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Article

Microplastic Pollution in Sewage Sludge from Wastewater Treatment Plants and Estimation of Microplastic Release

by
Soo-Jin Cho
1,2,
Ja-Hyung Choi
3,
Young-Sam Yoon
1 and
Nam-Il Um
1,*
1
Resource Recirculation Research Division, National Institute of Environmental Research, Incheon 22689, Republic of Korea
2
Devision of Environment and Energy Engineering, Yonsei University, Wonju 26493, Republic of Korea
3
Resource Recirculation Center, Korea Testing & Research Institute, Gyeonggi-do 13810, Republic of Korea
*
Author to whom correspondence should be addressed.
Water 2025, 17(3), 387; https://doi.org/10.3390/w17030387
Submission received: 2 January 2025 / Revised: 24 January 2025 / Accepted: 27 January 2025 / Published: 31 January 2025
(This article belongs to the Special Issue Microplastics Pollution in Aquatic Environments)
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Abstract

:
International efforts are being made to reduce environmental pollution caused by microplastics (MPs). Microplastics are released into the environment through sewage treatment sludge, and the use of sludge as a soil improvement agent is increasing rapidly, emphasising the importance of controlling microplastics in sewage treatment facilities. The release of microplastics into the environment is an increasingly significant concern, with sources including sewage treatment sludge. This study focuses on the analysis of microplastics in sewage sludge using optical (Fourier-transform infrared spectroscopy, FTIR) and thermal (Thermo Extraction Desorption–Gas Chromatograph–Mass Spectroscopy, TED-GC-MS) processing-based analytical equipment. The average amount of MPs in the sewage sludge analysed using FTIR was 228.5 microplastics/g of sludge (MPs/g), primarily of the polypropylene type. Approximately 75% of the MPs were 0.1 mm in size or smaller. However, the average amount of MPs in the sewage sludge determined using TED-GC-MS was 95.79 µg-MPs/g. For the systematic management of microplastics, it is important to estimate the amount of microplastics generated by sewage treatment plants. Therefore, a microplastic generation calculation formula was proposed and used to estimate the potential microplastic generation in sewage treatment plants. The total amount of MPs generated from sewage treatment plants in South Korea, calculated using the equation, was approximately 364 ton/yr; we further divided the total amount by administrative regions. The findings of this study can be applied to assess global trends in MP research.

Graphical Abstract

1. Introduction

Plastics are widely used across industries and in everyday life, owing to their versatility, ease of production, and cost-effectiveness. The global production of plastics has been continuously increasing—it increased from 1.5 million tons in 1950 to approximately 370 million tons in 2020, which is an increase of 250 times over 70 years [1]. At this rate, approximately 560 million tons of plastics is expected to be produced by 2030 [2]. The increase in production has resulted in increased generation of plastic waste. In recent years, the COVID-19 pandemic has increased the consumption of single-use plastics, resulting in the generation of an additional 8.4 million tons of plastic waste [3]. The increase in plastic waste is expected to be reflected in increased generation of microplastics (MPs).
MPs were first discussed by the National Oceanic and Atmospheric Administration (NOAA), and defined as ‘plastic materials smaller than 5 mm’ [4]. They can be divided into primary and secondary MPs. Primary MPs, such as microbeads and cosmetic products, are smaller than 5 mm for commercial reasons, whereas secondary MPs are plastics that have been reduced to sizes smaller than 5 mm through photo-degradation, abrasion, or other factors [5].
Sewage treatment plants are potential sources of MPs [6]. An investigation into MPs in areas upstream and downstream of sewage treatment plants revealed an increase of approximately 2.5 times in the abundance of MPs in the latter site—the average concentrations of MPs in the upstream and downstream areas were 3.14 MPs/m3 (min 0.49 MPs/m3, max 5.92 MPs/m3) and 5.81 MPs/m3 (min 0.80 MPs/m3, max 11.22 MPs/m3), respectively [7]. According to previous studies on the behaviour of MPs in sewage treatment plants, although the efficiency of the removal of MPs entering the plants varies between countries, it generally exceeds 95% during the water treatment process [8]. However, MPs removed during the water treatment process are not biologically or chemically degraded, and mostly accumulate in sludge [9]. Furthermore, in a recent study on the interaction between MPs and nitrifying microorganisms, it was found that MPs could reduce the efficiency of nitrification, even without being directly absorbed by nitrifying microorganisms, and ultimately affected the overall efficiency of the sewage treatment process [10]. In addition, sewage sludge is recycled in the form of fill and backfill material, which comes into direct contact with soil. In this case, MPs contained in sewage sludge are likely to enter nearby rivers through rainfall, agricultural water spraying, etc., and affect the cultivation of crops [11]. MPs accumulate in the environment through biological accumulation and food chains, and eventually reach the oceans through rivers, the atmosphere, and soil. The deleterious effects of MPs, which are ubiquitous in the environment, warrant the need for MP management.
In Korea, a new management strategy framework is being proposed for the management of microplastics. In particular, Um et al. identified the concept of microplastic waste and the source of microplastics during the waste treatment process based on the four-stage resource cycle of production, consumption, emission, and disposal [12]. As such, in Korea, which is recognising the importance of microplastic management, a new management policy is being prepared. This study aims to contribute to this policy development in Korea by conducting research on potential microplastic sources.
The recycling of sewage sludge as soil and cover materials is an increasingly prevalent practice. However, concerns have been raised regarding the potential for microplastics contained within sewage sludge to enter nearby rivers through processes such as rainfall and agricultural water spraying [13]. Efforts to reduce the generation of microplastics are urgently needed, as environmental impacts and human and ecological hazards caused by microplastics are increasing. This has led to a growing recognition of the urgent need to reduce the generation of microplastics, as the environmental impacts and human and ecological hazard implications of these particles are becoming increasingly apparent.
In this study, MPs were analysed in sewage sludge from South Korea sewage treatment plants using optical spectroscopic and pyrolysis analysis methods. Moreover, because emission estimates are required for reducing the generation of MPs, and current research on MPs is mainly focused on quantitative and qualitative aspects, with insufficient studies on the estimation of emissions [14,15], we also estimated the amount of MPs generated through sewage sludge based on the results of MP analysis.

2. Materials and Methods

2.1. Sampling

For sludge sampling, sewage treatment facilities following a typical sewage treatment flow, including a grit chamber, settling, a biological reactor, and tertiary treatment, were selected. Subsequently, we selected facilities that could ensure that the samples were representative, considering the facility capacity and sludge generation volume according to the statistics of the Ministry of Environment (MOE) on sewer systems. Two sewage treatment facilities near the Han River, ranking at the top of the sewer system statistics, were chosen for sampling. Facility W treats approximately 570,000,000 m3/yr (8%) of sewage, whereas Facility E treats 422,000,000 m3/yr (6%) of sewage [16]. Four sludge samples were collected separately as sludge cake (two samples) and sewage sludge (two samples) generated after all water treatment processes. Sludge cake is dried, reducing the moisture content from 80% to 10%, to produce sewage sludge, which is then recycled as fuel for thermal power plants. We aimed to compare the characteristics of MPs in the samples according to their physical impact. Sludge cake (SU-W and SU-E) and sewage sludge (SE-W and SE-E) generated after sewage treatment were collected safely from points where the mixture was sufficient, following the standard methods for the examination of waste in South Korea (ES 06130.c) [17]. Samples were taken from a wastewater treatment plant located at 37°33′ latitude 127°03′ longitude for SE-E and SU-E, and 37°34′ and 126°49′ longitude for SE-W and SU-W. Approximately 5 kg of sludge was collected per sampling point using a stainless steel scoop, and contained in a glass bottle. Additionally, considering the MP analysis method applied in this study, samples smaller than 5 mm in size were collected using a sieve.

2.2. Method of Microplastics Extration

In the absence of any standardised method for the pre-treatment of MPs, different methods are applied depending on the sample conditions and research objectives. In this study, we followed the preparation method suggested by the NOAA, which involves density separation, biological digestion, and filtration [18]. As a large amount of organic matter was expected to be present in the sewage sludge, we used 30% H2O2. H2O2 is known to be a strong oxidising agent, and was used in the biological digestion of the organics present in the sewage sludge. For density separation, the density of the solutions investigated in previous studies and the density of plastic types likely to be present in the sample were considered. MP extraction was performed using ZnCl2 (1.6 g/mL). The extraction solution was chosen based on sample properties; the MP extraction methods are presented in Figure 1 and Table 1 [19,20,21,22,23,24,25].
Four samples (dry weight, 0.5 ± 0.005 g) were placed in a beaker. Density separation was carried out using 100 mL of ZnCl2 (1.6 g/cm3) solution, focusing on controlling inorganic matter without time restriction. When the samples were mixed with the density separation solution and allowed to stand, the denser inorganic particles settled to the bottom of the solution, while the lighter plastic particles floated. The floating particles were taken and filtered out under reduced pressure using a 20 μm metal filter.
To remove organic matter that could interfere with the MP analysis, oxidising agents were used for the MP analysis. A volume of 100 mL of 30% H2O2 was added to the filtrate, and biological digestion was performed for at least 12 h. Thereafter, a second filtration step was carried out, as described above, to separate any impurities other than plastics, and the plastics were then extracted from the samples [26]. Care was taken to ensure that the samples on the filter formed a single layer after the extraction process, as accurate analysis using the Fourier-transform infrared spectroscopy method (FTIR) is only possible when a single layer is formed. The filter was subsequently dried for 24 h at room temperature in a sealed environment, to prevent contamination from external factors and to avoid plastic deformation associated with drying at high temperatures [27].

2.3. Identification of Microplastics

As mentioned for the pre-treatment methods, there is no standardised method for the analysis of MPs. Spectroscopic analysis using FTIR (59%) and the thermal extraction/desorption method (16%) are commonly used in MP analysis [28]. FTIR is the most commonly used analytical equipment for MP analysis. However, this method has some disadvantages; for example, the results are affected by the skill level of the analyst, and the analysis time is rather long, which makes it unsuitable for environmental samples expected to contain hundreds to thousands of MPs [29]. On the contrary, thermal extraction desorption–gas chromatograph–mass spectrometry (TED-GC-MS) offer advantages, such as the ability to analyse the composition and weight of MPs and no limitations on particle size [30]. However, TED-GC-MS cannot be used to perform qualitative analysis of some materials [31]. Therefore, in this study, we used both FTIR and TED-GC-MS for the analysis of MPs in sewage sludge to compensate for the disadvantages of both methods.

2.3.1. FTIR Analysis

FTIR analysis was performed to analyse the type and number of microplastics in the waste. In FTIR (Lumos II, Bruker, Billerica, MA, USA) analysis of the plastic type, the sample is irradiated with infrared rays, and the absorbance is measured. The metal filter used for MP extraction was divided into four parts, and multiple spectra from a single area were collected through focal-plane array (FPA)-based mapping to obtain optimal infrared spectroscopy images. The analysis was performed for plastics present on the entire 20 μm metal filter [32]. For MP particles with a concordance rate of 70% or higher between the analysis results and library data, qualitative confirmation of composition was performed. The concordance rate refers to the similarity between the sample and reference materials; for qualitative confirmation, the maximum concordance rate in IR analysis is defined as 100%. Cases for which the intensity of spectral peaks was too weak to distinguish or where the peak could not be determined due to interfering substances were excluded from the analysis results [33]. The spectra were obtained over a wavelength range from 400 and 7000 cm−1, with a spectral resolution of 0.075 cm−1.

2.3.2. TED-GC-MS Analysis

In the TED-GC-MS (Mettler Toledo, Gerstel, Agilent, Santa Clara, CA, USA) analysis, a balance in a thermal analyser was used to automatically measure the mass of the sample, then the sample was pyrolysed, the pyrolysis gas was adsorbed on an adsorption column, and it was subsequently thermally desorbed and introduced into a chromatography column. The products separated through the column were ionised in a mass spectrometer and cleaved; a unique spectrum for the molecular structure of each substance was obtained. Using this analysis, it was possible to identify the plastics and confirm their plastic composition, weight, and thermal characteristics [34]. The samples were weighed in a 900 μL crucible and heated under N2 to 600 °C, at a heating rate of 10 °C/min.
Therefore, TED-GC-MS, which can identify microplastics by weight, was used to determine the overall amount of microplastics. TED-GC-MS analysis was performed to detect characteristic ions for individual polymers: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polystyrene (PS), polymethyl methacrylate (PMMA), and styrene butadiene rubber (SBR).

2.4. Estimation of Microplastic Release

This study attempted to estimate the amount of microplastics generated by sewage treatment plants, a potential source of microplastics. However, there are currently no equations for calculating the amount of microplastics in waste. Therefore, in this study, we proposed an original calculation method by utilising proven calculation methods, such as the distribution of heavy metal content in waste and the UNEP mercury toolkit [35,36].
The proposed Equation (1) is based on the product of the amount of waste generated and the content of substances commonly used in the waste field. In addition, the calculation equation was developed with extensive input from Korean microplastics experts.
RMP = [Wsl × AMP × (1 − n)]
where Wsl (ton-sludge/yr) is the amount of annual sludge generated in the sewage system, investigated by MOE; AMP is the amount of MPs in the sludge (in units of MPs/g or ton-MPs/g, depending on the analysis method); n (%) is the sludge reuse rate in the process; and the sludge accumulation amount was corrected using this coefficient. RMP was expressed as MPs/g or ton-MPs/g.
In South Korea, the administrative set-up comprises a metropolitan government (1 region), metropolitan cities (6 regions), provinces (8 regions), a self-governing province (1 region), and a self-governing city (1 region). These 17 administrative regions are classified as metropolitan local governments [37]. We calculated the amount of MPs generated for all the sewage treatment plants located in these 17 administrative regions.

2.5. Quality Control

We took several measures to avoid cross-contamination by plastics present in the external environment. Such contamination can arise at all stages of MP analysis, for example, through sampling tools, during pre-treatment steps, and from the analytical equipment used [38]. We used stainless steel scoops and glass bottles to collect the samples, which were stored in dedicated MP analysis storage equipment. Glass, rather than plastic, tools were used in the extraction of MPs. Generally, when stirring is needed, a polytetrafluoroethylene-coated magnetic stir bar is used. However, considering the possibility of wear on the stir bar due to contact with samples and flask walls, which could affect the results, plastic stir bars were replaced with glass-coated magnetic stir bars. Also, during the extraction process, non-plastic materials, such as glass beakers and aluminium foil, were used to minimise potential cross-contamination. Lastly, caution was exercised to avoid oversight or loss of MPs while transferring and mounting the extracted filters [39]. A blank test was performed prior to sample analysis to improve confidence in the results. The blank test was performed using ultra-pure water, and the same analytical procedure was applied. The blank test confirmed the absence of microplastics. Therefore, the results of the microplastic analysis in this study are considered to have little influence from external sources other than microplastics in the sample. MP analysis methods are not standardised in terms of extraction, analytical methods, and QA/QC; therefore, to ensure the reliability of the analysis, we excluded environmental factors that could affect the MP analysis to as much of an extent as possible.

3. Results and Discussion

3.1. Extraction of Microplastics

The image mapping of the extracted MPs showed that the SU-W sample from Facility W apparently formed a single layer of MPs that were evenly distributed at the centre and on the periphery of the filter, whereas in the SE-W sample, MPs were mainly distributed on the periphery (Figure 2). In contrast, in the case of the sludge cake and sewage sludge samples from Facility E, MPs were mainly distributed on the periphery and centre of the filter, respectively. Larger particles were located at the centre, whereas smaller particles were distributed on the periphery of the filter. Based on these results, we expected the SU-W and SE-E samples to have relatively more MPs and larger particle sizes compared with the SU-E and SE-W samples. After image mapping, IR mapping was used to analyse the size, type, and formula of the plastics. Although contamination with organic and inorganic substances was sufficiently controlled during the extraction process, there is a possibility of the presence of non-plastic inorganic substances in environmental samples, and therefore, we performed the analysis accordingly [40].
For environmental samples in which a large number of MPs are expected, it is essential to predict the overall distribution and quantity of MPs based on optical mapping results. We believe that the prediction of MP characteristics through image mapping prior to the analysis allows for accurate determination and reduces the analysis time.

3.2. Results of Microplastics Identification

3.2.1. Results of FTIR Analysis

Using FTIR analysis, six types of plastics were detected in the sewage sludge, namely PP, PE, PET, PS, PMMA, PU, and PA. As shown in Figure 3, the MPs appeared fragmented, which is likely because they were irregularly broken into fragments by the mechanical equipment used in the water treatment process. The FTIR library spectra and the spectra of the plastics obtained after analysis showed a concordance rate of more than 70%. For the FTIR analysis, a concordance rate >70% between the analysis results and library values is considered reliable; thus, the results of MP analysis obtained in this study are deemed reliable.
The analysed MPs were classified by type, and ranked in the following order, according to their proportion: PP, PET, PE, PA, PU, and PMMA. PP, PET, and PE are used in various industrial sectors and products, and are produced and consumed in large quantities. This is probably why these three types accounted for a high proportion of the MPs found in the sludge. The abundance of PA is attributed to the use of polyamide-based coagulants in the sedimentation process during sewage treatment.

3.2.2. Results of Size Distribution

In order to confirm the possibility of progressive management of microplastics in the future, the size distribution was divided into the following three groups: MPs ≤ 0.1 mm (Range I), 0.1 < MPs ≤ 0.5 mm (Range II), and 0.5 < MPs ≤ 5.0 mm (Range III). The distribution of MPs in each size range is shown in Figure 3. The SU-W sample contained a total of 424 MPs/g, with 372 MPs/g (88%) in Range I and 52 MPs/g (12%) in Range II. The SE-W sample had a total of 150 MPs/g, with 106 MPs/g (70%) in Range I, 38 MPs/g (25%) in Range II, and 6 MPs/g (5%) in Range III (Figure 4). The SU-E sample had a total of 14 MPs/g, with 71% and 29% distribution in Ranges I and II, respectively. The SE-E sample showed a distribution trend similar to that of SU-E, with a total of 326 MPs/g.
The analysis of MPs in South Korean sewage sludge showed that the majority (75.25%) of the MPs were in Range I, 23.50% of MPs were in Range II, and 1.25% were in Range III. Plastics in the influent are initially screened through mechanical processes, such as screening and grit chambers, and are broken down into smaller sizes during sewage treatment. MPs generated after mechanical treatment processes remain in the sludge; as a result, most of the MPs are in the smaller size ranges. Generally, MPs are defined as ‘plastics with sizes less than 5 mm’. From a waste management perspective, MPs generated through sewage sludge require phased management according to their size ranges. Effective management can be achieved by prioritising measures for Range I MPs and expanding them in stages for Ranges II and III.

3.2.3. Results of TED-GC-MS Analysis

In TED-GC-MS analysis, a pyrolysis method is used to thermally decompose samples, and the gases produced are adsorbed onto an adsorption column. The gases are then thermally desorbed and introduced into a chromatography column. The products separated by the column exhibit unique spectra based on their substances, and the types of plastics can be analysed using these spectra. Unlike FTIR, TED-GC-MS cannot be used to differentiate MPs based on their sizes. To compensate for this limitation, a sieve was used during the sampling process to first sort the samples. All of the analysed samples were less than 5 mm in size. Thus, the plastics analysed in this study using TED-GC-MS can be considered to be MPs.
Using TED-GC-MS analysis of the sewage sludge, four types of MPs, namely PP, PE, PET, and PS, were detected. The proportion of the different plastic types was in the following order: PE > PP > PET > PS. The sludge cake and sewage sludge collected from Facility W had 119.11 and 46.10 µg-MPs/g, respectively. Facility E had 92.03 µg-MPs/g in sludge cake and 125.92 µg-MPs/g in sewage sludge (Figure 5).
The proportions of MPs in the sludge cake collected from Facilities W (68.7% PE, 15.6% PET, 14.4% PP, and 1.3% PS) and E (48.8% PE, 27.3% PET, 21.7% PP, and 2.2% PS) showed similar trends. In contrast, the sewage sludge showed opposing trends compared to those of the sludge cake. Facility W’s sewage sludge had 93.9% PP and 6.1% PS. In Facility W, sludge is produced after undergoing a total nitrogen treatment process using chemicals and fibre disc filters, which results in a higher proportion of PP MPs originating from fibres. Facility E’s sewage sludge had 47.4% PS, 22.9% PE, 22.8% PET, and 6.9% PP. Facility E uses a rubber belt press to dehydrate and dry sludge, which likely led to the significant amounts of PS originating from rubber materials during this process. Based on the results of this analysis, we can confirm that influent characteristics, treatment processes, and surrounding environments affect the amount and types of MPs. Sewage treatment plants must be managed as MP emission sources, rather than potential emission sources. Therefore, we believe that effective measures can be devised for reducing MPs generated in South Korean sewage treatment plants, by considering not only influent management, but also sewage treatment processes and equipment environments. The results of FTIR and TED-GC-MS analysis show some similarity. However, waste samples have the characteristic of it being difficult to maintain a completely homogeneous state. This characteristic of waste, and the peak overlap caused by the organic matter retained in the specimen, leads to differences between the two methods.

3.3. Estimated Release of Microplastics

Based on the results of our analyses, we estimated the amount of MPs expected to be generated through sewage treatment plants in South Korea. The estimates were made according to the South Korean administrative regions, using Equation (1) (Figure 6).
Wsl represents the regional sewage treatment volume in South Korea, according to sewage system statistics, and AMP is based on the results of TED-GC-MS analysis. The MP generation rate was calculated by assuming a 16% sludge reuse rate, according to the Environmental Statistics Yearbook [41].
An average of 364 ton/yr (min 175 ton/yr, max 479 ton/yr) of MPs is estimated to be generated through sewage treatment plants. The top three administrative regions (province, metropolitan government, metropolitan city) in terms of generation are proportional to the South Korean population, with 55% of the total MP generation predicted to occur in the top three regions. For better comprehension of the estimated MP amounts, we converted them into the equivalent production capacity of 500 mL PET bottles. When converting to production capacity, we applied a weight of 16.2 g, recommended by the MOE. Converting the MP generation into the production capacity of 500 mL PET bottles according to the MOE recommendation, the amount of plastics calculated in this study was found to be equivalent to 22.49 × 106 PET bottles: provinces 6.94 × 106, metropolitan government 3.23 × 106, and metropolitan city 1.15 × 106. Considering all the administrative regions in South Korea, the average was calculated as 1.32 × 106 (min 0.15 × 106, max 6.94 × 106). However, the calculated values are attributable to differences in generation due to the area and population of each administrative region, and are not related to the treatment efficiency of the sewage treatment facilities or sewage characteristics in each region.
We estimated the amount of MPs expected to be generated through sewage treatment plants, and converted this into the equivalent production capacity of plastic PET bottles. The results show that a significant amount of MPs are being generated. Therefore, measures for reducing and managing MPs generated from sewage treatment plants are necessary.

4. Conclusions

In this study, we analysed MPs in sludge samples from sewage treatment plants located in South Korea, using FTIR and TED-GC-MS methods. Using the analysis results, we estimated the amount of MPs generated through sewage treatment plants. Our results indicate that sewage treatment plants are no longer potential sources of MP emission, which emphasises the need for their management.
Through this study, it was confirmed that microplastics are contained in many wastes after water treatment. By utilising the results of this study in the preparation of future microplastic management policies, it is expected that effective management policies can be prepared. On average, 228.5 MPs/g (based on FTIR) and 95.79 µg-MPs/g (based on TED-GC-MS) of MPs were detected in the sewage sludge. The analysed MPs were mainly of the PP type, and approximately 75% were ≤0.1 mm in size. Because PP is a widely used plastic, reduction in plastic use must be prioritised to minimise MP emission. A phased management plan, focused on MPs ≤ 0.1 mm in size, is necessary.
MPs are a new source of pollution that impact the environment, and there is a lack of research on their amount and generation mechanisms, and on the technologies used for their removal in different countries. Therefore, this study can be useful for identifying global MP generation trends. In addition, for MP analysis, many countries and researchers use different pre-treatment methods and analytical methods. For these reasons, it difficult to compare the results of different studies. Applying the MP estimation equation proposed in this study in the field of MPs, which still lacks standardised research methods, could yield consistent and uniform research results, allowing for reliable comparisons.

Author Contributions

Writing-original draft, preparation, S.-J.C.; Formal analysis, J.-H.C.; Methodology, Y.-S.Y.; Project administration and Validation, N.-I.U. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Institute of Environmental Research, funded by the Ministry of Environment (NIER-2023-01-01-147).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

FPAFocal-plane array
FTIRFourier-transform infrared spectroscopy
MPsMicroplastics
MOEMinistry of Environment
NOAANational Oceanic and Atmospheric Administration
PAPolyamide
PEPolyethylene
PETPolyethylene terephthalate
PMMAPolymethyl methacrylate
PPPolypropylene
SESewage sludge
SUSludge cake
SBRStyrene butadiene rubber
TED-GC-MSThermal extraction desorption–gas chromatograph–mass spectroscopy

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Water 17 00387 g001
Figure 1. Specific flow of microplastic extraction in this study.
Figure 1. Specific flow of microplastic extraction in this study.
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Figure 2. Image mapping results of filter after extraction step; (A) SU-W, (B) SE-W, (C) SU-E, (D) SE-E.
Figure 2. Image mapping results of filter after extraction step; (A) SU-W, (B) SE-W, (C) SU-E, (D) SE-E.
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Figure 3. Comparison of spectrum match rates of analysis and library data.
Figure 3. Comparison of spectrum match rates of analysis and library data.
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Figure 4. Size distribution of microplastics.
Figure 4. Size distribution of microplastics.
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Figure 5. Results of microplastic abundance and ratio; (A) FTIR, (B) TED-GC-MS.
Figure 5. Results of microplastic abundance and ratio; (A) FTIR, (B) TED-GC-MS.
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Figure 6. Plastic bottle production capacity by administrative region, based on microplastics generation estimates.
Figure 6. Plastic bottle production capacity by administrative region, based on microplastics generation estimates.
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Table 1. Comparison of microplastic extraction methods in previous research.
Table 1. Comparison of microplastic extraction methods in previous research.
NationSampleMP Extraction SolutionMP Particles
/g-Sample		Ref.
Biological Digestion Density Separation
South KoreaSludge cake and sewage sludge30% H2O2ZnCl214–424This study
United StatesSewage sludgeNaClO-0.05[19]
GermanySewage sludge30% H2O2ZnCl224.00[20]
IrelandSewage sludge-ZnCl215.39[21]
ScotlandSludge cakeH2OH2O1.20[22]
NetherlandsSewage sludge30% H2O2NaCl0.76[23]
SwedenSludge cake--0.72[24]
ChinaSewage sludge30% H2O2NaCl56.39[25]
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Cho, S.-J.; Choi, J.-H.; Yoon, Y.-S.; Um, N.-I. Microplastic Pollution in Sewage Sludge from Wastewater Treatment Plants and Estimation of Microplastic Release. Water 2025, 17, 387. https://doi.org/10.3390/w17030387

AMA Style

Cho S-J, Choi J-H, Yoon Y-S, Um N-I. Microplastic Pollution in Sewage Sludge from Wastewater Treatment Plants and Estimation of Microplastic Release. Water. 2025; 17(3):387. https://doi.org/10.3390/w17030387

Chicago/Turabian Style

Cho, Soo-Jin, Ja-Hyung Choi, Young-Sam Yoon, and Nam-Il Um. 2025. "Microplastic Pollution in Sewage Sludge from Wastewater Treatment Plants and Estimation of Microplastic Release" Water 17, no. 3: 387. https://doi.org/10.3390/w17030387

APA Style

Cho, S.-J., Choi, J.-H., Yoon, Y.-S., & Um, N.-I. (2025). Microplastic Pollution in Sewage Sludge from Wastewater Treatment Plants and Estimation of Microplastic Release. Water, 17(3), 387. https://doi.org/10.3390/w17030387

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