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Article

Abundance and Characterization of Anthropogenic Microlitter in Effluent from Three Wastewater Treatment Plants in Gran Canaria (Canary Islands, Spain)

by
Marta Rodrigo Sanz
and
Juana R. Betancort Rodríguez
*
Water Department, Canary Islands Institute of Technology (ITC), Pozo Izquierdo, s/n, 35019 Santa Lucía, Spain
*
Author to whom correspondence should be addressed.
Water 2024, 16(1), 64; https://doi.org/10.3390/w16010064
Submission received: 29 November 2023 / Revised: 19 December 2023 / Accepted: 20 December 2023 / Published: 23 December 2023
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

:
Wastewater treatment plants (WWTPs) have been identified as significant point sources of microlitter particles (MPs) released into the environment. Simultaneously, they play a crucial role in effectively removing a substantial amount of MPs originated from domestic and industrial activities. This study evaluates the presence and chemical composition of MPs within the range of 5 mm to 25 µm in effluents from three WWTPs in the Canary Islands, each undergoing distinct treatment processes: pretreatment (PRET), activated sludge (AS), and membrane bioreactor (MBR) over an eight-month period. The concentrations of MPs exhibited substantial variations in the three effluent samples: 7461.50 ± 3843.87 MP/L in PRET, 32.71 ± 19.55 MP/L in AS, and 9.53 ± 5.21 MP/L in MBR. The predominant forms of MPs were fragments (58–66%) and fibers (27–33%), with the most prevalent size class being within the range of 150–25 µm. The mass concentrations of plastics, analyzed through pyrolysis gas chromatography–mass spectrometry (Pyr-GC/MS), were determined as 123.33 µg/L for PRET, 17 µg/L for AS, and 10 µg/L for MBR. This study revealed a diverse polymer profile, with polyvinyl chloride (PVC) and polyolefins (PE and PP) predominantly present. This work enhances our understanding of MP dynamics and provides a valuable reference for future assessments, aiding in the selection of effective removal technologies.

Graphical Abstract

1. Introduction

In simple terms, microlitter consists of pieces of litter (microlitter particles; MPs) including glass, metal, plastic, paper, etc.; ultimately, microlitter is litter at the micro end of the size range [1]. In relation to the size of the particles making up microlitter, the established criterion is for the particular case of microplastics, on which almost all research is focused. There is still no agreement on the upper and lower size limits for microplastics. The International Organization for Standardization (ISO) has defined microplastics as “any solid particle insoluble in water with any dimension between 1 and 1000 µm” (ISO/TR21960:2020) [2]. Additionally, the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection [3] has defined microplastics as “plastic particles < 5 mm in diameter, which include particles in the nano-size range (1 nm)”. Certainly, among most research found in the literature, microplastics are commonly defined as “plastic fragments below 5 mm that pass through a 5 mm mesh screen but are retained by a lower one” in what has emerged as an international standard [4,5,6]. This is the definition adopted in this document for MPs.
Microlitter in the marine environment is a complex problem with no simple solution. Once introduced into the aquatic environment, these MPs have the potential to undergo degradation and fragmentation, producing even smaller particles, including nanoparticles with sizes below 100 nm [7]. It is widely acknowledged that, in aquatic species, the primary route for the accumulation of MPs, especially microplastics, is through ingestion. Due to their persistent and buoyant characteristics, these small-sized MPs can be ingested by a diverse range of aquatic organisms (planktivorous, filter-feeding, etc.). But also, importantly, reducing the size of available MPs is pivotal as it increases the ability to cross biological barriers, the propensity for bioaccumulation, as well as food web transfer [8,9,10]. Laboratory studies suggest that, in certain cases, microplastics can interfere with feeding capacity, leading to internal damage or toxicological effects in specific species. Furthermore, the surface of microplastics may attract and accumulate both organic and inorganic contaminants, potentially resulting in adverse effects on biota [11,12].
Therefore, microlitter has become a strong environmental stressor in coastal marine ecosystems. Its introduction into these ecosystems includes, among others, specific discharge points, such as riverine input, stormwater, and sewage outflows [13,14].
Indeed, wastewater treatment plants (WWTPs) have been specifically identified as an important point source of small anthropogenic litter emissions and, in particular, for microplastics, which have been the focus of most of the studies carried out on their occurrence in effluents [15,16,17,18]. The sources of microlitter in untreated wastewater (WW) entering each WWTP primarily depend on the sewer system type (combined or separate) and the contributors of WW (residential, commercial, and industrial). Combined sewer systems tend to have higher fractions of fragments and anthropogenic litter as a result of surface runoff from urban areas and roads [19,20,21]. Meanwhile, household WW content, including litter, is influenced by people’s lifestyles, customs, awareness, and the proper use of the sanitation network. Key contributors to microlitter from urban and household sources include fiber losses from textiles during domestic laundering [22,23,24]; microbeads utilized in cosmetics and personal care products like toothpaste, facial cleansers, and body washes [25]; rubber particles produced through the abrasion of tires and carried by stormwater runoff [26,27]; and leachates from landfills [28].
Several studies indicate that in WWTPs, overall, microlitter removal rates are typically high, ranging from 80 to 95%, with most of the microplastics successfully eliminated [29,30,31]. Nevertheless, even with the typically low concentrations reported in these treated effluents, the residual fraction remains substantial and constitutes a significant source of microlitter entering into waterbodies due to the large volume of effluent continuously discharged by these facilities [5,15,16,17]. As a result, WWTPs are acknowledged as both recipients and potential contributors of MPs, making sewage treatment a crucial aspect that demands consideration for effective urban water cycle management. This significance is particularly pronounced in arid zones like Gran Canaria, where a substantial portion of treated WW is reused for irrigation. Consequently, the presence of MPs in WWTP effluents has become an issue gaining increased attention in recent years [32,33,34].
Coastal seawater constitutes a vital water resource in the Canary Islands, influencing not only tourism, the primary economic activity, but also serving as the seawater source for desalination plants, which provide water for human consumption to most of the islands’ inhabitants.
Despite these circumstances, research into microlitter pollution in WWTPs is still limited in the Canary Islands region. Little is known about the presence of microlitter in the substantial volumes of effluent that are constantly discharged into the marine environment. The decision to center this study on Gran Canaria Island was based on the fact that this island supports a large urban population nucleus equipped with a diverse range of WWTP facilities, which discharge substantial volumes of treated effluent into coastal marine waters.
From a regulatory point of view, microlitter is explicitly addressed in Section 4.4 of the Marine Strategy Framework Directive Descriptor 10—Marine Litter (MSFD, Directive 2008/56/EC) [35]. The specific marine strategy developed for the Canary Islands under Real Decreto 1365/2018 [36] incorporates two distinct targets related to Descriptor 10: (1) diminishing the contribution of nutrients, pollutants, and litter from wastewater and (2) decreasing the quantity of microplastics entering the marine environment. Most recently, microlitter has also been targeted in the ongoing revision of the Urban Wastewater Treatment Directive (Directive 91/271/EEC) [37], which forms the regulatory foundation for most European wastewater treatment plants (WWTPs). This revision includes the mandatory monitoring of microplastics at the inlets, outlets, and in the sludge of urban WWTPs exceeding 10,000 population equivalent. To ensure consistent implementation, a methodology for measuring microplastics in urban wastewater and sludge will be established.
Therefore, this work aims to study the presence of microlitter in the effluent of three WWTPs over an eight-month period. These WWTPs employ distinct treatment processes—complete pretreatment (PRET), activated sludge process (AS), and membrane bioreactor technology (MBR)—and consistently release their effluent into coastal marine water bodies. Particle counting, morphology identification, and chemical characterization results are combined to assess MP concentrations in the three effluent samples. This comprehensive approach aims to enhance the understanding of the contribution of MPs from three distinct WWTPs that can be applied toward developing effective strategies and treatment technologies to improve MP removal, aligning with the targets outlined in European Directives for reducing these inputs.

2. Materials and Methods

2.1. Wastewater Sampling

When choosing sampling facilities, our aim was to secure representative facilities of those existing on the Canary Islands, considering both their capacity and the diversity of treatments. Accordingly, three installations with three different types of treatments were chosen (see Table 1).
Specifically, grab samples of WW were collected from all the facilities on the same day, once a month, over an eight-month period (June 2022–February 2023, excluding December 2022). In detail:
  • PRET: This WWTP, one of the largest on the island, comprises a large pumping station that lifts WW. The samples were collected before the pumping stage, subsequent to the pretreatment steps, which include a 6 mm coarse + 3 mm fine screening followed by desanding and degreasing. The effluent is representative of the discharges that may occur occasionally through excessive storm flow, particularly when the sewerage system is at risk of being overwhelmed, such as during heavy rainfall or emergency situations (e.g., sewer blockages or equipment failures at wastewater treatment works). In such scenarios, the flow from the unitary sewage system that cannot be pumped undergoes pretreatment before being discharged into the sea through the submarine outfall;
  • AS: This WWTP is based on AS technology and has several treatment steps, briefly: screening, grit removal, desanding–degreasing, primary settling, aeration/activated sludge, lamellar settling, and chlorination. The treated effluent, with an annual mean flow of 345 m3/h, is representative of biological AS treatment, the predominant technology on the islands. Samples were collected after disinfection treatment, specifically in the outlet chamber just before discharge into the submarine outfall;
  • MBR: This WWTP operates on MBR systems. In recent years, due to its high treatment efficiencies, this technology has experienced significant growth, particularly in situations where discharge takes place in sensitive coastal areas or when its effluent is designated for irrigation. Given that the treated water from the sampled plant, with an annual mean flow of 230 m3/h, is primarily intended for irrigation, the effluent samples were collected just before reaching the storage tank. Any effluent not used for irrigation is directed through a submarine outfall.
Grab samples were collected in the morning, coinciding with the main peak flow rate. The collection of grab samples for the analysis of MPs in WW is a standard and well-documented procedure [15,39]. WW samples were immediately transported to the laboratory, where all parameters were analyzed within a maximum period of eight hours post collection. The sampling volumes varied based on the water type: 1 L for pretreated WW (PRET) and 5 L for effluent undergoing secondary treatments (AS and MBR).

2.2. Analytical Procedure for Wastewater Samples

The main physicochemical WW parameters were also analyzed for each sample. In situ measurements included acidity/basicity (pH, pH units), water temperature (T, °C), and electrical conductivity (EC, µS/cm), measured with Hach SensION portable electrodes (Hach Lange GMBH, Düsseldorf, Germany). Further analyses involved chemical oxygen demand (COD, mg/L), total nitrogen (TN) (mg/L), and total phosphorus (P-PO4) (mg/L) determined through Hach reagent kits (LCI 400 or 500, LCK348 or 349, and LCK 138 or 338, respectively, Hach Lange GMBH, Düsseldorf, Germany). All assessments of physicochemical parameters adhered to the ISO-UN-UNE Standards [40,41].
The specific physicochemical characteristics of the sampled WW effluent from the three facilities can be found in the Supplementary Materials (Table S1).

2.3. Analytical Procedure of Microlitter

All samples underwent a consistent processing method, following the procedure described in Edo et al. [5]. Briefly, WW samples were filtered (using a Millipore 47 mm stainless steel pressure filter holder; Merck KGaA, Darmstad, Germany) through wet sieving in a sieve cascade. Stainless steel meshes (47 mm Ø) with opening sizes of 5000, 1000, 500, 150, and 25 μm were employed. To eliminate organic matter and prevent microbial growth, the filters were immersed in glass beakers containing hydrogen peroxide (H2O2, 33% w/v; AppliChem ITW Reagents, Darmstadt, Germany) placed in a water bath at 60 °C for 16–20 h. Subsequently, they were washed with ultrapure water (18.2 MΩ·cm Milli Q; Millipore SAS, 67120 Molsheim, France) to eliminate any residual H2O2 and filtered through the same sequence of 5000, 1000, 500, 150, and 25 μm meshes. All filters were then dried at 150 °C and stored in precleaned glass Petri dishes.
All filters were analyzed visually under a trinocular light stereomicroscope (Nikon Eclipse 50i; Nikon Instruments Europe B.V. Badhoevedorp, The Netherlands) with magnifications of 4×, 10×, and 40× and digitally with image software (Nikon DSFi camera and its DS-L2 control unit ver.4.0) to identify and classify the microlitter particles. The whole set of particles was counted and classified according to their shapes into the following categories: spherical particles (pellets), fragments (small broken parts), fibers (length significantly longer than thickness and width), films (very thin particles), and others (not belonging to the other types) [42].
Since MPs, and especially fibers, are ubiquitous in ordinary human environments, meticulous attention was given throughout the entire sample preparation and analysis process to minimize laboratory contamination. All plastic materials were replaced by metal or glassware wherever possible. Moreover, all materials were carefully cleaned and rinsed three times with ultrapure water before use. Each of the stainless-steel filters was also previously washed with ultrapure water and the presence of potential residues of organic matter and MPs was checked by optical microscopy. Laboratory procedural blanks, involving a complete sample pretreatment with ultrapure water, were also analyzed alongside every batch of samples to verify for any laboratory contamination and rectify the results if needed.

2.4. Chemical (Polymer) Composition of Microlitter Particles

To identify the composition of the MPs found in this study, 80% of the previously examined filters from all fractions in the samples from the three facilities were sent to be analyzed in an external laboratory. Thermal extraction–desorption (TED) using a thermobalance coupled to a gas chromatograph (GC) with mass spectrometer (MS) detector (the TED-GCMS method) was used for polymer identification. Furthermore, a blank filter was included to verify that the filter used left no remaining residue when pyrolyzed up to 600 °C.
The samples were processed and quantified as described in [43]. In summary, the filters containing MPs underwent thermal treatment in a furnace. The temperature was raised from 25 to 600 °C (10 °C/min) in nitrogen flowing at a rate of 90 mL/min at the gas outlet. A solid sorbent (polydimethylsiloxane (PDMS) stir bar) was fitted downstream from the reactor to collect the gas compounds released upon microplastic thermal decomposition. The stir bar was then introduced into a thermal desorption unit connected to a cooling injection system coupled to a GC with quadrupole mass spectrometer detection. A nonpolar HP-5 column (Agilent, 60 m × 0.25 mm i.d.; 1 μm, 5% phenyl 95% dimethylpolysiloxane; Agilent Technologies, CA, USA) was employed for the separation of compounds. The identification of the compounds (PA, polyamide; PE polyethylene; PET, polyethylene terephthalate; PP, polypropylene; PS, polystyrene; and PVC, polyvinyl chloride) was based on the retention time and the characteristic ions (m/z) of each compound.

2.5. Statistics

The concentrations of MPs were calculated considering their total amount and the volume sampled, which are expressed as the number of total microlitter particles per liter (MP/L) of WW for the five specified fractions, along with the corresponding maximum and minimum values. The number and size categories of microlitter particles are summarized as mean ± standard deviation, minimum, and maximum values, in units of MP/L. Average and standard deviation values were calculated for each facility and median and interquartile range (IQR) values were determined to assess the data dispersion.
Student’s and Welch’s t tests were employed to identify significant differences among independent results when necessary. Statistical testing for differences was conducted at the α = 0.05 significance level, corresponding to a confidence level of 95%. This analysis was performed using Excel 2013 (Microsoft Office). Pearson’s correlation coefficient (r) was calculated between MP data and various background parameters using Jamovi software (www.jamovi.org, accessed on 10 June 2023, version 1.6.9, Sydney, Australia) to assess the linear correlation between the values of both parameters.

3. Results and Discussion

3.1. Occurrence and Distribution of Microlitter Particles in Wastewater Effluent Samples

3.1.1. Microlitter Particle Occurrence

MPs found in the WW samples were sorted in five size categories (fractions) by means of the steel mesh size opening (25–150, 150–500, 500–1000, 1000–5000, and >5000 µm). Additionally, MPs were classified into five main shapes: fibers, spherical particles, films, fragments, and others. The MP concentration observed monthly over the eight-month sampling period for each WWTP effluent is shown in Table 2.
The comparison of IQR values (Table 2) indicates the variability in microlitter concentration across various facilities; the greatest 50% dispersion of mean values was observed in the PRET installation dataset (IQR = 2663.50), followed by the AS facility (IQR = 31.83), while the smallest was recorded in the MBR effluent (IQR = 8.60). Boxplots for each sample of WWTP effluent are shown in the Supplementary Materials (Figure S1).
As expected, the abundance of MPs (MP/L) in PRET samples consistently exceeded that of the other two WWTPs by approximately two or three orders of magnitude. Specifically, PRET samples exhibited a range of 15,133–2340 MP/L, with an average concentration of 7461.50 ± 3843.87 MP/L over the sampling period. In contrast, AS concentrations ranged from 58.80 to 8.90 MP/L, averaging 32.71 ± 19.55, while the MBR effluent showed a range of 17.40 to 3.20 MP/L, with an average of 9.53 ± 5.21 MP/L. The MP concentration in the AS effluent was statistically significantly higher than in the MBR effluent (t test = 3.96, p = 0.005), indicating the lowest particle concentration per volume among the three samples of effluent throughout the sampling period.
While the European Commission is expected in the near future to establish a standardized protocol for microlitter monitoring in WW matrices, the methodologies used thus far have been diverse. This heterogeneity is one of the main factors limiting the comparability among the numerous studies on this subject found in the literature. Moreover, WWTP effluent samples represent complex matrices, characterized by a significant variability in composition. Consequently, maintaining consistency in extraction techniques for quantifying microlitter presence and distinguishing microlitter particles from other natural organic materials becomes more challenging [31]. This fact is evident in our study, where the behavior of WW samples during the filtration stage varied significantly based on their origin and, predominantly, the preceding treatments at the facilities. In many PRETR samples (raw WW subjected only to pretreatment), the presence of a high concentration of suspended matter (SS average = 254.49 ± 131.03 mg/L) posed a substantial challenge for the detection, quantification, and classification of microparticles, due to the many white (or nearly transparent) flat cellulose fibers, alongside sand grains and fatty fragments. In particular, on the 1 mm and 500 µm filters, these PRETR samples formed a “filter cake” of organic nature, mainly composed of these cellulose fibers. Although treatment time and temperature were optimized during the H2O2 oxidation digestion step to achieve maximum digestion of organic debris, persistent “white fiber tangles” remained. These findings are consistent with those reported by Ruiken et al. [44], who found that in WW, the length of cellulose fibers from toilet paper averages in the range of 1–1.2 mm, resulting in most of these fibers being retained by the 1 mm and 0.5 mm mesh sizes of the sieves we used. These cellulose fibers were not observed on any filters of the AS or MBR samples.
Approximately one-third of suspended solids in raw municipal wastewater (WW) consists of cellulose, primarily in the form of toilet paper [45,46]. Because of its anthropogenic origin, it should be regarded as litter according to the UNEP definition [47]. However, in the present study, these microfibers were not considered due to the challenge of accurate counting. In fact, when analyzing micro- and nanoplastics, it is recommended to remove cellulose before starting the detection, quantification, and identification process as the abundance of cellulose materials makes it possible for the materials to become “false positives” for microplastics [48]. This aspect represents one of the areas for improvement in future studies, especially when considering influent (raw WW) or WW subjected only to primary treatment.
On examining the results obtained in the current study, it is evident that all technologies, even the most efficient removal ones, were unable to completely prevent the contribution to the microlitter load in marine waters. The average concentrations of MPs observed in the final effluent of the sampled WWTPs were 7461.50 ± 3843.87 MP/L for the PRET facility, 32.71 ± 19.55 MP/L at the outlet of the AS treatment plant, and 9.53 ± 5.21 MP/L in the MBR facility effluent. These results indicate a clear influence of all the stages, particularly those involved in biological (secondary) treatment, on MP removal, whether through degradation by biological processes or through transfer to sludge [15]. Biological treatment is mainly based on the removal of organic matter, while additional or more sophisticated treatments, as in the case of MBRs, can enhance the removal of specific pollutants such as nitrogen (N), phosphorus (P), microorganisms, and suspended solids (SSs). Technologies for removing MPs and other micropollutants are currently under extensive investigation [49,50].
Recent publications and reviews have reported highly variable concentrations of MPs in effluent sampled from WWTPs [21,27,30,51,52,53]. A large proportion of the variability and discrepancies observed in these studies can be attributed to the diverse composition of raw wastewater (WW), variations in sampling techniques, isolation and detection methods, and differences in the lower size range considered when isolating and identifying MPs. In this study, all sampled effluent underwent the same analysis protocol and the lower size range for all samples was consistently set at 25 µm. Below this size, smaller microplastics, such as ultrasmall and nanoparticles, may remain in the effluent and be undetected, potentially leading to an underestimation of total abundance. Focusing just on studies where MP concentrations were determined in WWTP effluent subjected to secondary treatment with a lower size limit of 20–25 µm, concentrations ranged from 0.1 to 79.9 MP/L [5,27,54]. The results obtained in our study for activated sludge (AS) and membrane bioreactor (MBR) effluent fall within this range. It should also be considered that this variability has to do not only with the methodological differences discussed earlier but also with various intrinsic factors related to the WWTP itself. These factors include differences in capacities and layouts, specific characteristics of individual treatment units preceding discharge, the composition of raw WW, the characteristics of the served area, etc.
According to the findings of the present study, the effluent from the WWTP operated using MBR technology showed significantly lower average concentrations of MPs (9.53 ± 5.21 MP/L) compared to the final effluent from the AS process, at 32.71 ± 19.55 MP/L. These results were to be expected since several related investigations [27,55,56] have consistently shown that MBR technology, combining membrane filtration processes with suspended-growth biological reactors, exhibits high efficiency in MP removal, often approaching or reaching 100% (99.9%) and exceeding the results obtained with the AS process.
As anticipated, the effluent subjected solely to primary treatment exhibited the highest concentration of microlitter particles (MPs), at 7461.50 ± 3843.87 MP/L. Few studies in the literature report results for effluent undergoing only primary treatment, and the site-specific heterogeneity of WW characteristics and treatment processes (e.g., size of grids and screens, frequency of cleaning) makes comparisons challenging. Nonetheless, it has been reported that screening and grit removal in the primary treatment stages can remove between 69% and 79% of total MPs [57], implying a significant reduction in MPs from raw WW and their accumulation in screening residues and sand in our study. In particular, other studies [31,55] have suggested that skimming and sedimentation processes may play a crucial role in MP removal, as many MPs tend to adhere to suspended solids in WW. Primary treatment should therefore be the principal focus for enhancing MP removal in discharge (e.g., storm overflows) or in the reuse of any treated WW, as it plays a role not only in reducing MPs, but also in preventing the release of floating matter, especially large plastics, into the environment. These larger plastics may break down into smaller particles through various actions, such as thermal, photo-oxidative, and mechanical forces [58,59].

3.1.2. Microlitter Particle Sizes and Morphology Distribution

Figure 1 summarizes the distribution of the five divided fractions of MP sizes (>5000, 5000–1000, 1000–500, 500–150, 150–25 µm) at the three WWTPs.
As shown in Figure 1, the smallest-sized MPs in the 150–25 µm fraction constitute the highest proportion in all WWTP effluent. Specifically, this fraction accounts for 89.3% in PRET, 87.7% in AS, and 67.0% in MBR, out of the total MPs counted in each effluent. The 500–150 µm size range is the second most abundant across all the samples of effluent, representing 19.3% for MBR and 10.2% and 8.7% for PRET and AS, respectively. MPs in the 1000–500 µm size range vary from 0.2% in PRET to 11.9% in MBR and 3.6% in AS. MPs belonging to the fraction of 5000–1000 µm were only observed in effluent originating in two facilities, MBR and PRET, in which MPs of this size accounted for 1.8% and 0.3%, respectively. Throughout this study, only two samples displayed a negligible number of particles > 5000 µm, suggesting the absence of MPs in this fraction in the effluent of the three studied WWTPs.
When comparing these values, the distribution of microlitter size fractions differs among the three WWTPs. PRET and AS showed the highest concentration of microparticles in the smallest range (150–25 µm), with no statistical differences between them, but both significantly higher than MBR. Conversely, in the 500–150 µm size range, the MBR effluent exhibited a higher proportion of MP concentration compared to the other two treatment plants, which showed no differences between them. In the 1000–500 µm size range, all treatment plants exhibited differences, with the MBR effluent having the highest proportion, followed by AS and PRET.
Regarding the size and morphology of MPs, Figure 2 shows a selection of stereoscopic images illustrating the diversity of MPs recovered from the effluent of the three WWTPs.
The proportion of the different type classes of microlitter (fibers, spherical, films, fragments, and others) in each effluent is shown in Figure 3. The distribution is based on the average number of each particle type identified during this study for each WWTP.
According to our findings, fragments and fibers are the predominant shapes of MPs observed in all of the sampled effluent, regardless of the operational technology. The AS effluent exhibited a significantly higher average proportion of fragments, at 65.6%, compared to the other two facilities, which did not differ significantly from each other, registering 59.8% and 58.6% for MBR and PRET, respectively. However, the MBR effluent showed the highest proportion of fibers (33.3%), while the average percentages for the other two facilities did not exhibit a significant difference: 28.4% for PRETR and 27.3% for AS. Film-shaped microlitter had a much lower presence, with proportions ranging from 5.7% for PRET to 4.2% for AS and 2.8% for MBR.
Spheres were generally present in low proportions, and no significant differences were observed among the sampled effluent, registering at 2.9% for PRET and AS and 3.2% for MBR. Only the PRET effluent showed a significant proportion of MPs, averaging 4.4%, with shapes or characteristics that did not conform to the classification criteria and were therefore categorized as “other”.
In all three WWTPs studied, the smallest fraction analyzed (150–25 µm) constituted the highest proportion of MPs in the final effluent post-treatment. This observation corroborates the findings reported by others [5,15,51,54] who pointed out that the primary treatment stages in conventional WWTPs are more efficient in removing larger MP fractions, while the smallest fractions remain in the final effluent.
In general, these studies highlight that larger particles (>∼150 µm) are efficiently removed by pretreatment and primary WW treatment and, in particular, treatments involving sedimentation. This may explain the absence of MPs > 1000 µm in the effluent samples treated with AS. The potential fragmentation processes occurring during treatments must also be taken into account. Despite the low concentrations observed for MPs > 500 μm, their assessment is significant, as they could serve as a potential source of smaller MPs due to fragmentation processes and also contribute to the overall mass load.
In the PRET and AS effluents, most of the particles (>95%) are concentrated in the smaller fractions (<500 µm). In the case of the MBR effluent, which exhibits the lowest MP/L values, this concentration is more evenly distributed among all the studied fractions. In comparison to the other two facilities, the MBR effluent shows a significantly lower proportion of MPs (67.0%) in the smallest fraction (150–25 µm) and higher percentages in the other two size fractions: 19.3% in 500–150 µm and 12% in 1000–500 µm. As suggested by Egea-Corbacho et al. [56], this distribution may be due to the fact that medium-sized particles remain in the tank and concentrate, thus obtaining a higher percentage of these larger fractions in the MBR effluent. Although the nominal pore size (0.04 µm diameter) should prevent the passage of large particles, some may pass, possibly due to leakage, particularly in the case of fibers when appropriately oriented.
In terms of morphology, fragments consistently emerge as the predominant shapes of MP in all WW effluent samples, averaging between 58.6% and 65.6%. Fibers follow closely behind, with a range from 27.3% to 33.3%. Among fragments, many different irregular or rounded shapes and colorations were observed, suggesting the potential fragmentation of larger items as their origin [53,60].
While fibers and fragments consistently emerge as the dominant types of MPs in WW, the corresponding percentages do not always agree. Some studies report that fibers are largely the predominant MPs remaining in WWTP effluent [30,31,53,61,62]. However, these observations differ from studies such as the one by Murphy et al. [63], and Schell et al. [17], which report a similar % of fragments (average 67–69%) and fibers (average 18.5–19%) in WWTP final effluent, respectively. These discrepancies may arise from variations in treatment processes or differences in methodological approaches.
The presence of a lower proportion of fibers in the effluent, as observed in this study, could be attributed to their higher retention capacity compared to the other shapes, owing to their elongated and irregular form and tendency to form fiber bundles during WW treatment filtration steps [17,55]. Films and spheres were uncommon shapes observed during the whole sampling period, accounting for less than 6% of the total collected MPs, aligning with other studies [53,60].

3.1.3. Influencing Factors and Seasonal Variation

The assessment of key effluent parameters, such as SSs, revealed a statistically significant positive correlation with MP concentration solely in the case of the AS facility (r = 0.676, p = 0.007). For the primary treatment (PRET) facility, chemical oxygen demand (COD) also exhibited a positive correlation with MP concentration (r = 0.757, p = 0.06).
These correlations are difficult to interpret. In the case of the SS parameter, the pore size of the filters used for SS determination in WW samples is within the range of 0.4–1.2 µm. However, in this study, MPs smaller than 25 µm were not identified, thus excluding information related to these smaller fractions. The positive correlation between PRET effluent and the COD parameter is predictable, as a high pollutant load persists in the effluent after pretreatment.
The literature shows contrasting results in this respect and the relationship between MP data and background parameters does not seem to be clear [15,64]. In other studies, high MP and mass concentrations have been reported as being associated with increased suspended matter and turbidity during heavy rainfall events [62]. During the samplings in this study, no extreme rainfall events or abrupt flow changes that could explain the observed variations were recorded, especially in the case of the PRET facility effluent.
Table 2 shows that all three facilities exhibit peak concentrations in September, with generally slightly higher concentrations during the winter months. The occurrence of this peak in September could be linked to societal behaviors, especially the increased activity levels during this month. Following the August break, a significant holiday month in Spain, people return from vacations and schools and government offices reopen, which could potentially lead to increased wastewater production, carrying a higher MP load. The winter months mark the peak tourist season in Gran Canaria, and the influx of tourists during this period may contribute to an increased presence of MPs, consistent with findings in other studies conducted in areas with high tourist seasonality [65]. To better elucidate this aspect, a more extended study should be conducted at a selection of WWTPs affected by tourism, spanning a duration of at least one year. This would enable the observation of MP fluctuations attributable to the seasonality of tourism.

3.2. Analysis of Microplastic Polymer Composition

Table 3 shows the microplastic mass concentration (µg/L) of the detected polymer classes in the effluent from the three WWTPs. A comparison of the relative abundance of the polymer types is plotted in Figure 4.
Py-GC/MS analyses confirmed the presence of six of the most common plastic polymers (PVC, PS, PA, PET, PP, PE) in most of the effluent samples from the three WWTPs. When pyrolyzed up to 600 °C, no peaks were observed at any retention times when processing the blank filter. Among all the effluents, the PRET samples showed the highest mass concentrations for every analyzed polymer, except PP, whose value was below the limit of quantification (LOQ) (Table 3). Considering the total values of polymer mass concentration, ranked from highest to lowest, PRET presented 123.33 µg/L, followed by AS and MBR with 17 and 10 µg/L, respectively. For most polymers, no significant differences were observed when comparing the AS and MBR effluent, except in the case of PA, whose concentration in MBR was below the LOQ, and for PE, whose concentration in AS was significantly higher than in MBR.
Studies reporting data on polymer composition in WWTP effluent based on mass concentration are still very limited and comparable data are scarce. The recorded mass concentrations for the samples analyzed in this study were within the µg/L range, significantly exceeding the values reported in other studies [62,66], which both used 10 μm meshes to concentrate WWTP effluent samples. This variability may arise not only from different sample processing factors (filtered volume, pore size of filters, etc.) but also from site-specific influences and various intrinsic elements specific to the WWTP itself, such as catchment population habits or socioeconomics, raw wastewater composition, treatment processes, etc. The sum of all these factors could contribute to the observed variability in the total mass concentrations of plastics measured [18]. Further research is needed to address this issue comprehensively.
While acknowledging that not all particles collected in the samples are plastics [53,67], the polymer mass results obtained in the present study are consistent with the expected pattern based on the microparticle concentration data reported earlier in this paper. Specifically, MP concentrations in PRET samples consistently ranked as the highest, while the MBR samples exhibited the lowest abundance among the three effluent sources sampled during this study.
The proportion of the six analyzed polymers varied between the different sampling sources of effluent (Figure 4). In AS, the highest average percentage was for PVC (39.2%), followed by PET (19.4%), PE (17.8%), and PA and PP (both at 4.2%). In the case of MBR, neither PA nor PP polymers were quantified, but PVC and PS were the predominant plastic types, accounting for 61.1% and 19.4%, respectively, followed by PET (13.9%) and PE (5.6%). The PRET effluent showed PE as the dominant polymer (41.3%), followed by PVC (25.5%), PS (12.4%), and PA and PET (both 10.4%), but PP was not quantified.
The polymers targeted in this study are among the 10 most in-demand and manufactured plastics in the world (Plastics Europe, 2022) [68] and their presence in the effluent sampled in this study agrees with the numerous studies reporting on plastic types in WWTP effluent [5,15,62,69], though the relative abundance of each polymer varies. In the referenced studies, polyolefins (PE, PP) and PET were the dominant polymer types and PVC made up less than 10%. In the current study, none of the three WWTP effluents sampled showed a similar polymer composition profile. In both outlets subjected to a secondary treatment (AS and MBR), PVC was the dominant polymer type (AS: ~39%, MBR: ~61%), whereas in the PRET effluent, PE (~41%) was the dominant type, followed by PVC (26%). These results align with those obtained by Franco et al., 2021 [53], who conducted research in two WWTPs operated with secondary treatment in Cadiz (Spain) with different wastewater sources and which reported relative PVC abundances of 15% and 40% in the effluent.
Among the results, the notably high percentage of PVC (61%) in the MBR effluent, compared to 39.2% for AS and 25.5% for PRET, is remarkable. PVC is a versatile thermoplastic, and its presence can be traced to diverse sources within the urban and industrial settings contributing to the wastewater stream of these facilities. PVC is a commonly used material in everyday household items (water service pipes, fittings, flooring, fashion and footwear, certain types of packaging, etc.) and when these products are used, damaged, or discarded, they can end up in the sewage system, eventually reaching WWTPs. In particular, the MBR-operated WWTP collects wastewater from an industrial area with several industries engaged in PVC carpentry, manufacturing blinds and window and door frames. These activities may contribute to the clear difference in the MP composition of the influent entering this WWTP which is potentially not being effectively removed by the treatment process.
Likewise, the versatility and reasonable cost of polyolefins have made them the most widely used type of plastic, representing almost 50% of European plastic consumption [66]. It is therefore not surprising to find PE and PP among the most abundant plastics in the effluent of WWTPs. This can be attributed, on one hand, to the extensive use of these polymers in daily life. On the other hand, as suggested by Kim and Park in 2021 [70], plastic particles with lower density, such as PE and PP, tend to reach WWTPs in larger quantities due to their reduced settling from the source to the treatment plant.
In any case, variations in the abundance of these target polymers across different studies may be attributed to the habits and consumption patterns of the relevant catchment population regarding plastic products. The three WWTPs in this study collected wastewater from residential sources along with commercial and industrial areas (hospitals, shopping centers, greenhouses, food processing plants, manufacturing processes, etc.). Alternatively, variations can also be influenced by the behavior of plastic MPs in different treatment steps. For example, high-density MPAs like nylon (PA) tend to settle more easily than PE and have a higher likelihood of being deposited from wastewater into sludge [61].

4. Conclusions

The average MP concentration in the effluent treated with AS technology was relatively low (32.71 ± 19.55 MP/L), in contrast to the effluent subjected solely to a complete pretreatment, which recorded a much higher concentration at 7461.50 ± 3843.87 MP/L. Given that AS technology has been widely adopted among municipal Canarian WWTPs, particularly those located along the coast, it transpires that these facilities are preventing significant quantities of MPs from reaching marine environments and thus contributing to achieving the objectives established by the marine strategy of the Canary Islands territory.
Furthermore, this study confirmed that MBR-treated effluent exhibits a lower MP content compared to effluent treated with AS. This is one of the advantages of MBR technology in WW treatment, emphasizing its significance when specific effluent quality standards are necessary (e.g., discharges to bathing areas, special protection areas, etc.), regardless of the operational and maintenance costs. Primary treatments play a significant role in removing larger fragments and fibers from wastewater discharge; therefore, all WW facilities should, at least, be designed to provide such treatment for peak inflows that may be discharged directly into water bodies, such as stormwater overflow and runoff. This would help to mitigate the carryover and introduction of microplastics into marine environments.
This research reveals that the reuse of reclaimed WW can serve as a potential source of organic matter and essential nutrients for plants and microorganisms. However, it also highlights a significant concern: the potential transport of substantial amounts of MPs to agricultural soils when this water is used for irrigation. The assessment of the MP load reaching agricultural soils from this particular water source has been poorly explored in the Canary Islands and more studies are needed.
To examine the extent of plastic pollution in wastewater, this study used Pyr-GC/MS to investigate the mass concentrations of plastics with particle sizes ranging from 25 to 5000 μm in three WWTPs (123.33 µg/L for PRET, 17 µg/L for AS, and 10 µg/L for MBR), thus providing a useful approach for evaluating their origin and enabling limitations to be placed on their production directly at the source. Considering their impact on the aquatic environment, the polymer profile is noteworthy as the density of MPs can influence their distribution and bioavailability.
Implementing additional treatments at the end of the WW treatment process is a matter to be considered, although it could entail substantial costs. Alternatively, source control options and preventative measures including banning, eliminating, substituting, or reducing plastics in consumer products and better management through intensive education and awareness actions and campaigns should be prioritized.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16010064/s1: Table S1: The specific physicochemical characteristics of the sampled wastewater effluent of the three facilities during this study: acidity/basicity (pH, pH units); electrical conductivity (EC, µS/cm); temperature (T, °C); suspended solids (SSs, mg/L); chemical oxygen demand (COD, mg/L); total nitrogen (TN, mg/L); and total phosphorus (P-PO4, mg/L); Figure S1: Median concentrations of MPs (MP·L–1) in boxplots for each WWTP effluent subjected to different treatments: A—activated sludge (AS), B—membrane bioreactor (MBR), and C—pretreatment (PRET).

Author Contributions

J.R.B.R.: conceptualization, funding acquisition, data curation, project administration; J.R.B.R. and M.R.S.: formal analysis, investigation, methodology, validation, visualization, writing—original draft, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research is carried out under the ABACO project with code MAC2/4.6c/324, within the INTERREG V-A MAC 2014-2020 program, co-financed at 85% by FEDER.

Data Availability Statement

The data are not publicly available due to a confidentiality agreement signed at the beginning of the study.

Acknowledgments

The authors would like to thank the managers of the sampled facilities for access and the technical background data. Special thanks to the staff members at the three WWTPs for helping with the monthly sampling and for sharing their experience and functional knowledge of the facilities.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Water 16 00064 g001
Figure 1. Distribution (%) of MP size in the effluent sampled from the three WWTPs.
Figure 1. Distribution (%) of MP size in the effluent sampled from the three WWTPs.
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Figure 2. A selection of stereoscopic images recovered from the three WWTP effluents: (A) spherical, (B) fiber, (C) fragment, (D) fiber, (E) film, (F) fiber, (G) fragment, (H) spherical, (I) fragment, (J) fiber, (K) cellulosic fibers, and (L) fiber.
Figure 2. A selection of stereoscopic images recovered from the three WWTP effluents: (A) spherical, (B) fiber, (C) fragment, (D) fiber, (E) film, (F) fiber, (G) fragment, (H) spherical, (I) fragment, (J) fiber, (K) cellulosic fibers, and (L) fiber.
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Figure 3. The proportion (%) of MP shape categories in each effluent (AS, MBR, and PRET). The distribution is based on the average number of particles found during this study for each WWTP.
Figure 3. The proportion (%) of MP shape categories in each effluent (AS, MBR, and PRET). The distribution is based on the average number of particles found during this study for each WWTP.
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Figure 4. Comparison of averaged relative abundance (%) of polymer types in PRET, AS, and MBR WWTP effluent. Note: PA (polyamide), PE (polyethylene), PET (polyethylene terephthalate), PP (polypropylene), PS (polystyrene), and PVC (polyvinyl chloride).
Figure 4. Comparison of averaged relative abundance (%) of polymer types in PRET, AS, and MBR WWTP effluent. Note: PA (polyamide), PE (polyethylene), PET (polyethylene terephthalate), PP (polypropylene), PS (polystyrene), and PVC (polyvinyl chloride).
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Table 1. Sampled WWTPs with their main treatments and characteristics.
Table 1. Sampled WWTPs with their main treatments and characteristics.
LabelWastewater SourceSize (p.e.) 1Main Treatment
PRET 2Household-Pumping station; coarse and fine screening + degritting + degreasing
AS 3Household + industry (10%)171,600Pretreatment + settling +
activated sludge +
chlorination
MBR 4Household + industry (25%)50,000Membrane bioreactor +
chlorination
Notes: 1 p.e.: population equivalent; 2 PRET: pretreatment; 3 AS: activated sludge; 4 MBR: membrane bioreactor. Note: Effluent from AS and MBR discharges into the coastal waters complies with the current European regulations (Directive 91/271/EEC) and their transposition into Spanish law (Real Decreto-Ley 11, 1995) [38], which establishes the regulations applicable to the treatment of municipal WW.
Table 2. The microlitter concentration (MP/L) over the eight-month sampling period in each sample of WWTP effluent for all the size fractions considered. Maximum (Max), minimum (Min), average, standard deviation (SD), and median and interquartile range (IQR) values for each facility are also shown.
Table 2. The microlitter concentration (MP/L) over the eight-month sampling period in each sample of WWTP effluent for all the size fractions considered. Maximum (Max), minimum (Min), average, standard deviation (SD), and median and interquartile range (IQR) values for each facility are also shown.
PRET (MP/L)AS (MP/L)MBR (MP/L)
Jun-22781350.878.00
Jul-22781710.144.60
Aug-2239678.903.20
Sep-2215,13358.8013.00
Oct-22234023.604.40
Nov-22608619.2012.00
Jan-23942848.0613.60
Feb-23710842.1117.40
Max15,13358.8017.40
Min 23408.903.20
Average7461.5032.719.53
SD3843.8719.555.21
Median7460.5032.8610.00
IQR2663.5031.838.60
Table 3. Averaged mass concentration of microplastics (>25 µm) detected and quantified (µg/L) in the PRET, AS, and MBR WWTP effluent; LOQ, limit of quantification.
Table 3. Averaged mass concentration of microplastics (>25 µm) detected and quantified (µg/L) in the PRET, AS, and MBR WWTP effluent; LOQ, limit of quantification.
PolymerMP Average Mass Concentration (µg/L)
ASMBRPRET
Polyvinyl chloride (PVC)13.008.0043.33
Polystyrene (PS)0.671.0010.00
Polyamide (PA)0.33<LOQ6.67
Polyethylene terephthalate (PET)1.000.676.67
Polypropylene (PP)0.33<LOQ<LOQ
Polyethylene (PE)1.670.3356.67
Total ∑ polymer)17.0010.00123.33
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MDPI and ACS Style

Sanz, M.R.; Rodríguez, J.R.B. Abundance and Characterization of Anthropogenic Microlitter in Effluent from Three Wastewater Treatment Plants in Gran Canaria (Canary Islands, Spain). Water 2024, 16, 64. https://doi.org/10.3390/w16010064

AMA Style

Sanz MR, Rodríguez JRB. Abundance and Characterization of Anthropogenic Microlitter in Effluent from Three Wastewater Treatment Plants in Gran Canaria (Canary Islands, Spain). Water. 2024; 16(1):64. https://doi.org/10.3390/w16010064

Chicago/Turabian Style

Sanz, Marta Rodrigo, and Juana R. Betancort Rodríguez. 2024. "Abundance and Characterization of Anthropogenic Microlitter in Effluent from Three Wastewater Treatment Plants in Gran Canaria (Canary Islands, Spain)" Water 16, no. 1: 64. https://doi.org/10.3390/w16010064

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