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Article

Investigating the Potential of Coagulants to Improve Microplastics Removal in Wastewater and Tap Water

by
Claudio Casella
,
Daniel Sol
,
Adriana Laca
* and
Mario Díaz
Department of Chemical and Environmental Engineering, University of Oviedo, C/Julián Clavería s/n, 33006 Oviedo, Spain
*
Author to whom correspondence should be addressed.
Microplastics 2025, 4(4), 89; https://doi.org/10.3390/microplastics4040089
Submission received: 29 September 2025 / Revised: 22 October 2025 / Accepted: 10 November 2025 / Published: 12 November 2025
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Abstract

This study investigates the impact of using coagulants on the removal of microplastics (MPs) from wastewater and tap water. Before the settling step, coagulants commonly used in water treatment (FeCl3 or Al2(SO4)3) were added at different concentrations to samples taken from an activated sludge reactor and tap water. MPs initially contained in the water samples were chemically and physically characterized, resulting in most of them being fibres smaller than 500 μm, in both media. The use of coagulants improved MPs removal, and the best results were obtained with the aluminum salt, which allowed removal efficiencies of 43% and 62% for tap water and wastewater, respectively. These results demonstrated the potential of coagulants to improve the removal of MPs in treated waters and wastewaters. However, the necessary concentration of the assayed coagulants was quite high, highlighting the interest in investigating their combination with coagulant aids, such as organic polyelectrolytes, which might allow for reduced doses.

Graphical Abstract

1. Introduction

MPs are an important pollutant in municipal wastewaters due to their multifaceted origin [1]. Specifically, MP concentrations range from 10−3 to 105 MPs/L in the influent of municipal wastewater plants (WWTPs) [2,3,4]. During wastewater treatment, more than 90% of MPs are removed from wastewater, mainly being trapped in the sewage sludge [4,5]. However, MP concentrations in the range 10−3–103 MPs/L are still found in the treated water. Therefore, it is imperative that these facilities take measurements to increase the effectiveness of MP removal in order to reduce their discharge into the aquatic environment [6].
This problem also extends to tap water. According to Ren et al. [7], drinking tap water is responsible for the greatest percentage (>80%) of MPs intake into the human body. It has been estimated that each adult who drinks tap water may consume up to 7500 MPs/year [8]. This highlights the importance of having treatment techniques that reduce MP concentrations in drinking water to levels that avoid endangering human health [9]. However, the existence of MPs in drinking water treatment plants (DWTPs) has received less attention than its occurrence in WWTPs. In fact, the removal rates are lower in DTWPs, around 32%, with MP concentrations within the ranges of 10−3–103 MPs/L in DTWP effluent (similar to the range found in treated wastewater) [2]. Contrary to what might have been expected, concentrations found in tap water are usually higher, between 10−1 and 104 MPs/L, due to the use of plastics in the distribution network [2,10].
Membranes have been reported as the most effective techniques to remove microplastics from water. A removal efficiency of 90% has been reported for MPs when reverse osmosis was used as tertiary treatment, and the use of membrane bioreactors (MBRs) allowed removal efficiencies of above 99% in some WWTPs. Unfortunately, these technologies have high operating costs and do not achieve such high removals in all cases [11]. A less expensive option is the employment of dynamic membranes (DMs). However, it is not yet a fully mature technology and there is still considerable uncertainty regarding their large-scale performance, maintenance, and operational stability [12]. Additional proposals include other tertiary treatments, such as activated carbon adsorption, which at lab level has given variable results (efficacies between 40 and 90%) and requires further research to be implemented as an effective technique for MP removal [13].
Coagulation is a low-cost and effective method to enhance the removal of colloids and fine particles from wastewater and drinking water. In the same way, some authors have indicated that the efficacy of MP removal by settling could be improved by using suitable coagulants [10,11,14,15]. There are two types of coagulants: inorganic (i.e., FeCl3, Al2(SO4)3) and organic (i.e., polyacrylamide). In general, inorganic coagulants are more cost-effective and versatile, while organic coagulants are more expensive but more environmentally friendly, generating less waste [16]. Organic coagulants are frequently used as coagulation aids, that is, to complement inorganic coagulants. The effectiveness of each type depends on several factors, including pH, turbidity, suspended solids, and temperature. For example, high pH can reduce the efficiency of inorganic coagulants, high turbidity may require higher doses, and temperature influences coagulant solubility and reaction rates [17,18,19]. The use of inorganic coagulants usually requires additional separation steps to eliminate residual metals, which is one of the main drawbacks of the process. This need for extra treatment has driven interest in natural coagulant aids, such as modified clays and chitosans, which enable a reduction in coagulant dosage and, in some cases, an increase in removal efficacy [20].
It has been reported that the use of coagulants for urban wastewater achieved MP removals of between 30 and 98% [14]. For example, Zhou et al. [21] reported an MP removal of 29.7% when employing polyaluminum chloride (PAC). Nevertheless, removals of over 99% were reported with iron and aluminum salts in experimental simulations carried out with polystyrene particles and polyethylene terephthalate particles [22,23]. Combined coagulation methods are more effective for MP removal, with efficacies reported close to 100% for techniques such as electrocoagulation and coagulation–filtration [24]. It is well-known that during the biological treatment of wastewater, a portion of the MPs is eliminated with the secondary sludge [25], due to processes like physical entrapment in the flocs or adsorption on the active biomass. The MPs removed during secondary treatment are important because frequently this is the final treatment before the water is discharged into the environment. Coagulants may be added during activated sludge water treatment [26], although the previous trend in recent decades has been to avoid the addition of chemical products in the water line. With respect to MPs, the studies conducted so far have paid very little attention to the effect that the use of these coagulants combined with the presence of biomass flocs may have on the removal of these micropollutants.
Coagulation is commonly employed in DWTPs too. The material, size, shape, and type of MPs, water quality, and coagulant type and dosage are some of the variables that affect MP removal. In a Canadian DWTP, 45% of MPs were removed with 0.5 g/L of FeCl3 [27]. Better results were obtained at laboratory scale, using Al2(SO4)3 as coagulant, where removal efficacies of 40–62% could be obtained for tap and drinkable water [28,29,30,31]. This percentage increased up to 75% when the concentration of Al2(SO4)3 was higher than 10 g/L [32,33]. Using iron salts (FeSO4 or FeCl3) combined with MgSO4, 72% of polyethylene microplastics (PE-MPs) and 92% of polystyrene microplastics (PS-MPs) were successfully removed from tap water [34]. In addition, removals between 40 and 50% were reported [35] for different MPs (polyethylene, PE; high-density polyethylene, HDPE; polypropylene, PP; and polyvinyl chloride, PVC) in DWTP water samples when FeCl3 or Al2(SO4)3 was used.
The present work aimed to evaluate the possibility of adding a suitable dosage of coagulant (FeCl3 or Al2(SO4)3) prior to the secondary sedimentation as a way of improving the capture of MPs by the sludge flocs and the subsequent removal by settling. Additionally, the same reagents were assayed in tap water, in order to evaluate the possibility of including a simple step of coagulation-settling after the main distribution of water supply to reduce the quantity of MPs that reach the domestic taps. Several concentrations have been tested, and the physical and chemical properties of the MPs were examined in both cases, before and after the treatment.

2. Materials and Methods

2.1. Sample Collection

The sample of activated sludge used for the experiments was taken from a WWTP located in northwest Spain, which was designed to serve 86,500 equivalent residents and treats an average daily flow of 23,786 m3. As can be observed in Figure 1, the first step of the process consists of double mechanical screening (30 mm and 8 mm pore size), followed by the grit and grease removal system. Then, wastewater passes into the primary settlers (30 m diameter). After that, nitrogen and organic matter are biologically removed through the combination of anoxic and aerobic reactors. The activated sludge sample was collected from the outlet of the biological reactor, before settling (Figure 1).
The tap water sample was collected from a laboratory of the Department of Chemical and Environmental Engineering of Oviedo University. The water was allowed to run for 3 min before collecting the sample in a glass bottle. Both samples were stored at 4–6 °C. The same samples were used for all the experiments to prevent the results from being affected by variations between different batches.

2.2. Sample Characteristics

The physicochemical characteristics of the activated sludge used for the experiments assessed were humidity, TSS, and pH (Table 1). A pH meter (ORP sensION+ PH3, HACH, Loveland, CO, USA) was used to measure the pH. A gravimetric analysis was used to estimate the total suspended solids (TSS) and the humidity of the activated sludge sample.
In the case of tap water, TSS, pH and turbidity were analyzed. The turbidity of samples was measured using a nephelometer (Policontrol AP2000, AWWA member, São Paulo, Brazil) (Table 1).

2.3. Mixing–Settling Experiments

To prevent MP contamination, distilled water and reagents were filtered through a glass microfibre filter (pore size of 0.7 μm, Whatman, Florham Park, NJ, USA) before being used.
For the experiment with activated sludge, 400 g (wet weight) of the sample were placed in 500 mL beakers. Then, coagulants were added at different concentrations (0.2 g/L, 0.6 g/L, 1.2 g/L, and 2.4 g/L of FeCl3 or Al2(SO4)3) (Sigma Aldrich, Madrid, Spain). The mixtures were shaken in a flocculation tester (JLT6, VELPS Scientifica, Usmate Velata, MB, Italy) for 30 min at 30 rpm. This equipment has six positions for glass containers, an electronic speed control system (10–300 rpm), and a programmable timer. Afterwards, the sludge was allowed to settle for 30 min. Finally, the supernatant was separated from the settled solid by glass pipette.
For the experiments with tap water samples, 400 mL were placed in each beaker, and three coagulant concentrations were tested (0.2 g/L, 0.6 g/L, and 1.2 g/L of FeCl3 or Al2(SO4)3). The same mixing–settling procedure was followed. All the experiments were carried out in triplicate.

2.4. Treatments of Samples for Microplastic Isolation

In each beaker containing the settled sludge, 30 mL of 50% H2O2 solution (VWR, Chemicals, Briare, France) were added and the mixtures were left for 24 h at room temperature. Afterwords, the oxidation of the remaining organic contaminants were guaranteed by adding 40 mL of Fenton reagent and leaving for a further 24 h at room temperature. Then, a stainless-steel module (CISA Sieving Technologies, Barcelona, Spain) with overlapped 500, 250, 100, and 20 μm sieves was used to filter the sample. Filtered, distilled water was used to wash away the microparticles retained on each sieve and collected in beakers. Subsequently MPs were isolated from the inorganic contaminants by density separation using a ZnCl2 solution (d = 1.5 g/mL, 97% purity, VWR, Chemicals, Briare, France). MPs were then recovered in glass microfibre filters (0.7 μm pore, Whatman, Florham Park, NJ, USA).
In the case of tap water samples, the same treatment (oxidation, filtration, and density separation with ZnCl2) was used, with the exception that only H2O2 solution was employed for the oxidation. MPs were also recovered in glass microfibre filters (0.7 μm pore, Whatman, Florham Park, NJ, USA).

2.5. Microplastic Analysis

A semi-automatic stereomicroscope (Leica M205FA, Leica Microsystems CMS GmbH, Wetzlar, Germany) equipped with a high-resolution colour digital camera (Leica DFC310FX; 1.4 Mpixel, CCD, Leica Microsystems CMS GmbH, Wetzlar, Germany) was used to determine the number of MPs retained in the filters. Additionally, fibre and fragment sizes were estimated using ImageJ software 1.11 version (Confocal UniOvi ImageJ). A µ-FTIR spectrophotometer (Perkin Elmer Spotlight 200i FTIR, Springfield, IL, USA) from the Molecular Spectroscopy Unit of the Autonomous University of Madrid was used to determine the chemical composition of MPs. With this aim, isolated MPs were placed on transparent supports to be analyzed by transmission, and a spectral database of approximately 36,000 compounds was used to identify the polymers.

2.6. Quality Assurance and Quality Control (QA/QC)

Based on the methods presented in several studies, quality control and assurance (QA/QC) was carried out from MP sample collection to quantification [2,25,36], which made them reliable. QA/QC practices included employing glass microfiber filters (0.7 µm pore size), avoiding polymeric materials in the lab, and filtering chemical reagents before use. From sampling to analysis, good field and laboratory procedures (GLP) were used to minimize secondary contamination from MPs that were found in the air, on surfaces, and eventually on the equipment. The only plastic materials used were Falcon tubes and plastic bottles. Control experiments were conducted in both cases and the mean concentrations of MPs released from tubes and bottles were 1.5 ± 1.2 MPs/L and 2.4 ± 0.9 MPs/L, respectively. This represented less than 1% of the measured MP concentrations and was therefore considered negligible.
All experiments and analysis were carried out in triplicate. All sample filtration procedures were performed under strict supervision in a laminar flow cabinet. This type of cabinet provides an ISO 5 clean air working environment with unidirectional flow of sterile, particle-free air in order to prevent sample contamination. Other precautions were the use of fibre-free lab coats, blank controls, and keeping the filters protected at all times to minimize their exposure to the environment. These procedures assure that the MPs detected in the samples are not the consequence of contamination from external sources or artifacts derived from the experimental procedure.

3. Results

3.1. Native MPs

Different types of MPs found in the samples of tap water and activated sludge are shown, as examples, in Figure 2.

3.1.1. Tap Water

The average MP concentration found in the tap water used for the experiment was 213 ± 12 MPs/L. The quantity of MPs found in tap water varies widely throughout the world, with usual values between 1 and 1247 MPs/L [14,37,38,39,40,41].
The size, shape, and chemical composition of MPs identified in the tap water sample are shown in Figure S1 and Table S1. Most of the MPs corresponded with fibres and fragments with percentages of 60.3 ± 2.3% and 38.0 ± 1.0%, respectively. Films and microbeads accounted for only 1.1 ± 1.5% and 0.6 ± 0.2%, respectively. This is consistent with other authors, who have reported that fibres and fragments represent more than 98% of the total MPs found in tap water [9,36,42].
According to Figure S1b and Table S1, the distribution of MP size was as follows: 6.3 ± 2.3% (>500 μm), 27.0 ± 4.6% (500–250 μm), 40.0 ± 3.6% (250–100 μm), and 26.7 ± 5.9% (100–20 µm). It was expected that the percentage of MPs above 500 μm would be low because the last treatment in a DWTP is sand filtration, which retains the larger MPs [36,43,44]. Consistently with previous works, the vast majority of MPs fell within the 500–20 μm range (60–90%), with 73% of MPs higher than 100 μm.
In the tap water samples, nine main polymers were detected: rayon (RA): 20%, polyethylene (PE): 14%, polyamide (PA): 14%, polyester (PES): 11%, polyethylene terephthalate (PET): 11%, polypropylene (PP): 11%, and polyvinyl chloride (PVC): 11%. The rest of the polymers identified represented only 8% of the total. (Figure S3). Other authors also found similar concentrations of RA 10–26% in tap water [37,38,44]. RA-MPs and PA-MPs can be found in natural water bodies because of textile washing or fishing activities [9,36]. PES and PVC-MP come mainly from damaged pipes and water supply systems [45]. The rest of the identified polymers in important quantities may have originated from the disintegration of plastic wastes, such as packaging materials [34,35,36].

3.1.2. Activated Sludge

The average MP concentration found in the activated sludge was 445 ± 35 MPs/L. This value is within the 50–6260 MPs/L range reported in previous studies for activated sludge samples [46,47,48,49,50].
The size, shape, and chemical composition of MPs identified in the sludge samples are described in Figure S2a and Table S2. The abundance of fibres and fragments accounted for 60.7 ± 2.1% and 35.7 ± 3.2% of the total MPs, respectively, whereas films and microbeads accounted for only 2.0 ± 1.0% and 1.6 ± 0.6%, respectively. These predominant shapes are in line with those observed in the tap water samples, and consistent with other authors. For example, [51,52,53] reported that the sum of fibres and fragments represents more than 96% of activated sludges of WWTPs.
According to Figure S2b, the distribution of MP size was as follows: 22.0 ± 1.7% (>500 μm), 34.7 ± 5.5% (500–250 μm), 29.7 ± 0.6% (250–100 μm), and 13.7 ± 4.0% (100–20 µm). In relation to the sizes found in the tap water, it is observed that they are larger. Again, these results are comparable to those published for activated sludges [5,53].
In the activated sludge sample, 11 different polymers were identified, with the following 7 being the most abundant: RA 28%, PES 22%, PE 11%, PA 8%, PET 8%, PP 7%, and cellulose (CE) 7%. The other identified polymers contributed the remaining 9% (Figure S4). The majority of MP fibres detected in the sludge sample were made of RA, PES, and PE (Figure S4), which indicates their textile origin. It is well-known that small fibres of these synthetic polymers are discharged in wastewater during household washing [54,55,56]. Nonetheless, MP fragments are created when larger plastic particles (like packaging and disposable bags) break down into tiny bits due to mechanical, chemical, and biological processes [15,48]. Up to 30 distinct polymers have been found in municipal WWTPs [57,58], and their incidence is primarily influenced by the quantity and nature of the industrial wastewater discharged into the facility [5,48,59].

3.2. Effect of Coagulant on MP Removal

3.2.1. Tap Water

The addition of coagulants to the tap water greatly increased the percentage of MPs removed by settling, as shown in Figure 3. For all the assayed concentrations of coagulant, Al2(SO4)3 was more effective than FeCl3 at removing MPs from tap water. For example, with 1.2 g/L of coagulant, this efficiency increased from 1.1% without coagulants, to 34.3% and 43.2% with FeCl3 and with Al2(SO4)3, respectively. Although these efficacies might seem modest, they fall within the usual reported ranges in the literature for the use of inorganic coagulants in drinking water (i.e., 40–62% [28,29,30,31,60]).
MPs that remained suspended in the supernatant using 1.2 g/L of coagulant were classified based on their shape and size (Figure S2). When FeCl3 was employed, the average percentage of fibres and fragments were 63.9 ± 1.7% and 32.8 ± 3.5%, respectively, and when Al2(SO4)3 was used, the mean percentages of fibres and fragments were 68.3 ± 1.4% and 26.9 ± 3.3%, respectively. When comparing the percentages obtained in the supernatants with the percentages obtained in the sample of tap water (Figure S1), slight variations can be observed. In particular, when Al2(SO4)3 was added, the percentage of fibres changed from 60% to 68%. Thus, it seems that aluminum flocs have a greater capacity to trap fragments than fibres. The MP size distributions found in the supernatants were similar to those observed in other tap waters [61].

3.2.2. Activated Sludge

Results obtained with different coagulant doses are shown in Figure 4 and, again, for all the assayed concentrations of coagulant, Al2(SO4)3 was more effective than FeCl3 at removing MPs from activated sludge. The addition of FeCl3 as coagulant raised the MP retention percentage from 7.8% (without coagulants) to 51.9%, with the maximum doses assayed (2.4 g/L). Meanwhile, using Al2(SO4)3 the retention percentage increased to 61.6 ± 1.3% and the MP concentration decreased from 411 MPs/L to 171 MPs/L. These efficacies are within the ranges reported in the literature for coagulation in real and synthetic wastewaters (i.e., 30–82% [11,18], depending on the specific characteristics of the MPs), although not as high as those reported by other authors in experimental simulations [22,23]. The MPs that remained suspended in the supernatant of the activated sludge were characterized according to its shape and size, and significant differences were not observed with respect to the untreated sample (Figures S3 and S4).
The disparity of results obtained in different works is because the removal efficiency of MPs by coagulation depends on the type and dosage of coagulant used, but also on MP properties (size, shape, density) and other parameters specific to the aqueous matrix, such as pH, turbidity, temperature, and solids loading. Optimization involves adjusting dosage, using organic aids, or modifying operations, balancing cost, efficiency, and sludge production.

4. Discussion

Even though the two matrices were entirely different, results demonstrated that in both cases and at all doses tested, using Al2(SO4)3 was more successful than FeCl3 at removing MPs. When results obtained for the tap water and sludge were compared, for example, with an Al2(SO4)3 concentration of 1.2 g/L, the MP removals were higher for the activated sludge (50.1%) than for the tap water (43.2%). As previously reported, the formation of more porous, lighter, and more voluminous flocs improved the capture of small, low-density particles such as MPs [25,62,63].
Conversely, FeCl3 creates more compact and dense flocs that are less effective in holding onto tiny floating fragments. The presence of extracellular polymer substances (EPSs) in the activated sludge also makes easier to capture particles like MPs [64]. Proteins and polysaccharides form a matrix in the sludge that serves as an extra network to capture MP fragments. In addition, in previous works, a linear correlation was found between the percentage of MPs retained in sludge flocs and the concentration of humic acids [65]. The addition of coagulants enhanced biological flocs in their ability to hold particles. The changes brought about by coagulant addition in the complicated chemical environment of activated sludge (pH, presence of negative surface charges) encourages interactions between MPs and the formed flocs [17]. In comparison with the activated sludge, tap water is a much simpler medium, with very little organic matter or suspended particles, which notably reduces the quantity of “bridges” that may be formed between MPs and other solid species. Therefore, the coagulant–MP interaction is directly related to the efficiency of FeCl3 or Al2(SO4)3 in this scenario [66].
When comparing both salts’ behaviors, aluminum flocs with a lighter and more extended structure are better for capturing fragmented MPs because they are frequently smaller and less dense. Iron flocs, which are denser, settle more rapidly, with less success in trapping the suspended MPs, whereas aluminum flocs are better to swallow and stick to low-density MPs. In the case of activated sludge, it can be observed that, in comparison to the values reported in Table S2, there are significantly higher removal values for MPs of a size between 20 and 100 µm and for MPs of size > 500 µm (better removal efficiency). This is likely due to the presence of suspended particles and bioflakes, which give the coagulant additional chances to trap MPs. Electrostatic forces allow the smallest MPs (20–100 µm) to adhere to bioflakes or bioflocs of the sludge, which in turn are trapped by the coagulant. In addition, aluminum flocs are stable in a wide pH range (pH 5.5–8.5), which includes typical pH values of both tap water and activated sludge (7.5 and 7.2 in the samples used in the present study). However, the ideal pH range for FeCl3 is more acidic (pH 4.0–6.0), which makes the process more complicated due to the convenience of controlling the pH [24,67,68].
In addition, in drinking water, FeCl3 gives a metallic taste and smell, which is a drawback. Additionally, sludges produced by Al2(SO4)3 usually contain fewer harmful contaminants than those produced by FeCl3, because leftover iron may combine with other elements in the water to create harmful coagulants. With respect to the use of these coagulants in activated sludge systems, biological activity is less affected by Al2(SO4)3. In fact, FeCl3 could acidify the system and alter the biology of the process, decreasing its efficiency [24,67,68].
Considering that the coagulant doses used here are quite high, the metals should be recovered. The recirculation of the precipitated sludge could reduce, to some extent, the amount of fresh reagent to be added. In any case, it would also be necessary to precipitate the dissolved metals that remain in the treated water as hydroxides, especially in the case of drinking water, which must meet strict concentration standards. Another option would be the use of coagulant aids, which would allow us to achieve the same results with lower doses of coagulants.

5. Conclusions

The present study investigates the potential of coagulants to improve MP removal in wastewater and tap water. In particular, a sample of tap water taken from the lab and a sample of activated sludge from a municipal WWTP were used for the experiments. Previously, MPs contained in the samples were quantified and characterized. Concentrations of 213 MPs/L and 445 MPs/L were measured in tap water and activated sludge, respectively, and fibres smaller than 500 µm were the most abundant MPs in both cases. Ferric and aluminum salts were tested, and both coagulants notably improved = MP removal in tap water and activated sludge. However, the best results were obtained with Al2(SO4)3, which increased MP removal from 1% (without coagulants) to 43% in tap water and from 8% to 62% in sludge, with coagulant doses of 1.2 g/L and 2.4 g/L, respectively. The better performance of the aluminum salt in comparison with the ferric salt was due to the creation of lighter and more porous flocs that facilitated the capture of small low-density particles, such as MPs. The results obtained in this study highlight the potential of using coagulants to reduce MP concentration in drinking water and wastewater. In the case of tap water, a coagulation–sedimentation step could be considered just before the water reaches domestic taps. Likewise, the use of appropriate coagulants just before secondary decantation would positively contribute to both the sedimentation process and the removal of MPs. Finally, it is necessary to mention that the concentrations used here are too high for practical implementation, which underscores the interest in conducting further research to find reagents that would enable working at lower doses.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/microplastics4040089/s1, Figure S1: Relative abundance of MPs in tap water (before the treatment): shape (a), size (b); Figure S2: Relative abundance of MPs in activated sludge (before the treatment): shape (a), size (b); Figure S3: Relative abundance of MPs with different chemical compositions found in the tap water sample (before treatment); Figure S4: Relative abundance of MPs with different chemical composition found in the activated sludge sample (before treatment); Figure S5: µFTIR spectra of different types of MPs analyzed; Table S1: Physicochemical parameters of the activated sludge for the experiment; Table S2: Relative abundance of MPs in tap water (before treatment) according to their shape and size and percentage of MPs removed by settling; Table S2 Relative abundance of MPs in activated sludge samples (before treatment) according to their shape and size and the percentage of MPs removed by settling.

Author Contributions

C.C.: conceptualization, methodology, validation, visualization, writing—original draft preparation; D.S.: conceptualization, visualization, writing—review and editing; A.L.: conceptualization, methodology, formal analysis, writing—review and editing; M.D.: supervision, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the financial support from the Employment, Industry and Tourism Office of the Principality of Asturias (Spain) through the project SEK-25-GRU-GIC-24-010.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are grateful to the Photonic Microscopy and Imaging Processing Unit of the Scientific–Technical Services of the University of Oviedo (Confocal UniOvi ImageJ) for their assistance in the MPs analysis and to the Molecular Spectroscopy Unit of the Autonomous University of Madrid (UAM) for their assistance in FTIR analysis. This article is dedicated to Alexander Santiago Casella Flores.

Conflicts of Interest

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

Abbreviations

The following abbreviations are used in this manuscript:
CECellulose
DAFDissolved air flotation
DMDynamic membrane
DTWPPotable water treatment plant
EREpoxy resin
MBRMembrane bioreactor
MPsMicroplastics
NPsNanoplastics
PAPolyamide
PACPolyaluminum chloride
PAMPolyacrylamide
PANPolyacrylonitrile
PEPolyethylene
PEIPolyethylenimine
PESPolyester
PETPolyethylene terephthalate
PPPolypropylene
PVCPolyvynil chloride
RARayon
SEBSStyrene-ethylene-butylene
WWTPWastewater treatment plant

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Microplastics 04 00089 g001
Figure 1. Flowchart of the WWTP where the sludge sample was taken. Collection point is marked with an asterisk (Source: own elaboration).
Figure 1. Flowchart of the WWTP where the sludge sample was taken. Collection point is marked with an asterisk (Source: own elaboration).
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Microplastics 04 00089 g002
Figure 2. Several MP types identified in the tap water sample (a–c) and the activated sludge sample (d–i).
Figure 2. Several MP types identified in the tap water sample (a–c) and the activated sludge sample (d–i).
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Microplastics 04 00089 g003
Figure 3. MP concentrations in the tap water supernatant after the mixing–settling experiments (bars) and the percentages of MPs removed by settling (red line) for different doses of coagulants.
Figure 3. MP concentrations in the tap water supernatant after the mixing–settling experiments (bars) and the percentages of MPs removed by settling (red line) for different doses of coagulants.
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Microplastics 04 00089 g004
Figure 4. MP concentrations in the activated sludge supernatant after the mixing–settling experiments (bars) and the percentages of MPs removed by settling (red line) for different doses of coagulants.
Figure 4. MP concentrations in the activated sludge supernatant after the mixing–settling experiments (bars) and the percentages of MPs removed by settling (red line) for different doses of coagulants.
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Table 1. Physicochemical parameters of the activated sludge from the WWTP analyzed.
Table 1. Physicochemical parameters of the activated sludge from the WWTP analyzed.
SampleMonth (2024)TSS (g/L)pHMoisture (%)Turbidity (NTU)
Activated sludgeNovember2.3 ± 0.37.2 ± 0.297.7 ± 1.1-
Tap waterNovember0.02 ± 0.017.5 ± 0.5-<0.1
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Casella, C.; Sol, D.; Laca, A.; Díaz, M. Investigating the Potential of Coagulants to Improve Microplastics Removal in Wastewater and Tap Water. Microplastics 2025, 4, 89. https://doi.org/10.3390/microplastics4040089

AMA Style

Casella C, Sol D, Laca A, Díaz M. Investigating the Potential of Coagulants to Improve Microplastics Removal in Wastewater and Tap Water. Microplastics. 2025; 4(4):89. https://doi.org/10.3390/microplastics4040089

Chicago/Turabian Style

Casella, Claudio, Daniel Sol, Adriana Laca, and Mario Díaz. 2025. "Investigating the Potential of Coagulants to Improve Microplastics Removal in Wastewater and Tap Water" Microplastics 4, no. 4: 89. https://doi.org/10.3390/microplastics4040089

APA Style

Casella, C., Sol, D., Laca, A., & Díaz, M. (2025). Investigating the Potential of Coagulants to Improve Microplastics Removal in Wastewater and Tap Water. Microplastics, 4(4), 89. https://doi.org/10.3390/microplastics4040089

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