Science of the Total Environment 916 (2024) 170254

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Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

Review

Microplastic contamination in wastewater: Sources, distribution, detection

and remediation through physical and chemical-biological methods
Avishek Talukdar a, Pritha Kundu b, Sayan Bhattacharya b, *, Nalok Dutta c
a
Department of Zoology, University of Calcutta, Kolkata, West Bengal, India
b
School of Ecology and Environment Studies, Nalanda University, Rajgir, Nalanda, Bihar 803116, India
c
Biochemical Engineering Department, University College London, London WC1E 6BT, United Kingdom

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Wastewater receives microplastics from

both domestic and industrial sources.
• More than 30 types of microplastics are
detected in wastewater.
• Physical & chemical treatments are used
to remove microplastic from wastewater.
• Algal mass colonization and bioinspired
molecules are emerging treatment
methods.
• Marine animals & aquatic plants can be
explored in microplastic bioremediation.

A R T I C L E I N F O A B S T R A C T

Editor: Damia Barcelo Microplastics are tiny plastic particles smaller than 5 mm. that have been widely detected in the environment,
including in wastewater. They originate from various sources including breakdown of larger plastic debris,
Keywords: release of plastic fibres from textiles, and microbeads commonly used in personal care products. In wastewater,
Microplastic microplastics can pass through the treatment process and enter the environment, causing harm to biodiversity by
Wastewater
potentially entering the food chain. Additionally, microplastics can act as a vector for harmful pollutants,
Remediation
increasing their transport and distribution in the environment. To address this issue, there is a growing need for
Treatment
Technologies effective wastewater treatment methods that can effectively remove microplastics. Currently, several physical
and chemical methods are available, including filtration, sedimentation, and chemical degradation. However,
these methods are costly, low efficiency and generate secondary pollutants. Furthermore, lack of standardization
in the measurement and reporting of microplastics in wastewater, makes it difficult to accurately assess
microplastic impact on the environment. In order to effectively manage these issues, further research and
development of effective and efficient methods for removing microplastics from wastewater, as well as stan­
dardization in measurement and reporting, are necessary to effectively manage these detrimental contaminants.

* Corresponding author.
E-mail addresses: sbhattacharya@nalandauniv.edu.in, sayan.evs@gmail.com (S. Bhattacharya).

https://doi.org/10.1016/j.scitotenv.2024.170254
Received 20 November 2023; Received in revised form 2 January 2024; Accepted 16 January 2024
Available online 20 January 2024
0048-9697/© 2024 Elsevier B.V. All rights reserved.
A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

1. Introduction environment (Kole et al., 2017; Long et al., 2019; Mintenig et al., 2017).
Also landfill leachates can contribute to microplastic at WWTPs (He
Plastic pollution scenario has become extremely serious nowadays. et al., 2019). Microplastics can be retained in sludge by certain treat­
Globally plastic production has grown from 5 Mega ton (Mt) to >300 Mt ment process, but when this sludge is used in agriculture, microplastics
between 1950 and 2015 (Plastics Europe, 2015) and 60 to 99 Mt of can be transported to soil. Though we have extensive database about the
which results in waste in 2015 which is expected rise to 155–265 Mt per effects and removal methods for other pollutants (Khazaee et al., 2015;
year by 2060 (Lebreton and Andrady, 2019). Though the production of Taheri et al., 2013), however, information available about microplastics
plastics has increased but their recycling rate is still low in many parts of is still in its infancy. As of today, there are no specific treatment facilities
the world. For example, only 26.3 % of plastic garbage in all Europe was available that are designed solely for the removal of microplastics from
recycled in 2012 (Plastics-the facts, 2013 in Hamidian et al., 2021). wastewater. Although some WWTPs have been found to remove a
Microplastics are essentially debris pertaining to plastic of approx. 5 portion of microplastics through physical or biological treatment pro­
mm (Hidalgo-Ruz et al., 2012; Eerkes-Medrano and Thompson, 2018; cesses, these methods are not always effective in removing all types of
Lares et al., 2018; Zhang et al., 2018) and are classified in various cat­ microplastics from the wastewater (Gao et al., 2023). Further research
egories according to size, form, polymeric combination, and source. and development of more efficient and effective treatment methods for
There is evidence that microplastics are a worrisome new pollutant, microplastics are necessary to address this growing issue (Ou and Zeng,
executing significant health impacts on humans. They can be found in 2018; Estahbanati and Fahrenfeld, 2016; Gies et al., 2018; Gouveia
diverse types of environments, including freshwater, ocean, soil, and air et al., 2018; Mintenig et al., 2017; Ziajahromi et al., 2017).
(Tian et al., 2023; G.Z. Liu et al., 2019; He et al., 2019; Li et al., 2019; WWTPs discharge untreated microplastics which ultimately accrue
Eerkes-Medrano et al., 2015; Lechner et al., 2014; Clark et al., 2016; in the environment (Carr et al., 2016). Thus, it is of utmost importance to
Hammer et al., 2016; Hidalgo-Ruz et al., 2012; Li, 2018; Rochman et al., study performance and efficiency of different treatment technologies in
2015). Presence of microplastics in the aquatic environment is a major retaining microplastic for preventing them from entering natural sys­
concern owing to their harmful consequences to the aquatic ecosystems tems. Though methodologies for sampling and analysis are widely
and human health (Ziajahromi et al., 2016). Microplastics cause variety published for marine samples (Claessens et al., 2013; Masur et al., 2015;
of harms to aquatic animals, can also leach out toxins, transport toxic Nuelle et al., 2014), knowledge about standard sampling procedures
chemicals and heavy metals, poisoning aquatic biota (Xiong et al., 2023; from wastewater are still insufficient. The effluents from WWTPs are
Clark et al., 2016; Sussarellu et al., 2016; Hermabessiere et al., 2017; Li major pathways for the entry of microplastics into aquatic environments
et al., 2019; Teuten et al., 2009; Wardrop et al., 2016). In wastewater, (Krishnan et al., 2023; Okoffo et al., 2023). Even though WWTPs are
microplastics with a high specific surface area can adsorb pollutants designed to treat wastewater before it is released into the environment,
such as polycyclic aromatic hydrocarbons, heavy metals (Foshtomi microplastic particles can remain in the treated effluents and be dis­
et al., 2019), polybrominated diphenyl ethers (Singla et al., 2020), charged into waterways. This can have harmful effects on aquatic eco­
pharmaceuticals, and personal care products, resulting in chronic systems and organisms, as microplastics can be ingested by organisms
toxicity because of their bioaccumulation in organisms (G.Z. Liu et al., and accumulate up the food chain (Sun et al., 2019; Kalcikova et al.,
2019; Ziajahromi et al., 2017). Despite extensive investigations into the 2017). According to previous studies, microplastics are not completely
occurrence and distribution of microplastics in diverse environments, removed from wastewater. For example, a WWTP in UK after tertiary
their origins and fates in certain environmental matrices, such as treatment removed 96 % of microplastics from wastewater (Blair et al.,
wastewater, remain uncertain. 2019), while 99 % of microplastics were removed after mechanical,
Wastewater treatment plants (WWTPs) are the final stage of the chemical, and biological treatments (Ziajahromi et al., 2016). Thus, the
anthropogenic water cycle and receive microplastic contaminants from microplastic removal strategies should target the most dominant
various sources, including domestic and industrial activities. Due to this, microplastic types for removal (Hou et al., 2021).
WWTPs have become one of the largest point sources of microplastics in Despite the vast volume of wastewater treated by WWTPs, even a
the environment (Ziajahromi et al., 2017). While WWTPs are typically small percentage of microplastics remaining in the effluents can have a
viewed as the last line of defence in preventing contaminants from significant impact on the environment. A study by Browne et al. (14)
entering our natural environment, they also act as significant point found that WWTPs potentially played an important role in microplastic
sources of microplastic pollution (Hajji et al., 2023). WWTPs receive accumulation in the environment. During treatment processes, micro­
microplastic contaminants from various sources, such as domestic and plastics can also absorb many pollutants and toxic chemicals, making
industrial activities, and are not always able to effectively remove all them more dangerous for the natural ecosystem.
microplastics during treatment, leading to their release into the envi­ However, the wide range of microplastic concentrations in effluents
ronment (Bayo et al., 2020; Browne et al., 2011; Mourgkogiannis et al., can be attributed to various factors, including influent from different
2018; Talvitie et al., 2017a). WWTPs are considered to be the main re­ sources such as household discharges and human activities, dissimilar
cipients of terrestrial microplastics before entering natural aquatic sys­ treatment techniques, and the absence of standardized methods for
tems (Sun et al., 2019). Daily human activities are the primary sources of microplastic sampling and analysis (Kazour et al., 2019; Sun et al., 2019;
microplastics found in wastewater. For example, laundry processes and Murphy et al., 2016; Ziajahromi et al., 2017; Hidayaturrahman and Lee,
various personal care products that contain polyester and polyamide 2019; Priya et al., 2023). Majority of microplastics in WWTP ends in
components contribute significantly to the microplastics found in sludge, which is likely to get leached into the terrestrial environment
wastewater (Napper and Thompson, 2016; Magni et al., 2019). Fibres upon disposal in landfills or applied as fertilisers (Carr et al., 2016).
and plastic beads originate from textile washing (Hernandez et al., Local area characteristics also influence microplastic emissions; though
2017) and various personal care products respectively (Eriksen et al., urban areas have greater microplastic emissions than rural areas, but
2013; Gouveia et al., 2018; Magni et al., 2019), enters the wastewater owing to better waste treatment mechanisms urban areas release fewer
streams through domestic discharge. Plastic particles used in blasting microplastics than rural areas (Eckert et al., 2018; Yin et al., 2020;
cleaning, moulding or various other processes discharged into municipal Reddy et al., 2022).
wastewater streams also contributes to microplastics at WWTPs. Finally, The aim of this paper is to analyze the current knowledge regarding
small plastic particles created through the abrasion or breakdown of microplastics in wastewater, including their types, properties, and
plastic products (used in packaging, textiles, and tires) can enter characteristics, factors affecting their abundance, origin, and subse­
wastewater pipes through stormwater runoff via wet deposition. These quent transport to WWTPs. Different techniques available for treatment
particles can accumulate in the environment and potentially enter and removal of microplastics from wastewater are also discussed and
WWTPs, further contributing to the microplastic pollution in the aquatic compared in this context, and future research areas related to sources,

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

fate, and solutions for microplastics in wastewater are identified. et al., 2019; Liu et al., 2019; Ziajahromi et al., 2017). PP fibres have been
The amount of microplastic released into the environment from detected in wastewater and can originate from a variety of sources,
WWTP is shown in Table 1. including the washing of other fibrous materials like carpets and
clothings made from synthetic fibres. Therefore, reducing the use of
2. Sources of microplastics in wastewater synthetic materials and use of sustainable alternatives can also help to
reduce the amount of microplastics that end up in wastewater (Claessens
Various consumer products that could possibly release microplastics et al., 2011).
in wastewater systems include glitter (Napper et al., 2015), contact lens Natural exfoliants (e.g. grounded walnut husks) have been
cleaners (Gregory, 1996), small buttons and jewellery, while non- substituted by microbeads in different exfoliant washes causing less skin
domestic sources of wastewater microplastic includes: (a) plastic parti­ harm (Chang, 2015; Decker and Graber, 2012; Fendall and Sewell,
cles used during cleaning of paint and engines by airblasting (Gregory, 2009). Microbeads in toothpaste helps in removing plague owing to
1996); (b) transportation or manufacturing loss of pre-production pel­ their abrasive action (Vieira et al., 2016). Personal care products (such
lets (Sheavly and Register, 2007); (c) Styrofoam lost during fillings or as toothpaste, face or body scrub etc.) contain primary microplastics
shipping; (d) fibres released from synthetic textile industries; and (e) (microbeads) between 0.5 and 5 % having an average size of 250 μm
dust from drilling and cutting plastics (Prata, 2018), also cosmetic in­ (GESAMP, 2010; Zikto and Hanlon, 1991) and are major contributors to
dustries, plastic manufacturing plants, fishing industry, shipping line, microplastics in wastewater. A single exfoliant wash can release
wear and tear of car and truck tires contributes to microplastic abun­ 4500–94,500 microbeads (Napper et al., 2015) while toothpaste releases
dance in wastewater (Dey et al., 2021). Polyester (PES) (~90 %), around 4000 microbeads (Carr et al., 2016). Various studies have re­
Polyamide (PA) (~53 %), Polyethylene (PE) (~17 %), Polypropylene ported maximum up to 1,000,000 fibre particles being released by
(PP), alkyd, acrylic, and polystyrene (PS) are among the most commonly washing of a single garment (Browne et al., 2011; Almroth et al., 2017).
found types of microplastic polymers in wastewater. These polymers are Whereas, polyester fabrics and acrylic fabrics can release around
used in a wide range of products, such as packaging materials, textiles, 6,000,000 and 700,000 fibre particles (De Falco et al., 2018; Napper and
and consumer products like cosmetics and personal care items, which Thompson, 2016) having similar washing load. Sillanpaa and Sainio
can end up in wastewater because of human activities. Due to their small (2017) reported that annually washing machines releases an estimated
size, microplastics can pass through traditional wastewater treatment 154,000 to 411,000 kg of polyester and cotton microfibers (thickness:
processes and end up in the effluents (Murphy et al., 2016; Talvitie et al. 10–20 μm; length: 100–1000 μm) in Finland though it also depends on
2017). Variation in microplastic composition is directly related to the textile properties (polymer, knit), washing conditions (temperature,
influent sources. Polyethylene terephthalate (PET) is also a commonly friction, velocity, washing time), use and type of detergent and softener,
found type of microplastic polymer in wastewater. PET is a type of and garment weathering (Almroth et al., 2017; Cocca et al., 2017; De
polyester, and like other microplastics, it can originate from a variety of Falco et al., 2018; Napper and Thompson, 2016; Sillanpaa and Sainio,
sources, including synthetic clothing and textiles. In addition to PET, 2017).
polyacrylonitrile (PAN) is another type of microplastic polymer that can Various sources of microplastics in wastewater are shown in Fig. 1.
be found in wastewater, often originating from clothings made of acrylic
fibres. Polyamide (PA), which is commonly used in textiles and fishing 3. Microplastic transport and fate in wastewater
gear, is another type of microplastic polymer that can be found in
wastewater. Overall, the types of microplastic polymers found in 3.1. Transport of microplastics
wastewater can vary depending on the sources and activities that
contribute to microplastic pollution (Siegfried et al., 2017). PE is widely There are various ways by which microplastics might be transported,
used in facial or body cleansers, packing films, and water bottles (Fen­ including sewage treatment plant emissions, rainwater runoff, sewage
dall and Sewell, 2009; Schymanski et al., 2018). drainage, and deposition of airborne microplastics (Mason et al., 2016;
Acrylonitrile-butadiene (AB) is not typically considered a micro­ Nizzetto et al., 2016; Dris et al., 2017; Siegfried et al., 2017). While
plastic polymer, but rather a type of synthetic rubber. However, it is true treatment facilities in urban areas may be better equipped to filter some
that AB can be found in wastewater and can originate from a variety of of these particles, they are not always 100 % effective. Ultimately, it is
sources, including pipes and automotive body parts (Lai et al., 2013). up to individuals to be mindful of the products they use and to properly
Acrylate is a type of polymer that is commonly used in beauty and dispose of them to help reduce the amount of microplastic pollution in
personal care products, such as nail polish and hair spray. Like other our environment. Though the treatment plants have good removal ef­
microplastics, acrylate can end up in wastewater through human ac­ ficiency, however, huge volume of microplastics still ends up into the
tivities, and can ultimately contribute to microplastic pollution in the environment by avoiding different treatment stages (Murphy et al.,
environment (e.g. cosmetics, cleansers etc.) (Simon et al., 2018; Magni 2016). Microplastics from wastewater can get into terrestrial and

Table 1
Comparative efficiencies of different treatment methods for microplastic removal from wastewater streams. The amount of microplastic released into the environment
from Waste water Treatment Plants (WWTP) are mainly influenced by the volume of wastewater.
Concentration of microplastic before Concentration of microplastic after Total volume of waste water Amount of microplastic released into the References
treatment treatment treated environment

1.44 mp/L 0.48 mp/L 17 million L 8.16 × 106 mp particles Ziajahromi et al.,
2017
2.2 mp/L 0.28 mp/L (after tertiary treatment) 13 million L 3.6 × 106 mp particles Ziajahromi et al.,
0.21 mp/L (after RO treatment) 48 million L 1 × 107 mp particles 2017
3.4 mp/L 0.3 mp/L (after secondary – Murphy et al., 2016
treatment)
10,044 mp/L 450 mp/L – – Sun et al., 2019
2.6–3.2 × 105 1.4–5.0 × 104 (after secondary – – Dris et al., 2015
treatment)
1.0 × 103 0.88 (after tertiary treatment) – – Carr et al., 2016
1.5 × 104 2.5 × 102 (after secondary Murphy et al., 2016
treatment)

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

Fig. 1. Various sources of microplastics in wastewater, coming from atmospheric deposition, storm runoff, industrial discharges, domestic discharges and land­
fill leachates.

aquatic ecosystems through various pathways (Mahon et al., 2017; (Mahon et al., 2017). The best removal effectiveness (99.9 %) was
Zhang et al., 2018). WWTPs serves as the major pathway through which demonstrated using membrane bioreactors with 0.4 m pore sizes, fol­
MPs and NPs enters aquatic and soil environment (Ziajahromi et al., lowed by quick sand filters (97 %), and dissolved air flotation (95 %)
2016, 2017; Park et al., 2020; Xu et al., 2019; Gündoğdu et al., 2018; (Talvitie et al. 2017). Fragmentation of microplastics can occur due to
Akarsu et al., 2020; Naji et al., 2021). MPs and NPs released in WWTPs photo-oxidation (UV rays or sunlight), mechanical pressure also does the
effluents contaminate aquatic environment (Yu et al., 2018) and the same while turbidity and bio-fouling interrupts this process. Final
sediments downstream (Conley et al., 2019). Also, the application of degradation may take place by microorganisms depending on various
WWTP sludge containing MPs and NPs in agricultural fields as fertiliser factors like polymer type, organism type, environmental conditions etc.,
results in accumulation of MPs and NPs in the soil environment and which can also be accelerated by processes like ionization, hydrolysis
ecosystem (Bretas Alvim et al., 2020; van den Berg et al., 2020). etc. (Gu, 2003).
Most of the lower density microplastics floats on the water surface
and are effectively removed by skimming devices. Sometimes they also
3.2. Fate of microplastics form flocs and settle down to be finally removed by settling mechanisms
at the primary and secondary treatment stages. Biofilms developing on
Though the high-end technologies can remove microplastics effec­ the surface of microplastics can affect their physical properties, thereby
tively (Talvitie et al. 2017; Ziajahromi et al., 2017; Blair et al., 2019; affecting the removal efficiencies at treatment plants (Carr et al., 2016).
Talvitie et al., 2015; Lares et al., 2018; Michielssen et al., 2016) but a lot Though very few microplastics are present in treated effluents, but the
is being influenced by the microplastics physiochemical properties sheer volume of effluents released everyday results in substantial
(Cheung and Fok, 2017; Long et al., 2019), such as particle size, nature contamination of aquatic ecosystems (Murphy et al., 2016; Talvitie
of polymer, and shape etc. (Long et al., 2019) and individual settling et al., 2017; Ziajahromi et al., 2017).
efficiency (Estahbanati and Fahrenfeld, 2016). The removal efficiency of
microplastics is inversely related with particle size (Long et al., 2019), 4. Microplastic detection technique in wastewater
also directly related with polymer density (Wu et al., 2020). Chemical
flocculating agents also facilitates microplastic removal (Sillanpää et al., There are several steps involved in finding microplastic in waste­
2018). Talvitie et al. (2017a) showed that sludge retention time may water treatment facilities. Several procedures are used, depending on
also potentially affect removal efficiency of microplastics; longer the properties of microplastics. Moreover, distinct dimensions in the
retention time resulted in lower microplastic concentration (Li et al., outcome might be produced by different identification procedures.
2018). Chances of biofilm formation on microplastic surface increases
with increase in contact with wastewater, and as a result, facilitates
sedimentation (Sun et al., 2019). Nutrient levels in wastewater also help
in increasing removal efficiency by facilitating microbial growth

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

4.1. Collection of samples designed to specifically target microplastics, which are small plastic
particles found in wastewater and are resistant to traditional treatment
Microplastic sampling can be conducted in different ways including methods such as Fenton reagents. Therefore, other methods like
collection in containers (Magnusson and Noren, 2014; Murphy et al., advanced oxidation processes, biodegradation, or membrane filtration
2016; Tagg et al., 2015), collection using autosampler (Michielssen may be more suitable for removing microplastics from wastewater (Tagg
et al., 2016; Talvitie et al., 2017a), separate pumping and filtration et al., 2017). This method was recommended by the US National
(Mason et al., 2016; Mintenig et al., 2017; Talvitie et al., 2015; Zia­ Oceanic and Atmospheric Administration (NOAA) for analysing marine
jahromi et al., 2017) and surface filtration (Carr et al., 2016). Sampling microplastics (Masur et al., 2015).
with autosamplers or containers is easy but gets limited collection vol­ Enzymatic degradation is a new method that is gaining popularity for
ume per sampling event. Whereas, separate pumping and filtration can breaking down microplastics. This approach involves soaking micro­
increase the sampling volume effectively, depending on mesh size and plastic samples in a mixture of different enzymes, including lipase,
wastewater characteristics. Carr et al. (2016) designed a filtering device amylase, proteinase, chitinase, and cellulase, to facilitate their degra­
where a stack of 800- diameter stainless steel sieve pans with mesh sizes dation (Cole et al., 2014; Loder et al., 2015), removing all organic
ranging from 400 to 20 mm was employed, which allowed further in­ molecules (proteins, lipids and carbohydrates) specifically. Mintenig
crease of sampling volume; however, the device need to be placed at the et al. (2017) performed a several step plastic-preserving enzymatic
water falls and fugitive airborne contamination cannot be avoided. The maceration process, using protease, lipase and cellulase with sodium
method could probably underestimate microplastic counts by only dodecyl sulphate (SDS, 5 % w/vol) and H2O2 for wastewater treatment.
sampling the lighter microplastics through skimming on water surface. Loder et al. (2017) further optimised the performance by increasing SDS
Currently, for taking representative samples, sampling volume is concentration, changing buffer composition, increasing enzyme purifi­
increased with collection of 24-h composite samples (Mason et al., 2016; cation efficiency and removal efficiency of polysaccharide and lipids.
Talvitie et al., 2017a). Wastewater samples are filtered to concentrate Alkaline treatment and acid treatment are alternative methods for
the microplastics, hence pore sizes have significant effects on micro­ organic matter removal from wastewater (Ziajahromi et al., 2017),
plastic collection amount (Magnusson and Noren, 2014). Mesh sizes however, their application needs extra caution, since it could damage
vary from 1 to 500 mm, with some studies using a series of filter stacks microplastics due to corrosive chemicals and strong oxidising properties
for the process. This process allows to sample large volumes and sepa­ of some acids used in the process (Claessens et al., 2013; Cole et al.,
rated the microplastics according to sizes. Neuston plankton net and 2014). Bayo et al. (2016) used isopropyl alcohol for removing organic
manta trawl can also be used for microplastic separation (Li et al., 2018; matters but its removal efficiency is still untested. Ultrasonication have
Prata et al., 2019), though not used till date for microplastic sampling also been applied to treat sea water but was not applied for wastewater,
from wastewater. A sampling device with four meshes of equal may be because of formation even finer particles of microplastics from
sizes—500, 190, 100, and 25 mm—with a 12 cm diameter was built by brittle plastic particles by applying these methods (Cooper and Cor­
Ziajahromi et al. (2017). The largest mesh screen was placed at the top coran, 2010; Li et al., 2018). Salt solutions can be used to separate
of the stack of mesh units, which were then enclosed in a PVC cover so inorganic materials from wastewater and sludge based on density sep­
that water could readily flow through them. The design of the system aration. Saturated sodium chloride (NaCl) solution is widely used due to
enabled sampling large volume of water and different size of micro­ its low cost and non-toxicity (Leslie et al., 2017; Li et al., 2018), how­
plastics simultaneously with a flow meter at front accurately measuring ever, this could lead to underestimation of high density microplastics
the amount of wastewater sampled. The designed device was reported to like PVC, PET (Duis and Coors, 2016). As a result, denser salt solutions
have an efficiency of 92 % to 99 % for 25 mm and 500 mm mesh like NaI or ZnCl solution are used (Mintenig et al., 2017; Ziajahromi
respectively, including the complete removal of microplastic particles et al., 2017). Elutriation technique developed by Claessens et al. (2013)
>190 mm (Ziajahromi et al., 2017). was applied by Carr et al. (2016) for microplastic separation from
WWTP influent, which work by exploiting inherent buoyant properties
4.2. Pre-treatment and processing of samples of microplastics based on combination of water flow and aeration. Carr
et al. (2016) used grab samples from different stages, mixed it with 3 %
Pre-treatment is a stage, involving significant steps of treatment in sodium hypochlorite solution for disinfection, and finally examined
WWTPs, which is employed to remove easily separable fractions (Enfrin them under microscope.
et al., 2019; Pal, 2017). Various methods are applied for extracting
microplastics from sludge samples containing organic matter and inor­ 4.3. Characterization of microplastics
ganic solids. Organic matter removal is necessary for chemical identi­
fication of microplastics. In WWTPs, the catalytic wet peroxidation Till now, >30 types of microplastics have been detected in WTTP
(WPO) process is frequently used to remove organic wastes. The pro­ waters (influent and effluent). Polyethene (PE), polypropylene (PP),
cedure is also used for pre-treating collected microplastic samples. polyamide (PA), polyester (PES), polystyrene (PS) and polyethene
Several chemicals, such as Hydrogen Peroxide (H2O2), Sodium Hypo­ terephthalate (PET) are most widely detected microplastics type in
chlorite (NaClO), and Fenton reagents, are employed to oxidise organic wastewater, having highest abundances of 64 %, 32 %, 10 %, 75 %, 24
materials (Erni-Cassola et al., 2017; Karami et al., 2016; Masur et al., %, and 29 %, respectively (Long et al., 2019; Mintenig et al., 2017;
2015). Oxidising with H2O2 is an efficient process and doesn't cause any Talvitie et al., 2017a; Ziajahromi et al., 2017). PET fibres comprised 65
change in microplastics composition or morphology after exposure to % and 88 % of total microplastics in tertiary and RO samples respec­
H2O2 (30 %) up to 7 days (McCormick et al., 2014; Nuelle et al., 2014; tively (Ziajahromi et al., 2017). Numerous influent wastewater samples
Tagg et al., 2015). However, this method is not suitable for large samples contained a variety of different plastic kinds, but the most common ones
or with higher organic detritus owing to prolonged treatment time. were polyester, polyethene, polypropylene, polyamide, and polystyrene
Ziajahromi et al. (2017) used this method with addition of Sodium io­ (Gies et al., 2018; Magni et al., 2019; Yang et al., 2019; Liu et al., 2019;
dide (NaI) as density separator. After that, Rose-Bengal (4,5,6,7-tetra­ Lv et al., 2019; Wolff et al., 2019). After tertiary treatment, irregular
chloro-20,40,50,70-tetraiodo-fluorescein, Sigma-Aldrich, 95 % dye white PVC microplastics were detected with size range 25–100 mm, but
content) solution was used, which stains natural and non-plastic parti­ were absent in the primary treated sample (Ziajahromi et al., 2017).
cles for visual separation of microplastics, thus can reduce over­ Various plastic items like food packaging bags, plastic bottles, and
estimation (Ziajahromi et al., 2017). Fenton reagents are a type of plastic cutlery are the sources of PE, PP, and PS microplastics (Lares
treatment used to quickly break down organic pollutants in wastewater et al., 2018; Mintenig et al., 2017; Talvitie et al., 2017b); textiles and
by generating reactive hydroxyl radicals. However, they are not synthetic clothing gives rise to PA, PET, and PES microplastics

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

(Hernandez et al., 2017; Sun et al., 2019; Wei et al., 2019). PE is also microplastic particle smaller than 1 mm is significant for studying bio­
found use in various personal care products like body and facial scrubs logical toxicity and environmental transformation of microplastics.
(Carr et al., 2016), as well as food packaging films and water bottles
(Cheung and Fok, 2017; Lares et al., 2018; Mintenig et al., 2017; Shah 4.3.2. Shape
et al., 2008; Ziajahromi et al., 2017). Further potential sources of PE, PP, Shape of microplastics is an important indicator, affecting their
PS, and PES microplastics include mechanically crushed plastic items, removal efficiency and influencing the interactions between other
tyres, textiles, and rubber particles in road dust (Hidayaturrahman and wastewater contaminants or microorganisms (McCormick et al., 2014;
Lee, 2019; Nizzetto et al., 2016; Talvitie et al., 2017a). While PE and PP Wang et al., 2018). Various MP shapes are observed in wastewater
MPs were associated with personal care and home items, PET and samples including fragments, fibres, films, microbeads, foam, granules,
polyester MPs were mostly sourced from laundry facilities (Lv et al., pellets, and spherical and irregular shapes, of them both fibres and
2019). Wastewater also contained high concentrations of polymers such fragments are observed to be dominant (Abbasi, 2021; Liu et al., 2019).
as acrylate, alkyd, polypropylene (PP), polystyrene (PS), polyurethane Gies et al. (2018) reported abundance of fibres and fragments to be 65.6
(PU), PS acrylic, polyvinyl alcohol (PVAL), and polylactide (PLA). It can % and 28.1 % respectively, on the other hand Lv et al. (2019) reported
be concluded that a large fraction of the microplastics found in waste­ fragment abundance to be 65 % and fibre abundance as 21 %, while Liu
water are the results of regular activities of human population. In raw et al. (2019) estimated fibres in the range of 33.5–56.7 % and fragments
wastewater, MPs comes from a variety of sources, including stormwater, in the range of 30.4–45.6 %. Broadly, microplastics can be categorized
commercial, industrial, and residential settings, are present in raw into fibre and particles (Gouveia et al., 2018; Talvitie et al., 2015),
wastewater. MPs from cloth washing, cosmetics and personal care though some are also categorized into irregular shaped and spherical
products, fragmentation of household plastic items, etc., are the main bead/pellet and some others divided them into flake/film, foam and
sources of MPs entering wastewater in residential areas (Becucci et al., chip (Magnusson and Noren, 2014; Mason et al., 2016; Murphy et al.,
2022; Gündoğdu et al., 2018; Hidayaturrahman and Lee, 2019; Lv et al., 2016; Talvitie et al., 2017a). The most abundant microplastic is reported
2019; Magni et al., 2019; Ziajahromi et al., 2017). A considerable to be fibre having 52.7 % average, which is mainly due to huge amounts
amount of microplastics (MPs) are added to wastewater by vehicles, of discharges by domestic washing machine, containing fibres (Browne
textiles, road paints, tyres, food packaging materials, medical applica­ et al., 2011; Napper and Thompson, 2016; Pirc et al., 2016). The higher
tions, pipes, and cars (Blair et al., 2019; Yang et al., 2019; Magni et al., amount of fibres was in line with studies on secondary treated effluent
2019; Hidayaturrahman and Lee, 2019; Ngo et al., 2019). MPs are added from a WWTP in Paris (Dris et al., 2015) and from Sweden (Magnusson
to wastewater by a variety of industries, namely textile, polymer, and and Noren, 2014). Irregular shapes microplastics on average accounts
other processing sectors (Liu et al., 2019; Xu et al., 2019). Moreover, for 28.8 %. The irregular fragments could potentially result from eroded
rainwater enters the WWTPs as MPs passing via the airway (Bakaraki plastic products for the daily use or from personal care products like
Turan et al., 2021; Blair et al., 2019). Ziajahromi et al. (2017) found that toothpastes (Carr et al., 2016). Other microplastics like film, pellet and
secondary effluent contained mainly PET fibres and PE particles, com­ foam have an average abundance of 10 % or below, which can be
binedly comprising 66 % of total microplastics, while primary effluent sourced from plastic bag erosion and packaging materials, while pellet
from contained different types of microplastic, with PET (35 %) being were mostly primary microplastics added to personal products (Sun
most abundant followed by nylon (28 %), PE (23 %), PP (10 %) and PS et al., 2019). Fendall and Sewell (2009) found majority of the PE par­
(4 %). Recently Simon et al. (2018) found out that PP particles ticles were spherical in shape, which are similar to microbeads found in
contribute majority of the whole microplastic mass. Future studies different personal care products.
should look at increasing the accuracy of microplastic mass quantifica­ Microplastic shapes can affect their removal efficiency in WWTPs
tion, which can be a tool for developing better understanding on (McCormick et al., 2014). Fibres owing to their morphology are the most
microplastic pollution. difficult microplastics to remove from Wastewater (Ngo et al., 2019).
Till now, nine types of microplastic shapes are detected in wastewater
4.3.1. Size viz. fibre, fragment, film, pellet, foam, particle, ellipse, line and flake. Of
There are currently two ways employed to classify microplastic by them fibres, pellets, fragments, and films are most common having
size. The uneven shapes of microplastics make them difficult to retain by highest abundances of 91.32 %, 70.38 %, 65.43 %, and 21.36 %,
using different-sized sieves, and another way is to use imaging tech­ respectively (Bayo et al., 2020; Hidayaturrahman and Lee, 2019; Lares
niques to detect microplastics (Lares et al., 2018; Mintenig et al., 2017; et al., 2018). Fibres originate from domestic laundry activities and with
Simon et al., 2018). Describing microplastic size with a single value is increased washing and textile consumption fibres have becomes more
difficult due to their irregular morphology. Hence, for size classification, frequent (Cesa et al., 2017). Fibres are also produced during textile
dimensions of 25 mm, 100 mm and 500 mm are most frequently used production (Hidayaturrahman and Lee, 2019; Napper and Thompson,
(Dris et al., 2015; Lares et al., 2018; Mintenig et al., 2017; Simon et al., 2016). Fragments comes from various personal care products like
2018; Talvitie et al., 2017a; Ziajahromi et al., 2017). WWTP influent toothpaste, masks, and soaps (Carr et al., 2016) and microplastic films
sometimes contains over 70 % of microplastics of 500 mm size (Dris from plastic packing bags (Kazour et al., 2019).
et al., 2015; Lares et al., 2018), while in case of effluent, 500 mm sized Fig. 2 shows Scanning Electron Microscope images of different
microplastic abundance can reach over 90 %, and in some other effluent microplastics extracted from WWTPs, showing diverse morphological
samples, around 60 % microplastic are smaller than 100 mm (Mintenig features and surface texture.
et al., 2017; Simon et al., 2018; Ziajahromi et al., 2017). Mesh size also
have a huge role in microplastic sample collection; a large mesh is often 4.3.3. Colour
unable to separate small sized particles (Lares et al., 2018). Using large In the WWTP, the primary effluent contained PE (42 %), PET (36 %),
sized meshes could be problematic in microplastic removal from PS (15 %), and PP (8 %) and majority of them were white and blue
wastewater, as smaller sized particles (< 25 mm) has the highest particles within size range 100–190 mm., having irregular shape (Zia­
abundance in wastewater (Simon et al., 2018). Microplastics abundance jahromi et al., 2017). The PE particles have similar shape and size as in
smaller than 1 mm ranged from 65.0 to 86.9 % and 81.0–91.0 % in the personal care products, also reported in previous studies (Carr et al.,
influent and effluent respectively (Liu et al., 2021). Most of the times 2016; Fendall and Sewell, 2009; Napper et al., 2015). Blue and black
primary microplastics were further fragmented to form secondary fibres were most common followed by granular particles of red and blue
microplastics (Magni et al., 2019). The small sized microplastics are colour (Ziajahromi et al., 2017).
most likely to be ingested by various aquatic organisms causing various Types of polymers detected in WWTPs with their density and relative
toxicological impacts (Qiao et al., 2019). Therefore, research on abundance in wastewater are shown in Table 2.

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

Fig. 2. Scanning Electron Microscope images of different microplastics extracted from WWTPs, showing diversity in morphology and surface texture.
Reprinted with permission from Elsevier Gao et al. (2022), Licence number 5518351079305.

5. Factors affecting MP abundance and characteristics in sewage

Table 2
sludge (SS)
Types of polymers detected in WWTPs with their density and relative abundance
in wastewater.
The abundance and removal rate of MPs may vary depending on the
Sl. Polymer Density (g/ Relative sources and methods used to treat wastewater in various areas (Yang
no. cm3)a abundanceb
et al., 2021). Conesa and Ortuño, 2022 have classified MPs in SS into five
1 Acrylic 1.09–1.20 Medium categories: fragments or microflakes; microfibers; foams; microspheres
2 Alkyd 1.24–2.01 Medium
or microbeads; and nurdles or granules. Microbeads, which can have a
3 Polyethylen terephthalate (PET) 0.96–1.45 High
4 Polyamide (nylon) 1.02–1.16 High round or irregular shape, are the most prevalent type of MPs found in the
5 polyaryl ether 1.14 Low environment. These are followed by fibres, films, pellets, and polymer
6 Polyester 1.24–2.3 High fragments of all kinds (Hanun et al., 2021). Furthermore, the majority of
7 Polyethylene 0.89–0.98 High polymers in SS are polyethylene (El Hayany et al., 2022). Various factors
8 Polypropylene 0.83–0.92 Medium
9 Polystyrene 1.04–1.1 Medium
can influence the amount of microplastics present in wastewater, such as
10 Polyurethane 1.2 Medium agricultural and industrial activities (Eerkes-Medrano et al., 2015; Long
11 Polyvinyl fluoride 1.7 Low et al., 2019), seasonal variability (Bayo et al., 2020) and urban runoff. In
12 Polyvinyl acetate 1.19 Low addition to the source, factors influencing the amount of MPs in the
13 Polyvinyl chloride 1.16–1.58 Low
influent of WWTPs include population density, lifestyle choices and
14 Polytetrafluoroethylene 2.1–2.3 Low
15 Styrene acrylonitrile 1.08 Low cultural practices, environmental conditions and washing practices,
16 Ethylene vinyl acetate 0.92–0.95 Low plant location and operations, weather patterns and seasonal variations,
17 Polyvinyl alcohol 1.19–1.31 Medium and more (Magni et al., 2019; Edo et al., 2020; Raju et al., 2020;
18 Acrylonitrile butadiene styrene 1.04–1.06 Low Hidayaturrahman and Lee, 2019). For instance, in Thailand's WWTP,
19 Polylactide 1.21–1.43 Medium
20 Vinyl-acetate-acrylic copolymer 1.22 Low
MPs were found in the final effluent during the rainy season, but not in
21 Polyethylene-Polypropylene 0.94 Low the dry seasons (76–192 and 36–68 MPs/l, respectively) (Kittipongvises
copolymer et al., 2022). MP concentrations were found to be higher during dry
22 Poly(phthalimide) 1.10 Low seasons than during rainy ones. This might be because MPs break apart
23 Polycarbonate 1.2–1.22 Low
and evaporate during the dry season, increasing MP concentrations
24 Terpene resin 0.98 Low
25 Plexar resin 0.92 Low (Menéndez-Manjón et al., 2022). Additionally, the number of MPs is
26 Poly(oxymethylene) 1.41 Low correlated with economic advancements. For instance, the authors of the
27 Polysulfone 1.24 Low study of 48 WWTPS in China noted that economically developed loca­
28 Silicone 1.1–1.2 Low tions had higher MPs emissions per capita (Hu et al., 2022). According to
29 Polystyrene acrylic Medium
30 Polyvinyl acrylate Low
Azizi et al. (2022), the largest dimensions, form, and polymer type of
31 Polyvinyl ethelene Low MPs found in WWTPs are 375–600 μm for fibre and polyethylene,
a respectively. The MP burden on the environment can be decreased by
Density: Based on Hidalgo-Ruz et al. (2012), Duis and Coors (2016), Simon
controlling the amount of MPs discharged into WWTPs by identifying
et al. (2018), Rosato et al. (2010).
b
Relative abundance: Based on Mintenig et al. (2017), Murphy et al. (2016),
the sources of MPs for wastewater (Talvitie et al., 2017a).
Ziajahromi et al. (2017), Lares et al. (2018), Talvitie et al. (2017a), Li et al. Various sewage sludge treatment processes can impact the charac­
(2018), Simon et al. (2018). teristics of microplastics positively or negatively (Mahon et al., 2014).
Lime stabilization has some negative effect as it makes fibres more
brittle resulting in numerous smaller fibre particles (Mahon et al., 2014),
thus, in turn, can increase the bioavailability and adsorption/absorption
rates. The process also increases the potential risk to humans and biota.

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Researchers have evaluated the mechanical, thermal, chemical, and identified as 1 μm due to the absence of proper devices to identify and
surface morphology, mass variation, and degradation of MPs. While quantify the NPs (Ali et al., 2021; Kang et al., 2020). NPs and MPs
positive effects with respect to reducing abundance of microplastic in smaller than 20 μm are readily absorbed by living things (Cristaldi et al.,
sewage sludge is reported for mesophilic anaerobic digestion and com­ 2020; Sobhani et al., 2022). Because of its bigger surface area and
posting (Zubris and Richards, 2005; Mahon et al., 2014). Composting complex characteristics that encourage the adsorption of additional
reduces fibres abundance owing to extraction process or losses (Zubris hazardous pollutants, NPs pose a greater threat to human health than
and Richards, 2005). Microplastic surface morphology also changes MPs. NPs are quickly absorbed by the body through a variety of chan­
during sewage sludge treatment. Fibre length and physical appearance nels, and the environment is greatly concerned about their harmful
can be altered by advanced alkaline stabilization. Other characteristics consequences (Yee et al., 2021). Approximately 25 % of NPs can be
like shredding and flaking in lime stabilization (LS), melting and blis­ released into the environment by WWTPs (Mohana et al., 2021). How­
tering in thermal drying (TD) (Mahon et al., 2017), and highly abrasive ever, MPs smaller that measure <100 μm in any one dimension are
and brittle and hackly MP structures have been also reported (Li et al., difficult to remove; in contrast, particles bigger than 500 μm are easily
2018). Few studies also claimed that treatment process has no effect on removed (Freeman et al., 2020; Menéndez-Manjón et al., 2022). NPs are
microplastic abundance (Healy, 2018), the size of the WWTP (popula­ uncontrollable by conventional treatment methods, such as physical,
tion equivalent), the influent volume, or weather conditions (Lusher chemical, and biological therapy units. On the other hand, size exclusion
et al., 2017). A broad study in China by Li et al. (2018) found micro­ in membrane processes may lead to the removal of NPs. NPs can be
plastic spatial distribution in sludge is influenced by forest cover, eco­ efficiently removed by MBR, RO, ultrafiltration, and dynamic membrane
nomic prosperity, and urbanization, while seasonal factors including filtration. NP deposition on the membrane surface, however, blocks the
temperature and rainfall may impact MP temporal variability. membrane pores and results in fouling, which lowers the water flow
MPs have a selective influence on the microbial population and (Mohana et al., 2021). To practise getting rid of the NPs on the mem­
favourably promote denitrification, they can impede nitrification in the brane surface, frequent washing is required because this fouling might
activated sludge process (Bretas Alvim et al., 2021; Li et al., 2020). be either reversible or permanent. Disinfection procedures can also
Moreover, MPs disrupt genes that confer antibiotic resistance and the change NPs' characteristic, which has an impact on how they interact
UV disinfection of microbes (Yang et al., 2022). Additionally, it was with other chemicals.
shown that in MBR, higher concentrations of MPs improved fouling
mitigation and increased microbial community and diversity, whereas 6.1. Primary and secondary treatment
lower concentrations restrict the proliferation of microorganisms (Wang
et al., 2022). Preliminary treatment mainly separates the floating materials, the
heavy settleable inorganic solids, oil and greases from the sewage
6. Microplastic removal from wastewater (Michielssen et al., 2016). This process reduces the BOD of the waste­
water by about 15 to 30 % (Chaisar and Garg, 2022). The majority of the
Based on the concentrations of the microplastics in the influent and microplastics in the wastewater could be successfully removed during
effluent, the removal of the microplastics by the WWTP (wastewater the preliminary and primary treatment (pre-treatment). According to
treatment plant) was determined. Wastewater can be treated in a variety reports, 35 % to 59 % of the microplastics in wastewater could be
of ways, and it can even be divided into stages that are called pre­ removed by preliminary treatment, and between 50 % and 98 % could
liminary, primary, secondary, and tertiary treatments. With tertiary be removed after the primary treatment (Sun et al., 2019). The general
treatment, the overall microplastics removal efficiency of WWTPs steps in primary treatment are screening, gritting, primary sedimenta­
increased to over 97 %, whereas without tertiary treatment, removal tion, and flotation. Depending on the specific target, different treatment
efficiency is comparatively lower. According to Talvitie et al. (2017a, plants have some variations. The potential MP removal pathways of the
2017b), WWTPs can be a means of cleaning up MPs and NPs that are steps of primary treatment are discussed as follows:
getting into the environment. By focusing on WWTPs, significant MPs
release into aquatic habitats are reduced (Masiá et al., 2020). Low MP 6.1.1. Screening
removal efficiencies, on the other hand, have been documented in Screen can be of three major types, coarse screen (mesh sizes 50–100
WWTPs, suggesting that the technologies available are inadequate for mm), middle screen (mesh sizes 10–40 mm), and fine screen (mesh size
eliminating MPs since they are not intended to address the MPs/NPs 2.5–10 mm). Plastic with <5 mm-sized particles is known as MPS. It
(Van Do et al., 2022). According to Uddin et al. (2020), if all wastewater implies that MPs cannot be removed using the coarse or middle screen.
is sufficiently treated before to release, the global burden of micro­ With a fine screen, MPs with particles larger than 2.5 mm can be
plastics (MPs) in the aquatic environment may be reduced by 90 %. This removed (Sun et al., 2019). Another type of screen with considerably
statement is based on the present removal efficiency of WWTPs. Sig­ smaller sieve size is called an ultrafine screen (0.2–2 mm). It is typically
nificant levels of MPs continue to leak into the effluents even after the positioned prior to the membrane bioreactor (MBR). The necessity of
wastewater containing MPs has been treated using both conventional reusing water, especially in the context of emerging water crisis, has
and sophisticated treatment techniques (Bretas Alvim et al., 2020). Even increased the relevance and significance of upgrading wastewater
though the concentrations of MPs left in the effluents were significantly treatment technologies. MBR is regarded as an effective and efficient
lower than those of influent wastewater, the massive volume of effluent technology to produce effluent of high quality. MBR has thus been
discharged consistently (Talvitie et al., 2017a, 2017b; Ziajahromi et al., heavily utilised in upgrading wastewater treatment facilities. Instead of
2017; Zhang and Chen, 2020) resulted in the release of a significant primary sedimentation, an ultrafine screen is placed before the MBR to
amount of MPs into the environment (Sun et al., 2019). The fate and prevent membrane fouling. Therefore, using an ultrafine screen would
transport of fragmented or weathered plastic particles into the waste­ enable the removal of MPs from wastewater having particle sizes bigger
water or fragmentation inside the treatment system are the two main than 0.2 mm. In spite of the irregular shapes of MP, some MPs, bigger
causes of NPs in WWTPs. Shear stresses caused by mechanical opera­ than 0.2 mm. in size, could still able to cross the screen through their
tions inside WWTPs have a considerable probability of breaking down narrow ends (Zhang et al., 2020).
MPs into NPs in wastewater treatment systems (Enfrin et al., 2020;
Mohana et al., 2021; Pramanik et al., 2021). These small sized particles 6.1.2. Gritting
can easily escape through the treatment facility and enter the soil and Grit are contaminants with high density (1.5 g/mL), including sand,
aquatic environments through sludge and effluent discharge (Murray gravel, cinder and other heavy solid materials (density around 2.65 g/
and Örmeci, 2020). Plastic particles of minimum size in WWTPs are mL). However, most MPs present in wastewater have a density of 0.8 to

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

1.6 g/mL. There are a few exceptions, though. For instance, the densities related processes with modification, such as anaerobic-anoxic-oxic
of Alkyd and PS range from 1.2 to 2.0 or 2.3 g/mL, and those of poly­ (A2O), oxidation ditches, and sequencing batch reactors, are included
tetrafluoroethylene (PTFE) are 2.1 to 2.3 g/mL (Zhang et al., 2020). The in the category of processes related to activated sludge (SBR). Biofilm
two main types of grit used in wastewater are horizontal flow grit and includes rotating biological contactors, filters, bio-trickling filters, flu­
aerated grit. The number of MPs removed in this process depends on the idized biological beds, and biological reactors with moving beds. In
water's flow velocity (V) and distance travelled (D) (Blair et al., 2019). addition to the two most common process types, there are some recently
The MPs will be eliminated if their settling time is less than D/V; developed technologies, including MBR, anaerobic ammonium oxida­
otherwise, they would stay in the effluent of grit. Some MPs might attach tion reactor (ANAMMOX), and shortcut nitrification and denitrification.
to the sand, disperse into grit with the sand, and then be removed. Sand Different biological processes remove MPs in different ways. As a result,
and cinder removal is the primary goal of grit in most treatment facilities the removal of MPs is covered in terms of the biological process type
in order to maximise economic efficiency. It implies that particles with (activated sludge, biofilm, etc.) (Yang et al., 2019; Zhang et al., 2020).
densities lower than 2.6 g/mL won't be removed from the stream The accumulation of the remaining plastic debris, which would then
effectively. As mentioned previously, the densities of alkyd, PS, and settle in the secondary clarification tank, is likely to be aided in this state
PTEE are low when compared to sand and coal cinder. In fact, according by sludge flocs or bacterial extracellular polymers in the aeration tank.
to some researchers, the removal of MPs during grit was even lower than Additionally, sludge flocs containing microplastics may form as a result
6 %. For aerated grit, in addition to the heavy MPs that are removed, of protozoa or metazoans ingesting them. As the suspended particulate
scum and MPs with densities that are slightly higher than or lower than matter congregates into a “floc,” chemicals like ferric sulphate or other
water are removed as well. However, it's possible that aeration will flocculating agents used during the secondary treatment may also have a
cause the MPs attached to the sand to fall off, increasing the MPs in the positive impact on the removal of microplastics (Murphy et al., 2016;
water. According to findings from Yang et al. (2019), aerated grit Phu et al., 2022; Rummel et al., 2017). It was not yet clear how precisely
removed about 60 % of the MPs present in the wastewater. In waste­ the microplastics interact with microbial or chemical flocs or how much
water, heavy MPs typically make up a small portion, and only PS is this might aid in the removal of microplastics. A dynamic redistribution
found among all the heavy types. Assuming that the PS is at its maximum of these particles in the aqueous phase and subsequent evasion of
content (37.5 %) and has a density of 2.3 g/mL, it can be completely removal during the settling stage are also likely outcomes of some
removed through settling. microplastics being trapped in unstable flocs and possibly failing to
settle effectively (Rummel et al., 2017). The duration of contact between
6.1.3. Primary sedimentation microplastics and wastewater in the treatment train is another aspect
By adjusting the water flow velocity, primary sedimentation removes that is thought to be crucial for the removal of microplastics from sec­
the suspended solids (SS). According to Zhang et al. (2020), it is ondary discharges. Recently it was shown that a longer contact time was
necessary for the SS settling time to be less than the tank's water travel linked to a higher risk of the microplastics developing a surface biofilm
time. The target contaminants to be removed during primary clarifica­ coating. The surface characteristics or relative densities of the micro­
tion are the low density SS (1.1 g/mL and 1.5 g/mL) or smaller size with plastics may be changed by the wetting effects of such bio-coatings
high density (1.5 g/mL). PET, PE, and PP are the three MPs that are most (Fazey and Ryan, 2016). Neutrally buoyant particles often can escape
prevalent in the wastewater. PET has a density that ranges from 0.96 to both the skimming and settling processes, so such changes could
1.45 g/mL. It is anticipated that the MPs made of PET would be caught in measurably affect the removal efficiency of microplastics. Therefore, it
the primary sedimentation. Due to their lower density than water, it has may be worthwhile to conduct more research into the effects of contact
been a challenge to remove PE and PP. As a result, primary treatment time and nutrient contents in wastewater on the surface fouling and
can be helpful in removing various moderate-density MPs from removal efficiency of microplastics (Besseling et al., 2017; Fazey and
wastewater. Ryan, 2016; Rummel et al., 2017).
The secondary treatment is capable of removing more fragment
6.1.4. Floatation particles than fibres, in contrast to the pre-treatment processes. Studies
Flotation may be added in some treatment facilities after the initial demonstrating a relative decrease in microplastic fragment abundance
sedimentation process. In the flotation tank, aeration creates the bub­ and an increase in fibre abundance following secondary treatment
bles. The generated bubbles will attract contaminants like oil and small provided support for this. One explanation is that during the pre-
SS, which will float to the top and can be removed after skimming. PE treatment, the fibres that were easily settled or skimmed were largely
and PP are frequently and significantly available MPs in wastewater. removed, whereas the leftovers may have some characteristics, like
They have about 0.89 g/mL of density. According to previous studies, neutral buoyancy, that were difficult to remove further (Talvitie et al.,
selective flotation can effectively remove fine plastic. Therefore, it is 2017). Due to the secondary treatment's ability to further remove large
possible to assume that flotation is an effective method for removing microplastics particles of various sizes, the secondary effluent's abun­
MPs. However, there hasn't been any reported specific work on the dance of these particles is relatively low (Mintenig et al., 2017). Studies
evaluation of MPs removal from wastewater with flotation, which needs revealed that microplastics with a size larger than 500 mm were not
considerable attention in future researches (H. Wang et al., 2019; Zhang present in the secondary effluent (Ziajahromi et al., 2017). According to
et al., 2020). Talvitie et al. (2017), only 8 % of microparticles larger than 300 mm.
remain after secondary treatment. However, Dris et al. (2015) discov­
6.2. Secondary treatment ered that even after secondary treatment, 43 % of the microplastics still
accounted for the size range of 500 mm–1000 mm. It was unclear why
Secondary treatment mostly involves treatment of effluents coming this high proportion existed. It should be further investigated in the
from primary sedimentation tanks. This step is generally accomplished future to determine whether it has anything to do with the specific
by biological decomposition of organic matter, which can be carried out microplastic removal efficiency attained by various secondary treatment
either under aerobic or anaerobic conditions. MPs from sewage can be processes under various operational conditions.
removed with an efficiency of 99 % during primary and secondary
treatment processes (Talvitie et al., 2017), however, a considerable 6.3. Tertiary treatment
amount of effluent that is released can cause plastic pollution (Murphy
et al., 2016). Two types of biological processes are common in secondary This stage makes use of a variety of technologies, including mem­
treatment, the process related to activated sludge, and the process brane bioreactors (MBR), ozone technology, biologically active filters
related to biofilm. Traditional activated sludge processes and some (BAF), sand filters, sequence batch reactors (SBR), media processes

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

(which create anoxic, anaerobic, and aerobic basins using filled car­ performed better than sand filtration (73 % MPs removal). Ozone
riers), etc. (Lee and Kim, 2018). According to records, primary treatment oxidation is even more effective at removing MPs than membrane
procedures remove between 70 % and 98 % of microplastics. Following filtration. It is anticipated that ozone will break down MPs as polymers
that, secondary treatment is capable of reducing the amount of plastics (Chen et al., 2018; Hidayaturrahman and Lee, 2019).
in wastewater effluent by another 20 %, and tertiary treatment can Adsorption with activated carbon has been used in tertiary treatment
further remove plastics from the effluents by another 2 % (Okoffo et al., for the removal of persistent organic pollutants (POPs) and colourants in
2019). Ou and Zeng (2018) have shown that efficiencies of plastic several cases. Because of the adherence, adsorption with activated car­
removal in various treatment processes follow the order of pre-treatment bon (AC) differs from adsorption with EPS. Adsorption with AC could be
> secondary treatment > tertiary treatment. used to adsorb gases, liquids, and solids that resembled jelly, depending
Particle removal can be accomplished effectively through coagula­ on the activated site. As a result, it is not anticipated that adsorption
tion. Coagulation has been shown to be capable of removing >35 % of with AC will remove MPs from water; however, filtration would be one
MPs from water (Ma et al., 2019). According to other sources, the method for doing so as MPs pass through an adsorption column packed
removal of MPs through coagulation was up to 81.6 % (Hidayaturrah­ with AC (Zhang et al., 2020).
man and Lee, 2019). In actuality, because solo coagulation only attaches The smallest sizes fractions (20–100 mm and 100–190 mm) were
or captures MPs in the flocs, it is unable to remove MPs. After coagu­ discovered to be the most abundant following tertiary treatment. In
lation, the solution was allowed to settle, and the removal efficiency was some instances, the relative abundance of fibre in the final effluent may
assessed. It is hypothesized that secondary sedimentation effluent could be higher than in the secondary effluent. This may be because fibres
therefore be removed by coagulation and sedimentation jointly by up to could more easily pass a membrane or filter when they were oriented
81.6 % MPs. Coagulation is typically used in conjunction with other longitudinally (Ziajahromi et al., 2017). This emphasises the importance
treatments like ozone oxidation, rapid sand filtration, and membrane and use of final stage technologies to eliminate small sized and fibre-like
filtration. Following coagulation, flocs are created to capture micro­ microplastics from the effluent. Normally, solids removed by back­
scopic particles like MPs and stop during filtration. As a result, it was washing filters are returned to the WWTP's. Because of this, the micro­
discovered that rapid sand filtration and membrane filtration were both plastics retained by tertiary treatment might not be eliminated from the
very effective at eliminating MPs (Hidayaturrahman and Lee, 2019). WWTP and might even increase its microplastics loading. This portion of
Due to the size advantage, membrane filtration (79 % MPs removal) the microplastics can be removed by pretreatment or secondary

Fig. 3. comparison of removal efficiency in different unit operation and processes (Preliminary, primary, secondary and tertiary treatment processes) in WWTPs.
Reprinted with permission from Elsevier (Cheng et al., 2021), Licence number 5515880501373.

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

treatment as the contact time increases (Michielssen et al., 2016). et al., 2018). The removal effectiveness of various microplastics was
Microplastics returning to the wastewater stream, however, might also influenced by their zeta potential and particle size. With the designed
make it more likely for them to elude the treatment procedures. MOF, a removal rate of up to 95.5 + 1.2 % could be attained. It also has a
Fig. 3 shows the comparison of removal efficiency in different unit high rate of recycling (Chen et al., 2020).
operation and processes in WWTPs.
7.4. Photocatalytic micromotors
7. Microplastic specific treatment technology
The use of micromotors in the removal of oil, metals/metalloids, and
There are some microplastic specific emerging technologies such as various organic materials has drawn the attention of researchers from all
algal masses colonization, bio-inspired molecules, metal organic over the world (Singh et al., 2021). In order to remove microplastics,
framework (MOF)-based foams, photocatalytic micromotors, advanced Wang et al. used photocatalytic TiO2-based micromotors (Au@mag@­
oxidation process. TiO2, mag14Ni, Fe) (L. Wang et al., 2019). These micromotors, which
are photocatalytic, could move themselves in water because of the
7.1. Algal masses colonization photocatalytic reactions occurring on the particles. The removal process
involved interactions between phoretic forces and shoveling/pushing
Since it has been observed that microalgae colonise microplastic forces. When using a chained assembly of micromotors, the shoveling
particles, altering the buoyancy of the aggregates, microalgae can be mechanism took control as opposed to phioretic interaction. The
employed for microplastic removal. Compared to the unaggregated chained assembly of micromotors is a better option for use in environ­
particles, this causes different sedimentation rates (Lagarde et al., 2016). mental systems because it operates effectively without fuel (Singh et al.,
By incorporating the microalgae, this property may be used for the 2021; L. Wang et al., 2019).
removal of microplastics. Fucus vesiculosus, a marine seaweed, was used
by (Sundbæk et al., 2018) to investigate whether microplastics could be 7.5. Advanced oxidation processes
removed through translocation in the algal tissues. The movement of
microplastics was constrained by the algal cells' small channels, which One of the best techniques for mineralizing a variety of resistant
led to the plastic particles' capture. Efficiency of up to 94.5 % has been organic contaminants is advanced oxidation. Typically, reactive oxygen
recorded, particularly in the algae's dissected regions (Martins et al., species are used to mineralize a variety of organic moieties, such as the
2013). The adhesion of plastic particles is improved as anionic poly­ hydroxyl and sulphate radicals. As a result, this approach was also used
saccharide substances leak out of the dissected areas. Electrostatic to study microplastic degradation. The degradation of cosmetic micro­
charge is a key factor in microplastic sorption onto the algal surface. The plastics was accomplished using integrated carbocatalytic oxidation and
presence of anionic polysaccharide components in the algal cell wall hydrothermal hydrolysis (Kang et al., 2019). In this method, perox­
causes the positively charged microplastic particles to sorb more onto ymonosulphate was activated by manganese carbide nanoparticles
the algae (Bhattacharya et al., 2010; Nolte et al., 2017). enclosed in N-doped carbon nanotubes, which led to microplastic
degradation. Depending on the various reaction parameters, including
7.2. Bioinspired molecules the catalyst dose, the concentration of microplastics, the temperature,
and the amount of peroxymonosulfate used, removal efficiencies of up to
Some researchers have created bioinspired molecules to remove 50 % were attained. Using the n-TiO2 semiconductor, ArizaTarazona
microplastic particles. Herbort and Schuhen showed how the use of both and colleagues showed how polyethylene microplastics can be degraded
organic and inorganic molecular building blocks could remove hydro­ photocatalytically in both aqueous and solid matrices. The weight loss
phobic microplastic particles (Herbort and Schuhen, 2017). The inclu­ served as a proxy for the degradation. Microplastic photocatalytic
sion compound (IC) of these bioinspired molecules is made up of an degradation was observed in that the total mass of the particles
inclusion unit (IU) and a capture unit (CU). This molecule's IU subunit is decreased when light and n-TiO2 semiconductor were present (Ariza-
an alkoxysilyl functionalized bioinspired component, and its CU subunit Tarazona et al., 2019). During the first 18 h of visible irradiation, the
can form bonds with various materials thanks to functional groups. The mass loss of polyethylene particles in an aqueous medium was reported
embedded water molecules are moved when the guest molecules to be 6.40 %, whereas in a solid matrix, the mass loss was 1.85 % during
(microplastics) are captured in the inclusion cavity. Through van der the first 16 h of irradiation. The higher concentration of hydroxyl rad­
Waals forces, these released water molecules further combine with other icals, which accelerated the degradation process, was held responsible
nearby water molecules (Tu et al., 2016). As a result, the guest molecules for the higher mass loss of polyethylene particles in the aqueous me­
fill up the cavity left by the release of water molecules, enabling the IC to dium. Similar to this, Tofa and colleagues used zinc oxide nanorods to
aid in the expulsion of the guest molecules. photocatalyze the degradation of low-density polyethylene (LDPE) res­
idues (Tofa et al., 2019). Increased brittleness, the presence of cavities,
7.3. Metal Organic Framework (MOF)-based foams wrinkles, and cracks on the surface of the LDPE after photocatalysis
served as evidence of degradation. It was discovered that the catalyst
Metal and organic ligands are combined to form porous structures surface area directly correlated with the degree of degradation (Singh
known as metal organic frameworks (MOFs). These chemical moieties et al., 2021).
aid in the entrapment of diverse pollutants due to their high surface
area, porosity, and versatile functionality (Singh et al., 2021). The ma­ 8. Bioremediation of microplastics in wastewater: potentials
terial must have enough porosity, a suitable framework to capture the and future possibilities
pollutant, and high durability in order to entrap the microplastics
(Zhang et al., 2016). Since MOFs have these qualities, they might Bioremediation has been cited in many research articles as a prom­
function well. Melamine foam was used by Chen et al. to cover it with ising technique, however, there are certain difficulties in implementa­
zirconium (Zr) MOFs. In order to create Zr-based UiO-66-X (X1H, NH2, tion of bioremediating microorganims inside the WWTP. Millions of
OH, Br, and NO2) MOFs, various 1,4-dicarboxybenzene ligands with microplastics are released into the environment every day by both the
functional groups were used. In a simulated suspension of microplastics discharge of treated water from WWTPs and the use of sewage sludge for
made of polyvinylidene fluoride (PVDF), polymethyl methacrylate soil improvement, despite the fact that MPs are not intended to be
(PMMA), and polystyrene (PS), this framework was used to test the removed and that the retention efficiency of these facilities is around 90
removal of microplastics (Chen et al., 2020; Kobielska et al., 2018; Mon %. As a result, these sites are regarded as a significant source of MP

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A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

release into aquatic environments. There is a chance that some higher 8.3. Environment friendly strategies
eukaryotes may be able to remove MPs from WWTPs. Potential fauna for
this purpose could include annelids (sandworms), echinoderms (sea Till now, a variety of technologies have been developed, including
cucumbers), and possibly other species that have not yet been fully membrane filtration, coagulation, chemical oxidation, and biological
researched. Seagrasses and macrophytes appear to be other potential degradation to remove MPs. The promotion of large-scale application of
candidates, but with some precautions to keep species propagules in these technologies are constrained by a few unresolved issues with them,
check. including a relatively high energy consumption, varying removal effi­
ciencies, and the potential for secondary pollution discharge, even in
cases where removal effectiveness has been demonstrated in research or
8.1. Potentials of marine animals small-scale application (Gao et al., 2022). Some eco-friendly and green
strategies need to be emphasized, such as:
MPs are quickly expelled by gastropods as faecal pellets (Gutow
et al., 2016). Amphipods and copepods both effectively expel MPs in A. Currently, the removal of MPs requires the addition of several
faecal pellets (Au et al., 2015; Blarer and Burkhardt-Holm, 2016), while chemicals to the treatment reactor, including coagulants, oxidation
the Cladocera Daphnia magna does so at varying rates depending on its catalysts, and absorbents. Nevertheless, these chemicals' potential
shape (Frydkjær et al., 2017). However, hybrid predators like toxicity to aquatic life has not been given much thought. These
Hymenaster pellucidus (0.48 0 to 9.10 4.21 microplastics/g) accumulate technologies would eventually pose environmental risks due to their
less MPs fragments and fibres than deposit-feeding and predatory spe­ widespread use. Thus, the use of biologically based or ecologically
cies like the echinoderm Ophiomusium lymani (1.96 0.66 to 3.43 1.35 friendly materials and reagents as adsorbents and coagulators is
microplastics/g). This suggests that filterfeeding or deposit-feeding or­ recommended, such as biochar and starch. An important index for
ganisms will better ingest and retain MPs. Mytilus mussels are useful for removal technologies should be the assessment and quantification of
bioremediation in natural ecosystems because they can hold onto pol­ secondary pollution risks (Gao et al., 2022). Future developments
lutants (Broszeit et al., 2016). Although MPs fragments can stay in the would undoubtedly focus on developing environmentally friendly
circulatory system of animals for 48 days (Browne et al., 2008), the methods and technologies with zero pollution discharge.
majority of MPs fibres, which are abundant, are excreted after 24 h, B. Reverse osmosis, ultrafiltration, nanofiltration, and filtration are a
decreasing the effectiveness of their elimination process (Chae and An, few treatment technologies that have clear drawbacks, like high
2020). The scientific community became interested in other filter- costs and removal efficiency flux. Thus, more work is required to
feeders like cnidarians because adhesion to the coral surface appears create highly effective and affordable abatement technologies to
to be a useful mechanism for MPs retention. Tropical corals are not at all remove MPs. It is important to support and progressively implement
recommended for bioremediation as tropical coral reefs are severely environmentally friendly technologies and green strategies on a large
affected by climate change (Hoegh-Guldberg et al., 2007). The sea cu­ scale. Furthermore, single technologies typically have very variable
cumber, which has been suggested for pollution monitoring (Mohsen removal efficiencies; therefore, integrated technologies can be
et al., 2019), can be a suitable organism for the removal of PCB- created to decrease costs and increase removal efficiencies (Singh
contaminated plastic as it selectively binds to PCBs (Van Cau­ et al., 2021).
wenberghe et al., 2015). The sandworm, Arenicola marina, has a MP C. Reducing the quantity of plastic waste is the green strategy of MPs
retention rate of 240–700 over its lifetime (1.2 2.8 particles/g), possibly pollution abatement, which is based on source control. This can be
without influencing its metabolism. Because MPs have an effect on the accomplished by employing substitute materials, like recycled or
embryonic development of other echinoderms, including sea urchins biodegradable products, to reduce the production of traditional
(Nobre et al., 2015), it is inconceivable to suggest using MPs for biore­ nondegradable plastic. Reusing and recycling plastic products is
mediation without first examining how they affect holothurian health. another eco-friendly and sustainable approach that can help the
Sandworms and holothurians show potential for MPs bioremediation, environment by reducing pollution emissions and saving energy and
but concerns about animal welfare still exist. Before recommending resources (Gao et al., 2022). To reduce plastic pollution, more sig­
applications for WWTP treatment, additional studies should look into nificant measures should be taken to promote the market for recycled
the effects of MPs on them. plastics in addition to outlawing the use of some single-use plastics,
such as bags, food packaging, bottles, and containers.

8.2. Potentials of aquatic plants 8.4. Status of research on MP mitigation using microalgae

Several plants with diverse taxonomic features can be applied for Numerous research endeavors have examined the interaction be­
effective phytoremediation of microplastic. The potential of algae, and tween microalgae and microplastics (MPs), but the majority of these
more specifically microalgae, for bioremediation in water has been investigations have primarily focused on assessing microalgal growth
investigated. Unicellular microalgae, by themselves or in combination and toxicity effects. In recent times, there has been a growing interest in
with bacteria, are able to break down endocrine disrupting substances in exploring the possibility of eliminating MPs through the formation of
wastewaters (Roccuzzo et al., 2021). According to Gutow et al. (2016), hetero-aggregates with microalgae. Notably, Cunha et al. (2019) noted
seaweeds like Fucus vesiculosus can hold onto suspended MPs on their that freshwater algae such as Scenedesmus sp. and Microcystis panniformis
surface. Aquatic macrophytes do not appear to be at ecological risk from produced fewer extracellular polymeric substances (EPS) and displayed
nano- and microplastics at concentrations that are realistic for the reduced hetero-aggregation compared to marine algae like Gloeocapsa
environment. These plant systems could be researched further to explore sp. and Tetraselmis sp. (Cunha et al., 2019). It has been ascertained that
their potential for MPs bioremediation. Seagrasses and aquatic macro­ the interaction between microalgae and MPs is influenced by the size of
phytes with their associated microbiota could be such potential candi­ the MPs. Larger-sized MPs exceeding 25 μm exhibited diminished
dates for MPs bioremediation in marine and brackish WWTP, hetero-aggregation with microalgae (Cunha et al., 2019).
respectively. Additional research should focus on the most effective Furthermore, the majority of research has employed polystyrene (PS)
techniques for raising resilient seagrasses in sludge waters, the ability of to investigate the impact of MPs on microalgae, as demonstrated in
local species to retain MPs, and strategies to guarantee the containment several studies (Bhattacharya et al., 2010; Cunha et al., 2019; Long et al.,
of species propagules. In order to prevent diversity disturbances outside 2017). In contrast, Lagarde et al. (2016) observed that high-density
the WWTP, the latter goal is crucial. polyethylene (HDPE) microplastics failed to form hetero-aggregates

12
A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

with microalgae despite prolonged interaction (Lagarde et al., 2016). In becomes less cost-effective when large quantities of MP-contaminated
another study, Cheng and Wang (2022) effectively removed various groundwater require extraction. Additionally, in off-site remediation,
types of MPs using the freshwater microalgae Scenedesmus abundans the expenses related to constructing pipelines to transport pumped
(Cheng and Wang, 2022). Specifically, three types of MPs—poly(methyl groundwater to treatment plants must also be taken into account.
methacrylate) (PMMA), polystyrene (PS), and polylactide (PLA)—were Currently, investigations into the migration of microplastics (MPs)
subjected to the microalgae, resulting in removal efficiencies exceeding from soil to groundwater primarily rely on small-scale indoor experi­
84 %. PMMA exhibited the highest total removal efficiency at 98 %. ments, which struggle to capture the intricate dynamics of the authentic
Moreover, Cunha et al. (2020) explored the removal of PS nano and soil-groundwater environment. Consequently, an urgent need exists to
microplastics using the freshwater microalgae Cyanothece sp. (Cunha scrutinize the legal and mechanistic aspects of MP migration on a field
et al., 2020). This study not only evaluated the impacts of lower and scale. While recent research has delved into the influence of various
higher concentrations of PS nano and microplastics on microalgal physical, chemical, and biological factors on the movement of MPs from
growth, extracellular sugars, and EPS, but also examined the formation soil to groundwater, the future should prioritize exploring methods to
of hetero-aggregates. Within the context of India, only a limited number curtail this migration by manipulating these factors. Notably, the po­
of studies have addressed the removal of MPs using microalgae. A ma­ tential for MPs to serve as carriers of other pollutants is a critical
jority of these studies have predominantly employed physical and concern. However, there is a paucity of research on the interaction be­
chemical techniques for MP removal, with biological methods remain­ tween MPs and co-carried pollutants during migration and the resultant
ing largely unexplored. Notably, Sarmah and Rout (2018) demonstrated impact on MP movement, highlighting an imperative area for future
the potential of algae species such as Oscillatoria subbrevis and Phormi­ inquiry.
dium lucidum to effectively degrade MPs (Sarmah et al., 2018). These Turning to the remediation of groundwater tainted with MPs,
algae were found to utilize low-density polyethylene (LDPE) as a source existing studies are limited in their exploration of viable remediation
of carbon and energy for degradation (Sarmah et al., 2018). techniques. The methods discussed earlier (adsorption, constructed
wetland, coagulation, and photodegradation) are largely confined to
9. Challenges in microplastic removal from wastewater experimental water samples (Tian et al., 2023). Hence, a forward-
looking approach would involve employing actual MP-contaminated
Examining the scientific literature and numerous studies reveals that groundwater to investigate the efficiency of these techniques for MP
there is a pressing need for more information, particularly on useful removal. These strategies are potentially instrumental for addressing
numbers, such as accurate estimates of the fate, quantity, and exposure MP-contaminated groundwater. In this context, determining the precise
to MPs in humans (Alexy et al., 2020). Due to the lack of a compre­ location of the contamination and employing suitable methods for
hensive definition of what kind of MPs can be considered, it is chal­ groundwater extraction is of paramount importance and warrants
lenging to compare the results of different surveys. Another obstacle to further investigation.
obtaining comparable results is the absence of a standardized method­ A plausible approach involves transporting the extracted wastewater
ology for detection and identification. Therefore, the comparability of to sewage treatment plants, where the removal of MPs is integrated into
the outcomes produced by these methods must be established (Klein the established treatment processes: preliminary treatment, primary
et al., 2018). Each MPs analysis step, including sampling, extraction, treatment, secondary treatment, advanced treatment, and disinfection.
separation, and identification, takes time to reveal a significant barrier This approach capitalizes on filtration, sedimentation, air flotation,
to large-scale monitoring, particularly for MPs for which there are no adsorption, absorption, coagulation, oxidation, degradation, and other
current identification protocols. A more accurate assessment of the processes inherent in sewage treatment plants to effectively eliminate
prevalence of MPs in WWTPs and distribution of MPs in aquatic envi­ MPs from the wastewater. Currently, there's a dearth of research focused
ronments would be possible with the use of appropriate and effective on the environmental and economic analysis of pumping MP-
methods of identification, improving comparisons between studies (Yu contaminated groundwater and treating it within sewage treatment
et al., 2018). MPs concentrations exposure in studies should be closer to plants, indicating a vital avenue for future exploration.
ecologically realistic concentrations to prevent incorrect interpretation In the context of in-situ groundwater remediation, adsorption and
of environmentally unrealistic results. Although some of the cutting- coagulation emerge as potential strategies. However, the entirety of the
edge technologies used in wastewater treatment processes have ach­ adsorption and coagulation processes for in-situ remediation warrants
ieved good removal efficiencies, the mechanism of MPs removal in deeper investigation, particularly in terms of controlling various tech­
wastewater treatment processes has not yet been studied. Utilizing and nical parameters throughout the subterranean process. Furthermore,
researching additional aspects of environmentally friendly removal tackling the challenge of filtering wastewater treated via adsorption and
methods as well as additional options for effective disposal in widely coagulation is an area ripe for further inquiry.
accepted low-cost sewage processing are necessities of the hour. Despite their efficacy, techniques like adsorption, coagulation, and
photodegradation for remediating MP-contaminated groundwater may
10. Evaluation of economic viability introduce secondary pollution risks due to the addition of adsorbents,
coagulants, or photocatalysts. Consequently, future research must delve
In general, preventing microplastics (MPs) from entering ground­ into the potential secondary pollution posed by these additives during
water proves more cost-effective than remediating already polluted the remediation process. While promising results have been obtained for
groundwater containing MPs. The adoption of measures like enacting the removal of MPs via adsorption, constructed wetland, coagulation,
relevant regulations and promoting public awareness to curtail plastic and photodegradation in controlled experiments, the actual perfor­
product usage entails minimal economic expense. Furthermore, the mance might vary when these methods are applied individually in
utilization of recycled plastics aligns with the principles of the circular practical scenarios. Hence, future investigations should concentrate on
economy, centered on reduction, reuse, and recycling (Swetha et al., assessing the efficacy of combined methods for MP removal. Regarding
2022). For introducing cationic salt into the soil-groundwater system, the regeneration and reusability of adsorbents and coagulants, the cur­
the associated costs mainly encompass materials and operational ex­ rent research landscape is inadequate. More studies are necessary to
penses, maintaining an economical aspect. The expense linked to off-site formulate regeneration methods for these materials and to evaluate their
remediation of MP-contaminated groundwater typically surpasses that adsorption and coagulation capacities. Discrepancies in temperature
of on-site remediation due to the high costs involved in groundwater effects on MP removal by various adsorbents underscore the need for
pumping. Factors like terrain, geological structure, and the volume of extensive research into the temperature's influence on different
polluted groundwater influence pumping costs. Off-site remediation adsorption processes. Additionally, the complexity of real-world sewage

13
A. Talukdar et al. Science of the Total Environment 916 (2024) 170254

with its diverse ions demands the development of novel adsorbents globally would be a significant step towards sustainability, especially in
resilient to interference from these ions or other pollutants. the contexts of treatment and removal of emerging contaminants like
For constructed wetlands, optimizing the plant species and substrate microplastics.
materials could yield more efficient compositions. The interaction be­
tween living organisms and MPs remains a significant knowledge gap, CRediT authorship contribution statement
and future research should focus on the metabolic and transformation
processes of MPs within plants, animals, and microorganisms. Avishek Talukdar: Writing – original draft. Pritha Kundu: Writing
In the domain of large-scale electrocoagulation, plate passivation – original draft. Sayan Bhattacharya: Writing – review & editing,
and excessive power consumption can hinder MP removal from Writing – original draft, Validation, Supervision, Conceptualization.
groundwater. Hence, future efforts should concentrate on harnessing Nalok Dutta: Writing – review & editing, Writing – original draft,
renewable energy sources like wind and solar power. Furthermore, the Validation.
exploration of alternative photocatalysts beyond TiO2 or TiO2-based
materials is a promising avenue, accompanied by comprehensive as­ Declaration of competing interest
sessments of the impacts of diverse factors on MP removal through
photodegradation. Due to the intricate nature of MP photodegradation, The authors declare that they have no known competing financial
an imperative task is elucidating the roles played by the principal interests or personal relationships that could have appeared to influence
reactive radical species in various types of MP degradation. the work reported in this paper.
The expenses associated with adsorption or coagulation mainly
revolve around materials and operational costs, which remain relatively Data availability
modest. Constructed wetlands, as a treatment approach, primarily
involve costs related to aquatic plants and operational requirements, No data was used for the research described in the article.
also maintaining an economical profile.
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