Estimating Microplastics related to Laundry Wash and Personal Care Products released to Wastewater in Major Estonian Cities: a comparison of calculated and measured microplastics

Abstract

Microplastics (MPs) research still at the budding stage in Estonia. A theoretical model build on substance flow analysis principles was developed. The goal of this study is to broaden understanding of MPs-types in wastewater and their contribution from known sources, quantify their presence based on model prediction and in-situ measurements. The authors estimate MPs from laundry wash (LW) and personal care products (PCPs)) in wastewater in Estonia. We found out that total estimated MPs load per capita from PCPs and LW in Estonia were between 4.25 – 12 tons/year, 3.52 – 11.24 tons / year respectively, and estimated load ended up in wastewater were between 700 – 30,000 kg/yr. and 2 – 1500 kg/yr. in WWTPs influent and effluent stream respectively. Finally. We conducted a comparison between estimated MPs load and on-site sample analysis and observed a medium-high level of MPs being discharged into the environment annually. During quantification and chemical characterization using µFTIR analysis, we found that microfibers with a length of 0.2-0.6 mm accounted for over 75% of the total MPs load in the effluent samples collected from four coastal WWTPs in Estonia. The estimation avails us broader overview about the theoretical MPs load in wastewater and gain valuable insight into developing process methods that prevent MPs accumulation in sewage sludge for safe application in agriculture.

Highlights

• Microplastics load in the influent and effluent of selected WWTPs is predicted.

•Estimated microplastics load in the effluent of selected WWTPs is compared with onsite measurements.  

•Microfibers make up the majority of MPs found in the effluent of the studied WWTPs.

Introduction

Synthetic plastic waste in the environment has generated enormous concerns in recent decades and is regarded as emerging micropollutants. It acts as vector of contaminants, thereby pose a threat to both the environment and humans [25]. In all households and enterprises, plastic-based products are used for different purposes. On a global scale, more than 367 million metric tons in 2020 were produced [14].

According to European chemical Agency (ECHA) in 2019, defines microplastics (MPs) as a material consisting of solid polymer-containing particles, to which additives or other substances may have been added, and where ≥ 1% w/w of particles have (i) all dimensions 1 nm ≤ x ≤ 5 mm, or (ii), for fibers, a length of 3 nm ≤ x ≤ 15 mm and length to diameter ratio of > 3. Polymers that occur in nature that have not been chemically modified (other than by hydrolysis) are excluded, as are polymers that are (bio)degradable. 

MPs can be divided into two categories based on their raw materials. Primary MPs are manufactured directly from industry and designed for specific purposes, such as medical equipment, liquid, and gas exploration, and exfoliants in cosmetics and personal care products [50], while secondary MPs resulted from the degradation of bigger plastic waste through mechanical and environmental processes such as wind, water, and sunlight [22]. This category includes debris from landfills, plastic mulching, car tire wear, stormwater debris and synthetic fibers released from washing machines [24, 49].  Both categories of MPs were quantified and reportedly found on the fish bodies in the range between 0.153 and 40.880 particles per fish living in freshwater [3]. In a different study, Li et al., [32] reported that short length microfiber in the environment, are easily mistaken as food by marine habitats. Similarly, the production of plastic involves the use of chemical additives, such as plasticizers, to improve strength and durability. Unfortunately, these additives can be toxic and cause endocrine disruption if ingested or bioaccumulate in human bodies [56, 34].

Research studies have reported about the abundance of MPs in influent and effluent stream in wastewater treatment plants (WWTPs) at varying concentrations [52, 54, 57, 58], under microscopic examination it was observed that majority of the MPs-type are from human source, especially microfibers released during laundry wash [32, 37, 52]. Bakaraki Turan et al. [5] observed that in wastewater sewer networks, more than 35% of identified MPs were of synthetic fibers origin emanated from households and enterprises heading towards WWTPs. Everyday laundry wash is considered main contributor of secondary MPs in the ecosystem [8]. The level of similarity in reported microfiber (MF) release rate from synthetic textile in different studies is low due to differences in fiber length, calculation approach, and experimental setup. A Recent study by Belzagui et al. [7] showed that MF with an average length of 0.3 mm and a diameter of 20 mm had detachment release rate ranging from 2 to 29 mg/garment, with estimated masses ranging from 23 to 73 mg/kg of garment

Despite the implementation of zero use of MPs in PCPs by several countries (EU, Canada, USA, UK) which has resulted into significant reduction of MPs commonly added to PCPs release into the environment [39], different studies have reported reasonable amount of MPs contents in PCPs and cosmetics after the European Union (EU) ban on microplastics in cosmetic and PCPs in 2018 [26]. Piotrowska et al. [41] analyzed the compositions of MPs contents in cosmetic product of 130 body scrub type samples obtained from Polish market. The result shows that 55% of the analyzed samples contain polyethylene (PE) as active ingredients for skin repair.

Earlier investigation on marine and beach litter in the Baltic Sea region reported a significant number of MPs. Approximately 1000 tons of MPs loads from different sources (PCPs, laundry textile, car tyre and household dust) reach Baltic Sea annually [47]. Human anthropogenic activities are the source of MPs, and likely to reach environment via treated and untreated wastewater [42].

In WWTPs, the wastewater treatment processes are implemented in three stages viz. primary, secondary, and tertiary. During the process, most MPs particles are separated into solid phase (sewage sludge) by density and the remaining 1–5% remain floating in the liquid phase (effluent stream) [4]. To a large extent, WWTPs can retain MPs relatively good between 85–95% for WWTPs in Baltic Sea region [6]. Despite high performance, WWTPs remain point source emitting MPs particles via effluent stream to surface water and from sewage sludge to land as used in agriculture [6, 10, 23, 33, 38].

To date, there have been rather few studies in Estonia that reported about microplastics in surface water [46], and none has given attention to MPs relevant to wastewater. This study is the first of its kind to estimate MPs load reaching and leaving WWTP in Estonia.

The objective of this study is to estimate MPs release by human source (microfibers from laundry wash (LW) and PCPs)), the quantity generated per capita that likely to end up wastewater and the corresponding concentrations in WWTPs effluent stream with treatment capacity for at least 2000 population equivalents (PEs) in major Estonian cities to lay the foundation for future studies in Estonia.

Method and description

Modelling approach

The model approach used in this study builds on substance flow analysis principles commonly used to develop a systematic overview of substance within a well-defined system in terms of space, time, and substances [2, 15]. The household and enterprises connected to municipal WWTPs in each city are set to be the geographical boundary considered in the model. In terms of substance, PE is the major microplastics ingredient being added to PCPs. In the same vein, synthetic fibers emit PE-type plastic particles [31]. Hence, it is taken as a reference sample of MPs particle substance considered in this model study. However, the current knowledge on the microplastics contents in PCPs and synthetic clothing are limited, which constraints us in using complex emission models. Therefore, using a generic approach, theoretical emission scenarios for microplastics were modelled.

Description of location

The Estonia population is sparsely distributed across the 45,288 km², with addition of 1520 islands. According to Statistic Estonia 2021, approximately 84% of the population are connected to municipal wastewater treatment plants that are capable of biological wastewater treatment processes up to secondary treatment stage. Until now, there are 664 municipal WWTPs across the country. However, 49% are small scale WWTPs designed for less than 300 population equivalents (PEs) while 51% has capacity to treat wastewater for more than 100 000 PEs [29].

A theoretical model was formulated to estimate the MPs concentrations of laundry wash (LW) and PCPs sources in the wastewater influent and effluent of the major cities in Estonia (Fig. 1): Tallinn, Tartu, Narva, Pärnu, Rakvere, Viimsi, Viljandi, Kuressaare, Paide, Võru, Sillamäe, Valga, Keila, Põlva, Põltsamaa, Rapla, Haapsalu, Elva, Haljala WWTPs. Notably, approximately 760,000 inhabitants in these chosen settlements, comprising 57% of whole Estonian population. The largest settlement is Tallinn with over 390,000 people followed by Tartu, Narva. Pärnu, and Viljandi with more than 100,000, 66,000, 40,400, and 20,309 people respectively and the rest are occupied by less than 20,000 inhabitants [48].

Fig. 1.

Study location indicating wastewater treatment plants where effluent samples were collected in Estonia

Estimating MPs load per capita in Estonia

Personal care products (PCPs)

We estimated the MPs release from personal care products load per capita in Estonia based on estimated values for European Union including Norway and Switzerland as reported by Gouin [20]. The published data were used as a yardstick in our own estimation. In Europe, the overall volume of plastic microbeads used in PCPs was estimated to be 4130 tons per year with an average usage value of 18.00 mg/day per capita and an equivalent amount of 6.57 g/year (18.00 mg × 365) across Europe in 2012 [20]. Hence, a total amount of 8.50 tons (6.57 g × 1.30 × 10⁶) of MPs from PCPs load are released per year in Estonia.

Based on literature studies, the average weight content of microplastics found in a PCP is 2.50%, and assuming a normal portion weight of about 2 g for a PCP (Q. [50], then each portion would contain 0.05 g microplastic. Then assuming individual in Estonia uses a microplastic-laden products in the form of facewash, shaving foam or toothpaste once a day it would amount to 24 tons a year (0.05 g × 1.30 × 10⁶ × 365). Therefore, despite the lack of sales volume, and market share, the estimate of 8.50 tons seems within a realistic range.

According to European Chemical Agency (ECHA), the approved legislation restricting the use of MPs in PCPs became effective in 2022. There is no study yet to ascertain the degree of compliance by the industries.

Therefore, considering the volume of plastic microbeads used in PCPs in 2012, we assumed the total MPs load per year might have been reduced over the years, at least by 50%. Also, during a market survey conducted in Estonia, we observed that the arrays of personal care products displayed on the shelves have included MPs ingredients and, in some cases, not all PCPs itemized microplastics contents on their ingredient list, but this does not mean that they are free of microplastics [9]. Therefore, in this study, we assumed that by now, there will be at least 50% reduction of the estimated values (8.50 – 24 tons) of MPs load release from PCPs, which amounts to 4.25 – 12.00 tons of MPs in PCPs released to wastewater in Estonia annually.

Laundry Wash (LW)

Estimating microfiber release from textile during LW per country has been reported in different studies, in Denmark [31], Norway [51], Sweden [36], and Germany [13], they have estimated quantitative data based on transparent assumptions and plausible calculations. The recent report published on this topic is from Spain by Belzagui et al. [7], which based their calculations on detailed laboratory experiment study and analysis. The report gives an account of microfiber detachment rate from textile garment during LW. We take que on the available data and basic information from neighboring countries to estimate annual microfiber release from textile during LW in Estonia. The essential information is as follows.

1. According to Organization for Economic Co-operation and Development (OECD) [40], 84% of the Estonian population are connected to municipal sewage treatment plants,

2. Washing habit, data from Norwegian Environment Agency. For Norwegians, washing habits implies 70 washing cycles / (capita, year), about 4 kg washing / cycle and emissions of 60 L of water / by cycle [51]

3. More than half of all clothes produced are made of synthetic material according to the fiber year consulting firm annual report [12].

4. The mass loss of microfibers during LW is between 23 – 73 mg/garment with an average length between 0.2 and 0.4 mm from domestic washing machines with a normal load [7].

The following assumptions were adopted to estimate MPs release from LW to wastewater in Estonia:

• Assumption 1: Approximately 1.1 million inhabitants will contribute MPs-type of synthetic fibers source from laundry to wastewater.

• Assumption 2: Based on Norwegian washing habits, we considered these conditions to be the same with Estonian washing habits. An Estonian is therefore expected to wash 70 cycles / (capita, year) * 4 kg (load/ cycle) which implies washing at least 280 kg / (capita, year) and use 4200 L of water / (capita, year), that is, 70 cycles / (capita, year) * 60 L / cycle for textile washing.

• Assumption 3: we assumed 50% of all Estonian textiles are made of synthetic materials.

Microfiber released during LW from household to wastewater in Estonia thus estimated based on assumption (1–3) as follow:

• (23 – 73) mg fibers / (kg garments) * 280 kg / (capita, year) * 1.1 million * 50% = 3.52 – 11.24 tons / year

MPs emission scenarios

Microplastics concentration release from PCPs and LW to wastewater were predicted by estimating the quantities within a stipulated range in 3 scenarios (low-medium–high) to have the representative quantities of MPs arriving and leaving WWTPs. A theoretical model was prepared in MS excel to predict MPs emission from LW and PCPs in the influent and effluent streams of WWTPs in Estonia.

Emission from personal care products

PCPs scenario parametrization

Wastewater produced (WWprod): The Estonian population data and average water consumption were obtained from the following government agencies [48, 18]. The average domestic water consumption per inhabitants in Estonia is 90 (L capita-1 day-1) and the population stood at 1.32 × 10⁶ in 2022. Due to individual differences in water use, we assumed that for the low scenario, all water consumed ends up in wastewater. Water used for food preparation and not end up in wastewater account for 1% loss be medium scenario, while on the worst-case loss of water during use in kitchen faucet and spilled by accident take into consideration of at least 2% loss for high scenario.

MPs removal in WWTPs (R_eff)

The efficiency of MPs removal during wastewater treatments process varies differently, these differences can be as a result of the characteristics of MPs-types and treatment (primary, secondary and tertiary) steps and technologies (CAS, MBR,) applied [35]. Research studies reported efficiencies ranging between 64.40 – 99.9% [52, 53, 4]. Then we assumed that for low emission scenario should be 96% while medium and high scenarios represent 90%, 40% removal efficiency respectively.

Personal care products (PCPs)

Based on literature data, we assumed that all MPs intentionally added to PCPs are washed and rinsed to wastewater. In 2012, 1.09 × 10⁶ L of liquid soap and PCPs were consumed in Estonia, equivalent to 0.002 g/capita/day [20]. For medium and high scenarios, we assumed 10 and 20 percent increase respectively.

For the microplastic volume in plastic-containing products, recent data showed that MPs contents in PCPs is in the range between 6 – 7% [19]. Therefore, we assumed the high scenario to be 10% and the medium and lowest assumptions for microplastic content are 7% and 0.10%, respectively.

Earlier studies reported that, the market penetration for liquid soaps reported to be about 1.50% in United states [20], Due to lack of market penetration data for Estonia, we believe that the market penetration declined gradually due to increased public awareness to boycott MPs-laden liquid soaps usage. Then for different scenarios, 0.50%, 1%, and 1.50% for low, medium, and high market penetration respectively.

By using the model, emission from PCPs to wastewater in Estonia was calculated in Eq. (1) and showed in Table 1 as followed:

$${\mathit{EC}}_{\mathit{pcps}}=\frac{CxUxMx\left(1,-,R_{\mathit{eff}}\right)}{{\mathit{WW}}_{\mathit{prod}}}$$ 1

where ECpcps is the estimated concentration release to wastewater (g/L), C is the concentration of MPs in a product (grams/gram), U is the daily usage of the product (g/ca/day), M is the market penetration of products with MPs ingredients, R_eff is the fraction of MPs removed in WWTPs, and WWprod amount of wastewater produced (L cap⁻¹ day⁻¹).

Table 1.

Parametrization under 3 scenarios predicting concentrations of MPs from PCPs in the influent and effluent of WWTPs

Parameters	Low	Medium	High
General
WWprod (L capita−1d −1)	90.00	89.10	88.20
Emission factor
C (g g−1)	0.001	0.07	0.10
U (g capita−1 d−1)	0.002	0.0022	0.0024
M	0.005	0.01	0.015
Influent concentration
ECpcps (gL−1)	1.11 × 10–4	1.73 × 10–2	4.08 × 10–2
1-Reff	0.04	0.10	0.60
Effluent concentration
ECpcps (µgL−1)	4.4 × 10–6	1.73 × 10–3	2.45 × 10–2

Emission from Laundry wash (LW)

LW scenario parametrization: Washing habit

We assumed that every single person would do laundry per week with a normal washing machine load 3 – 4 kg/week based on the Scandinavians washing habit culture. A person in the Northern Europe country washes approximately 1.50 cycles of laundry per week. For different scenarios, we assumed 3 kg per week washing machine load for low scenario, 6 kg per week and 8 kg per week for medium and high scenarios respectively.

Polymer distribution

There is no import data about the types of textile materials and corresponding polymer distributions imported to Estonia. Information from literature studies established that polyester fabric material releases more microplastics during laundry compared to other synthetic materials (PA, Acrylic) and set to be major source of microfibers commonly detected in WWTPs. We assumed 50% of fabrics used by Estonians are made of polyester synthetic materials plus or minus 5% for low and high scenarios respectively.

Removal in WWTPs

Microfiber interaction and removal in activated sludge treatment plant was investigated to have removal efficiency between 95 – 99% [44] and 98% was assumed for medium scenario.

Emission factor

Data from literature estimated the emission factor for microplastic fibers release during laundry to be between 23 – 73 mg/kg for polyester fabrics and set for low and high scenarios [7], while the mean value was taken to represent our medium scenario.

Emission from LW to wastewater in Estonia was calculated in Eq. (2) and shown in Table 2 as followed:

$$E_{\mathit{laundry}}=\frac{T_{\mathit{washed}}\times P_{\mathit{textiles}}\times EFx\left(1,-,R_{\mathit{eff}}\right)}{{\mathit{WW}}_{\mathit{prod}}}$$ 2

Table 2.

Parametrization under 3 scenarios predicting concentrations of MPs from LW in the influent and effluent of WWTPs

Parameters	Low	Medium	High
General
WWprod (L capita−1d −1)	90.00	89.10	88.20
Emission factor
Twashed (kg capita−1d −1)	0.43	0.86	1.14
Ptextiles (%)	0.45	0.50	0.55
EF (mg/kg)	23.00	48.00	73.00
Influent Concentration
ELaundry (µg L−1)	49.00	232.00	519.00
1-Reff	0.01	0.02	0.05
Effluent Concentration
ELaundry (µg L−1)	0.49	4.64	25.95

E_laundry is the emission release from laundry activities (grams per liter). T_washed is the textiles washed in kg per capita per day (kg per capita per day). P_textiles is the distribution of polymer among textiles. EF is the emission factor for microplastic fibers during laundering. R_eff is the fraction of microplastics removed during wastewater treatment. WWprod is the quantity of wastewater produced (L cap⁻¹ day⁻¹).

WWTP effluents sampling

Wastewater effluent samples were collected from five WWTPs in Estonian coastal cities, in two separate years in 2015 at Sillamäe, and in 2018 at Tallinn, Pärnu, Haapsalu, Kuressaare respectively. These treatment plants collect and treat wastewater from domestic households with the exception to Tallinn WWTP in city capital designed for 450,000 people equivalent., with an average influent flow 350,000 m³/day receiving storm water from combined sewer system. While Sillamäe, Pärnu, Haapsalu, and Kuressaare can treat wastewater with maximum capacity of 2700, 14,000, 1300 and 3600 m³/day respectively. In all selected treatment plants, the treatment process in use includes mechanical step (particle-screening, degreasing and sand removal), biological and chemical treatment step removing organic matter, nitrogen, and co-precipitation of phosphorous, and a sedimentation step separating the activated sludge and treated effluent. At the end of the treatment processes the effluent water is discharged into the Baltic Sea. Prior reaching the sea, 200 L of water from WWTPs were sampled with stainless steel bucket and sieved through Manta trawl cod end (mesh size 330 µm). The samples were collected in the cod end of the net. Cod ends with the samples were covered with aluminum foil and transported to the lab (under 4 degrees C).

MPs Identification and Quantification

In the lab the content of each cod end was rinsed with ultrapure water to a metal bucket and sieved through a set of stainless-steel sieves (5,000 µm, 1,000 µm, and 300 µm). Thereafter, particles from each sieve were flushed into a separate glass jar with ultrapure water. Formaldehyde (37%) was added in the proportion of 1 to 100 ml of sample. Samples were kept at room temperature in the dark until analysis in the lab.

If the samples contained a lot of organic material, they were left to settle. The solution on top of the settled organic material was pipetted and vacuum filtered onto a 1.6-µm pore size (VWR) glass fiber filter (47 mm diameter). Hydrogen peroxide (34.5%–36.5%) was added to the settled organic material in a proportion of 1:1 and left for oxidation under the ventilation cabinet for up to 7 days maximum. After oxidation, the samples were diluted with ultrapure water and vacuum filtered as described above. The filters were dried in glass Petri dishes in a drying oven (SANYO MOV-212F) at 60 °C for 15 min, and the particles remaining on the filters were analyzed using a stereomicroscope (Leica M205 C or Olympus SZX16). All MP particles were counted, partially photographed, and tested with a hot needle to distinguish plastics from other microlitter particles [11].

From every sample, all particles having plastic-like appearance were manually selected from the filters with tweezers and placed on transmission windows. Spectra were measured from each plastic-like particle. µFTIR microscope (PerkinElmer Spectrum Spotlight 400; PerkinElmer, USA) were used in point mode with an MCT detector. Measurements were performed in transmission form, spectral resolution 8 cm-1, spectral range 4000–650 cm-1, number of scans 8. Each suspected particle spectrum that matched a quality index greater than 70% was accepted as a plastic material. The reference spectral library was composed from in-house (Tallinn University of Technology, Department of Marine Systems) measured spectra and library developed in Leibniz Institute for Polymer Research Dresden and in Denmark (Aalborg University, Department of the Built Environment). The libraries contained the most common types of plastic and natural cellulose polymers.

In this study, we divided all MP particles into fibers and fragments. The fragments category includes all non-fiber MPs: films, foam, and pellets, and in general, we have identified fibrous plastics. As the MPs sampling was carried out using the Manta trawl with a mesh size of 330 µm, we categorized them into two size classes (330–999 µm and 1,000–4,999 µm) according to their longest dimension. The results were calculated by summing the number of MP particles in the analyzed water sample.

Contamination control

The procedures in the lab were conducted under the laminar flow to minimize sample contamination from the lab. The examinations under stereomicroscope and µFTIR could not be carried out under a laminar flow. That is why blank samples (set of filters) were left standing in the laboratory at each step and were investigated with a stereomicroscope as done for samples. Results presented in this work were not blank-corrected because they contained mainly organic fibers (cotton, linen). To minimize contamination in all steps, cotton lab coats and cotton clothes were worn and only metal and glass instruments were used as much as possible. All laboratory instruments and dishes were washed with ultrapure water and rinsed with ultrapure water three times before use. All samples were closed with aluminum foil or glass lids while processing. Finally, only essential lab personnel were in the laboratory during the sediment analysis to minimize the introduction of any contamination with fibers from clothes or air into the samples.

Results and discussion

Predicted MPs emission to wastewater

In this study, the estimated MPs concentrations in the influent and effluent of WWTPs were predicted, the summation of the influent and effluent concentrations for PCPs and LW in three scenarios are summarized in (Table 3). Based on our estimations, we predicted that MPs concentration of LW and PCPs source in the influent and effluent of Estonian WWTPs will fall within the predicted range, otherwise MPs removal efficiencies need to be further investigated. We observed that if accurate data on the MPs content added to PCPs, and its market penetration in Estonia from the appropriate authorities were available, it will improve the model estimate and give detail representation of MPs concentration reaching WWTPs and eventually discharged through the effluent stream to surface water. Also, the mathematical algorithm does not consider other categories (road paint, tire abrasion, urban grits) microplastics that can enter wastewater via storm water, infiltration, and illegal sewer dumping.

Table 3.

Summary of the total predicted MPs concentration in the influent and effluent stream of WWTPs

Parameter	Low	Medium	High
MPs Influent load (µg L−1)	49.00	232.02	519.04
MPs effluent load (µg L−1)	0.49	4.64	25.97

Estimated MPs load in the influent of Estonian WWTPs

MPs load to selected WWTPs in Estonia modelled with corresponding wastewater flowrate data is shown in Figs. 1 and 2 below. In WWTPs above 20,000 PEs, the modelled influent loads to Tallinn, Tartu, Narva, Pärnu and Viljandi are outlined (Fig. 2). It was estimated that, average MPs load to Tallinn WWTP could be 12,000 kg/yr. and it can be expected to reach a maximum load 30,000 kg/yr. but not below 2400 kg/yr. This estimation for Tallinn alone represents 60% of the total predicted MPs load to all WWTPs examined in this study while Tartu, Narva, Pärnu and Viljandi together account for 25% of the total high MPs load predicted in this work. On the other hand, in other small municipalities having WWTPs capacity below 20,000 PEs (Fig. 3). It was estimated that Rakvere, Viimsi, Kuressaare and Paide being the major recipient in this category will get high MPs load more than 1500, 1100, 760, 700 kg/yr., respectively and the rest facility expected to receive maximum load below 700 kg/yr.

Fig. 2.

Predicted amount of microplastics in wastewater influent expected in major cities of Estonia with a population greater than 20,000 people

Fig. 3.

Predicted amount of microplastics in wastewater influent expected in major cities of Estonia with a population lesser than 20,000 people

Estimated MPs load in the effluent of Estonian WWTPs

The modelled MPs concentrations in the effluent stream for all the twenty municipal WWTPs in Estonia are shown in figures below with the basic assumption of high MPs retention > 95% for WWTPs in Baltic Sea region [6]. MPs discharge via effluent stream was predicted for major treatment plants in Tallinn, Tartu, Narva, Pärnu and Viljandi (Fig. 4). The highest average concentrations are in Tallinn 1500 kg/yr., and it was observed that the effluent concentrations per year can be quite low as 25 kg/yr. in Viljandi., and approximately 100 kg/yr. in Tartu, Narva and Pärnu. In other cities with WWTPs capacity < 20,000 PEs (Fig. 5), MPs load release via effluent are between 2 – 80 kg/yr. provided that the WWTPs operating at maximum efficiency. The significant reduction in effluent concentrations of MPs is connected to partial removal during wastewater treatment mechanical and biological processes steps.

Fig. 4.

Predicted microplastics load in wastewater effluent stream from major Estonia cities > 20,000 PE

Fig. 5.

Predicted microplastics load in wastewater effluent stream from major Estonia cities < 20,000 PEs

MPs identification and quantification in WWTPs

An in-situ sampling campaign was carried at five WWTPs in Estonian coastal cities, in two separate years in 2015 at Sillamäe, and in 2018 at Tallinn, Pärnu, Haapsalu, Kuressaare respectively (Fig. 1). The total abundance number of MPs particles from multiple sources counted in each of the WWTPs effluent sample were 395, 783, 835, 845, and 2055 MPs/m³ at Kuressaare, Sillamäe, Haapsalu, Pärnu and Tallinn respectively. The highest MPs concentration in wastewater was found in the effluent of Tallinn WWTP, while the lowest concentration was observed in Kuressaare WWTP. The MPs abundance in other three WWTPs were relatively within the same range despite different sampling period. In all investigated WWTPs in this study, it was obvious that reasonable load of MPs particles was conspicuously released into the Baltic Sea. Particle separation and categorization further revealed that the number of MPs particle released is directly proportional to the size of the WWTPs. The extracted MPs samples in five WWTPs were mainly categorized in two forms as shown in Fig. 6. The fibers, which are longer than the width and fragments which are smaller fractions of larger plastic particles as seen under visual and stereo zoom microscopic examination. Fibers were major forms of MPs in Pärnu, Kuressaare and Sillamäe WWTP accounting for more than 35%, 55% and 70% contribution respectively while Tallinn and Haapsalu had less than 20% microfibers, but the samples were largely dominated with MPs fragments of heterogenous shapes. The treatment processes of the selected WWTPs exhibited varying retention capacities dealing with MPs pollution because they were not originally designed to capture MPs pollutants as it is observed today [21].The results of this study are in accordance with the previous studies done in and near WWTPs. It was observed that MPs particles and fibers were dominant microplastic-types in effluent of different WWTPs [16, 52, 58]. A team of Finish researcher reported that about 79% of the total amount of MPs samples collected were polyester fibers [30]. Relatively high concentration of fibers dominated MPs samples collected in surface water near WWTPs in Denmark [1] and Sweden [45] respectively.

Fig. 6.

The occurrence of microplastics type in the effluent of WWTPs

Chemical composition of MPs

The chemical characterization of MPs by $\mu$ FTIR spectroscopy was accurate in predicting exact polymer content of individual particles material analysed. The polymer-types sampled from wastewater effluent of Kuressaare, Haapsalu, Pärnu and Tallinn WWTPs is presented in Fig. 7. Unfortunately, chemical analysis for Silllamäe WWTP were unsuccessful due to equipment failure, and it is therefore excluded from our results. PE including LDPE and HDPE was dominant polymer component in all WWTPs, but much higher in Tallinn and Kuressaare representing about 39% and 52% of the total polymer component respectively. However, PP component was the second largest most common polymer in all the examined WWTPs, the concentration was at peak 34% in Pärnu WWTP followed by Kuressaare, and Haapsalu WWTPs, approximately 22% and 24% respectively. This result in agreement with earlier study that considered PP, PE, and PET as dominant polymers in WWTPs [49].Interestingly to note that the polymer distributions observed in Tallinn WWTP effluent sample being the biggest object were evenly distributed, except that PA and PBT components were significantly low. The distribution follows decreasing order PS < PP < PE < HDPE < PMMA < LDPE < EVA < PBT < PA < PAN < PET.Overall, the polymer components distribution in other WWTPs followed relatively similar decreasing order. Pärnu WWTP: PP < PET < PE < PAN < PS < PA < HDPE < LDPE < PMMA < EVA < PBT,Kuressaare WWTP: PE < PP < HDPE < PET < PAN < PS < LDPE < PMMA < EVA < PBT < PA and Haapsalu WWTP: PE < PP < PAN < PMMA < PET < PA < HDPE < PS < LDPE < EVA < PBT. It was observed that PP, PE, and PA were emitted largely from laundry-wash discharge, while PCPs remain the major contributor of PE.The results obtained in this study is in tandem with Lv et al. [35] wastewater analysis results who detected varying concentration of microplastics polymers 47% PET, 20% PS, 18% PE and 15% PP while Koelmans et al. [27] concluded that PE (25%), PET (16.5%) and PP (14%) are the most frequently found MPs polymer types in the aquatic environment. The abundance of these common MPs polymer types in the environment is tied to their production volume. Research studies confirmed PE, PP, PVC, PS, and PET as the sixth most produced polymers globally [17, 28, 55].

Fig. 7.

The distribution in percentage between microplastic types in the effluent of four WWTPs. EVA = ethylene–vinyl acetate, PA = polyamide, PE = polyethylene, PLA = polylactic acid, PP = polypropylene, PS = polystyrene, PMMA = Poly (methyl methacrylate)

Comparison with measured MPs data in effluent stream

Based on MPs chemical characterization results, we extracted, counted, and convert the numbers of polyethylene (PE) in samples collected from WWTPs to mass. PE is the major MPs-type added to PCPs [43, 49]. Also, the microfibers from LW are of an average length 0.2 mm, 0.6 mm and 3 mm from three size groups were counted and converted to mass. The total MPs from PCPs and LW per cubic meter were compared with the predicted effluent load in kilogram per year as shown in Fig. 8. The measured MPs concentrations are otherwise expressed by mass instead of number of particles or number of fibers per cubic meter using corresponding particle’s volume and density to make data immediately understandable.

Fig. 8.

Comparison between predicted and measured MPs-type (LW and PCPs) load in WWTPs effluent stream

It was assumed that the particle size will take on the filter sieve diameter deployed during sampling d_sieve (µm), for the estimation of the particle volume. The PE density ρ (g/cm³). of microplastics components identified by µFTIR spectroscopy were used. For the microfiber conversion, a 5 mm fiber length has corresponding weigh of 0.15 mg/fiber [49]. Therefore, the weight of the fibers in this study were extrapolated with the available data in literature.

The results in Fig. 8 shows that, the measured MPs concentration were within the predicted range in all investigated WWTPs. Tallinn treatment plant releases medium–high load of MPs to the environment annually, while other emits between average low and medium MPs loads relevant to LW and PCPs sources. Other studies reported that WWTPs release considerable number of MPs to the waterbodies despite relatively high removal efficiency [6].

The estimations were based on Estonia, and neighbouring Nordic countries data on the volume of PCPs use, MPs contents in PCPs, market penetration, washing habit, population density and wastewater flowrate. These data vary globally with individual country specifications, we believe that model estimation approach followed in this study may be relevant worldwide. Hence, future study will improve the model to accommodate broader categories of both primary and secondary MPs relevant to wastewater and will increase the predicted MPs concentrations in the influent and effluent of WWTP.

Efforts has been made to understand the removal of microplastic particles and microlitters in wastewater globally, but none is done in Estonia yet. The potential contribution to marine environment from WWTPs in Estonia also missing. Liu et al. [34] reported more microfibers from laundry wash constitute major type of MPs in wastewater and yet remains difficult to remove during conventional treatment processes.

Inability of the conventional WWTPs to eliminate microplastics pollution is a concern and urgent call to wastewater service providers to rise to the challenge by supporting the development of end-of-pipe technologies to capture MPs fragments and synthetic fibres in WWTP.

Conclusions

A mathematical model was developed to estimate the release of microplastics from PCPs and LW considering the different factors that influence the release of microplastics, such as the type and concentration of microplastics in the products, the wastewater flow rate and removal efficiency in wastewater treatment plant. Microfibers released during domestic washing are evasive, with poor retention in wastewater treatments process steps and subsequently dominate MPs samples. In this study we quantify MPs release from LW and PCPs by the population of household connected to WWTPs in major cities in Estonia. The total estimated MPs load per capita from PCPs and LW in Estonia were estimated to be between 4.25 – 12 tons/year, 3.52 – 11.24 tons / year respectively, and estimated load relevant to wastewater were between 700 – 30,000 kg/yr. and 2 – 1500 kg/yr. in WWTPs influent and effluent stream respectively. The most dominant group of microfibers were short length 0.2– 0.6 mm, which accounted for more than 75% of total microplastics at sampling point in four Estonian coastal WWTPs. The results avail us a good overall picture and understanding of the need to intensify our investigation and broaden this line of research study, and in the same way for water service provider in Estonia to upgrade their treatment units to enable them to capture these MPs-types.

Author contribution

Conceptualization, visualization, writing—original draft was done by Ayankoya Yemi Ayankunle, & Natalja Buhhalko. Supervision and writing—reviewing was done by Karin. Pachel, & Erki Lember. Supervision and validation were done by Vallo Kõrgmaa, Arun Mishra & Kati Lind. Data curation and formal analysis were performed by Ayankoya Yemi Ayankunle, Natalja Buhhalko. & Vallo Kõrgmaa. All authors read and approved the final manuscript.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.