Review: Microplastic Mitigation in Biosolids from WWTPs

Review
A review of methods for mitigating microplastic contamination in biosolids
from wastewater treatment plants before agricultural soil application
Sadique Anyame Bawa
a,*
, Andrew Chan
a
, Anna Wrobel-Tobiszewska
a
, Marcus Hardie
b
,
Carmel Towns
a
a
School of Engineering, University of Tasmania, Australia
b
Tasmania Institute of Agriculture (TIA), University of Tasmania, Australia
HIGHLIGHTS GRAPHICAL ABSTRACT
•WWTP biosolids are a signicant source
of MP contamination in agricultural
soils.
•MPs accumulate in sewage sludge and
fragment signicantly during treatment.
•Source, secondary (in-plant), and post-
contamination interventions for miti-
gating MP contamination in biosolids.
•Froth otation, HGMS, pyrolysis, and ES
technologies show promise for miti-
gating MPs in biosolids.
ARTICLE INFO
Editor: Kevin V. Thomas
Keywords:
Microplastics
Biosolids
Agricultural soil
Mitigation technologies
Wastewater treatment plants
ABSTRACT
Wastewater treatment plants (WWTP) are recognized as major sources of microplastic (MP) particles in terres-
trial environments, particularly in agricultural soils through biosolids application. While many reviews have
focused on the distribution, detection, and mitigation of MPs in wastewater efuent to limit their discharge into
oceans, our understanding of methods to mitigate biosolid contamination remains limited. This review focuses on
methods for mitigating MPs contamination in biosolids at various intervention points, including sources, WWTP
including the primary and secondary treatment stages where sludge is generated, and post-contamination. These
methods are categorized as physical, physicochemical, and biological approaches, and their advantages and
limitations are discussed. For instance, physicochemical methods, especially froth otation, are cost-effective but
are hindered by contaminants and reagents. Physical methods like microbre ltration devices (MFD) are safe
but their efciency depends on the lter pore size and design. Biological methods, particularly microbial
degradation, are limited by the varying efciencies of microorganisms in breaking down MPs and the extended
time required for their effective degradation. Other physical methods including dissolved air otation, and
ultrasonication already exist in WWTPs but may require retrotting or optimization to enhance MP removal from
biosolids. As each method inherently has limitations, the key to achieving MP-free biosolids, and thus preventing
their release into agricultural soil, lies in integrating these methods through multi-coupling strategies.
* Corresponding author.
E-mail address: [email protected] (S. Anyame Bawa).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
https://doi.org/10.1016/j.scitotenv.2024.177360
Received 19 June 2024; Received in revised form 17 October 2024; Accepted 31 October 2024
Science of the Total Environment 957 (2024) 177360
Available online 16 November 2024
0048-9697/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (
http://creativecommons.org/licenses/by/4.0/ ).

1. Introduction
The United Nations Environmental Programme (UNEP, 2014) iden-
tied microplastics (MP) as one of the top ten environmental issues,
highlighting their signicant threat to global biodiversity, ecosystems,
and human health. The emergence of MP pollutants is attributed to
widespread plastic use and production, along with inadequate waste
management practices, including improper disposal and recycling
(Geyer et al., 2017;SAPEA, 2019;Zhu et al., 2019). Although the
toxicity of MPs in humans is not fully understood, their presence in the
food chain and persistence in the environment, including soil, fresh-
water, air, and oceans, raises concerns (De Souza Machado et al., 2019).
MPs are plastic particles smaller than 5 mm in diameter, although
recent denitions have been extended from 1 nm to 5 mm. These par-
ticles are either intentionally produced for specic purposes in consumer
products such as exfoliants or result from the fragmentation of larger
plastic materials (Barboza et al., 2019;Frias et al., 2018;Frias and Nash,
2019;GESAMP, 2016, 2019;Surendran et al., 2023;Van Cauwenberghe
et al., 2015). Microplastics enter various environmental compartments
through multiple pathways including wastewater treatment plants
(WWTPs), surface runoff, atmospheric deposition, and agricultural
practices (e.g., plastic mulching, irrigation, and fertilizer usage) (Bl¨ asing
and Amelung, 2018;Corradini et al., 2019;Huang et al., 2020;Piehl
et al., 2018;Zhang et al., 2020a, 2020b). MPs are known to leach
chemically unbound additives that are responsible for inertness, exi-
bility, and colour properties within plastic polymers as they weather and
degrade (Hahladakis, 2018). MPs can adsorb a variety of pollutants,
such as pharmaceuticals, heavy metals, Polychlorinated Biphenyls
(PCBs), and poly-uoroalkyl substances (PFAS) in wastewater. This
adsorption process can lead to the mobilization and transport of these
toxic substances into the ocean environment, resulting in harmful effects
on aquatic biota and bioaccumulation in the food chain (Ziajahromi
et al., 2016;Bryant et al., 2016;Groh et al., 2019;De Souza Machado
et al., 2019;Mammo et al., 2020). While the risks of MPs in aquatic
environments are well documented, understanding the long-term effects
of MPs in agricultural soil, particularly their potential impact on soil
organisms and food safety remains a signicant knowledge gap (Feng
et al., 2020;Horton et al., 2017;Nizzetto et al., 2016;Rillig, 2012, 2018;
Rillig et al., 2023).
Wastewater treatment plants have become a major source of
microplastic pollution in the environment. This is because WWTPs play
a crucial role in receiving and intercepting terrestrial MPs from indus-
trial and domestic wastewater, as well as stormwater (Ziajahromi et al.,
2017). However, treatment systems within these plants are not always
able to completely remove microplastics, leading to their release into the
environment. While MPs primarily enter the ocean through treated
wastewater efuent discharge, in agricultural soils, they enter through
the application of biosolids (Okoffo et al., 2020). Biosolids are treated
organic solid waste (sludge) from WWTPs that are applied to soils to
enhance fertility and promote a circular economy between the agricul-
ture and wastewater treatment sectors. However, the application of
biosolids poses a signicant threat due to the high accumulation of MPs
in the sludge during the primary and secondary wastewater treatment
stages. WWTPs have been reported to concentrate approximately 78-99
% of MPs in the sludge (Gies et al., 2018;Prata, 2018;Ziajahromi et al.,
2016). This has led to the input of substantial amounts of MPs into
agricultural soils, with estimates suggesting that between 1241 and
26,042 t of microplastics are applied annually in the United States,
Canada, Australia, and China (Mohajerani and Karabatak, 2020). Given
the persistent nature of MPs, their gradual accumulation in agricultural
soils over time has been reported, especially in areas with a history of
biosolid application (Corradini et al., 2019;Okoffo et al., 2021;Rios
Mendoza et al., 2021;Rolsky et al., 2020).
Studies have indicated that soil microplastic pollution poses a greater
threat than the aquatic environment because of the direct accumulation
pathway and the extended contact time of MPs within the soil. Unlike in
the ocean, where MP particles exhibit high mobility and can be easily
transported vertically or horizontally by currents, MPs in the soil are less
mobile, leading to a longer contact period that enables the leaching of
toxic chemicals. This extended contact time can decrease microbial ac-
tivity, alter soil pH, and lead to the formation of “plastic-rock com-
plexes”as MPs adhere to inorganic soil particles (Rillig et al., 2023;
Wang et al., 2023). Additionally, MPs have been detected in earth-
worms, where they impair growth, cause gastrointestinal damage, and
even lead to mortality (Huerta Lwanga et al., 2016;Rezaei Rashti et al.,
2023). Porous soils can also facilitate the vertical migration of smaller
microplastic particles to deeper depths, potentially posing a threat to
groundwater security (Mintenig et al., 2019;Okutan et al., 2022).
Furthermore, MPs have been observed to have adverse effects on the soil
water cycle by reducing water inltration, increasing evaporation rate,
and decreasing soil bulk density (Guo et al., 2022;Wan et al., 2019).
Changes in soil properties due to microplastic pollution have been re-
ported to have a cascading effect on plant growth, root traits, and
nutrient uptake (de Souza Machado et al., 2018, 2019;Huang et al.,
2023;Rillig et al., 2019;Zhang et al., 2019). There is also concern that
MPs may accumulate and translocate to the edible portions of plants,
potentially entering the food chain (Aydın et al., 2023). Despite the
growing concern, research in this area still presents notable gaps, with
only 9.2 % of global MP studies focusing on biosolids-amended soils
(Ziajahromi and Leusch, 2022).
Currently, there are no wastewater treatment facilities specically
designed to eliminate MPs. Despite the efciency of current conven-
tional treatment systems in removing MPs from wastewater, small
fractions still escape through the efuent discharge (Gao et al., 2022).
The main challenge lies in managing sludge, where the removed MPs
tend to accumulate. Several removal methods have been proposed to
address MPs in biosolids. Thermal destruction, such as high-temperature
incineration, is considered the most effective method for removing
microplastics from sewage sludge, including biosolids (Vahvaselk¨ a and
Winquist, 2021). However, studies have shown that this approach may
lead to the formation of persistent MPs, with bottom ash potentially
serving as a new source of contamination (Shen et al., 2021;Yang et al.,
2021a, 2021b). An alternative approach involves the separate treatment
of otation-skimmed sludge from dissolved air otation (DAF). This
process is thought to remove grease and oils in wastewater along with
MPs, forming a scum layer that can be treated separately from other
sludge generated at the treatment plant to prevent further contamina-
tion (Sun et al., 2019). Although promising, this approach does not
completely solve this issue, as it still requires addressing the removal of
MPs before disposal. Moreover, the otation method primarily targets
less dense particles, leaving denser MPs to contaminate the settled
sludge (Vahvaselk¨ a and Winquist, 2021).
To ensure the safe reuse of biosolids in agriculture, more research
and development of microplastics removal methods are needed to pre-
vent agricultural soil contamination. Early intervention methods are
needed to separate and remove MPs from wastewater before they reach
treatment facilities. Other potential methods include oating MPs dur-
ing wastewater treatment to prevent them from settling into solid waste,
removing them from sludge during treatment or biosolids, and in some
instances, remediating contaminated agricultural soil.
This review discusses various methods for mitigating MPs in WWTP
biosolids before agricultural soil application, with a specic focus on
their implementation at key points along the source-to-biosolids
pathway. We begin by discussing the sources of MPs in biosolids and
their fate within WWTPs, with a focus on the primary and secondary
treatment stages where sludge is generated as well as the subsequent
sludge treatment processes. Following this, we discussed and compared
methods for mitigating MP contamination in biosolids at three main
intervention points: source interventions, secondary (in-plant) in-
terventions, and post-contamination interventions. These methods are
further classied into physical, physicochemical, and biological ap-
proaches, and their effectiveness and limitations are discussed. Finally,
S. Anyame Bawa et al. Science of the Total Environment 957 (2024) 177360
2

we identied areas for future research to better mitigate MP contami-
nation in biosolid, contributing to broader environmental protection
efforts.
2. Sources of MPs in biosolids
MPs in WWTPs originate from various sources, including household
activities, industrial processes, and stormwater runoff. While a portion
of these MPs are released into the aquatic environment through efuent
discharge, the majority are captured in the sludge. When converted into
biosolids and applied to agricultural soils, this sludge acts as a pathway
for MPs to enter terrestrial environment (Ben-David et al., 2021;
F¨ altstr¨ om et al., 2021;Okoffo et al., 2019;Suaria et al., 2020;Sun et al.,
2019). Common household sources of MPs include cleaning products
and personal care items such as toothpaste, facial scrubs, body wash,
and cosmetics. These products often contain micro-sized plastics, known
as primary MPs, which are intentionally manufactured for specic
functions, particularly exfoliation. Industrial activities, especially the
production of plastic goods, contribute signicantly to the presence of
primary MPs in wastewater through the release of plastic pellets.
Additionally, wastewater often contains secondary MPs, which result
from the breakdown of larger plastic into smaller fragments. This frag-
mentation occurs through mechanical, thermal, and biological degra-
dation processes that plastics undergo throughout their lifecycle (Kong
et al., 2020). Synthetic textiles are a major source of secondary MPs that
shed bres during laundering. Other sources include the degradation of
food packaging and the use of industrial abrasives in surface blasting
(Jan Kole et al., 2017;Magni et al., 2019;Parashar and Hait, 2022;
Siegfried et al., 2017;Suaria et al., 2020;Sun et al., 2019;Zahra et al.,
2022). Stormwater runoff further exacerbate this issue by introducing
additional secondary MPs. Particles from construction materials, poly-
vinyl chloride (PVC) pipe wear, tyre wear, road markings wear, and
littered plastics, especially in combined sewer systems, contribute to the
inux of MPs into WWTPs (Acarer, 2023;Dris et al., 2015;Zhang et al.,
2021a, 2021b, 2021c, 2021d, 2021e). Fig. 1 shows a simplied illus-
tration of the source and pathway of MPs.
The variety of plastic materials used in everyday products is reected
in the types of MPs found in sludge and biosolids. Environmental frag-
mentation inuences the size and shape of MPs, leading to signicant
spatial and temporal variations in secondary MPs (Andrady, 2017). The
characteristics of these particles also depend on several factors,
including the catchment area of the WWTP, wastewater sources, and
specic treatment processes applied (Edo et al., 2020;Raju et al., 2020;
Talukdar et al., 2024). For instance, higher concentrations of foam and
bead-shaped MPs have been observed in wastewater from economically
developed regions, likely due to the extensive use of personal care
products and insulation materials (Hu et al., 2022a, 2022b).
In biosolids, MPs appear in various forms including spheres, frag-
ments, lms, foam, microbeads, pellets, and granules. Fibres and frag-
ments are the most commonly observed types, often originating from
plastic packages and synthetic textiles during laundering (Gies et al.,
2018). The predominant plastic detected in biosolids include poly-
ethylene (PE), polypropylene (PP), and polyester (PET) (Murphy et al.,
2016;Ziajahromi et al., 2021;Azizi et al., 2022). These materials are
commonly used in packaging, textiles, and consumer products, resulting
in their signicant presence in wastewater and biosolids. Table 1 sum-
marizes studies on MPs detected in WWTP sludge and biosolids.
3. MPs pathway and fate in WWTPs
WWTPs consist of two main components, wastewater treatment
process and sludge (solid waste) management. Understanding the
movement of MPs through these processes is essential for identifying
and optimizing treatment methods at each stage to mitigate MP
contamination in biosolids.
A typical WWTP consist of a preliminary wastewater treatment stage,
such as screening and grit removal, which have been reported to remove
a signicant percentage (35.1–58.6 %) of MPs. Further removal occurs
during the primary and secondary treatment stages, with removal rates
ranging from 56.8 % to 98.3 % (Michielssen et al., 2016;Ziajahromi
et al., 2021). However, although WWTPs can remove signicant amount
of MPs from wastewater, they do not eliminate MPs from the resulting
Fig. 1. Simplied diagram of MP sources and pathway into WWTPs biosolids.
S. Anyame Bawa et al. Science of the Total Environment 957 (2024) 177360
3

sludge (Chen et al., 2020a, 2020b). The removal of MPs during waste-
water treatment is inuenced by their physicochemical characteristics,
such as polymer type, size, and shape. For instance, smaller MPs tend to
remain suspended in the wastewater, increasing their likelihood of
escaping removal, whereas larger MPs settle more effectively into the
sludge (Long et al., 2019). The hydrophobic nature of MPs allows them
to form complexes with exopolymeric substances (EPS) or biolms
during treatment, facilitating their settling and removal from
wastewater into the sewage sludge matrix (Melo et al., 2022;Summers
et al., 2018;Zhang and Chen, 2020). Following the wastewater treat-
ment, the generated sludge undergoes further processing to create
biosolids.
This sludge treatment process typically involves several stages
(Fig. 2), each of which can signicantly affect the characteristics and
distribution of MPs. The initial stage of the process involves thickening,
in which sludge from the primary and secondary treatment stages is
mixed and thickened to reduce volume (Di Giacomo and Romano,
2022). During this thickening step, the physical and mechanical forces
applied to the sludge can cause microplastics to fragment. Studies have
suggested that the mechanical thickening method, where sludge is
subjected to pressing, may lead to microplastic fragmentation and size
reduction. In contrast, gravity thickening, which relies on sedimentation
to concentrate organic solids, may not signicantly impact MPs frag-
mentation, and could potentially remove only around 6 % of, predom-
inantly, lower-density MPs through supernatant removal (Alavian
Petroody et al., 2021;Mahon et al., 2017;Xu and Bai, 2022).
Following the thickening stage, the sludge is stabilised by chemical,
biological, or thermochemical methods to eliminate or reduce patho-
gens, odours, and contaminants. Chemical approaches, such as alkaline
stabilization, can cause shredding of MPs because of the high alkalinity
(pH >12) and mechanical mixing of the sludge slurry. In addition,
biological stabilization methods such as aerobic digestion does not
signicantly degrade MPs but can induce surface alterations such as
irregular cracking and the generation of smaller particles (Xu et al.,
2019;Zhang and Chen, 2020). Hyperthermophilic composting has been
found to degrade specic MPs, while thermal treatments such as incin-
eration can alter the surface characteristics of MPs and enhance their
adsorption to pollutants (Cydzik-Kwiatkowska et al., 2022;Hooge et al.,
2023).
Mechanical dewatering is used to further reduce the volume of sta-
bilised sludge using techniques such as lter or screw pressing and
centrifugation. During the centrifugation process, an estimated 20 %-54
% of microplastics were reported to be removed along with the centrate
(press water). The removed centrate is cycled back into the inuent
stream before entering the primary treatment stage of the wastewater
treatment plant. This exposes the MPs in the centrate to further abrasion
and breakdown, as they go through the treatment process again (Alavian
Petroody et al., 2021;Hatinoglu and Sanin, 2022;Hutli et al., 2019). The
recirculation process can potentially contribute to more diverse MP
sizes, reducing their chances of being effectively removed from the
wastewater. Mechanical dewatering of sludge often leaves it partially
wet, necessitating thermal drying to further reduce moisture through
evaporation. However, thermal drying can cause microplastics to melt
and blister, subjecting them to further breakage into ner particles (Edo
et al., 2020;Mahon et al., 2017).
Throughout the sludge treatment process, up to 82 % of larger MPs
reportedly fragment into sizes ≤500
μ m in biosolids due to the combined
effects of mechanical, chemical, and biological processes. The most
common MP size detected after treatment is approximately 250
μ m,
which has been reported to inuence sludge behaviour (Chen et al.,
2020a, 2020b;EL Hayany et al., 2020;Koutnik et al., 2021). Following
treatment, biosolids are transported and applied to agricultural soil,
where they accumulate.
3.1. Impact of MPs on sludge rheology
The abundance of MPs can signicantly inuence the rheological
properties of sludge, which refers to their ow and deformation
behaviour under stress. Although research on the direct impact of MPs
on sludge rheology is still limited, studies on lake sediments suggest that
the size and type of MPs can signicantly affect the sediment behaviour.
For example, variations in MP size and composition have been shown to
inuence viscosity, ow properties, and yield stress, potentially leading
to sediment resuspension even under minor disturbances from wind or
Table 1
A summary of studies on MPs contamination of WWTPs sludge.
Matrix type Size range
(
μ m)
Polymers
isolated
Shape Reference
Primary
sludge
>25
μ m PET, PA, PE,
PP
Fibres,
fragments,
granular
Ziajahromi
et al. (2021)
>1.0
μ m–Fibres, foams,
pellets
Gies et al.
(2018)
<500
μ m,
500–2000
μ m,
≥2000
μ m
PP, PA, PE,
PES
Fibres,
fragments,
spheres
EL Hayany
et al. (2020)
300–5000
μ m PP, PE, PES Fragments,
lm lines,
glitters
Pittura et al.
(2021)
<100
μ m,
100–1000
μ m,
≥1000
μ m
PES, PVA, PE,
others
Fibres,
fragments
Harley-
Nyang et al.
(2022)
Secondary
sludge
>25
μ m PET, PA, PE,
PP
Fibres,
fragments,
granular
Ziajahromi
et al. (2021)
>1.0
μ m–Fibres,
fragments
Gies et al.
(2018)
10–5000
μ m–Fibres, foils,
spheres
Leslie et al.
(2017)
<250
μ m,
≥250
μ m
PET, PS, PA,
PVC
Fragments,
Fibres, Foams,
Films
Men´ endez-
Manj´ on et al.
(2022)
>50
μ m PE, PP, PES,
acrylic bres
Fibres,
fragments
Schell et al.
(2022)
<100,
100–1000,
≥1000
μ m
PES, PVA, PE, Fibres,
Fragments
Harley-
Nyang et al.
(2022)
45–400
μ m–Films, Fibres Carr et al.
(2016)
Biosolids 37–5000
μ m Polyolen,
acrylic, PA,
alkyd resin, PS
Fibres, shafts,
lms, akes,
spheres
Li et al.
(2018)
60–4200
μ m PA, PP, PE, PC Fibres,
fragments,
microbeads,
lms, foams,
Liu et al.
(2019)
–PBA, Rayon,
PA, PBMA, PE,
PET, PP, PPE,
CP, PVC etc.
Fibres, pellets,
microbeads,
fragments,
lms
Xu et al.
(2020)
200–5000
μ m,
<200
μ m
PP, PET, PE,
PB, EVA
Flakes, bres Zhang et al.
(2020a,
2020b)
80–1700
μ m PVC, PB,
PFTE, PE,
PAN,
Fragments,
spheres, lms,
bres
Ren et al.
(2020)
–PP, PA, PE,
PES
–EL Hayany
et al. (2020)
20-100
μ m,
100-200
μ m,
200-500
μ m
and 500-5000
μ m
PE, PET, PU,
Acrylate,
PMMA, PC,
PA, PVC, PS,
PP
bres,
fragments, or
beads
Ziajahromi
et al. (2024)
<25
μ m, 63-
1000 >
μ m
PE, PVC, PS,
PP, PC, PET
–Okoffo et al.
(2022)
20
μ m-5000
μ m
PE, PP, PA,
PVC, PET,
PMMA
Fibres and
fragments
Rezaei
Rashti et al.
(2023)
Polyethylene (PE), polypropylene (PP), polyethersulfone (PES), polyamide (PA),
polyester (PET), polystyrene (PS), polyvinyl chloride (PVC), polyurethane (PU)
and polycarbonate (PC), Polybutylene (PB), Polymethyl Methacrylate (PMMA).
S. Anyame Bawa et al. Science of the Total Environment 957 (2024) 177360
4

waves (Wu et al., 2024).
In sludge, MPs are known to inuence key characteristics such as oc
formation, settling, and EPS composition, all of which are essential for
dening its rheological properties. MPs may fragment ocs, reduce their
size, and alter the composition of microbial communities and extracel-
lular polymeric substances, both of which are critical for sludge aggre-
gation and sedimentation (Hatinoglu and Sanin, 2022). Additionally,
MPs are considered a component of the total suspended solids (TSS) in
sludge, and research has demonstrated a direct positive correlation be-
tween increased TSS levels and higher sludge viscosity (Collivignarelli
et al., 2019). These changes may strongly inuence the rheology of
sludge, potentially reducing the treatment efciency by affecting ow
dynamics, particularly during processes such as anaerobic digestion and
digested sludge dewaterability (Hong et al., 2018;Xu et al., 2021).
From a microplastic mitigation perspective, an understanding of the
complex relationship between the presence of MPs, sludge rheology, and
treatment efciency is needed. This is essential to enhance the ability to
effectively remove MP contamination, as methods for their removal may
need to be adjusted to account for potential changes in sludge rheo-
logical properties.
4. MPs accumulation in agricultural soils through biosolids
application
Biosolids are treated sludge that meet regulatory standards and can
be safely applied to land, including agricultural soils. In the European
Union (EU), approximately 35 % of arable land receives biosolids, with
even higher usage in the United States (55 %), Canada (60 %), and China
(45 %) (Lu et al., 2012;Huˇ sek et al., 2022;Mohajerani and Karabatak,
2020;Raheem et al., 2018). In Australia, the utilization of biosolids in
agricultural land has increased from 55 % in 2010 to 73 % in 2021
(Vero, 2022).
Continuous biosolid application leads to the persistence and accu-
mulation of MPs in agricultural soils due to their inert nature. In
Australia alone, it is estimated that 4700-372,300 metric tonnes of
plastic particles enter agricultural soils annually via biosolid applica-
tions (Okoffo et al., 2021;Ziajahromi et al., 2021). Similarly, in Canada,
approximately 8740 metric tonnes of MPs are introduced into agricul-
tural land through biosolid reuse annually (Mohajerani and Karabatak,
2020).
Van Den Berg et al. (2020) showed that repeated biosolid applica-
tions lead to a threefold increase in MPs accumulation in soils compared
to untreated soils. MPs have been found in the soil 15 years after the last
biosolid application, indicating their persistence (Zubris and Richards,
2005). Similar patterns of MP contamination and accumulation in
agricultural soils due to biosolid use have been reported by Keller et al.
(2020),Tran et al. (2023),Yang et al. (2021a, 2021b), and Colombini
et al. (2022).
The widespread use of biosolids globally indicates their value as
sustainable resources for enhancing soil quality. However, this extensive
application also reects the scale of the potential risk of soil pollution
due to microplastic contamination in biosolids from WTTPs.
5. Microplastics-targeted mitigation methods
Wastewater treatment facilities vary signicantly in terms of infra-
structure, operational parameters, and wastewater inuent character-
istics. Thus, a one-size-ts-all approach to mitigate microplastics,
especially from biosolids, may not be effective across treatment facil-
ities. However, there are opportunities along the microplastics pathway,
spanning from the wastewater source through the various stages of
wastewater treatment, sludge processing and biosolids, to capture or
transform microplastics before they are transferred to agricultural soils
for application.
This section categorises microplastic mitigation methods based on
specic intervention points along the pathway from the source of
wastewater to biosolids (Fig. 3). This highlights three key intervention
stages: source, secondary (during wastewater treatment), and post-
contamination.
Research on many of these methods is still in its initial stages,
focusing primarily on bench-scale testing to improve quantitative
analysis for extracting MPs from various environmental media. These
methods have the potential for MP pollutant mitigation. Additionally,
some wastewater treatment methods, although not originally designed
for MP mitigation, have demonstrated the capacity to reduce MP
pollution. However, these methods have inherent limitations, including
incomplete removal, potential for MP fragmentation, or the need for
additional pretreatment steps. The economic advantage of these
methods is that they require less feasibility testing for large-scale
adoption, as they may only require optimization to effectively target
and remove MPs.
MPs mitigation approaches can be broadly classied as physical,
physicochemical, and biological methods. Physical methods of micro-
plastics mitigation operate through non-chemical mechanisms, such as
capture, physical separation, otation, and sedimentation, whereas
physicochemical methods involve chemical reactions accompanied by
physical removal mechanisms. Biological methods, however, involve
the conversion and mineralization of MPs into CO
2
and H
2
O through
Fig. 2. About 92 % of MPs in sludge enter the thickening tank (Gies et al., 2018). 6 % of MPs are removed with supernatant water during thickening (Alavian
Petroody et al., 2021). 20-54 % further reduction of MPs in sludge occurs during mechanical dewatering (centrifugation) (Alavian Petroody et al., 2021;Talvitie
et al., 2017). About 79 % of MPs in biosolids are transported to soil (Gatidou et al., 2019) (source: author).
S. Anyame Bawa et al. Science of the Total Environment 957 (2024) 177360
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