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Abstract and Figures

Plastic pollution of the aquatic environment is a major concern considering the disastrous impact on the environment and on human beings. The significant and continuous increase in the production of plastics causes an enormous amount of plastic waste on the land entering the aquatic environment. Furthermore, wastewater treatment plants (WWTPs) are reported as the main source of microplastic and nanoplastic in the effluents, since they are not properly designed for this purpose. The application of advanced wastewater treatment technologies is mandatory to avoid effluent contamination by plastics. A concrete solution can be represented by membrane technologies as tertiary treatment of effluents in integrated systems for wastewater treatment, in particular, for the plastic particles with a smaller size (< 100 nm). In this review, a survey of the membrane processes applied in the plastic removal is analyzed and critically discussed. From the literature analysis, it was found that the removal of microplastic by membrane technology is still insufficient, and without the use of specially designed approaches, with the exception of membrane bioreactors (MBRs).
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Molecules 2019, 24, 4148; doi:10.3390/molecules24224148
Membrane Processes for Microplastic Removal
Teresa Poerio *, Emma Piacentini and Rosalinda Mazzei
National Research Council—Institute on Membrane Technology (ITM–CNR) c/o University of Calabria,
Cubo 17C, Via Pietro BUCCI, 87036 Rende (CS), Italy; (E.P.); (R.M.)
* Correspondence:; Tel.: +39-0984-492076
Academic Editor: Alberto Figoli and Francesco Galiano
Received: 11 October 2019; Accepted: 12 November 2019; Published: 15 November 2019
Abstract: Plastic pollution of the aquatic environment is a major concern considering the disastrous
impact on the environment and on human beings. The significant and continuous increase in the
production of plastics causes an enormous amount of plastic waste on the land entering the aquatic
environment. Furthermore, wastewater treatment plants (WWTPs) are reported as the main source
of microplastic and nanoplastic in the effluents, since they are not properly designed for this
purpose. The application of advanced wastewater treatment technologies is mandatory to avoid
effluent contamination by plastics. A concrete solution can be represented by membrane
technologies as tertiary treatment of effluents in integrated systems for wastewater treatment, in
particular, for the plastic particles with a smaller size (< 100 nm). In this review, a survey of the
membrane processes applied in the plastic removal is analyzed and critically discussed. From the
literature analysis, it was found that the removal of microplastic by membrane technology is still
insufficient, and without the use of specially designed approaches, with the exception of membrane
bioreactors (MBRs).
Keywords: plastic removal; wastewaters treatment; membrane processes; ultrafiltration; dynamic
membranes; reverse osmosis; membrane bioreactors; membranes reuse; membranes recycling
1. Introduction
The world plastic production is constantly growing, with production rising from 335 million
tons in 2016 to 348 million tons in 2017. [1]. Asia is the largest producer of plastics (50.1%), followed
by Europe (18.5%), North American Free Trade Agreement (17.7%), Middle East, Africa (7.71%),
Latin America (4%) and Commonwealth of Independent States (2.6%). This significant increase and
widespread in worldwide production of plastics produces a huge amount of plastics waste on land
that enters the aquatic environment causing growing concerns [2].
Different papers report that the presence of a large part of microplastic fibers in the aquatic
environment is due to the washing of synthetic clothes [3,4]. The ingestion of microplastic, besides
causing the obstruction of digestive tract, can facilitate the transfer of contaminants adsorbed by the
plastic, with unclear consequences to the health of aquatic organisms and humans [5–7]. Indeed a
major problem with microplastic is their ability to adsorb other common environmental
contaminants, such as metals [8–10], pharmaceuticals [11,12], personal care products [12] and others
[13,14]. Consequently, the microplastic can potentially cause diseases such as cancer, a malformation
in animals and humans, impaired reproductive activity, and reduced immune response [15].
Microplastic removal from the aquatic environment represents a new urgent challenge in the
last decade considering the disastrous impact for aquatic species and human beings. These
contaminants have been detected in various aquatic environments such as lakes, rivers, oceans,
urban wastewater effluents.
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Based on the particle size, plastics are defined as microplastic (MP) and nanoplastic (NP).
According to the National Oceanic and Atmospheric Administration (NOAA), the definition of
microplastic is particles of synthetic polymers (less than 5 mm in diameter), which resist (bio)
degradation [16]. On the other hand, nanoplastic is defined as particles (nanospheres,
nanowires/nanotubes, and nanofilms) with smaller dimensions, between 1 and 100 nm [17–20].
Based on their origin, MP and NP are divided into two classes, primary and secondary plastic
[21]. The first includes small pieces of specially manufactured plastic, such as hand and facial
cleansers, shower gels, toothpaste, industrial scrubbers, and plastic micro-nanospheres, etc., while
the latter are small pieces of plastic derived from the deterioration of larger plastic waste both at sea
and on land.
The more common plastic materials founded in the effluents are polypropylene (PP),
polyethylene (PE), Polystyrene (PS), polyvinyl-chloride (PVC), polycarbonate (PC), polyamides (PA)
Polyester (PES), and polyethylene terephthalate (PET), these are reversible thermoplastic polymers,
highly recyclable materials that can be heated, cooled, and shaped repeatedly [22].
In addition to these recyclable materials, also thermoset polymers have been found, they are
irreversible materials that after being heated and formed they cannot be re-melted, reformed, and
recycled; the most common thermoset materials are epoxy resins, vinyl ester, silicone, melamine
resin, unsaturated polyester, phenolic resins, polyurethane, formaldehyde, acrylic resins, etc. [22].
Nowadays, 98% of MP is retained from wastewater treatment plants (WWTPs) but MP with a
size smaller than 20 μm and NP is not retained; therefore, WWTPs plants are supposed to be one of
the major responsibility for the plastic pollution in wastewater effluents [22–24]. The wastewater
processing can be grouped into four main treatments: preliminary treatment, primary treatment,
secondary treatment, and tertiary treatment, also named final or advanced treatment [25] (Figure 1).
Figure 1. Classification of wastewater treatment methods.
These treatments include different methods and technologies (some of them are reported in
Figure 1) whose use depends on the nature of the wastewater to be treated, as well as on the
different quality of the water to be obtained (drinking water, swimming, agriculture, industries,
Preliminary treatment is often required to protect equipment and enhance the performance of
subsequent treatment processes. Primary treatment consists of removing large suspended organic
solids, but the liquid effluent coming from this treatment still contains a large amount of suspended
organic material, indeed the microplastic removal efficiency is approximately 25%. The secondary
treatment, despite more efficient, can reduce microplastic concentrations by 75% [26]. Removal
Molecules 2019, 24, 4148 3 of 15
efficiency of 98% could be reached by the not always used tertiary treatment, producing an effluent
of almost drinking-water quality [23–26]. The limited application of tertiary treatments in the
WWTPs, coupled with the huge amount of treated wastewaters to obtain water with different
quality, are a source of plastic in the effluents. The application of advanced final stage wastewater
treatment technologies is mandatory to avoid effluent contamination by plastics. Among the tertiary
treatment processes, membrane operations can offer an effective solution to the microplastic and
nanoplastic pollution in the effluents.
To date, only a few papers report the application of membrane processes for microplastic
removal. In this review, an analysis of the documents in this field has been carried out to highlight
the growing interest of the scientific community towards the problems of plastic pollution as well as
to demonstrate the still insufficient knowledge and experience in the removal of plastic, with specific
emphasis on the application of the membrane technologies. The membrane processes actually
applied to plastic removal have been reported and critically discussed.
2. Literature Analysis on the Microplastic and Its Removal by Membrane Processes
A document search was conducted using Scopus ( searching
“microplastic removal” as a specific keyword in “Article Title, Abstract, Keywords” and selecting
“Article” as document type in order to perform the search as general as possible, excluding the
review. A total of 79 documents published from 2015 to 2020 were collected and used as the database
for analysis. The chronological distribution of publications related to the microplastic removal by
membrane technology and other treatments, as well as the incidence of membrane treatments in the
field, are reported in Figure 2.
Figure 2. A) The distribution of publications related to microplastic contaminant removal from 2015
to 2020 and B) the incidence of the research on membrane technology applications with respect to
other existing treatment processes.
In the last five years, the removal of microplastic received increased attention, and the
distribution in Figure 1 shows one peak corresponding to 2019. Although the application of
membrane technology in the removal of microplastics is still limited, the last year registers a certain
increase of studies in which the conventional membrane separation process and membrane
bioreactor (MBR )are combined with the other existing treatment processes to reach a more effective
removal of microplastic contaminants from wastewaters. The removal of the plastic was found to
strictly depend on some parameters used as indicators to classify it, such as the shape, size, and mass
of plastic particles. The main influencing factors that can affect the performance of membrane
processes for MP removal are listed in Table 1.
Total documents: 79
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Table 1. Influencing factors and membrane process parameters to be considered for microplastic
(MP )removal by membrane processes.
Influencing Factors Membrane Process Parameters
Membrane process
Membrane material
-Transmembrane pressure
-Polarization concentration
-Cake layer formation and
-Specific energy consumption
Membrane pore size
Membrane thickness
Membrane surface properties
Source of polluted water (Seawater,
surface water, municipal water,
industrial wastewater etc.)
Chemical composition
It should be highlighted that the comparison of the literature data in terms of removal efficiency
of MP is not so simple considering the different composition of the treated wastewater, the different
characterization procedures of MP, and also the range size of the MP considered. In particular, the
inexistence of standardized protocols of characterization has led to a lot of information not directly
comparable due to the use of different units (e.g., mass per volume, number per volume, etc.) [19].
The removal efficiency of microparticles in some WWTPs was reported by Sun et al. 2019 [27] and
summarized in Table 2. In some cases, a very high MP removal was reported, probably due to the
size range of the MP considered. However, despite the high plastic removal, the huge amount of
treated wastewaters is considered a source of plastic in the effluents [22].
Table 2. Microplastic removal by different wastewater treatment plants (WWTPs) (Data elaborated
from Sun et al. 2019).
Treatment Processes Microplastic Removal (%) WWTP location
Primary, Secondary 99.9 Sweden
Primary, Secondary (Biofilter) 88.1 France
Primary, Secondary 99.9 United States
Primary, Secondary 98.4 Scotland
Primary, Secondary 11–94 Netherlands
Primary, Secondary 95.6 United States
Primary, Secondary 98.3 Finland
Primary/AnMBR 99.4 United States
Primary/MBR 99.3 Finland
, Secondar
, Tertiar
97.2 United States
Primary, Secondary, Tertiary (BAF) 97.8 Finland
Secondary treatment: conventional activated sludge process; AnMBR: anaerobic membrane
bioreactor, MBR: membrane bioreactor; GF: granular filter; BAF: biological aerated filter.
The shape of the plastic particles affects their removal efficiency in WWTPs and can determine
the interaction between other contaminants or microorganisms [28]. The shape is categorized as
fiber, granular, fragment, film, and foam (Figure 3).
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Figure 3. Common microplastic shapes and related materials (Elaborated from [19] and [22]).
The most abundant shape of plastic particles found in the wastewaters is into fiber due to the
discharge of domestic washing machines containing synthetic polymers for clothing [29]. Some
studies report that pretreatment removes fibers more effectively than fragments, while secondary
treatment removes more fragment particles than fibers [27-31].
Regarding the plastic size dimensions, 25 μm, 100 μm, and 500 μm are most frequently used for
size classification [21,30,31], but recent studies showed that plastic particles with nanometer size are
also present in the wastewater due to the fragmentation of synthetic fibers or to the degradation of
polymers [19,32]. Fragmentation and degradation are the two main steps in the breakdown process
of polymer particles [33]. Fragmentation is the process of breakdown of larger polymer chains into
smaller polymer fragments. Degradation is a process of bond-break, followed by a chemical
transformation that changes the polymer properties. This process can occur by hydrolysis,
photodegradation, mechanical/physical degradation, thermooxidative degradation, and
biodegradation [34]. The reduced size allows the nanoparticles to pass through biological barriers
[35] to penetrate tissue, to affect the behavior and metabolism of organisms to accumulate in organs,
and to enter the base of a food chain more easily than larger. Furthermore, the degradation processes
increase the surface area of its degradation products, causing an enormous biological impact, for
example, the 40 nm nanoparticles deriving from the degradation of a common plastic bag have a
surface area of 2600 m
[36]. A more detailed physical-chemical characterization of plastic, such as
size, mass, shape, and chemical composition is necessary, since not only can it help for a better
understanding of their real threat [19,20] but it can also allow selecting appropriate methodologies
that guarantee a more efficient plastic removal from effluents.
Regarding the amount of microplastic contained in wastewater, some information was found in
studies conducted in Northern Europe, the United States, and Australia. The detected amounts of
microplastic varied between 1–3160 and 0.0007–125 particles L
for raw and treated wastewater,
respectively. However, data for the Mediterranean area, Asia and South America are lacking [37].
Jenna R. Jambeck et al. calculated that 275 million metric tons (MT) of plastic waste were produced
in 192 coastal countries in 2010, with 4.8 to 12.7 million MT entering the ocean. Furthermore, this
quantity of plastic waste is expected to increase by order of magnitude by 2025 [38].
2.1. Ultrafiltration
Ultrafiltration (UF) represents a feasible alternative for the water treatment since it permits to
attain drinking water of high quality in an economic manner thanks to low energy consumption,
high separation efficiency, and compact plant size [39–41]. It is a low-pressure process (1–10 bar)
that, using asymmetric UF membranes having a pore size between 1–100 nm, can reject particulates
and macromolecules such as proteins, fatty acids, bacteria, protozoa, viruses, and suspended solids.
In particular, it can reduce organic matter and BOD (biological oxygen demand) by at least 95%,
Molecules 2019, 24, 4148 6 of 15
greatly reduce turbidity, exceed regulatory standards of water quality, achieving 90–100% pathogen
removal. Moreover, many municipal water treatment facilities use UF treatment against
contamination from cryptosporidium, giardia, and other organisms that could cause serious illness
if ingested [42,43]. Therefore, UF is used to replace existing secondary (sedimentation, flocculation,
coagulation) and tertiary filtration (sand filtration and chlorination) methods employed in WWT. In
particular, it can permit the reuse of water coming from the industries that consume huge volumes
of water or discharge highly toxic effluent such as chemicals, steel, plastics and resins, paper and
pulp, pharmaceutical and the food and beverage industries, water and wastewater treatment plants,
and etc. [39].
UF, despite a broad molecular weight cut off (MWCO) range, is less active in removing low
molecular weight organic matters. In many cases, UF is integrated into the process, using primary
(flotation and filtration) and some secondary treatments as pretreatment stages and used for
pre-filtration in reverse-osmosis plants to protect the reverse-osmosis process.
Today, the ultrafiltration process coupled with the coagulation step is one of the main water
treatment technologies in the current water plants proving a significant removal of organic matter in
water. However, these technologies are not properly designed for the microplastic removal that
remains in the final effluents [22,44]. Indeed, most of the papers report the removal of the natural
organic matter (NOM) that is a complex matrix of organic compounds with a wide variety of
chemical properties, chemical composition, and molecular weight [45,46].
However, the worrying levels of microplastic in freshwaters make a mandatory a-depth
investigation of the behavior of microplastic removal during coagulation and ultrafiltration (UF)
processes, also considering that they are water treatment technologies used in the production of
drinking water [47,48].
To date, a few papers report the study of microplastic removal by coagulation and UF process
for the production of drinking water [49,50]. In particular, the study reported by Ma et al. 2019
focused on the removal behavior of polyethylene (PE) in drinking water treatment by ultrafiltration
and coagulation processes by using an Fe-based coagulant. PE is the most abundant plastic pollutant
detected in the water, and moreover, its density (0.92–0.97 g/cm
) very close to that of water makes it
difficult to remove by water treatment processes. Low removal efficiency of PE particles (below 15%)
was observed after coagulation, indicating the ineffectiveness of the sole coagulation process with
respect to microplastic removal. However, when the Polyacrylamide (PAM) was added to enhance
the coagulation performance, removal efficiency of small-particle-size PE (d < 0.5 mm) significantly
increased from 13 to 91% (Figure 4).
Figure 4. Scheme of the process for removal (A) and removal efficiency (B) of polyethylene (PE)
using FeCl
O and anionic polyacrylamide (PAM)(elaborated from Ma et al. 2019).
For what concerns the UF performance, an interesting result was that the membrane fouling
was progressively reduced after coagulation with PE. In particular, by increasing the dosage of
coagulant, the porosity of the floc cake layer increased due to the presence of PE particles, especially
the large ones. As a result, less severe membrane fouling was induced compared to that with flocs
Molecules 2019, 24, 4148 7 of 15
alone. The presence of larger PE particles had positive effects on the membrane fouling. The
membrane flux decreased by only 10% in the presence of large-particle-size PE (2 < d < 5 mm) after
the coagulation with 0.2 mmol/L PAM and 2 mmol/L FeCl3·6H2O, respectively [50]. However, this
behavior may not be a general rule but can depend on different parameters related to the membrane
process, as well as to the plastic characteristic (chemical composition, size, and shape).
The final comment is that as a general principle, UF could be used to remove PE particles
totally, but many efforts are required to understand how the cake layer formation and then the
fouling is influenced by the presence of plastic particles. Indeed, also the shape of microplastic can
affect their removal in different water treatments;. As reported in Talvitie et al. 2017, a portion of
plastic in “fiber shape” is not retained by the WWTPs. Therefore, the final stage treatments have to
be properly designed for the removal of the fiber to increase the removal efficiency of plastic.
2.2. Dynamic Membranes Technology
Recently, DM is emerging as an attractive technology for municipal wastewater treatment
[51,52], surface water treatment [53], oily water treatment [54], industrial wastewater treatment [55],
and sludge treatment [56].
This technology is based on the formation of a cake layer (dynamic membrane, DM), which acts
as a secondary membrane/barrier created when particles and other foulants in the wastewater are
filtered through a supporting membrane. Since the filtration mechanism of the DM is quite different
compared to the MF/UF processes, in the sense that the fouling and foulants are necessary to create
the DM layer, the resistance to filtration is caused exclusively by the layer of the cake. However,
thicker layers and dense fouling cause a loss of membrane performance. The parameters that must
be taken into consideration to limit the formation of fouling are the same that are involved in the DM
formation [57].
The DM formation process depends on various parameters relating to the supporting
membranes (membrane materials, membrane pore size), to the deposited material (particle size,
concentration) and to the operating conditions (operating pressure, cross-flow velocity) [52].
DM technology attracted great attention because:
i) it employs relatively lower-cost materials compared to traditional membranes (such as mesh,
non-woven fabric, and woven filter cloth, and stainless-steel mesh);
ii) extra chemicals or other contaminants are not introduced considering that the filtration layer
is formed by the contaminants of the influent;
iii) the experimental setup is generally more compact than the traditional membrane processes
(e.g., for ultrafiltration (UF) and microfiltration (MF)), since DM permeation flux is much larger, the
membrane module quantities are saved;
iv) the energy supply is lower since DM operates under gravity driving mode, and lower
transmembrane pressure is required compared to traditional membranes.
The application of DM technology for micro-plastics removal has been also studied [58] because
DM is suitable to remove low-density/poorly settling particles. DM technology was applied for
micro-particle removal from synthetic wastewater under a gravity-driven operation by using a
lab-scale DM filtration setup. The DM was formed on a 90 μm mesh and the synthetic wastewater
was prepared with diatomite (AR, Tianjin BASF, D90 = 90.5 μm, meaning that over 90% of the
particles in this study are within the defined size range of micro-particles) and tap water. The
effluent turbidity was reduced to <1 NTU (Nephelometric Turbidity Unit) after 20 min of filtration,
verifying the effective removal of micro-particles by the DM. The transmembrane pressure (TMP)
during the DM filtration process (in the range from 80 to 180 mm of water) was lower than that
observed for conventional microfiltration and ultrafiltration (16 times lesser than the value obtained
for wastewater microfiltration) also reducing the energy consumption. Different influent flux was
used (in the range from 9 to 21 L/h), and the linear increase of TMP (at a constant rate) was observed
with DM filtration time. At an influent flux of 9 L/h, the effluent turbidity was 4.94 NTU at 10 min
and 1.41 NTU at 20 min of filtration time while, at an influent flux of 21 L/h, the effluent turbidity
was reduced to 7.14 NTU after 3 min and 1.53 NTU at 5 min of filtration indicating that a higher
Molecules 2019, 24, 4148 8 of 15
influent flux facilitated the rapid formation of the DM. The DM formation process was strongly
affected by the influent particle concentration. Higher influent particle concentrations resulted in
more microparticles being filtered through the supporting mesh, laying the foundation for the rapid
formation of the DM layer and a faster effluent turbidity reduction. As a result, increasing flux and
influent particle concentration can be used for the control of the DM formation process.
2.3. Reverse Osmosis
Reverse Osmosis (RO), is actually used in municipal and industrial water treatment systems to
purify water using nonporous or nanofiltration membranes (pore size > 2 nm) by removing salts,
contaminants, heavy metals, and other impurities. It works by applying a high pressure (10–100 bar)
to a concentrated water solution that forces the water through a semipermeable membrane, leaving
all the other substances essentially in a more concentrated water solution. It is currently applied also
in food and beverage production, biopharmaceutical manufacturing, power generation, production
of high purity water, and desalination of brackish waters and seawater, as well in the recovery of
industrial and municipal wastewater [59].
RO membrane fouling is a major challenge for reliable membrane performance [60]. A
pretreatment stage is mandatory to maintain the flux rates, to control membrane fouling at industrial
scale RO desalination systems, minimizing the membranes cleaning frequency, and prolong the
useful life of the RO equipment. Usually, some common pretreatment involves the use of chemicals
such as coagulants, antiscalants, oxidizing agents, and disinfectants [60,61]. Other strategies for
fouling mitigation include cleaning, surface modification, and the use of novel membrane materials
[61]. Nowadays, steady performance in terms of water quality and flux was achieved by the
combination of an UF pretreatment with RO in the desalination process [62]. A growing trend in the
application of combined RO-UF plants for desalination at an industrial scale is reported by Ashfaq et
al. 2019. Some plants are reported here: Tuas, Singapore (Capacity: 318,000 m3/day; Year: 2013),
Ashdod, Israel (Capacity: 275,000 m3/day; Year: 2013), Ajman, United Arab Emirates (Capacity:
115,000 m3/day; Year: 2012), Tangshan, China (Capacity: 110,000 m3/day; Year: 2012), Teshi, Ghana
(Capacity: 60,000 m3/day; Year: 2014), Accra, Ghana (Capacity: 60,000 m3/day; Year: 2014), Red Sea
Coast—Saudia Arabia (Capacity: 30,000 m3/day, Year: 2016), Gwangyang, South Korea (Capacity:
30,000 m3/day; Year: 2015) [63].
The performance of the RO process with respect to MPs removal was reported by Ziajahromi et
al. 2017 [31]. They characterized and quantified the microplastic in samples coming from a WWTP
that produce a highly treated effluent, including screening and sedimentation, biological treatment,
flocculation, disinfection/de-chlorination processes, ultrafiltration, and finally a reverse osmosis
(RO) process. Results indicate the presence of microplastic fibers in the samples after RO process. In
particular, irregular shaped microplastic were detected and identified by Fourier transform infrared
spectroscopy analyses in attenuated total reectance (ATR-FTIR) as alkyd resin (modified polyester)
commonly used in paints. This microplastic detection was attributed by the authors to the
occurrence of some membrane defects or simply small openings between pipework, indicating the
necessity to ad-hoc design the processes for microplastic removal.
Most of the more performant applications of RO in the microplastic removal are obtained when
coupled with membrane bioreactor technology that is hereafter discussed.
2.4. Membrane Bioreactor (MBR)
Membrane bioreactor (MBR) are systems in which catalysis promoted by biological catalysts
(bacteria, enzymes), is coupled to a separation process, operated by a membrane system (generally
microfiltration or ultrafiltration) [64].
Thanks to the different compartments created by the membrane, a controlled heterogeneous
(organic/water)/multiphase (liquid/gas) reaction system can be developed. The different phases can
be kept separated (as for example, in a membrane-based solvent extraction process), or they can be
dispersed into each other (as in a membrane emulsification process). Besides, the versatility of the
technology permits an easy integration with other process (e.g., pervaporation, reverse osmosis)
Molecules 2019, 24, 4148 9 of 15
perfectly in line with green chemistry principles, within the logic of process intensification, which
offers new and much more opportunities in terms of competitiveness, product quality improvement,
process or product novelty and environmental friendliness [65].
Nowadays, MBR is deemed as one of the most powerful technologies for efficient municipal
and industrial wastewater treatment around the world; however, new emerging fields of application
are vastly growing, such as food, pharmaceutical, biorefinery, and biodiesel production [66].
In these last years, MBRs received an extensive academic interest and a very rapid growth in
practical municipal and industrial wastewater treatment applications. The great success is given by
the significant improvement given by this technology, respect to the traditional methods of water
treatment, such as high effluent quality, small footprint, complete separation of hydraulic retention
time (HRT), and solids retention time (STR), easy scale-up, etc. Regarding fouling control, various
methods have been developed in this technology. The most recent methods are based on
mechanically assisted aeration scouring, in-situ chemical cleaning enzymatic and bacterial
degradation of foulants electrically assisted fouling mitigation, and nanomaterial-based membranes
[67]. The demonstration of the exponential interest of this technology is the significant increase on
both large (10,000 m
/d) and super large-scale (100,000 m
/d) plants worldwide. The Beijing
Wenyu River plant in China was the first to be built, with the capacity of a super-large scale (100,000
/d) and subsequently many new plants were built in China (around 200) and in all the world and
at the following link is possible to see them It is expected that the
plant under construction in Sweden (Henriksdal plant) will be the largest MBR in the world with a
treatment capacity of 864,000 m
/d [64,65].
In MP treatment, the role of MBR is the decrease of solution complexity by the biodegradation
of the organic matter; this will permit the purification of MP and its further treatment. The process
generally starts when a pre-treated streams enters in the bioreactor, where the process of
biodegradation of organic matter is carried out. The produced mixed liquor is then pumped along
with semi-crossflow filtration system for the separation process (Figure 5). Thanks to the membrane
process, the MP is concentrated in the retentate stream.
Figure 5. Schematic representation of a MBR process.
In recent work [68], the performance of MBR was compared with other final-stage wastewater
treatment technologies (disc-filter, rapid sand filtration, and dissolved air flotation) for MP removal.
The MBR used is located in Finland and consisted of a submerged membrane unit (8 m
, 0.4 μm) and
ultrafiltration process.
Molecules 2019, 24, 4148 10 of 15
Respect to the other advanced treatment processes, the used MBR showed a significant
improvement in MP removal (99%), higher quality of final effluent, and a great potentiality in
decreasing the number of process stages, replacing the conventional secondary clarifiers
(conventional activated sludge, CAS). A comparison among the tertiary treatments is shown in
Figure 6. From this figure, it is possible to assess that MBR allowed the highest reduction of MP in
the final effluent, demonstrating that the membrane-based technology is the most efficient.
Figure 6. The number of microplastic particles per liter in the final effluent of each wastewater
treatment plant (Data elaborated from Ziajahromi et al. 2017 and Talvitie et al. 2017). RO: Reverse
Osmosis; DF1: Disc Filter with pore size 10 μm; DF2: Disc Filter with pore size 20 μm; RSF: Rapid
Sand Filters; DAF: Dissolved Air Flotation; MBR: Membrane Bioreactor.
However, although MP in the wastewater treatment could be removed through different
wastewater treatment plants (which in some cases include a step of MBR) [22], any full-scale plant
has been developed, or specifically designed for this purpose. So, tuned MP treatment technologies
or strategies which can support MP removal in existing wastewater treatment plant are currently at
the preliminary stage of research.
Among the recently developed strategies, the use of a mixture of different enzymes, as a
preliminary “plastic preserving” maceration step, seems to be very promising. The aim of “plastic
preserving enzyme maceration” is the degradation of all the organic matter in order to produce a
purified MP solution for further treatment [27].
The biodegradation is an alternative strategy in which MBR was used for plasticizer treatment.
Different studies have indicated the possibility of complete phthalate esters degradation by a wide
class of bacteria and action mycetes [69–71]. The most well-known bacteria for phthalate esters
degradation are reviewed in Gao et al. (2016) [70].
Also, in this case, the combination of biodegradation in the integrated process seems more
appropriate than individual treatment step. At present, MBR technology in phthalate esters
biodegradation is studied in the laboratory alone or coupled with activated sludge, starting from
different wastewater origin (synthetic [72], paper mill wastewater [73], municipal solid waste
leachate [74], etc.). In WWTP, the use of MBR showed about 70% higher removal of Di(2-ethylhexyl)
phthalate (DEHP) of conventional treatment (3%) [72] further improved (83%) if associated with
preliminary adsorption. A complete MP degradation was reached if MBR was associated with a
preliminary anaerobic treatment and followed by an RO filtration [73].
Microplastic particles/L
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However, the biodegradation efficiency in the MBR is also strictly related to physicochemical
properties of the phthalate esters and operative conditions such as hydraulic retention time (HRT),
initial feed concentration, etc.
A very promising discovery, which could be in the future associated with MBR technology, is
the isolation of a novel bacterium (Idonella sakaiensis) able to use polyethylene terephthalate (PET)
as its major energy and carbon source [75]. In particular, this bacterium produces two different
enzymes, when in contact with PET, which can efficiently convert PET in the less dangerous
monomers (terephthalic acid and ethylene glycol). Dawson and co-workers (2018), recently reported
in Nature Communications the size reduction of microplastic (from 31.5 μm to less than 1 μm) when
exposed to Antartic Krill (Euphasia superba) [76]. An in-depth study of the mechanism by which
Antartic Krill can reduce the size of MP will show the enzyme complex capable of performing this
process. The enzymes can be easily integrated with the MBR, so in the future, it will probably be
possible to degrade the MP in the enzymatic membrane reactor, as already demonstrated by Barth et
al. for PET degradation [77].
2.5. Polymeric Membranes as Source of Plastic Waste: Recent Advances in Their Reuse and Recycling
Nowadays, membrane technology is widely used in water and wastewater treatment with a
well-established market. The world market for Membrane Filtration is expected to grow over the
next five years, reaching 7030 million US$ in 2024, from 4710 million US$ in 2019, according to a new
GIR (Global Info Research) study [78]. The huge spread of membrane processes has raised the need
to develop methods to reuse and recycle these materials [79]. Many efforts have been made the
LIFE+ TRANSFOMEM research project (LIFE13 ENV/ES/000751) [80], “Transformation of disposed
reverse osmosis membranes into recycled ultra- and nanofiltration membranes”, in which the
recycling process of disposed reverse osmosis membranes and their reuse in nanofiltration and
ultrafiltration processes have been studied. Project results demonstrated that almost 70% of the
membranes are recyclable, and the use of recycled membranes can save between 85% and 95%
compared to the acquisition of new commercial membranes. Furthermore, there is a company called
“MemRe, RO Membrane Recycling”, based in Germany, which deals with the recycling and reuse of
membranes at the end of their life [81]. In addition, it should also be noted that the production of
membranes is increasingly oriented to the use of new bio-based polymers (recyclable and
biodegradable) as an alternative to petrochemical polymers [82].
3. Conclusions
The analysis of the literature has shown that ad hoc designed microplastic treatment processes
must be developed to limit plastic pollution. The water industry and the WWTPs do not currently
have experience or technologies to efficiently separate MP from effluents. From several papers, it
emerged that advanced tertiary treatment is needed to remove the plastic for wastewater. Among
the tertiary processes, the membrane processes, MBR in particular, appear to be the most promising
with MP removal of 99.9%, also offering the possibility to decrease the number of process stages in
the WWTPs.
Furthermore, a more detailed and uniform chemical-physical characterization of the plastic is
mandatory to select appropriate methodologies that guarantee a more efficient removal of the
plastic from the effluents. From literature emerged that the data are not easily comparable to each
other due to the lack of standardized characterization protocols. This characterization should also
include nanoplastics that could have a more serious biological impact.
The constructive actions for the reduction of pollution from microplastic that can act in synergy
are the implementation of an environmental pollution awareness policy that leads to a reduction in
the use of single-use plastic materials and the design of operational processes and production based
on the use of biodegradable materials to prevent accumulation in the environment.
Molecules 2019, 24, 4148 12 of 15
Author Contributions: Teresa Poerio analyzed all the data and wrote the article. Emma Piacentini analyzed the
literature data and contributed to the writing of the article, Rosalinda Mazzei analyzed the literature data
related to the MBR process and contributed to the writing of the article.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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Membrane fouling is a major drawback in the membrane filtration industry for water treatment. Mixed-matrix membranes (MMMs) are well known for their enhanced antifouling and antibacterial properties, which could offer potential benefits for membrane filtration processes in the water treatment field. In this work, three electrospun nanofibrous MMMs (P, CP, and MCP, which were, respectively, the pristine polysulfone membrane and mixed-matrix membranes (MMMs) consisting of GO–ZnO and GO–ZnO–iron oxides) were studied for antifouling and antibacterial properties with respect to the arsenic nanofiltration process. The effects of these composites on the antifouling behaviour of the membranes were studied by characterising the bovine serum albumin (BSA) protein adsorption on the membranes and subsequent analysis using microscopic (morphology via scanning electron microscopy) and Brunauer–Emmett–Teller (BET) analyses. The antibacterial properties of these membranes were also studied against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). The composite nanoparticle-incorporated membranes showed improved antifouling properties in comparison with the pristine polysulfone (PSF) membrane. The excellent antimicrobial properties of these membranes make them appropriate candidates to contribute to or overcome biofouling issues in water or wastewater treatment applications.
... Recently an additional treatment stage based on membrane separation was introduced to the WWTPs (tertiary treatment) and an increase in the microplastics removal up to 98% was observed. The 2% of nonrejected microparticles corresponded to either microplastics with sizes below 20 μm or to nanoplastics (Poerio et al., 2019;Silva et al., 2018). Coagulation steps combined with, for example, ultrafiltration (UF) membranes (pore sizes between 1 and 100 nm) result in a rejection of organic matter as well as microplastics. ...
Microplastic and nanoplastic are still an invisible problem on a global scale. Recognized as a contaminant only in 2004, microplastic and nanoplastic can have both instantaneous and long-term effects on living organisms at all levels-from molecular and genetic to population. This study examines the primary sources of microplastics and nanoplastics, their environmental hazards to the environment and humans. The factors influencing the degradation of plastic in natural conditions, methods of qualitative and quantitative detection of micro and nanoplastics, and methods of their removal from water are considered.
Mismanagement of plastic waste results in its ubiquitous presence in the environment. Despite being durable and persistent materials, plastics are reduced by weathering phenomena into debris with a particle size down to nanometers. The fate and ecotoxicological effects of these solid micropollutants are not fully understood yet, but they are raising increasing concerns for the environment and people’s health. Even if different current technologies have the potential to remove plastic particles, the efficiency of these processes is modest, especially for nanoparticles, for which integrated and combined technologies are usually required. Metal-organic frameworks (MOFs), a class of crystalline nano-porous materials, have unique properties, such as strong coordination bonds, large and robustus porous structures, high accessible surface areas and adsorption capacity, which make them suitable adsorbent materials for micropollutants. This review examines the preliminary results reported in literature indicating that MOFs are promising adsorbents for removal of plastic particles from water, especially when MOFs are integrated in porous composite materials or membranes, where they are able to assure high removal efficiency, superior water flux and antifouling properties, even in presence of other dissolved co-pollutants. Moreover, a recent trend for the alternative preparation of MOFs starting from plastic waste, especially polyethylene terephthalate, as sustainable source of organic linkers is also reviewed, as it represents a promising route for mitigating the impact of the costs deriving from the widescale MOFs production and application. This connubial between MOFs and plastic has the potential to contribute at implementing a more effective waste management and the circular economy principles in the polymer life cycle.
For many years, global plastic pollution has been a severe concern, and micro (nano) plastics (MNPs) have attracted the attention of researchers all over the world. Because MNPs can exhibit toxicological and interact with potentially toxic elements (PTEs) in the environment, soil toxicity can occur. Despite the fact that MNPs may accumulate in plant roots and have deleterious impacts on terrestrial environments, their impact on soil systems and plant crops has been overlooked. Human and animal use of MNP-contaminated plants or fruits will eventually result in health problems. Because identifying and measuring MNPs in diverse soil samples is difficult, knowledge of their fate, environmental, and ecological effects in terrestrial ecosystems are limited. As a result, it is critical to use an innovative strategy to remove MNPs from the natural environment. Microbial remediation is regarded to be a greener option amongst the many MNPs remediation processes. Enzymatic processes, substrates and co-substrates concentration, temperature, pH, oxidative stress, and other biotic and abiotic variables all impact the microbial breakdown of plastics. As a result, it is critical to understand the fundamental routes that microorganisms use to consume plastic particles as their only carbon source for growth and development. The benefits and downsides of different MNP remediation methods, such as enzymatic, advanced molecular, and biomembrane technologies, in stimulating the bioremediation of MNPs from diverse environmental compartments, as well as future research prospects, were discussed in this review.
The occurrence of microplastics (MPs) in aquatic environments has been a global concern because they are toxic and persistent and may serve as a vector for many legacies and emerging pollutants. MPs are discharged to aquatic environments from different sources, especially from wastewater plants (WWPs), causing severe impacts on aquatic organisms. This study mainly aims to review the Toxicity of MPs along with plastic additives in aquatic organisms at various trophic compartments and available remediation methods/strategies for MPs in aquatic environments. Occurrences of oxidative stress, neurotoxicity, and alterations in enzyme activity, growth, and feeding performance were identical in fish due to MPs toxicity. On the other hand, growth inhibition and ROS formation were observed in most of the microalgae species. In zooplankton, potential impacts were acceleration of premature molting, growth retardation, mortality increase, feeding behaviour, lipid accumulation, and decreased reproduction activity. MPs and additives could also exert some toxicological impacts on polychaete, including neurotoxicity, destabilization of the cytoskeleton, reduced feeding rate, growth, survivability and burrowing ability, weight loss, and high rate of mRNA transcription. Among different chemical and biological treatments for MPs, high removal rates have been reported for coagulation and filtration (>86.5 %), electrocoagulation (>90 %), advanced oxidation process (AOPs) (30 % to 95 %), primary sedimentation/Grit chamber (16.5 % to 58.84 %), adsorption removal technique (>95 %), magnetic filtration (78 % to 93 %), oil film extraction (>95 %), and density separation (95 % to 100 %). However, desirable extraction methods are required for large-scale research in MPs removal from aquatic environments.
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Cesium is a radioactive fission product generated in nuclear power plants and is disposed of as liquid waste. The recent catastrophe at the Fukushima Daiichi nuclear plant in Japan has increased the 137 Cs and 134 Cs concentrations in air, soil and water to lethal levels. 137 Cs has a half-life of 30.4 years, while the half-life of 134 Cs is around two years, therefore the formers' detrimental effects linger for a longer period. In addition, cesium is easily transported through water bodies making water contamination an urgent issue to address. Presently, efficient water remediation methods towards the extraction of 137 Cs are being studied. Prussian blue (PB) and its analogs have shown very high efficiencies in the capture of 137 Cs + ions. In addition, combining them with magnetic nanoparticles such as Fe3O4 allows their recovery via magnetic extraction once exhausted. Graphene and carbon nanotubes (CNT) are the new generation carbon allotropes that possess high specific surface areas. Moreover, the possibility to functionalize them with organic or inorganic materials opens new avenues in water treatment. The combination of PB-CNT/Graphene has shown enhanced 137 Cs + extraction and their possible applications as membranes can be envisaged. This review will survey these nanocomposites, their efficiency in 137 Cs + extraction, their possible toxicity, and prospects in large-scale water remediation and succinctly survey other new developments in 137 Cs + extraction.
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The use of zinc oxide nanoparticles (ZnO NPs) and polybrominated diphenyl ethers (PBDPEs) in different products and applications leads to the likelihood of their co-occurrence in the aquatic system, making it important to study the effect of PBDPEs on the fate and transport of ZnO NPs. In this study, we determine the influence of PBDPEs (BDPE-47 and BDPE-209) on the colloidal stability and physicochemical properties of ZnO NPs in different aqueous matrices. The results indicated the shift in ζ potential of ZnO NP from positive to negative in the presence of both PBDPEs in all tested waters; however, the effect on the NPs surface potential was specific to each water considered. The lower concentration of the PBDPEs (e.g., 0.5 mg/L) significantly reduced the ζ potential and hydrodynamic diameter (HDD) of ZnO NP, even in the presence of high content of dissolved organic matter (DOM) in both freshwater and industrial wastewater. Moreover, both BDPE-47 and BDPE-209 impede the agglomeration of ZnO NP in simple and natural media, even in the presence of monovalent and polyvalent cations. However, the effect of BDPE-47 on the ζ potential, HDD, and agglomeration of ZnO NP was more pronounced than that of BDPE-209 in all tested waters. The results of Fourier transform infrared (FT-IR) and X-ray Photon Spectroscopy (XPS) further confirm the adsorption of PBDPEs onto ZnO NP surface via aromatic ether groups and Br elements. The findings of this study will facilitate a better understanding of the interaction behavior between the ZnO NPs and PBDPEs, which can reduce the exposure risk of aquatic organisms to both pollutants.
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At present, the pollution of microplastic directly threatens ecology, food safety and even human health. Polyethylene terephthalate (PET) is one of the most common of microplastics. In this study, the micro-size PET particles were employed as analog of microplastic. The engineered strain, which can growth with PET as sole carbon source, was used as biocatalyst for biodegradation of PET particles. A combinatorial processing based on whole-cell biocatalysts was constructed for biodegradation of PET. Compared with enzymes, the products can be used by strain growth and do not accumulated in culture solution. Thus, feedback inhibition of products can be avoided. When PET was treated with the alkaline strain under high pH conditions, the product concentration was higher and the size of PET particles decreased dramatically than that of the biocatalyst under neutral conditions. This shows that the method of combined processing of alkali and organisms is more efficient for biodegradation of PET. The novel approach of combinatorial processing of PET based on whole-cell biocatalysis provides an attractive avenue for the biodegradation of micplastics.
In the current study, the indirect recycling of discarded reverse osmosis modules as i) membrane support for the preparation of anion exchange membranes and ii) polypropylene components for the assembly of an electrodialysis stack have been investigated for the first time. In such a way, 51% of the discarded module could be recycled into an electrodialysis system composed of 54% of recycled components. Recycled anion exchange membranes have been prepared by casting and phase inversion methods using discarded reverse osmosis membranes, preconditioned as support. The influence of casting thickness and solvent evaporation time in membrane properties has been studied. Besides, a complete membrane characterization has been carried out (SEM, water content, ion exchange capacity, permselectivity, electrical resistance, diffusion coefficients and mechanical properties) to select the optimal membrane preparation conditions, obtaining recycled anion exchange membranes with a high permselectivity (87%, similar to the commercial membranes). Finally, the technical viability of the recycled membranes has been tested by brackish water desalination experiments in the assembled electrodialysis stack, achieving 84.5% of salt removal. This study could open an alternative within the recycling of discarded reverse osmosis membranes, avoiding their disposal in landfills and moving membrane technology into a circular economy.
Microplastics (MPs), are tiny plastic fragments from 1 μm to 5 mm generally found in the aquatic environment which can be easily ingested by organisms and may cause chronic physical but also toxicological effects. Toxicological assays on fish cell lines are commonly used as an alternative tool to provide fast and reliable assessment of the toxic and ecotoxic properties of chemicals or mixtures. Rainbow trout liver cell line (RTLW-1) was used to evaluate the toxicity of pollutants sorbed to MPs sampled in sandy beaches from different islands around the world during the first Race for Water Odyssey in 2015. The collected MPs were analyzed for polymer composition and associated persistent organic pollutants: polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT). In addition, DMSO-extracts from virgin MPs, MPs artificially coated with B[a]P and environmental MPs were analyzed with different bioassays: MTT reduction assay (MTT), ethoxyresorufin-O-deethylase (EROD) assay and comet assay. Microplastics from sand beaches were dominated by polyethylene, followed by polypropylene fragments with variable proportions. Organic pollutants found on plastic from beach sampling was PAHs (2–71 ng g ⁻¹ ). Samples from Bermuda (Somerset Long Bay) and Hawaii (Makapu'u) showed the highest concentration of PAHs and DDT respectively. No toxicity was observed for virgin microplastics. No cytotoxicity was observed on cells exposed to MP extract. However, EROD activity was induced and differently modulated depending on the MPs locations suggesting presence of different pollutants or additives in extract. DNA damage was observed after exposure to four microplastics samples on the six tested. Modification of EROD activity level and DNA damage rate highlight MPs extract toxicity on fish cell line.
Microplastics have aroused increasing concern as they pose threats to aquatic species as well as human beings. They do not only contribute to accumulation of plastics in the environment, but due to absorption they can also contribute to spreading of micropollutants in the environment. Studies indicated that wastewater treatment plants (WWTPs) play an important role in releasing microplastics to the environment. Therefore, effective detection of the microplastics and understanding their occurrence and fate in WWTPs are of great importance towards microplastics control. In this review, the up-to-date status on the detection, occurrence and removal of microplastics in WWTPs are comprehensively reviewed. Specifically, the different techniques used for collecting microplastics from both wastewater and sewage sludge, and their pretreatment and characterization methods are reviewed and analyzed. The key aspects regarding microplastics occurrence in WWTPs, such as concentrations, total discharges, materials, shapes and sizes are summarized and compared. Microplastics removal in different treatment stages and their retention in sewage sludge are explored. The development of potential microplastics-targeted treatment technologies is also presented. Although previous researches in microplastics have undoubtedly improved our level of understanding, it is clear that much remains to be learned about microplastics in WWTPs, as many unanswered questions and thereby concerns still remain; some of these important future research areas are outlined. The key challenges appear to be to harmonize detection methods as well as microplastics mitigation from wastewater and sewage sludge.
Microplastics are plastic fragments lower than 5 mm that are detected in the environment causing various effects on organisms. Several research articles have recognized Sewage Treatment Plants as important sources of polyethylene and polypropylene beads, polyester, polyamide and other types of microplastics. For their determination, techniques such as visual identification using microscope, Fourier-transform infrared and RAMAN spectroscopy are used, while chemical oxidation, enzymatic maceration and density separation are applied as pretreatment methods for the removal of the inorganic and organic content. Microplastics’ concentrations range up to 3160 particles L-1, 125 particles L-1 and 170.9 x 103 particles Kg-1 TS dw in raw, treated wastewater and sludge, respectively. Their removal during wastewater treatment ranges between 72% and 99.4%; the main processes that contribute to their removal are primary and secondary treatment, while the effect of tertiary treatment depends on the applied technology. Entrapment in suspended solids and accumulation to sludge are the major mechanisms governing their fate. A standardized protocol for samples’ collection and pretreatment as well as microplastics’ isolation and characterization is needed; future reseach should investigate the possible chemical and physical changes of microplastics during treatment, and their role as carriers for the transfer of emerging micropollutants.
Microplastics have garnered much attention worldwide as a new emerging pollutant, especially because of their eco-toxicological effects in marine environments. As they are gradually detected in freshwaters, understanding how microplastics, with their small particle size and low density, will behave during current drinking water treatment processes is urgently needed. In this study, Al- and Fe-based salts were used in the presence of polyethylene (PE), which is suspended/floats easily in water and is the main constituent of microplastics. Results showed that Al-based salts performed better in PE removal efficiency than Fe-based salts. The smaller the PE particle size, the higher the removal efficiency. However, a low removal efficiency was observed, even with a high Al-based salt dosage of 15 mM (below 40%). Additionally, water conditions, such as ionic strength, turbidity level, barely influenced the removal efficiency. In comparison to pH, polyacrylamide (PAM) addition played an important role in removing PE; especially anionic PAM addition, because of the positively charged Al-based flocs it generates under neutral conditions. For ultrafiltration, although PE particles can be completely rejected, slight membrane fouling was induced after coagulation with conventional Al-based salts. With increasing dosage, membrane fouling was gradually aggravated owing to the thick cake layer formed. However, the larger the PE particles, the greater the roughness of the Al-based floc cake layer, leading to less severe membrane fouling. Based on this investigation, the microplastic removal behaviors exhibited during coagulation and ultrafiltration processes have potential application in drinking water treatment.
Microplastics have caused great concern worldwide recently due to their ubiquitous presence within the marine environment. Up to now, most attention has been paid to their sources, distributions, measurement methods, and especially their eco-toxicological effects. With microplastics being increasingly detected in freshwater, it is urgently necessary to evaluate their behaviors during coagulation and ultrafiltration (UF) processes. Herein, the removal behavior of polyethylene (PE), which is easily suspended in water and is the main component of microplastics, was investigated with commonly used Fe-based salts. Results showed that although higher removal efficiency was induced for smaller PE particles, low PE removal efficiency (below 15%) was observed using the traditional coagulation process, and was little influenced by water characteristics. In comparison to solution pH, PAM addition played a more important role in increasing the removal efficiency, especially anionic PAM at high dosage (with efficiency up to 90.9%). The main reason was ascribed to the dense floc formation and high adsorption ability because of the positively charged Fe-based flocs under neutral conditions. For ultrafiltration, although PE particles could be completely rejected, slight membrane fouling was caused owing to their large particle size. The membrane flux decreased after coagulation; however, the membrane fouling was less severe than that induced by flocs alone due to the heterogeneous nature of the cake layer caused by PE, even at high dosages of Fe-based salts. Based on the behavior exhibited during coagulation and ultrafiltration, we believe these findings will have potential application in drinking water treatment.