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Abstract

Objective

This study aims to assess the concentration and removal efficiency of microplastics (MPs) at a major wastewater treatment plant in Jordan, a region with limited data on MP pollution.

Methods

A field-based experimental study was conducted. Grab samples of 14 L were collected from both influent and effluent streams on a single sampling day. Wet sieving was used to isolate MPs in the 38–1000 µm range, followed by wet peroxide oxidation and optical microscopy for particle quantification and morphological classification.

Results

The influent contained an average of 62.6 MPs/L, while the effluent had 23.8 MPs/L, resulting in an overall MP removal efficiency of 62%. Fibers and fragments were the most prevalent types in the effluent, indicating partial resistance to the treatment process.

Conclusion

The results underscore the presence of residual MPs in treated wastewater and suggest a need for more robust filtration technologies and plastic management strategies to reduce environmental MP release.

Keywords

Microplastics wastewater wastewater treatment plant microplastics removal

Introduction

Plastic, a strong, affordable, and durable synthetic polymer, is widely used in consumer goods, construction, packaging, and medical devices.¹ Global plastic production reached a staggering 400.3 million metric tons in 2022, with China leading at 32% of output.² However, plastic waste poses significant environmental challenges due to its non-biodegradability. Discarded plastics fragment into microplastics (MP), which affect marine and freshwater ecosystems.³ Understanding plastic's lifecycle is vital for mitigating its environmental impacts.⁴

The term “microplastic” was defined by the EPA as plastic particles smaller than 5 mm, formed by the degradation of plastic debris, including primary microbeads and secondary MPs from larger items.⁵ Advances in detection prompted broader classifications, with NOAA recognizing particles smaller than 1 mm, reflecting evolving scientific and regulatory frameworks.^6,7 MPs pose significant environmental and health hazards, affecting diverse species and ecosystems. They can translocate through biological membranes, leading to inflammation and toxicity.⁸ MPs carry heavy metals and pollutants, threatening marine biota, and human health, particularly through contaminated seafood and respiratory exposure.⁹

MPs are classified by morphology into fibers, fragments, films, and flakes. Fibers are thin, elongated particles commonly originating from textiles or fishing gear, such as polyester threads. Fragments are irregularly shaped, rigid pieces formed by the breakdown of larger plastic items like packaging materials. Films refer to thin, flexible plastic layers typically derived from plastic bags or wraps. Flakes are flat, plate-like particles often resulting from degraded plastic containers or paint coatings. This classification approach aligns with previous studies on MP characterization.¹⁰

MPs enter wastewater treatment plants (WWTPs) from various sources, including microbeads, synthetic textile fibers, tire wear, and urban dust.^11,12 Secondary MPs primarily originate from larger plastic debris fragmentation and agricultural practices, emphasizing the need for thorough sampling and analysis to assess their environmental impact.^13,14 Preliminary and primary wastewater treatments remove MPs through physical mechanisms, achieving 35%-59% and 50%-98% removal rates, respectively, for particles typically ranging from 20 µm to 5 mm.¹⁵ Methods like air flotation enhance removal, particularly for polyethylene microbeads (size range: 100–500 µm).¹⁶ Removal efficiency varies by MP size, shape, and wastewater characteristics.¹⁷ Secondary wastewater treatment utilizes clarification and biochemical methods to further reduce MPs that escape earlier treatments, achieving 0.2–14% MP levels, generally for MPs in the 10 µm to 5 mm range.^18,19 Techniques like the activated sludge process enhance MP removal rates, but effluents still risk plastic pollution.^20,21 Tertiary wastewater treatment significantly reduces MPs to 0.2%–2% of initial concentrations, with membrane bioreactors, targeting MPs as small as 20 µm, achieving up to 99.9% removal efficiency.^20,22 Other techniques like ultrafiltration and sand filtration also enhance MP removal, but operational challenges persist.^23,24 MPs levels in WWTPs vary based on served population, wastewater type, and local activities.²⁴ MPs are generally detected in both influent and effluent of WWTPs, with reported influent concentrations varying from 1 to 10044 particle/L and effluent concentrations in the range of 0 to 447 particle/L based on data collected from 24 different WWTPs treating MPs between 0.01 mm and 5 mm.¹² Standardized methods are needed for consistent MP pollution comparisons.²³ Over 30 types of MPs polymers have been identified in WWTPs, with polyester (28%-89%), polyethylene (4%-51%), and polyethylene terephthalate (4%-35%) being the most common.^21,22 Fibers dominate in both influent and effluent, complicating accurate quantification.²⁰ WWTPs contribute approximately 25% of global MPs inputs to marine environments. Despite advanced treatments, small particles, particularly those <100 µm, often escape, especially during combined sewer overflows.^25,26 Enhanced WWTP designs are needed to effectively capture and reduce MP discharges.²⁷

Representative sampling is crucial for detecting low MP concentrations in wastewater. Techniques include manual grab sampling, autosamplers, and surface filtration. Grab sampling is simple but limited in volume, while autosamplers and separate pumping allow for larger volumes but require specialized equipment.^20,28 Variability in MP concentrations depends on sampling methods, influencing study results.²⁹ Each method's suitability depends on the WWTP's characteristics and analysis goals. High MP loads in sludge and biosolids highlight the need for accurate sampling to ensure feasibility and relevant results.³⁰

Various methods are employed to purify and separate MPs from wastewater and sludge, facilitating accurate quantification and identification. These include wet peroxide oxidation (WPO),³¹ alkaline hydrolysis,³² enzymatic digestion,³³ density-based methods,³⁴ and oil extraction and magnetic-based methods.³⁵ The selection of a suitable method depends on study objectives and sample characteristics.

The rapid rise of MPs pollution has driven the development of innovative detection and quantification methods. While current techniques are reliable, they remain labor-intensive, time-consuming, and often only semi-quantitative. Spectroscopy and microscopy are commonly employed to identify and quantify MP in wastewater.³⁶ MP analysis is split into physical characterization (size, shape, color) and chemical analysis (composition).¹² Stereomicroscopy was used to visually assess and count all MP particles on each filter based on size, color, and shape, performing a full count rather than extrapolation. This approach maximizes data reliability, despite inherent limitations in polymer identification and potential bias with fibers.³⁷ Visual methods such as the hot needle test aid in distinguishing plastic particles, but the technique is size-dependent and risks misidentification²⁵ and limited magnification.³⁸ Despite these drawbacks, it remains a fundamental method for preliminary MP analysis, providing essential data for further investigation.

WWTPs significantly contribute to the release of MPs into the environment.¹² This release poses severe environmental and health risks, as MPs can carry heavy metals, toxic substances, and pathogens, potentially entering food chains and affecting human health, soil, and agricultural crops.³⁹ The interaction of MPs with WWTPs presents a challenging research area, particularly as technological advancements have enhanced MP detection in freshwater and marine ecosystems.²⁶ Understanding the sources and behaviors of MPs is crucial for evaluating their impacts on treatment processes and the environment.^40,41 This study is vital for quantifying MP loads in treated wastewater, facilitating discussions on using treated wastewater for agricultural irrigation, and addressing knowledge gaps about MPs in Jordan's wastewater systems.

This study was conducted to assess the presence and removal efficiency of MPs at the Samra WWTP in Jordan (Figure 1). In addition to quantifying MP concentrations in influent and effluent, we analyzed the morphologies and classifications of detected MPs. To our knowledge, this is the first study of its kind conducted in Jordan—and possibly in the broader Middle East—where limited data exists on MPs in wastewater systems. The region's distinct climatic conditions, infrastructure design, and waste management practices may influence MP behavior and removal efficiency differently than in moderate or industrialized regions. This work thus fills a critical knowledge gap and establishes a foundation for future research and policy development in arid and semi-arid zones.

Figure 1.

Areal view of the Samra WWTP. Left image is for Jordan. Base map imagery: ^© Google Maps.⁴²

Methods and materials

Study site description

The Samra WWTP, located in the Zarqa Governorate of Jordan, is one of the largest and most advanced wastewater treatment facilities in the Middle East. It serves the Greater Amman and Zarqa regions, with a population equivalent of approximately 3.5 million people. The plant receives influent primarily from urban and suburban residential zones, as well as from light industrial and commercial sources, treating mixed domestic and industrial wastewater. It processes approximately 370,000 cubic meters of wastewater per day.⁴³ The preliminary treatment stage includes mechanical screening with screens of 6 mm in size and grit removal units. The surrounding region features mixed land use, including residential settlements, agricultural fields, and industrial areas. Treated effluent from the Samra WWTP is discharged into the Zarqa River, which supports downstream agricultural irrigation. The main crops cultivated in the vicinity include olives, citrus fruits, vegetables, and forage crops. The treatment process begins with preliminary screening and grit removal, followed by primary sedimentation to eliminate solids. Secondary treatment uses activated sludge systems to reduce biochemical oxygen demand and suspended solids. Tertiary treatment employs advanced filtration and disinfection methods, such as sand filtration and UV disinfection, ensuring the effluent is pathogen-free and meets reuse standards. Potential routes of MP transport in this environment include domestic wastewater discharge, industrial effluents, urban runoff via rain water systems, atmospheric deposition, and human activities such as improper plastic waste disposal and tourism-related littering. The Zarqa River itself acts as a key pathway, carrying both treated and untreated residues toward agricultural zones, where MPs can accumulate in soil and crops.⁴⁴

Sampling method

At the Samra WWTP, wastewater from two sampling points was collected which are designated as follows: S1 for the influent point of the plant, and S2 for the effluent point of the plant, as shown in Figure (2). The sampling event took place in July 2023. For each sampling point, a volume of 14 L of wastewater was collected using a grab sampling method with a stainless steel bucket attached to a telescopic rod to reach a depth of approximately 30–50 cm below the surface. Samples were collected after the mechanical screening stage to focus on MP content without large debris interference. The hydraulic retention time (HRT) of the Samra WWTP is approximately 2 days. We did not consider HRT in the sampling procedure, as our primary aim was to quantify the absolute concentrations of MPs in the influent and effluent streams. However, we acknowledge that variations in influent MP loads may not be immediately reflected in the effluent due to the plant's HRT. This temporal mismatch may influence the accuracy of removal efficiency estimates and represents a limitation of the current study.

Figure 2.

Process flow schematic of Samra WWTP and sampling points S1 and S2.

Samples processing

Samples wet sieving

Wet sieving was conducted in the laboratory to separate MPs from wastewater matrices, following the sampling event.⁴⁵ Different mesh sizes were used, and the sieve tower was mounted on an electromagnetic sieve shaker (CISA BA200N, Barcelona) to accelerate the filtration process and ensure accurate and consistent results.

For each of samples collected from S1 and S2, a volume of 14 L was used. The wet sieving process employed mesh sizes of 38 μm, 200 μm, 500 μm, and 1000 μm to characterize the particle size distribution of MPs. Although MPs are generally defined as particles ≤5000 μm, particles >1000 μm were excluded from analysis to focus on the smaller, more mobile, and bioavailable fractions. This range also aligns with the detection capabilities of our analytical setup and enhances consistency across samples. This range of mesh sizes allowed for the separation and analysis of various particle sizes present in the wastewater. Repeating this process ensured consistency and comparability in the data collected across different sampling periods. After that, samples were washed from sieves into 500 mL glass beakers and dried in oven at 60°C. The lower detection size limit for MPs in this study was 38 µm, determined by the smallest mesh size used during the wet sieving process. Particles smaller than this threshold were not captured or analyzed, and results should be interpreted accordingly.

Organic matter digestion

WPO, as described by Masura et al.,⁴⁶ was utilized to remove organic residues contaminating the MP samples. To enhance the digestion rate, the samples were gently heated to 60 °C during the Fenton reaction, a method consistent with prior studies for treating samples with substantial organic content.³¹ Fenton's reagent was prepared by mixing 20 mL of 30% H₂O₂ (Bio Sane, Fischer Chemical (HK) Limited Co., China) with 20 mL of an iron catalyst (FeSO₄.7H₂O) (AZ Chem for chemicals, China). This solution was added to the samples, which were then heated to 60°C on a hotplate for 30 min, using a stir bar for mixing and a watch glass to cover the beaker. An additional 20 mL of H₂O₂ was added to the samples until no visible organic matter remained. Finally, the samples were transferred to beakers and dried at 60°C.

Microparticle extraction

Density-based separation was used to isolate MPs from inorganic materials such as sediment and sand. In this study, a sodium iodide (NaI) (Proto Chemical Industries, India) solution with a density of approximately 1.5 g/cm³ was utilized. The NaI solution was mixed with the dried samples and stirred continuously. The floating particles were then isolated and transferred to a set of sieves to remove any salt residue. After washing away the NaI, the floating particles were collected, and suction filtration was performed using cellulose nitrate filters (0.45sμm).

Optical microscopy analyses

After wet sieving, particles from each size fraction (38–200 µm, 200–500 µm, and 500–1000 µm) were collected onto separate filter papers. These filter papers were then examined under a Leica DM 750 light microscope (Leica Microsystems, Switzerland) at magnifications of ×4, ×10, and ×40. Images of the MPs were captured using a Leica MC 190 HD camera with Leica LAS software. MPs on each filter were counted and classified by shape; fiber, fragment, flake, film, and bead; and by color, including black, blue, green, orange, purple, red, transparent, white, and yellow.

Quality assurance and quality control

A strict protocol was followed to prevent MP contamination and disease from wastewater pollutants. This included vaccinations, wearing protective gear, using non-plastic containers, sterilizing equipment with ethanol, and conducting experiments under a fume hood. Background blank filters were used to assess contamination, and matching MPs were excluded from the analysis.²¹

Abundance, balance, and removal efficiency of microparticles

The average number of MPs at each sampling point (S1 and S2) in the liquid fraction was reported in units of MP particles per liter of wastewater. The removal efficiency of the Samra WWTP was calculated using the following equation:

R E = \frac{M P_{i n f} - M P_{e f f}}{M P_{i n f}} \times 100 %

(1)

where RE is the removal efficiency in %, $M P_{i n f}$ is MPs in the influent in (MP/L) and $M P_{e f f}$ is MPs in the effluent in (MP/L)

Statistical analysis

Statistical analysis was performed using Microsoft Excel (Microsoft 365) with a two-sample t-test to compare MP concentrations before and after purification. A p-value < 0.05 indicated significant differences, with T_stat > T_critical leading to rejection of the null hypothesis (H0). H0 stated no significant difference in MP concentrations, while the alternative hypothesis (Ha) proposed purification reduces MP concentration. If the p-value was greater than 0.05, H0 was not rejected, indicating no significant effect from purification on MP concentration.

Recovery efficiency

In this study, a positive recovery control using spiked MP particles was not performed due to logistical constraints and the exploratory nature of the investigation. Nonetheless, to minimize potential MP loss during sample processing, we adhered to established protocols that emphasize gentle handling, careful filtration, and laboratory practices designed to reduce contamination and particle loss. While this approach helps maintain data integrity, we acknowledge that the inclusion of recovery efficiency assessments through spiked samples would provide more robust quality assurance. Future work will incorporate such controls to enhance the reliability and comparability of MP quantification.

Results and discussion

Microparticles concentrations and properties

The MP concentrations in the influent and effluent of the Samra WWTP were determined from samples collected on 24 July 2023, during a single-day sampling event. Each sample had a volume of 14 L and was analyzed across different size fractions of 38–200 µm, 200–500 µm, and 500–1000 µm, as presented in Figure 3. For the 38–200 μm size fraction, the influent concentration is 30.4 MP/L, while the effluent concentration is 11.9 MP/L. For the 200–500 μm size fraction, the influent concentration is 17.4 MP/L, and the effluent concentration is 7.6 MP/L. This indicates a significant decrease in MP concentration, highlighting the plant's ability to effectively remove MPs in this size range.

Figure 3.

Percentage distribution of the concentration of different size fractions of MPs from influent (S1) and effluent (S2) points within the sampling event. Data are based on full count stereomicroscopy.

For the 500–1000 μm size fraction, the influent concentration is 14.9 MP/L, and the effluent concentration is 4.2 MP/L. The reduction is particularly notable in this size range, showcasing the plant's higher removal efficiency for larger MPs. The data indicates that the Samra WWTP is effective at reducing MP concentrations across all size fractions, especially for larger MPs (500–1000 μm).

The influent wastewater volume measured 370,785 m³ per day and the effluent volume was 371,939 m³ per day on 24 July 2023.⁴³ This slight increase in volume may be influenced by HRT and fluid dynamics within the plant, as well as minor measurement variations. The influent carried 23,227 MPs/day into the plant. This highlights the significant presence of MPs in the raw sewage entering the treatment facility. After treatment, the effluent discharged from the plant contained 8847 MPs/day. This indicates that while the WWTP was effective in removing a substantial portion of the MPs from the wastewater, a considerable number of MPs still passed through the treatment process and were released into the environment. It still means that approximately 38% of the MPs entering the plant were not captured and were subsequently discharged. These results are in line with other studies conducted in urban WWTPs.^47–49

Influent concentrations are higher across all size fractions compared to effluent concentrations, demonstrating the plant's capability in filtering out MPs. The large variations in MPs concentrations could be partially related to different analysis methods applied. For example, a higher MPs concentration might be observed when a finer mesh size was applied.^50,51

The influent MPs distribution at the Samra WWTP on 24 July 2023, shows a variety of shapes (Figure 4). Fibers dominate the influent, constituting 52.5% of the total MPs. This high percentage suggests that a significant portion of MPs entering the plant is fibrous, likely originating from textiles and clothing. Fragments make up the second largest portion, with 35.8%, indicating a substantial presence of broken-down plastic pieces. Films, which are thin and flexible pieces of plastic, account for 6.8% of the influent MPs. Beads, small spherical plastics often found in personal care products, represent 2.9% of the influent. Flakes, which are irregularly shaped plastic pieces, constitute the smallest portion at 2.1%. This distribution highlights the diverse sources and forms of MPs entering the wastewater treatment process.

Figure 4.

Percentage distribution of shape categories in the influent (S1) and effluent (S2) points within the sampling event. Data are based on full count stereomicroscopy.

In the effluent, the distribution of MP shapes changes somewhat after treatment at the Samra WWTP. Fibers still dominate, increasing slightly to 56.2% of the total MPs in the effluent. This indicates that fibers are particularly resistant to removal during the treatment process. Fragments remain a significant portion, though they decrease to 32.4%, suggesting some effectiveness in their removal. The percentage of films decreases to 4.5%, showing that the treatment process is more effective at removing this shape of MPs. Beads increase slightly to 3.6%, indicating a potential challenge in their removal. Flakes also show an increase to 3.3%, which might be due to their small size and irregular shape making them harder to filter out. This effluent distribution highlights the varying effectiveness of the WWTP's treatment processes in removing different shapes of MPs. The reported percentages indicate the relative abundance of these types of plastics in the influent and effluent.

The predominance of fibers over particles has been reported previously for wastewater effluents and influents.⁴⁸ Browne et al. reported that every single wash of polyester clothing can release over 1900 MP fibers/L and over 100 fibers/L of effluent.¹³ The low density and high length-to-width of the fibers are likely to play a role in maintaining these MPs in freshwater for long periods of time, which prevents them from being captured.²⁰ For these reasons, fibers are named the most difficult form of MPs to be captured in WWTPs and the most evacuated in effluents.²⁰

The comparison between influent and effluent distributions shows that while the Samra WWTP effectively reduces the overall concentration of fragment, flake, film and bead shapes of MPs, while fibers remain the most persistent, increasing their share in the effluent. Films see the most significant reduction, while beads and flakes increase slightly, suggesting areas where the treatment process could be improved. In addition, the degradation of plastic particles has been proposed as the origin of the high concentration of fibers in the effluent. The fragmentation of large plastic particles during transport or cleaning leads to the presence of fragments and films in the wastewater.¹²

Additionally, the color of MPs is significant because marine organisms are more likely to ingest brightly colored plastics or particles that resemble their food.⁵² The influent MPs color distribution at the Samra WWTP on 24 July 2023 show that blue MPs dominate the influent, constituting 29% of the total. This high percentage suggests that blue-colored plastics, likely originating from various consumer products and packaging, are a significant part of the MPs entering the plant. Red MPs make up the second largest portion at 16%, followed closely by black and white MPs, each at 15%. Transparent and yellow MPs each account for 8% of the influent, while green MPs make up 8%, indicating diverse sources of these pollutants. Purple MPs represent 0.8%, and orange MPs are the least prevalent, at just 0.2%. This distribution highlights the wide range of plastic products contributing to MP pollution in the influent.

In the effluent, the distribution of MP colors changes slightly after treatment at the Samra WWTP. Blue MPs remain the most prevalent, still constituting 29% of the total, indicating that they are resistant to removal during the treatment process. White MPs increase to 20%, suggesting that these particles are less effectively removed. Black MPs see a slight increase to 16%, remaining a significant portion of the effluent. Red MPs decrease slightly to 14%, indicating some effectiveness in their removal. Green MPs decrease to 5%, showing that the treatment process is relatively effective at removing these particles. Transparent and yellow MPs remain constant at 8%, suggesting they are neither significantly removed nor increased during treatment. Purple MPs decrease to 0.3%, and orange MPs see a slight increase to 0.3%, indicating minor fluctuations in their removal efficiency.

The color of MPs in wastewater varies widely depending on the habits of each region, with different studies reporting varying percentages. For instance, Takdastan et al. (2021) found that the highest abundance of MP colors was clear and green.⁵³ Similarly, a study in Wuhan, China, reported a significant abundance of transparent MPs in analyzed wastewater, which may be attributed to the local population's habits.⁵⁴

The comparison between influent and effluent distributions shows that while the Samra WWTP effectively reduces the overall concentration of some colors of MPs, others persist or even increase. Blue and white MPs are particularly resistant to removal, maintaining or increasing their presence in the effluent. Green and purple MPs are more effectively reduced, suggesting that the treatment process has varying effectiveness based on color.

Microparticles removal efficiencies

The concentration of MP in the influent and effluent of the Samra WWTP depended on the different size fractions. In the influent, the total concentration of MPs is 62.6 MP/L, with the 38–200 µm, 200–500 µm, and 500–1000 µm fractions having concentrations of 30.4 MP/L, 17.4 MP/L, and 14.9 MP/L, respectively. In the effluent, the total concentration is significantly reduced to 23.8 MP/L, with the same size fractions having concentrations of 4.2 MP/L, 7.6 MP/L, and 11.9 MP/L, respectively. The results are summarized in Table 1.

Table 1.

MP concentrations (MP/L) and removal efficiency (%) related to the sampling event.

Fraction size (μm)	S1 (MP/L)	S2 (MP/L)	Removal efficiency
38–200	30.4	11.9	61.8%
200–500	17.4	7.6	56.0%
500–1000	14.9	4.2	72.6%
All	62.6	23.8	62.0%

MP: microplastic.

The removal efficiency, calculated using the difference between the influent and effluent concentrations, is approximately 62.0% for the total MPs. When looking at the data in detail, the removal efficiency for the 500–1000 µm fraction is about 72.6%, for the 200–500 µm fraction it is around 56.0%, and for the 38–200 µm fraction it is approximately 61.8%. The efficiency varies significantly across different size fractions, with the smallest fraction having the highest removal efficiency and the largest fraction having the lowest. These results suggest that the treatment process is more effective at removing larger MPs.

The presence of MP in wastewater treatment processes can vary based on factors such as wastewater flow and peak usage times.²⁹ However, reported removal efficiencies from previous studies exhibit some inconsistencies. These differences may be attributed to differences in sampling strategies, the heterogeneous nature of MPs, variations in influent type and load, and the quantification methods used, which complicates direct comparisons between studies.^29,55

A comparative analysis of MP concentrations and removal efficiencies (RE%) between the Samra WWTP and various other (WWTPs) is presented in Table 2. The current study of Samra WWTP, a secondary treatment plant, shows an influent concentration of 62.6 MPs/L and an effluent concentration of 23.8 MPs/L, resulting in a removal efficiency of 62%.

Table 2.

Comparison of MPs concentrations and removal efficiencies in various WWTPs.

Location	Treatment level of WWTP	Influent (MPs/L)	Effluent (MPs/L)	Removal efficiency %	Sampling Volume (L)	MP Size Range (µm)	References
Samra WWTP	Secondary	62.6	23.8	62	14	38–1000	Current study
Morocco	Tertiary	188	50	72	1–10	290 to 4200 µm	Hajji et al.⁵⁶
Morocco	Tertiary	519	86	81	1–10	290 to 4200 µm	Hajji et al. ⁵⁶
China	Tertiary	23	8	66.1	10	100–500	Tang et al.⁵⁴
Italy	Tertiary	3	0.4	84	1–10	100–500	Magni et al.⁴⁷
France	Secondary	293	35	88.1	20	100–5000	Dris et al.¹⁸

MP: microplastic; WWTP: wastewater treatment plant.

Hajji et al. reported in Morocco for two tertiary treatment plants an influent concentration of 188 MPs/L and 519 MPs/L, and effluent concentration of 50 MPs/L and 86 MPs/L, respectively.⁵⁶ The removal efficiencies for these plants are 72% and 81%, demonstrating the varying capabilities of tertiary treatment in different contexts. In China, the tertiary treatment plant studied by Tang et al. has an influent concentration of 23 MPs/L and an effluent concentration of 8 MPs/L, resulting in a removal efficiency of 66.1%.⁵⁴ This is the lowest removal efficiency among the tertiary plants listed, suggesting potential room for process optimization. In Italy, the tertiary treatment plant studied by Magni et al. has a significantly lower influent concentration of 3 MPs/L and an effluent concentration of 0.4 MPs/L, yielding a high removal efficiency of 84%.⁴⁷ This indicates the effectiveness of tertiary treatment processes in reducing MP concentrations. The secondary treatment plant in France, studied by Dris et al.,¹⁸ exhibits an influent concentration of 293 MPs/L and an effluent concentration of 35 MPs/L, achieving a high removal efficiency of 88.1%. This is notable as it surpasses the removal efficiency of some tertiary treatment plants, indicating the effectiveness of its treatment processes.

Compared to other WWTPs, the Samra WWTP, being a secondary treatment plant, performs relatively well with a removal efficiency of 62%, which is comparable to some tertiary treatment plants. Despite this, there is room for improvement, particularly in light of the higher efficiencies achieved by other plants. Upgrading to tertiary treatment, optimizing existing processes, and adopting best practices from high-performing WWTPs could potentially enhance MP removal at Samra WWTP. Moreover, it is evident that tertiary treatment plants generally have higher removal efficiencies and lower effluent concentrations compared to secondary treatment plants. However, the French secondary treatment plant's performance suggests that certain secondary treatment processes can also achieve high removal efficiencies. A two-sample, two-tailed t-test was performed to assess significant differences in MP concentrations between influent and effluent samples, as well as across different particle size fractions. Due to the exploratory nature of the study and limited number of samples, no formal sample size calculation was conducted. While assumptions of normality and equal variance were not tested statistically, the data were treated under the assumption of approximate normal distribution for comparative purposes. The aim was to provide indicative results rather than definitive statistical conclusions. No corrections for multiple comparisons were applied. Results were considered statistically significant at p < 0.05.

Environmental concerns related to microplastic pollution

Our results demonstrate a significant presence of MPs in both the influent and effluent of the Samra WWTP, with concentrations varying across particle size fractions. The persistence of MPs in treated effluent, with approximately 38% of the total MPs remaining post-treatment, raises concerns regarding their release into the environment. The downstream discharge of these MPs into natural water bodies may contribute to ecological risks, including bioaccumulation in aquatic organisms and disruption of ecosystems. Given the wide variety of particle sizes detected, the environmental fate and transport of MPs should be further studied, as smaller MPs (<200 µm) are more readily bioavailable and potentially more harmful. Continuous monitoring and advanced treatment technologies are crucial to minimize environmental loading of MPs.

Energy and safety conditions of the Samra wastewater treatment plant

The Samra WWTP operates with a secondary treatment process, effectively reducing MPs by 62%. However, the energy requirements for advanced treatment stages such as activated sludge and UV disinfection are substantial and represent an important consideration for sustainable operation. Optimization of influent and effluent flow management can improve energy efficiency without compromising treatment quality. Furthermore, ensuring rigorous safety protocols during sampling and treatment is essential to protect personnel from exposure to wastewater contaminants, including MPs. Our adherence to strict contamination prevention protocols during sampling highlights the importance of worker safety and data integrity in such studies.

Potential reuse of recovered microplastics under circular economy principles

Although MPs are typically regarded as pollutants, their recovery from wastewater streams presents opportunities within the circular economy framework. Emerging technologies for recovering and reprocessing MPs could transform waste materials into secondary raw materials for industries such as construction or manufacturing. However, practical application faces challenges related to the purity of recovered MPs, potential contamination, and economic viability. Integrating MP recovery processes into WWTP operations could contribute to resource recovery and pollution reduction, aligning with global sustainability goals and circular economy principles.

Conclusions

This study evaluated MP concentrations and removal efficiencies at the Samra WWTP through systematic sampling of influent and effluent streams. The applied methods included multi-stage filtration, chemical treatment, and optical microscopy. Results showed variable removal efficiencies across different MP shapes: fibers were partially removed; fragments persisted in the effluent but accumulated in sludge, beads showed minor fluctuations indicating removal challenges, while films were more effectively eliminated. Color distribution analysis revealed that black, red, and blue MPs slightly increased in the effluent, suggesting resistance to current treatment processes, whereas green MPs decreased, indicating some removal effectiveness. Overall, the Samra WWTP demonstrated moderate effectiveness in reducing MP content, with the primary clarification stage playing a crucial role in removal. However, direct comparisons with other studies remain challenging due to the lack of standardized detection protocols, variations in mesh sizes and separation solutions, and differing WWTP capacities and technologies. Building on these findings, we propose the following priority actions to improve MP management in wastewater treatment facilities, especially in Mediterranean countries with similar contexts. Firstly, the upgrade and optimize treatment technologies, then the use of standardize monitoring protocols such as the establishment of regional guidelines for MP sampling, size fractionation, and analytical methods. Finally, regional collaboration and policy development should be strengthened. Implementing these actions will contribute to safeguarding water resources and ecosystems across the Mediterranean basin, supporting sustainable wastewater management and environmental protection.

Footnotes

Acknowledgements

The authors highly acknowledge the University of Jordan for providing support to perform this research.

ORCID iDs

Ehab AlShamaileh

Author contributions

Both authors contributed equally to the manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

Data available upon reasonable request.

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Microplastics removal efficiency in wastewater treatment plants in Jordan