Removal of microplastics from wastewater: available techniques and way forward

The occurrence of a number of emerging contaminants in various environmental matrices has become a serious concern for the scientific community and policy makers across the globe. Widespread occurrence, bioaccumulation, toxicity, and persistence are the major issues accompanying these emerging contaminants (Richardson & Kimura 2017; Sarkar et al. 2021). Microplastic particles are one such contaminant that has gained attention in the recent decade. Small plastic particles, less than 5 mm in size, are termed microplastics and may be in the form of fragment, fibre, pellet, foam, and/or film (Thompson et al. 2004; Arthur et al. 2009) (Figure 1). These microplastic particles originate either through intentional manufacturing (primary microplastics) or after the breakdown of large-sized plastics (secondary microplastics) due to the action of abiotic and biotic factors in the environment. The occurrence of these microplastic particles has been reported in variety of matrices such as surface water, ground water, marine water, bottled water, soil, air, biotic organisms, food items, honey, sugar (Liebezeit & Liebezeit 2013; Barboza & Gimenez 2015; Galloway & Lewis 2016; Wright & Kelly 2017; Karthik et al. 2018; Kosuth et al. 2018; Blettler & Wantzen 2019; Conti et al. 2020). According to one estimate, approximately 14 million tonnes of microplastics are present on the ocean floor (Barrett et al. 2020). The presence of microplastics has serious repercussions for the ecosystem as well as for human health. Table 1 summarizes the impact of microplastics over ecosystems and human health.

The occurrence of microplastics (both primary and secondary) in water primarily takes place through the discharge of sewage/wastewater treatment plant effluent and surface run-off (Iyare et al. 2020). There are large numbers of industries that use (primary) microplastics for various applications such as medicines or cosmetics (Lechner & Ramler 2015; European Commission Report 2017). After the use of these products at domestic/commercial level, these primary microplastics are washed off and become part of the sewage/wastewater. Conversely, breakdown of large-sized plastic particles into smaller pieces (secondary microplastics) often takes place in landfill sites/dumping grounds/oceans, which further becomes one of the sources of microplastics in the water bodies (Kyrikou & Briassoulis 2007; Galloway & Lewis 2016). As sewage/wastewater treatment plants are not equipped for the complete removal of microplastics, the effluent released from these plants contains substantial quantities of microplastics (Amrutha & Warrier 2020). Upon mixing of this effluent with the freshwater sources, microplastics become part of the fresh/drinking water supply chain as well (Magnusson & Noren 2014; Novotna et al. 2019; Okoffo et al. 2019). For example, increase in the concentration of microplastics in the Chicago River has been reported to be due to local wastewater treatment plants' effluent (McCormick et al. 2014). It is also important to note that many components of water treatment plants and water distribution systems are usually made up of plastic materials such as high density polyethylene, polyvinyl chloride, and polypropylene (Mintenig et al. 2019) and, hence, these further contribute towards microplastic generation in the water they carry. The occurrence of microplastics in various water sources is summarized in Table 2.

These plastic particles are of concern owing to their toxicity upon ingestion/inhalation (Table 1). Furthermore, various contaminants, additives/plasticizers, microbes adsorbed onto the surface of microplastics also cause serious health effects (Ensign et al. 2012; Deng et al. 2017; Schirinzi et al. 2017; Smith et al. 2018; Prata et al. 2020). Preliminary studies have shown that microplastics may result in genotoxicity, carcinogenesis, fibrosis, exaggerated inflammatory response, oxidative stress etc. (Deng et al. 2017; Schirinzi et al. 2017). According to some studies, microplastics have the potential to cross the placental barrier as well (Grafmueller et al. 2015; Ragusa et al. 2021) which results in various reproductive and teratogenic abnormalities (Swan et al. 2005; Swan 2008; Lang et al. 2008).

Notably, one of the most important routes of microplastic transfer from environmental matrices to human is through the ingestion of contaminated water. As sustainable development goals target for good health and well-being, there is a requirement to focus on the availability of clean water, free of any contamination to everyone (SDG 2015). Although there are certain laws and regulations targeting curtailment of production and use of primary microplastics (Kentin & Kaarto 2018), there are many more challenges ahead. Effective strategies for microplastic removal from wastewater need to be developed so that surface water sources are not contaminated upon receiving discharge from wastewater treatment plants. In this paper, a short review of various existing removal options for microplastics in wastewater treatment plants has been done.

It is important to understand that toxic effects of microplastics are not only due to their inherent nature but also due to various additives and plasticizers that are added during manufacturing. Furthermore, when these microplastics enter the environment, adsorption of various chemicals, pollutants, and microbes takes place on its surface. Thus, it is not only the microplastics which are of concern; rather, the adhered contaminants also pose significant health risks.

Regulation for primary microplastics, which is mostly found in pharmaceuticals and cosmetic products, is scarce, especially in developing countries (Perren et al. 2018). Lack of suitable plastic waste management systems further leads to the dumping of plastics mixed with other wastes at landfill sites, resulting into secondary microplastics in the long run. Removal of microplastics becomes challenging owing to their wide occurrence and small size, which usually allows them to escape the sieving/filtration processes. Conventional wastewater treatment plants are not fully efficient for their removal and hence final effluent contains significant amounts of microplastics. Discharge of this effluent into the surface water bodies and/or surface run-off allows microplastics to become mixed with the drinking water supply chain (Murphy et al. 2016). In addition, the amount of microplastics that is entrapped in sludge often gets no further treatment and is mostly disposed off onto the land (Yuan et al. 2022). This results in the contamination of groundwater and soil as well, through percolation (Tagg et al. 2021). Moreover, as the groundwater is generally considered safe for drinking and hence does not pass through any treatment process, microplastics present in the groundwater do not even get the chance of removal and thus makes humans more prone to ingestion of the same. Where low- and middle-income countries are still struggling for full-scale solid waste and wastewater management, addition of new technologies for removal of microplastics is an extra burden in the supply chain. Therefore, the new treatment options need to be cost effective as well as simple to incorporate.

Wastewater treatment processes involve treatment mainly at three stages viz. primary, secondary, and tertiary. In many instances a preliminary step is also employed to remove the large floating materials and grits, however this step is generally considered under the primary treatment. Overall, primary treatment involves physical settling of insoluble solids from the wastewater stream through various mechanisms such as screening, grit removal, oil and grease removal, and sedimentation (with coagulation). At this stage, the contaminants which can readily float or settle under the action of gravity are removed. Secondary (biological) treatment targets degradation of the biological content of the wastewater using either the attached growth system or the suspended growth system. Tertiary (advance) treatment is the final treatment stage in which chemical disinfection, filtration (ultrafiltration/microfiltration/nanofiltration), reverse osmosis, ozonation etc. are employed. The removal efficiency in wastewater treatment plants is calculated based upon the particle concentration of microplastics (viz. the number of microplastic particles/litre) in the influent and effluent.

Considering the stage-wise removal of microplastics, approximately 35–59% of the microplastics are removed during the preliminary treatment and 50–98% are removed during the primary treatment in wastewater treatment plants (Sun et al. 2019). The main mechanism involved in removal of microplastics at this stage is the skimming and settling of the entrapped microplastics during gravity separation. The shape of the microplastic particles also matters, as the fibres are removed more easily than the fragments in these treatments (Magnusson & Noren 2014; Talvitie et al. 2016; Ziajahromi et al. 2017). Plants that discharge the effluent only after the secondary treatment are more prone to release microplastics into the environment (Carr et al. 2016), although some advanced techniques such as membrane bioreactors may reduce the microplastic concentration to some extent (Lares et al. 2018). It has also been reported that overall microplastic removal can be achieved up to 88% in the absence of tertiary treatment and up to 97% with tertiary treatment (Sun et al. 2019). With time, the performance of reactors also goes down, which results in lower removal efficiency (Leslie et al. 2017). A summarized evaluation of microplastic removal processes is shown in Table 3.

Removal of microplastics using existing water treatment systems is highly desirable compared to any new technology because of the familiarity with the system, ease of operation and low costs involved. Therefore, Ma et al. investigated the aluminium (Al)- and iron (Fe)-based salts (AlCl₃·6H₂O and FeCl₃·6H₂O, respectively) for coagulation during the wastewater treatment process and achieved microplastic removal up to 36.89% for the particle sizes <0.5 mM using Al-based salts (Ma et al. 2019). A striking feature was that higher efficiency could be obtained for small-sized particles, while it decreased gradually for large-sized microplastics (4.51% for sizes between 2 and 5 mm). Fe-based salts, however, were poor in performance for microplastics removal. The main mechanisms involved in removal were charge neutralization and sweep flocculation. As the size of floc formed using Al-based salts was less compared to that of Fe-based salts, it rendered more surface area and hence higher removal efficiency. Furthermore, Al-based salts provided higher positive zeta potential as compared to Fe-based salts, which rendered easy charge neutralization of the microplastic particles with a negative zeta potential. The microplastic particles entangled over the flocs were then removed using ultrafiltration membranes. The removal efficiency of Al-based flocs is also affected by the pH and polyacrylamide (PAM). It was seen that pH did not play much of a role in the removal of microplastics at the low dose of Al salt (0.5 mM AlCl₃·6H₂O), however, at high coagulant doses (5 mM AlCl₃·6H₂O) the removal efficiency decreased upon increasing the pH. Similarly, addition of PAM did not affect the removal of microplastics at low doses of AlCl₃·6H₂O, but it resulted in enhanced removal at higher doses. Nevertheless, the addition of PAM or other similar additives should be minimal in water treatment processes, considering their non-biodegradable nature and enhanced risk upon increase in acrylamide monomer content (Stephens 1991).

Electrocoagulation is another viable solution for the removal of microplastics. Potentially, electrocoagulation does not require chemicals thus making it environment friendly. Electrocoagulation involves the production of coagulants electrically. Usually, Fe⁺² and/or Al⁺³ ions of the metal electrodes react with the OH̄ ions produced after the electrolysis and generate metal hydroxide coagulants. Microplastics, being solid particles, become destabilized in the presence of these coagulants and subsequently are entrapped in the sludge blanket produced by the coagulants. Flocs containing microplastic particles can then be removed from the water. Perren et al. have reported that approximately 90% of the microbeads were removed using the Al-based electrocoagulation technique (Perren et al. 2018). The major mechanism involved was charge neutralization and flocculation (Table 3).

Microplastics can also be removed during the filtration stage of the wastewater treatment (Michielssen et al. 2016). Microfiltration (0.1–1 μm), ultrafiltration (2–100 μm), and nanofiltration (∼2 nm) have been utilized for the removal of microplastics (Poerio et al. 2019). Wang et al. integrated the low-cost biochar in the sand filter systems to improve the efficiency of the filtration, which can result in removal of microplastics (Wang et al. 2020a). Up to 10 μm diameter size of microplastic particles could be removed with efficiency of more than 95%. This immobilization was achieved through three major mechanisms viz. ‘stuck’, ‘trapped’, and ‘entangled’ as visualized through environmental scanning electron microscopy. ‘Stuck’ represented the containment of the microplastic particles in the gaps between the filter particles owing to the small size of the latter. ‘Trapping’ is achieved when the filter material is porous in structure and flow of the water is slow. Furthermore, ‘entanglement’ occurs when the deep holes of the filter paper provide restriction for the microplastic particles against the water flow. Similarly, Wang et al. employed granular activated carbon filters in the wastewater treatment plants and achieved the reduction in microplastics' abundance in treated effluent of up to 56.8–60.9% (Wang et al. 2020b). Moreover, the performance was more suitable for the small-sized microplastic particles, as approximately 73.7–98.5% of the captured particles were in the size range of 1–5 μm. The main mechanism involved here was postulated to be synergistic combination of physical adsorption and biodegradation (Zheng et al. 2018).

Pulse clarification is another technique which helps to remove microplastics in wastewater treatment plants. Sarkar et al. demonstrated that pulse clarification and filtration are cumulatively able to remove up to 85% of microplastics (Sarkar et al. 2021). The mechanism involved here is the entrapment of microplastic particles in the sludge blanket formed due to the coagulation (H₂SO₄, Al₂(SO₄)₃.24H₂O). Pulsation helps to keep the sludge blanket in expansion which eases the entrapment. Finally, filtration further adds in the removal of microplastics.

However, filtration processes have some inherent shortcomings such as: (1) the process is slow and overtime it results in obstruction of the pores, (2) the process works under pressure and hence costly, (3) membrane regeneration is a cumbersome process. Furthermore, although conventional wastewater treatment processes can help in the removal of microplastics to some extent, none of these processes are specifically targeted to remove/degrade the microplastics (Iyare et al. 2020). Significant amounts of the removed microplastics remain in sludge, which further contaminates the land and subsequently groundwater. Therefore, alternative options that are targeted for the microplastics' removal need to be explored.

Microalgae may offer a possibility for the removal of microplastics, as it has been seen that microalgae colonize microplastic particles thus altering the buoyancy of the aggregates. This results in differential sedimentations rates as compared to the un-aggregated particles (Lagarde et al. 2016). This property may be utilized for the removal of microplastics by incorporating the microalgae. Sundbæk et al. employed a marine seaweed, namely Fucus vesiculosus, for the possible removal of microplastics through translocation in the algal tissues (Sundbæk et al. 2018). Due to narrow channels in the algal cells, the movement of microplastics was restricted and thus the plastic particles were captured. Efficiency of as high as ∼94.5% was observed, especially in the dissected areas of the algae. As the dissected areas ooze out anionic polysaccharide substances, the adherence of plastic particles is enhanced (Martins et al. 2013). Sorption of microplastic particles has also been reported by the other researchers (Bhattacharya et al. 2010; Nolte et al. 2017). The major mechanism for the sorption of microplastic particles onto the algal surface is the electrostatic charge. The positively charged microplastic particles tend to sorb more onto the algae owing to the presence of anionic polysaccharide substances in the algal cell wall (Nolte et al. 2017).

In order to remove microplastic particles, bioinspired molecules have been developed by some researchers. Herbort and Schuhen demonstrated the concept for the removal of hydrophobic microplastic particles through the combination of organic and inorganic molecular building blocks (Herbort & Schuhen 2017). These bioinspired molecules consist of an inclusion unit (IU) and a capture unit (CU), which are combined together to form an inclusion compound (IC) (Figure 2). In this, IU is the alkoxysilyl functionalized bioinspired component of the molecule, and the CU component has the ability to bond with the various materials through functional groups. Upon capture of the guest molecules (microplastics) in the inclusion cavity, the embedded water molecules are displaced. These released water molecules further combine with other surrounding water molecules through van der Waals forces (Tu et al. 2016). Thus, the cavity created by the release of water molecules is filled up by the guest molecules, thus allowing the IC to help in the removal of guest molecules.

Metal organic frameworks (MOFs) are porous structures that are the combination of metals and organic ligands. Having high surface area, porosity, and versatile functionality, these chemical moieties help in the entrapment of a variety of pollutants (Zhang et al. 2016; Kobielska et al. 2018; Mon et al. 2018). In order to entrap the microplastics, the material should have sufficient porosity, a suitable framework to capture the pollutant, and high durability. Thus, MOFs might work well as these have such properties. Chen et al. employed melamine foam to load zirconium (Zr) MOFs over it. Zr-based UiO-66-X (X=H, NH₂, OH, Br, and NO₂) MOFs were prepared using 1,4-dicarboxybenzene ligand with different functional groups (Chen et al. 2020). This framework was used to test microplastic removal in a simulated suspension of polyvinylidiene fluoride (PVDF), polymethyl methacrylate (PMMA), and polystyrene (PS) microplastics. The particle size and zeta potential of different microplastics affected the removal performance. As high as 95.5±1.2% removal could be achieved using the designed MOF along with having high recyclability (Chen et al. 2020).

Micromotors are the miniature self-propelled devices that have attracted researchers across the globe owing to their application in removal of oil (Guix et al. 2012; Gao et al. 2013; Mou et al. 2015), metals/metalloids (Uygun et al. 2016; Vilela et al. 2016; Wang et al. 2016; Villa et al. 2018), and various organics (Soler et al. 2013; Mushtaq et al. 2015; Simmchen et al. 2017; Zhang et al. 2017). Therefore, Wang et al. employed photocatalytic TiO₂-based micromotors (Au@mag@TiO₂, mag=Ni, Fe) for the removal of microplastics (Wang et al. 2019). Being photocatalytic, these micromotors could move themselves in water due to the photocatalytic reactions taking place on the particles. Removal was achieved by phoretic interaction and shovelling/pushing interactions. Phoretic interaction prevailed for individual micromotors, while the shovelling mechanism dominated when a chained assembly of micromotors was used. For the application in environmental systems, the chained assembly of micromotors is the better choice as it works efficiently independent of the fuel (Wang et al. 2019).

Advanced oxidation processes are one of the most suitable methods for the mineralization of various recalcitrant organic contaminants. Reactive oxygen species, such as hydroxyl radical, sulphate radical are usually employed to mineralize a variety of organic moieties. Therefore, microplastic degradation was also studied using this method. Integrated carbocatalytic oxidation and hydrothermal hydrolysis was used for the degradation of cosmetic microplastics (Kang et al. 2019). In this process, manganese carbide nanoparticles were encapsulated in N-doped carbon nanotubes, which activated peroxymonosulphate resulting in degradation of microplastics. Removal efficiency of up to 50 wt% was achieved depending upon the various reaction parameters, such as catalyst dose, microplastic concentration, temperature, and peroxymonosulfate dosage. Ariza-Tarazona and colleagues demonstrated the photocatalytic degradation of polyethylene microplastics in aqueous and solid matrices using the n-TiO₂ semiconductor. The degradation was estimated by the weight loss. It was observed that in the presence of light and n-TiO₂ semiconductor, the total mass of microplastic particles was reduced, indicating the microplastic photocatalytic degradation (Ariza-Tarazona et al. 2019). In aqueous medium, the mass loss of polyethylene particles was reported to be 6.40% during the first 18 hours of visible irradiation, while in the solid matrix the mass loss was 1.85% in the first 16 hours of irradiation. The higher mass loss of polyethylene particles in aqueous medium was attributed to the higher concentration of hydroxyl radicals, which promoted the degradation process. Similarly, Tofa and co-workers used zinc oxide nanorods for the photocatalytic degradation of low-density polyethylene (LDPE) residues (Tofa et al. 2019). The degradation was confirmed by the increased brittleness, presence of cavities, wrinkles, and cracks on the surface of LDPE after photocatalysis. The degree of degradation was found to be directly proportional to the catalyst surface area.

Microplastic pollution in environmental matrices is a global challenge. Where it is very obvious that reduction in the production and usage of plastics, and safe disposal of the same are the keys to administer this pollution, it is also a reality that it will take time to deal with the immensity and urgency of the problem at present. Therefore, development of efficient techniques for removal of microplastics from water is essential to avoid a number of health problems.

Although there are some prevailing treatment techniques, the efficacy thereof is still a challenge. Moreover, it is also necessary to make these technologies available at minimal cost to make it affordable. Photocatalytic treatment techniques are effective for the removal of microplastics; however, application of this techniques might not be feasible in every case owing to high cost and complexities involved. Mechanical removal of microplastics through self-propelled micromotors is yet to be tested for practical purposes. Similarly, requirement of power for electrocoagulation techniques might be a challenge in low-income settings. In this context, development of small filtration units may prove to be a suitable approach. These units should be able to fit into domestic water treatment equipment so that microplastics can be removed along with all the other impurities of water. These types of filtration units are also recommended for purifying groundwater. Furthermore, realizing that the sources and sinks of microplastics are varied, any single technology will not be able to provide the complete solution. Based on the source from where the microplastic is to be removed, a combination of methods such as biological entrapment (algal masses) and filtration may be used.

The authors declare there are no competing interests.

Authors are thankful to the Indian Council of Medical Research (ICMR), New Delhi for their financial support (project grant number ICMR-NIREH/BPL/IMP-PJ-44/2021-22/469: Principal Investigator – Surya Singh, ICMR – NIREH, Bhopal).

All relevant data are included in the paper or its Supplementary Information.