in the natural environment, but also enter WWTPs via domestic and industrial wastewater discharges (Hajji et al., 2023; Ruffell et al., 2023) (Figure 4). Concentrations of MP in these environments have been shown to range from 0.0009 to 15 MPs/L. It is estimated that nearly 520,000 tons of MPs are discharged from WWTPs into freshwater rivers in Europe (Hajji et al., 2023). For these reasons, the numerous studies that have been conducted to investigate the fate of plastics in WWTPs have mainly focused on three key aspects: removal efficiency, fate of MPs and impacts on treatment processes (Carr et al., 2016; Ziajahromi et al., 2017). But the main focus, the fate of plastics in WWTPs is a complex issue that is influenced by various factors such as size, composition and treatment processes. MPs can take different pathways after their release into the environment and enter the three main environmental compartments: water, air and soil. There are many variables that influence the behavior of MP in the different environments. The physico-chemical properties of MP, such as size and density, as well as environmental conditions, such as temperature and solar radiation, play a crucial role. According to Gkika et al. (2023), the unique properties of MPs polymers have a significant impact on their durability, fate, degradation and ability to absorb or release organic pollutants.

Figure 4. MPs removal efficiency in different treatment stages.

Several studies have investigated the removal efficiency of MP at different treatment stages in different WWTPs and identified their removal mechanisms. In a study by Iyare et al. (2020), it was found that although WWTPs are not designed to remove MPs, an average removal value of 88% was achieved at secondary treatment and 94% removal efficiency at tertiary treatment. The authors also reported that most of the MPs, 72% on average, were removed during the pre- and primary treatment steps. Similarly, Parashar and Hait (2023) reported that the overall removal efficiency of MPs during primary, secondary and tertiary treatment in WWTPs ranged from 57% to 99%, 78.1% to 99.4% and 90% to 99.2%, respectively. Furthermore, Gkika et al. (2023) found that the efficiency of MP removal in primary treatment may depend on several variables. A key factor is the presence of an aerated grit chamber, which plays a role in improving removal efficiency. The exact categories of polymers present in the influent also influence removal rates. Studies in the literature have shown that removal efficiencies vary widely, ranging from 40.7% to 91.7%. The study also highlighted the influence of the different treatment steps on the relative size distribution of the MPs. Pre-treatments were found to have a significant impact on the removal of larger particles. For the secondary treatments, such as activated sludge and sedimentation, their configuration and retention time played a role in determining the removal efficiency, which ranged from 28.1% to 66.7%. These results show the importance of considering different treatment strategies and their specific configurations in order to effectively reduce MP pollution during wastewater treatment.

Numerous studies have reported that the conventional treatment process are not always effective in removing MPs larger than 500 μm resulting discharge of MPs into aquatic environments along with the effluent (Bilgin et al., 2020; Schmidt et al., 2020; Xu et al., 2021).

While typical WWTPs are capable of removing up to 90% of MPs from wastewater, it is important to consider that smaller MPs, such as the microbeads in facial cleansers and synthetic textile fibers, may escape from these treatment processes (Ziajahromi et al., 2017; Raju et al., 2020; Funck et al., 2021). This highlights the need for improved treatment steps and the application of state-of-the-art technologies to address the problem of tiny MPs escaping conventional treatment processes. Advanced wastewater treatment methods, including membrane bioreactors, are mentioned in the literature as possible options to increase effectiveness in removing small size MPs (less than 100 μm). However, research on the fate of MPs in WWTPs has shown that they can interact with the solid fraction of wastewater treatment, causing them to accumulate in the sludge (Mahon et al., 2017). If the sludge is subsequently used in agriculture or disposed of in landfills, MPs can enter terrestrial habitats (Zettler et al., 2013). However, little is known about the fate of MPs when used in agriculture (Tu et al., 2020; Casella et al., 2023). The use of sewage sludge as an additive in agriculture is considered an important source of MP in soil matrices. Similarly, MP can serve as colonization sites for specific microorganisms in both terrestrial and aquatic ecosystems (Zettler et al., 2013). Recent research has shown that MP in soil matrices have the potential to disrupt fungal community diversity (Zhang et al., 2021b). Moreover, the favorable growth of Actinobacteria and Bacteroidetes was found on polyethylene (PE) surfaces (Zhang et al., 2019, 2021b; Ren et al., 2020). Indeed, the potential transfer of MPs through the application of sewage sludge in agriculture still raises questions about the long-term consequences for soil health and crop safety (Zhang et al., 2020; Weber et al., 2022).

Possible strategies for robust management plan

The global campaign against MP pollution has gained significant momentum in recent years. Calls have been made to strengthen the capacity to manage plastic waste, particularly through increased recycling initiatives (Diggle and Walker, 2022). While effective recycling can help address the broader problem of plastic pollution, its impact on reducing MP pollution remains rather limited (Kumar et al., 2021). This study highlights the critical shortcomings of the current waste management system that allow MP to escape. Given the inability of the existing waste management system to adequately address the scale and severity of MP pollution, it is unrealistic to expect significant improvements without fundamental changes. It is unlikely that simply intensifying efforts under the existing framework will lead to significantly better results. There is therefore an urgent need to re-evaluate and adapt current waste management practices to effectively block the pathways through which MP escapes into the receiving environment.

In order to prevent ecosystems degradation, the implementation of strong legislative measures to monitor and regulate the overuse of plastics is essential (Prata et al., 2019). Effective management, recycling practices and the establishment of environmentally friendly disposal systems are crucial in the pursuit of a plastic-free environment. Developing countries have introduced extensive measures to combat the proliferation of plastics, including comprehensive bans on plastic bags and bottles and the imposition of fines for plastic violations (Gopinath et al., 2020). Over the past three decades, global efforts have led to the formulation of laws aimed at addressing the hazards and impacts of increasing plastic consumption and waste (Table 3) (Bhardwaj et al., 2020; Wen et al., 2021; Usman et al., 2022). As can be seen from the previous discussion and highlighted in Table 3, many international laws lack a comprehensive framework and the necessary global mechanisms to effectively monitor and evaluate progress toward the established goals. Although these legal frameworks often include sound governance strategies, they generally rely on individual countries to design and develop their own strategies for implementation. The effectiveness of these strategies in turn depends on a country’s political will and allocation of resources to address the problem. The United Nations (UN) has recognized the problem of plastic pollution by including it in 11 SDGs alongside SDG-14 (Walker, 2021). However, the fact that only one indicator out of a total of 247 is dedicated to the impact of plastic in the ocean is woefully inadequate given the alarming rate at which plastic pollution is increasing globally. This allocation urgently needs to be reconsidered. Moreover, the European Parliament and the Council have failed to include MPs in their directives, while the measures proposed by WHO to mitigate the problem remain out of reach in many countries (Usman et al., 2022). This underline the need for a concerted commitment to provide resources that can support research efforts and the development of practical, measurable tools to comprehensively address this pressing global problem.

Table 3. Different governance strategies to control MP pollution (adapted from Usman et al., 2022)

A comprehensive global assessment of national laws and regulations introduced by countries to restrict the production, import, use and disposal of single-use plastics and MPs, which contribute significantly to the spread of marine pollution, found that in 2018, about 60% (127 out of 192) of countries had enacted various forms of legislation concerning plastic bags (Xanthos and Walker, 2017). These laws specifically aim to regulate aspects such as production, distribution, use, trade, taxation, levies and disposal. While landfills remain the primary method of disposing of plastic waste, the gradual release of MPs and toxic additives poses a significant threat to the environment, inevitably leading to a phase-out of traditional landfill practices (Shen et al., 2022). Although incineration is an option for the disposal of plastic waste, the significant release of greenhouse gases remains a critical issue. With approximately, 79% combustible carbon per ton, this method generates an estimated 2.9 tons of CO₂ emissions, highlighting the environmental impacts associated with this approach (Hamilton et al., 2019).

In this context, recycling is widely recognized as the optimal long-term solution to address the current MP problem and ensure sustainable plastic use (Kassab et al., 2023). Reusing 1 ton of plastic waste by recycling instead of producing new materials can save approximately 130 million kilojoules of energy (Ramirez and George, 2019). However, the recycling rate for plastic waste, especially for secondary (recycled) plastics, remains low. Therefore, promoting environmentally friendly and cost-effective alternatives to plastics is essential in the long term. Researchers also need to explore methods to break down the basic components of plastics so they can be converted into new materials. Researchers have discovered a mutant enzyme capable of breaking down plastic bottles in days –a significant improvement over the centuries it takes in the oceans (Lamichhane, 2023).

In recent years, several studies have underlined the effectiveness of plastic-degrading bacteria. In particular, Ideonella sakaiensis, a member of the Ideonella genus and Comamonadaceae family, has shown that it is able to consume plastic polyethylene terephthalate (PET) as a sole source of carbon and energy, effectively contributing to plastic recycling (Yoshida et al., 2016). With the help of two successive enzymes, these bacteria can break down PET into terephthalic acid and ethylene glycol, both environmentally friendly substances (Bornscheuer, 2016). Similarly, research by Gao and Sun showed that a mixed culture of Exiguobacterium sp., Halomonas sp. and Ochrobactrum sp. degrades both PET and PE films better than individual isolates, as evidenced by observations with SEM (Gao and Sun, 2021).

Given the ubiquitous presence of plastic, there is an urgent need for more than one alternative solution that is renewable, environmentally sound and biodegradable. Unregulated and poorly managed bioplastics could potentially lead to environmental damage comparable to that of conventional plastics. It is therefore crucial that legislators set strict criteria with high standards for the classification of bioplastics to promote consumer and business confidence (Bhagwat et al., 2020). Biodegradable plastics based on cellulose and polyolefins should be actively promoted due to their cost-effectiveness, high mechanical strength and ease of decomposition in the environment (Ammala et al., 2011). Recent advances have led to the development of plant-based materials capable of replacing single-use plastics in various consumer products and a polymer film that mimics the properties of spider silk. This innovative material has comparable durability to conventional plastics and can be produced on an industrial scale from sustainable components thanks to an energy-efficient technology that fuses plant proteins into silk-like materials (Kamada et al., 2021). Most importantly, this substance is compostable at home, so no special industrial composting facilities are required. Furthermore, due to its natural composition, the material can be safely biodegraded in most natural environments, providing a promising alternative to single-use plastics and MPs on the commercial market.

Future perspectives and conclusions

Aquatic ecosystems play a crucial role in the transport and deposition of plastic waste from terrestrial storage to surface waters (Margenat et al., 2021). All recent studies consistently identify urban areas, transportation, infrastructure and WWTPs as major sources of micro-, meso- and macroplastics (van Emmerik, 2021; Cowger et al., 2022). Recent scientific articles have shown that the movement of plastics over land and rivers is influenced by human activities, flood and storm events, hydrodynamics and their combinations. Interestingly, the majority of plastics do not reach the open sea but end up on beaches, float in coastal waters or accumulate on land and in river systems (Köklü et al., 2023; Sönmez et al., 2023). To address this problem, several innovative technologies have been developed and deployed to reduce pollution from MaPs. These technologies mainly include innovative devices placed along rivers and streams to effectively collect MaPs and other litter. Prominent examples include the Ocean Cleanup Foundation’s Interceptor Project, which has been successfully implemented in Indonesia, Malaysia and the Dominican Republic (The Ocean Cleanup, 2021). The Bubble Barrier, which Waternet has installed in a number of Amsterdam canals, is another notable tool (Waternet Annual Report, 2021). This bubble barrier diverts floating litter, such as MaPs, so that it can be cleaned up on the riverbank.

Further research needs to focus on the pragmatic use of area-specific expertise on rivers, with particular attention to the collection and examination of extensive and original datasets. In addition, it is essential to identify the origins and entry points of plastic pollution and to understand the basic mechanisms of transport. This priority issue not only offers new perspectives on the production, distribution, fate and impact of plastics, but also highlights the urgent need for a thorough investigation of plastic pollution in the aquatic environment. Furthermore, there is a notable lack of comparative research between technology-based enhanced treatment strategies and conventional treatment methods (Iyare et al., 2020).

In addition, the accumulation of plastics in WWTPs can lead to operational challenges and potential environmental impacts. Further research and the development of innovative treatment technologies are needed to effectively address the challenges posed by plastics in WWTPs effectively. Understanding the fate of plastics in WWTPs is critical to developing strategies to contain their presence and minimize their potential negative impacts on the environment. The United Nations Environment Programme considers plastic pollution to be a major environmental problem that, along with climate change, is becoming a threat to biodiversity and human health. Metrics such as models or ecological footprints can be useful tools for decision-making, public engagement and policy development. Nevertheless, contingency plans should be continuously adapted and developed to consider the future of waste management and plastics (Klemeš et al., 2020). For example, to achieve the best environmental performance through recycling, it is important to improve pre-treatment methods in line with the most appropriate recycling technology for a given polymer. Some of the studies emphasize the importance of polymer quality (e.g., mixed origin and mixed materials), which affects the overall environmental performance of a technology, but does not change the performance rating of the technology (Schwarzer et al., 2022).

Effective waste management is critical to achieving the SDGs that aim to address environmental, social and economic challenges. Governments and organizations around the world have adopted waste management plans that are aligned with the SDGs and emphasize the need for sustainable waste management practices. Under the SDGs, waste management plans and regulatory improvements focus on several key areas. These include reducing waste generation through waste prevention and promoting sustainable consumption patterns. Recycling and recovery efforts aim to increase waste prevention, conserve resources and reduce greenhouse gas emissions. Above all, improving regulation plays a crucial role in creating a regulatory framework that supports sustainable waste management. By adopting comprehensive waste management policies that include waste prevention, recycling and safe disposal, countries can minimize environmental impacts, conserve resources and contribute to the achievement of the SDGs. These regulations cover the classification, collection, treatment and disposal of waste, as well as extended producer responsibility (EPR). EPR requires producers to take responsibility for the entire life cycle of their products, including the disposal of post-consumer waste.

To further advance waste management in line with the SDGs, cooperation and knowledge sharing between countries and stakeholders is crucial. Sharing best practices, innovative technologies and scientific research can improve waste management systems and promote sustainable solutions worldwide. In addition, financial incentives, capacity building and public awareness campaigns are integral components to drive behavioral change and ensure the successful implementation of waste management plans.