Research Progress and Prospects of Microplastics Treatment in

Wastewater

Jingyu Zhang

School of Resources and Environmental Engineering, East China University of Science and Technology,

Shanghai, 200030, China

Keywords: Wastewater Treatment, Microplastics, Sludge Emissions.

Abstract: With the wide application of plastics, it leads to a large amount of plastic waste. In particular, microplastics,

which are difficult to degrade, have become an important pollution problem in the global ecological

environment. This paper provides a detailed review of the classification and characteristics of microplastics

and their environmental and health impacts, and a systematic summary of existing microplastic treatment

methods, including physical, chemical and biological methods. This paper concludes that microplastics are

widespread in water bodies and have far-reaching impacts on the environment, including soil and groundwater.

Among them, the issue of the potential toxicity of microplastics to living organisms (including humans) has

also attracted widespread attention. In addition, this paper analyses the effectiveness of different treatment

processes and the challenges they face, using the treatment of microplastics in wastewater treatment plants as

an example. It is shown that the existing treatment technologies, although effective in reducing the discharge

of microplastics, still have certain limitations, such as the problem of secondary contamination of

microplastics. For this reason, future research should focus on more efficient and sustainable microplastic

degradation technologies to better address this global environmental challenge.

INTRODUCTION

Plastics are synthetic polymers extracted from

organic products such as coal, natural gas, salt,

cellulose and crude oil (Kasmuri, 2022), which are

widely used in food, textile, construction, industry,

medical and other production and life due to their

lightweight, stable nature, low cost, and long-lasting

durability. However, with time, a large number of

plastics become solid waste, entering the corners of

the city to affect human life, and even entering the

ecological environment to hinder balanced

development.

Studies have shown that the degradation cycle of

plastic waste is so long that it can be considered

almost non-degradable. Given that the degradation of

plastics occurs through photodegradation rather than

biodegradation, it implies that plastics can only

undergo a gradual decomposition from a substantial

mass into microplastics with a reduced particle size

(< 5 mm) under environmental conditions (Geng,

https://orcid.org/0009-0005-5872-4110

2024). Microplastics are highly adsorbent and

resistant to biodegradation, accumulating in the

environment and, due to their small size and

widespread presence in the ecosystem, causing many

adverse effects, and have become one of the most

serious emerging environmental problems. If

microplastics are prevalent in the marine

environment, they could pose a significant threat to

biota. Their composition and relatively large surface

area render them prone to the adsorption of aqueous

organic pollutants and the leaching of toxic

plasticizers (Cole, 2011). Additionally, microplastics

can adsorb hydrophobic organic pollutants from the

water, leading to complex contamination issues and

potential toxicity-enrichment problems within

ecosystems. Therefore, the treatment of microplastics

is urgent.

At present, the treatment of microplastics exists in

the physical, chemical, biological, and other

traditional methods, as well as the more emerging

advanced oxidation and catalyst methods. For

microplastics in the water body, the current main need

172

Zhang, J.

Research Progress and Prospects of Microplastics Treatment in Wastewater.

DOI: 10.5220/0013246800004558

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 1st International Conference on Modern Logistics and Supply Chain Management (MLSCM 2024), pages 172-176

ISBN: 978-989-758-738-2

to rely on the treatment process of sewage plant,

through the screen grid composed of primary

treatment and secondary biological treatment, can

remove microplastics with a particle size greater than

500 microns (Park, 2021), after the tertiary treatment,

most of the use of coagulation settling tanks to make

the microplastics settle, but also studies have shown

that the use of membrane technology (such as nano-

filtration membranes) to achieve a better removal

effect (Sharma, 2022). In practice, wastewater plants

also introduce air flotation tanks, biological

treatment, advanced oxidation and other methods for

deeply treating microplastics in wastewater.

However, this method also has certain defects, that is,

the microplastics removed by sedimentation will

accumulate in the sludge, along with the solid waste

excluded from the sewage plant, if not properly

handled will produce secondary pollution.

In conclusion, microplastics are an environmental

problem caused by human production and life, which

spreads widely until it threatens our lives. Compared

with the situation of microplastic pollution, the

treatment of microplastics in wastewater still lacks a

clear generalized system, and microplastics

discharged with sludge have not been given due

attention. The purpose of this paper is to study the

current treatment methods of microplastics, firstly, to

introduce the classification and characteristics of

microplastics, secondly, to summarize the existing

physical, chemical and biological methods for the

treatment of microplastics, and finally, to take

microplastics treatment in wastewater treatment as an

example of the systematic application and analysis of

the above methods.

OVERVIEW OF

MICROPLASTICS

The more common microplastics can be divided into

two main categories. Plastics manufactured to

microscopic sizes are defined as primary

microplastics, commonly used in facial cleansers and

cosmetics or as air spray media, and as carriers for

certain medicines. Secondary microplastics, on the

other hand, are defined as tiny fragments of plastic

produced in the oceans and on land by the

decomposition of larger plastic fragments. Over time,

the culmination of physical, biological and chemical

processes reduces the structural integrity of plastic

fragments, leading to fragmentation.

The physical and chemical properties of

microplastics are highly prone to change. Research

has shown that under the influence of environmental

factors such as runoff, tides, wind, radiation, and

biological activity, microplastics not only undergo

physical alterations, such as cracking and foaming but

also experience chemical changes, including bond

breakage and molecular fragmentation. These

transformations directly impact the migration and

distribution patterns of microplastic particles in

aquatic environments, as well as their adsorption and

desorption processes, ultimately affecting their

ecological and environmental impacts (Ma, 2024).

Additionally, microplastics exhibit migratory

behaviour, enabling them to travel long distances

under the influence of wind, runoff, and tides.

Due to the certain mobility of microplastics, they

are not only prevalent in the marine environment and

surface water ecosystems, but also significantly

impact the subsurface environment including soil and

groundwater (Geng, 2024). Studies have shown that

large quantities of plastic wastes seep into the

subsurface environment through different pathways,

and subsequently migrate and accumulate under the

action of sedimentation and surface runoff to form

reservoirs in the subsurface.

Research has demonstrated that microplastics

have detrimental effects on both the environment and

living organisms, including humans. Toxicological

studies on microplastics are generally classified into

in vitro and in vivo investigations. In vitro studies are

laboratory-based experiments using isolated cells or

tissues to examine the toxic effects of microplastics at

the cellular or tissue level. In contrast, in vivo studies

involve experimentation on living organisms to

assess the impact of microplastics on entire organisms

or specific organs. The results of these studies have

categorized the toxic effects of microplastics on

human cells into several biological endpoints,

including cytotoxicity, immune response, oxidative

stress, barrier integrity, and genotoxicity (Osman,

2023). Four of these five endpoints were confirmed

as significant biological effects of microplastics on

human cells, with irregularly shaped microplastics

exhibiting particularly pronounced effects. Moreover,

the concentration of microplastics and the duration of

exposure were found to significantly influence

cytotoxic and immune responses (Danopoulos,

2022). These findings suggest that human exposure to

microplastics may pose health risks. Additionally,

microplastics have been shown to induce harmful

alterations in the gastrointestinal physiology of

marine organisms and disrupt normal metabolic

processes following prolonged exposure (Qiao,

2019).

Research Progress and Prospects of Microplastics Treatment in Wastewater

173

TREATMENT OF

MICROPLASTICS

Since the initial identification of microplastics in

aquatic environments in 2004, there has been an

increasing focus among researchers on artificial

methods for degrading microplastics. Advances in

technology and equipment have led to the

development of various treatment approaches, which

can be broadly classified into three primary

categories: physical, chemical, and biological

methods. This section will provide an overview of

these treatment methodologies.

3.1 Physical Method

The use of physical principles to remove

microplastics is a more common filtration method.

This method relies on the driving force or other

external forces so that the liquid in suspension

through the filtration medium, solid particles, and

microplastics are retained to separate from the liquid.

According to this principle, in the operation of

wastewater treatment plants, often in the primary

treatment of grids and sand filters, which has a simple

structure, less investment and can intercept other

suspended pollutants, the advantages of the

wastewater suitable for microplastics concentration is

high. Studies have shown that up to about 70% of

microplastics can be removed by conventional sand

filters (Talvitie, 2017), but in practice, only the larger

particle size can be removed due to the limitations of

the filter mesh, which requires the cooperation of

subsequent processes.

In addition, air flotation is an effective method

based on the hydrophobic separation principle. In this

method, hydrophobic particles are attached to the

surface of bubbles generated by foam floating, which

are then brought to the gas-liquid interface. In water

treatment systems, air flotation is employed to

remove insoluble substances from water by

introducing air under high pressure. This method

offers the benefits of relatively low capital investment

(due to reduced chemical costs) and high operational

efficiency. Studies have shown that the removal of

microplastics can reach 80% by treatment with air

flotation process (Vo, 2024). However, the fact is that

air flotation is not inherently suitable for analyzing

plastics separation because of its poor bubble

predictability, which may lead to high particle losses.

3.2 Chemical Method

Chemical methods use chemicals and chemical

reactions to reduce the concentration of microplastics

in the water column, and currently include advanced

oxidation and photocatalysis. Advanced oxidation

has an important role in pollutant removal methods,

where the addition of peroxides to the system using

the Fenton method enhances its ability to break down

organic pollutants, and is particularly suited to the

recovery of specific polymers from water. It has been

shown that during exposure to Fenton treatment,

large-sized microplastics gradually decompose to

smaller sizes and small-sized microplastics gradually

degrade (Wang, 2017). However, the drawback of

this method is that once the microplastics become

smaller in size, the degradation process becomes slow

and often takes more than a month for the treatment

to reach the standard.

It is also possible to use photocatalytic methods,

which form electrons by shining light from holes into

a semiconductor; later, these holes combine with

H2O or OH- to produce OH- and O2. This process

will affect the microplastics, causing them to break

down or even mineralize into CO2 and H2O.

Photolysis in natural environments occurs in the C-C

backbone of the plastics. The process is divided into

three steps: initiation, propagation and termination.

The formed radiation particles generate peroxyl

radicals throughout the process, which play an

important role in the photodegradation process

(Gewert, 2015). The advantage of this method is that

its decomposition products are pollution-free and

more environmentally friendly, but in practice the

cost is higher.

3.3 Biological Method

Traditional biological methods for the treatment of

pollutants often lend themselves to the natural process

of helping microorganisms (mainly fungi or bacteria)

to clean the environment, in which plastic materials

degrade into larger plastics and microplastics due to

different environmental factors. Traditionally,

biodegradation occurs in four steps, (1)

biodeterioration, i.e. the formation of a biofilm

around the plastic polymer; (2) microorganisms

undergo bio-fragmentation, producing extracellular

enzymes that act on the polymer to convert it into

oligomers/dimers/monomers, making it more readily

available for uptake; and (3) assimilation of the

oligomers/dimers/monomers occurs, where they are

assembled on microorganisms and taken up by the

microbial cells. (4) Mineralization occurs when

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metabolites such as CO2, H2O, and CH4 are

produced (Sharma, 2022), thus providing energy for

the degradation process. In practical studies,

microbial degradation is often combined with

physicochemical processes to achieve higher

efficiency. The advantages of this approach are cost-

effectiveness, low energy consumption,

environmental friendliness, high specificity of

remediation using bacteria, fewer harmful by-

products, etc., but at the same time it has some

disadvantages, such as the degradation process is very

slow and takes a great deal of time, sometimes years,

to complete.

However, some studies have also shown that there

also exist emerging methods of degrading

microplastics by direct action of organisms, such as

the mucus obtained from a jellyfish, Aurelia-aurita,

which can bind to and reduce the toxicity of

nanoplastics (NPs). Its mucus can capture up to 90%

of polystyrene NPs within 30 min (Geum, 2022). The

treatment of the jellyfish for the removal of

microplastics is done without the addition of any

harmful chemicals. There is no emission problem

given, which fully meets the needs of sustainable

development. However, there are limitations because

jellyfish only reproduce in large numbers during

certain seasons, and it is not possible to capture

jellyfish in their natural state in a sustainable manner.

3.4 An Example of Microplastics

Treatment in a Wastewater

Treatment Plant

The effluent treatment process in wastewater

treatment plants (WWTPs) is typically divided into

three stages. The primary treatment involves coarse

and fine screening, grit and oil removal, skimming,

and primary settling (sedimentation). To optimize the

biochemical properties of the effluent for subsequent

secondary biological treatment, most microplastics

and fibrous pollutants entering the WWTP are

removed through skimming and settling in the

primary treatment system. Research indicates that

approximately 70% to 98% of microplastics are

removed during primary treatment (Iyare, 2020), with

grease traps being the main removal mechanism.

Microplastics are predominantly removed by

adsorption onto surface solids or sludge during

sedimentation.

The secondary treatment is primarily biological,

targeting the removal of chemical oxygen demand

(COD), biological oxygen demand (BOD), nitrogen,

and phosphorus. Although not specifically designed

for microplastic removal, the physical processes

involved still contribute to microplastic reduction.

Studies suggest that the microplastic removal rate

during secondary treatment ranges from 70% to 98%.

However, Talvitie et al. observed an increase in the

proportion of secondary microplastics with higher

levels of treatment. After activated sludge treatment,

primary and secondary microplastics constituted 19%

and 81% of total microplastics in the secondary

effluent, respectively. Following advanced treatment,

these proportions shifted to 9% and 91%,

respectively. This increase in secondary microplastics

may be attributed to the escape of synthetic fibers

during treatment due to their small size and

morphology, which allows them to pass through fine

pores (Amir, 2021). Consequently, if water is

discharged after the secondary treatment stage,

microplastic concentrations may remain above

acceptable levels, necessitating further reduction via

tertiary treatment.

A variety of technologies are available for tertiary

(or advanced) treatment, including Biological

Aerated Filters (BAFs), sand filters, Membrane Bio-

Reactors (MBRs), and ozone technology. It has been

documented that primary treatment can remove about

70% to 98% of microplastics, while secondary

treatment can reduce plastic concentrations in

wastewater to below 20%. Tertiary treatment further

decreases plastic concentrations to less than 2%

(Talvitie, 2017). The effectiveness of tertiary

treatment technologies, such as sand filters,

sequencing batch reactors (SBRs), and media

filtration processes, has been demonstrated, as

tertiary-treated effluent generally contains fewer

microplastic particles compared to effluent that has

not undergone tertiary treatment. However, the

efficiency of tertiary treatment remains a topic of

debate. Research has shown that microplastic

fragments, ranging from 20 to 100 micrometers, can

bypass all stages of treatment, including tertiary

processes, and be released into the environment

through the effluent.

Thus, despite the availability of advanced

treatment technologies, some microplastics will

inevitably enter the environment through effluent

from WWTPs, while others may enter the soil via

sludge produced during primary and secondary

treatment. Addressing these emissions remains a

critical challenge for current microplastic degradation

efforts.

Research Progress and Prospects of Microplastics Treatment in Wastewater

175

CONCLUSION

This paper analyses in depth the classification and

characteristics of microplastics and their migration

and ecotoxicity in the environment, and summarizes

the current main methods for microplastic treatment.

It is found that although the more traditional methods

can reduce the harm of microplastics to the

environment to a certain extent, there are still

problems such as incomplete treatment and easy to

cause secondary pollution. In the emerging research

methods (such as photocatalysis and the use of

Aurelia-aurita jellyfish degradation method), there

are more immature technology, high cost and difficult

to industrialize shortcomings.

These shortcomings are particularly evident in

wastewater treatment plants. The combined use of

tertiary treatment has improved the removal

efficiency of microplastics. However, some

microplastics are still able to enter the environment

through the treatment process or accumulate in the

sludge. During sludge disposal, due to the lack of a

targeted process and the sensitivity to climate and

other factors, the impact on the concentration of

microplastics is uncertain, and may be either positive

or negative, resulting in doubtful compliance with the

final discharge standards.

To sum up, in order to effectively deal with

microplastic pollution, future research should focus

on developing more efficient and environmentally

friendly treatment technologies, considering the

feasibility of industrialization of the technology, and

at the same time, strengthening the research on the

long-distance migration of microplastics in the

environment and the complex pollution mechanism.

At the same time, the solution to the microplastic

problem not only relies on technological means, but

also requires global policy support and the

enhancement of public awareness in order to achieve

more comprehensive environmental protection and

sustainable development goals.

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