Characterization of microplastics in wastewater
Lina Sabienski
Supervisor: Anna Rotander 2020-06-06
List of content
Abstract 2
1 Introduction 3
1.1 Background 3
1.2 Aim and limitations of the study 4
1.3 Description of the selected WWTP 5
2 Methods 6
2.1 Sampling 6
2.2 Analysis of 300 μm filter 8
2.3 Analysis of 50 μm filter 8
2.4 Visual characterization of microplastics 9
2.5 FTIR analysis 11
2.6 Quality assurance & contamination control 11
3 Results 12
3.1 Amount of microplastics per m3 sampled water 12
3.2 Distribution of different microplastic types in wastewater effluent 12
3.3 Fibers 13
3.4 FTIR analyses of microplastic particles 14
4 Discussion 15
4.1 General discussion 15
4.2 Reflections on the applied method 16
4.2.1 Sampling 16
4.2.2 Analysis 17
5 Conclusions and recommendations for the future 18
6 References 19
2 Abstract
This study aims to detect how many microplastics and what kind are released from the wastewater treatment plant (WWTP) Skebäck, in Örebro. The study was limited to the analysis of three filters with 50 μm mesh size and one filter with 300 μm mesh size. The samples were taken at different times, two in the fall of 2019 and one in the spring of 2020. Visual characterization was used for the quantification of microplastics, and a lower and upper bound was used. The lower bound represents particles that were deemed identifiable as plastic with high certainty, while the upper bound also includes particles that may have been microplastic. An additional ATR-FTIR analysis was performed on selected microplastics >300 μm.
The presence of microplastics in the effluent from Skebäcks WWTP could be confirmed. The quantity of microplastics per m3 (MP/m3) trapped on the 50 μm filters were quantified in a range
between 0 MP/m3 to 291 MP/m3 for the lower bound, and 72 MP/m 3 to 435 MP/m3 for the upper
bound. The 300 μm filter had considerably less microplastics than the 50 μm filter with 1.8 MP/m3.
The quantification of fibers on the 50 μm filter and 300 μm filters was not possible due to high blank contaminations. According to the concentration of 63 MP/m3 of the lower bound count on the
50 μm filters and the amount of water flowing through Skebäck in 2019, 17 818 935 m3, 1.1 billion
microplastic particles were released into Svartån that year. In comparison the highest value of the upper bound count, 435 MP/m3, gave a release of 7.7 billion microplastic particles. Using the
concentration of the 300 μm filter 1.8 MP/m3, 32 million microplastics/year were released from
Skebäck in 2019. The amount of spheres 50-300 μm released in the effluent from Skebäck was estimated to be 3.7 kg in 2019.
Keywords: ATR-FTIR spectroscopy, Microplastic, Plastic pollution, Visual characterization, Wastewater effluent, Wastewater treatment plant (WWTP)
1 Introduction
1.1 Background
Since plastics were first produced in the 1960s a certain amount of plastics have been entering the marine environment. However, the consumption of plastic continues to increase and so do the quantities entering the environment, making plastics one of the most common and persistent pollutants in ocean waters and beaches worldwide1. 359 million tonnes of plastic were produced in
2019 worldwide, with merely 29 million tonnes (0.8%) recollected from consumers2. Despite the
widespread recognition of the problem, it is one that is still growing and even if stopped immediately will persist for centuries.
Although reports on findings of plastic pellets in the sea were published as early as the 1970s the debate only really took off with the article by Thompson published 2004, where data on
microplastics in zooplankton from the Atlantic were presented3, 4, 5. Now, microlitter, in particular
microplastics <5 mm, have been internationally recognized as a serious problem to the marine environment and the Marine Strategy Framework Directive (MSFD) requires EU Member States to ensure that, by 2020, "properties and quantities of marine litter do not cause harm to the coastal and marine environment"6. Plans for reducing the amount of marine litter in the Baltic Sea and
North-East Atlantic are also being developed within HELCOM and OSPAR7, 8.
The past few years the research on microplastics has grown significantly, with on-going research on microplastics in water and the effect on biota as well as on methods for detecting, analyzing and monitoring microplastics9. Wastewater treatment plants (WWTPs) have been researched
extensively as a possible source of microplastics into the waters9. The WWTPs receive wastewater
from households, institutions, commercial establishments and industries, and sometimes also rainwater run-off from urban areas which leads to the accumulation of anthropogenic particles, including microplastic. The treatment of wastewater focuses mainly on the elimination of large objects and reducing the concentration of nutrients and organic material through physical, chemical and biological processes. With the addition of a disc filter or Membrane Bio Reactor (MBR) before the effluent water is discharged it increases the removal of microplastic particles in wastewater from 90% to 99.9%10. However, many Swedish WWTPs lack this kind of final treatment of wastewater11.
Domestic wastewater is a significant pathway of microfibres to the WWTPs which are the most commonly found microplastic in aquatic and terrestrial environments12. Personal care products such
as facial scrubs and toothpaste and cleaning agents contain granulated polyethylene (PE),
polypropylene (PP) and polystyrene (PS) particles, which have replaced for instance walnut shells as scrubbing agents and can be found in WWTPs13, 14. Plastic fragments, microbeads and a high
proportion of microfibers are also found in wastewater effluents14, 15, 16.
WWTPs may not be the biggest source of microplastics to the environment, however, the potential for microplastics to act as a contaminant vectors in the environment and to aquatic organisms is considerably more significant in WWTPs than for example atmospheric deposition and
4 organic pollutants such as PBDEs, PCBs, DDE and nonylphenols (NP) as well as heavy metals to the plastic surface17, 18, 19. In addition microplastic may leach toxic plastic additives such as
brominated flame retardants, bisphenol A and phthalates20. This may be of great concern as
wastewater can contain relatively high concentrations of harmful compounds from e.g. household products, cleaning and personal care products as well as pharmaceuticals21, 22. The Baltic Sea
countries such as Denmark, Finland and Poland have started processes to restrict the use of intentionally added microplastics in different products, however Sweden is the only Baltic Sea country that currently has a law concerning “the placing of cosmetic products that are intended to be rinsed off and contain microplastics on the market”. Sweden is also compiling specific guidelines to minimize emissions of microplastics from industrial production.
Although wastewater treatment systems are not designed to specifically metabolise plastic materials the microplastic particles may be removed through the screening, sedimentation, flotation,
coagulation-flocculation and filtration processes. Most of the plastics are settled in the primary sedimentation process, and the efficiency in removing them becomes higher at the secondary and tertiary levels with more advanced treatment processes16, 10, 23. Based on the results from different
studies, the capability of advanced WWTP using tertiary treatment technologies to capture
microplastics and fibers is as high as 95-99%9, 13, 24, 25. However, the retained microplastics are still
present in the sludge which is used as fertilizer on the fields. WWTPs are still considered to be important sources of microplastics to the environment due to the large volumes of treated wastewater constantly being released and the sludge used as fertilizer13.
1.2 Aim and limitations of the study
The overall aim of the study was to investigate how much microplastics Skebäck’s sewage
treatment plant in Örebro discharges per m3 (MP/m3) treated wastewater. The distribution of fibers
and microplastics such as filaments, fragments, films, foam, particles and spheres was also of interest. The study was limited to three 50 μm mesh sized filters and one 300 μm mesh size filter, which were used to sample wastewater effluent at different times, two in the fall of 2019 and one in the spring of 2020.
1.3 Description of the selected WWTP
The WWTP in this project was Skebäck in Örebro, which takes in wastewater from approximately 140 000 persons from households/industries with a total capacity for 220 000 persons. The
wastewater is treated in three steps; mechanically through skimming and settling processes, biologically through degradation with microorganisms and chemically with precipitants which get rid of as much phosphor as possible. The effluent water is discharged into Svartån which has its outlet in Hjälmaren, see figure 1. On average the WWTP can handle 45 000 m3 wastewater/day,
with a total capacity of 90 000 m3 wastewater/day26.
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2 Methods
2.1 Sampling
All samples were collected at the same location, about 200 m away from the outlet of the WWTP, see figure 2 and figure 3. For this project filters made of stainless steel, with mesh sizes 300 μm and 50 μm were used. Prior to sampling the filters were cleaned in an ultrasonic water bath for one hour and thereafter rinsed with MQ water. The samples were collected at different times and with
different sampling durations as described in table 1. The pump used for collecting the samples was made up of a Grundfos submersible Unilift KP 350 (0.7 kW/230V) with a mechanical flowmeter which has an uncertainty of ± 1%. Its maximum volume is 4 500 L/h and has a filter stack with positions for 3 filters. The pump was placed on the bottom with the water inlet approximately 25 cm below the water surface as seen in figure 2.
A field blank was done for sample P3 by putting a 300 and 50 filter into the pump but without submerging it under water. The filters were stored in lidded metal jars dressed with aluminum foil and stored at 6 °C before analysis.
Table 1. Description of the sampling time, filters and volume.
Date Sample ID Mesh size Sampling duration Volume water filtered WWTP water flow
25/9 - 2019 P1 50 μm 20 min 800 L 14 m3/min
3/10 - 2019 P2 50 μm 15 min 346 L 12 m3/min 22/4 - 2020 P3 50 μm 10 min 657 L 25 m3/min 22/4 - 2020 P3 300 μm 30 min 2 227 L 25 m3/min
Figure 2. The red dot represents the sampling location situated 220 meters away from the outlet of the WWTP, which is represented by the blue dot on the map.
Figure 3. Sampling site 220 meters away from the WWTP. The pump samples the water which comes around the corner as depicted by the red line.
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2.2 Analysis of 300 μm filter
The 300 µm filters could be analyzed directly under a light microscope as not much organic material was present. The light microscope used was a Zeiss Stemi 508 microscope, mounted with an Axiocam ERc. Anthropogenic particles were identified using shape, texture and color. The identified microplastics were then transferred from the 300 µm filter into two 30 mm petri dishes. One petri dish was only for fibers, and the second petri dish for fragments, particles and films. Afterwards, a few drops of 30% hydrogen peroxide (H2O2) were added onto the particles and fibers
in the petri dishes to remove any organic material. They were then put into an oven at 50oC. This
would also tell if there were any misidentified microplastics.
The petri dishes were weighed before and after the analysis to see if the microplastics were enough to be quantified by weight. Any particles >300 µm were analysed with ATR-FTIR to verify the visual analysis and determine what type of polymer the plastic is.
2.3 Analysis of 50 μm filter
The material on the 50 µm filters was carefully rinsed off into glass jars with MQ water (deionized water) and a spatula while inside a fume hood. A spray bottle with MQ water was used to soften up the material while also rinsing off anything that should have gone through the filter. Everything that went through the filter into the petri dish was discarded. Two spatulas were used, one for scraping off the material from the filter and one for cleaning the first spatula. This was done until no more material could be scraped off.
30% Hydrogen peroxide was poured into a 50 mL falcon tube. Using a disposable glass pipette hydrogen peroxide was dropped into the glass jars and onto the spatulas to clean them as well as possible of any remaining residue. The glass jars stayed in the fume hood for an hour to ensure no violent reaction happened with the organic material before adding the rest of the hydrogen peroxide. The glass jars were covered with aluminium foil and put into an oven at 50oC overnight. A
procedure blank was made for all samples by adding 50 mL hydrogen peroxide into a separate glass jar, undergoing the same steps as the sample.
The next day the glass jars were taken out of the oven and into the fume hood for filtering. A white membrane filter (PP, 10 µm pore size) was secured under the filtrating funnel before pouring the liquid from the glass jar into the filtrating funnel. The glass jar was rinsed twice with MQ water from a spray bottle as well as the walls of the filtrating funnel to get everything onto the membrane filter.
A cylindrical separation funnel was set up in a tripod and the filter was moved to the separation funnel with a tweezer. Using NaCl solution (1.2 g/mL) in a spray bottle and a spatula, the filter was scraped and rinsed clean into the separation funnel. The separation funnel was then filled with NaCl and left to separate in the fume hood with the opening covered with aluminum foil. After 4 hours
the valve was opened carefully and the lower part of the liquid and precipitate were collected into a 50 mL falcon tube and shaken well. The upper part of the liquid when collected into another falcon tube also had some particles left because a little precipitate was stuck on the sides of the funnel, see figure 4. Therefore both the upper and lower part of the first separation underwent a second density separation in two separate separation funnels for ~20 hours. The upper layer from the second separations were pipetted over into 50 mL falcon tubes with a glass pipette in order to avoid the precipitate on the sides. The lower part was discarded. The transferred liquid from both density separations was then filtered onto a grey membrane filter with grid pattern (cellulose nitrate, 0.8 µm pore size) and carefully placed with a tweezer into a 60 mm petri dish. It was then analyzed visually under a light microscope.
Figure 4. Separation funnel with the liquid from the upper layer after a second density separation. As shown some precipitate also gets stuck on the sides of the funnel.
2.4 Visual characterization of microplastics
To avoid misidentification of microplastics during the visual characterization, a particle was only counted as microplastic if it fulfilled at least two of the following criteria: 27
1. No cellular or organic structures are visible
2. Fibres are equally thick throughout their entire length and should not be tapered at the end.
3. Coloured particles are homogeneously coloured.
4. Fibres are not segmented, or appear as twisted flat ribbons.
5. Particles are not shiny.
The identified microplastics on the filters were categorized according to shape and grouped into six groups: Fragments, films, particles, filaments, foams and spheres. Fibers were in a category of their own.
10 Fragments are defined as irregular shaped particles with torn edges and have the appearance of being broken down from a larger piece. Films are flat and flexible particles with smooth or angular edges and thinner than fragments. Particles are three dimensional, round and irregular shaped particles and lack torn edges. Fibers and filaments are long fibrous materials that are equally thick throughout their entire length and should not be tapered at the ends. Filaments have substantially thicker diameters than fibers, which was evaluated visually from case to case. Foams are spongy fragments which deform under pressure and can be elastic depending on the weathering state. Spheres are hard particles with smooth, almost spherical in shape. See figure 5 for a visual representation.
As there was some uncertainty with visual characterization of such small particles an upper and lower bound was introduced. The lower bound represents the lowest amount of microplastics in the sample, as only particles that were deemed to be plastic with high certainty were included. The upper bound represents the highest amount of microplastics counted in the sample as even particles that were small and more uncertain were counted as plastic.
Ideally the size of microplastics found on the 50 µm filter should be between 50-300 µm while the 300 µm filter would catch everything between 300-5 000 µm. However, particles over 300 µm in size were found on the 50 µm filters and were still counted as part of the 50 µm filter as it would take a lot of time to measure every individual particle, which was not possible with the time constraints of this project. Therefore the results are presented as the amount of microplastics found on 50 µm filters and 300 µm filters respectively.
Figure 5. The measured microplastics in the left picture are a particle in the upper left corner (169.428 µm), a fragment on the left side (668.159 µm), a particle in the middle (427.533 µm) and in the right lower corner a fiber (598.751 µm). An additional unmeasured two particles and 3 fragments were counted in the left picture. The picture in the upper right corner depicts a sphere (222.622 µm). The picture in the lower right corner depicts a film (774.607 µm).
2.5 FTIR analysis
To verify the visual characterization as well as identify what kind of polymer the plastic particles were made of, individual particles with a size >300 µm were picked out for analysis with
ATR-FTIR. Attenuated total reflectance (ATR) is an accessory unit that can be used with Fourier transform infrared (FTIR) spectrometers. It makes it possible to measure directly onto a solid sample surface by pressing the sample towards an ATR crystal (diamond). This allows for analysis of small sample sizes and without sample preparation, which greatly speeds up sample analysis time. The analysed particles are exposed to infrared radiation from which a spectrum is obtained that has characteristic peaks for specific chemical bonds between atoms28. This makes it possible to
identify the exact composition of the particle and the spectrum can be compared to other spectrums with known composition. The characteristic peaks for plastic polymers are around 2 800-3 000 cm-1
and 1 400-1 500 cm-1. Water may show around 3 500 cm-1 and any organic material left is seen as a
peak around 1 000 cm-1. The analysed microplastics were selected based on size and how
commonly occurring they were in the samples. In this project a correlation of >95% with a spectrum in a library was accepted, however some exceptions were made for particles <95% because the characteristic peaks for plastic polymers were well matched with a reference spectrum. The spectra contained in the library included polyeten (PE), Polypropylene (PP), polystyrene (PS), polyamide (PA), polyethylene terephthalate (PET), polyvinyl chloride (PVC),
polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB) and polylactic acid (PLA) plastic polymers.
2.6 Quality assurance & contamination control
To avoid the risk of plastic particles being introduced to the samples during sample collection, preparation and analysis, the time the sample was exposed to the air was minimized as much as possible. The samples were stored in lidded metal jars and the openings of glass jars and separation funnels were covered with aluminium foil during the sample preparation. Glassware (glass jars, separation funnels) and stainless-steel equipment (pincette, spatula) was used during the sample processing. A laboratory coat and nitrile gloves were worn while working with the samples in an enclosed laboratory with at most three people in it.
One field blank for the 300 μm filter and three procedural blanks for each 50 μm filter were done to identify possible contamination. However, no recovery tests were done to assess the accuracy and reproducibility of this method.
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3 Results
3.1 Amount of microplastics per m
3sampled water
The amount of microplastics found on the 50 μm filters were blank corrected by subtracting the average of three procedure blanks, and a limit of detection (LOD) was calculated for each category of microplastics (mean of all blanks + 3 * standard deviation). The final concentrations of
microplastic per m3 effluent water for the 50 μm filters ranged from 0 MP/m3 to 291 MP/m3 for the
lower bound, and 72 MP/m3 to 435 MP/m3 for the upper bound, see table 2. However only the filter
of sample P1 had all concentrations above LOD, while sample P2 only had particles and spheres over the LOD value. The true amount of microplastics for the categories below LOD may be just below the detection limit or none at all. Therefore the lower bound was set as 0 and the LOD value as the upper bound for the categories of microplastics that were below LOD.
In the 300 μm filter only 3 out of 5 fragments were found after the hydrogen peroxide treatment, and 1 film. As the blank didn't go through a hydrogen peroxide treatment the suspected plastic fragment found in the blank was not deducted from the results as it was deemed too uncertain to be counted as a plastic fragment through visual characterization. The resulting concentration from the analysis of the 300 μm filter was 1.8 MP/m3.
The procedure blanks were contaminated with microplastic fragments with an average of 9-22 for lower and upper bound respectively, which gave a high LOD of 35 for fragments, see appendix 1 for more details.
Table 2. Results from visual characterization of fragments, films, particles, filaments and spheres in all 50
μm samples after blank correction (sample - average of all blanks) with lower and upper bound. LOD values are presented as the upper bound in the categories of microplastics that were below the LOD, with the lower bound being set at 0.
Sample ID
& Volume Fragment Film Particle Filament Sphere Total
Concentration (MP/m3) P1, 800 L 135-208 10-16 77-101 0-6.2 11-17 233-348 291-435 P2, 364 L 0-35 0-2.3 16-27 0-6.2 7-9 23-79 63-217 P3, 657 L 0-35 0-2.3 0-3 0-6.2 0-0 0-47 0-72
3.2 Distribution of different microplastic types in wastewater effluent
Because all types of microplastics were below LOD in sample P3 it was not included in figure 6, which represents the compositions of sample P1 and P2 with the lower and upper bound. The distribution of microplastics in sample P1 consisted of ~60% of fragments and ~30% particles, with 7 times more particles than spheres. In sample P2 the amount of particles was 2 times as big as the amount of spheres. Foam particles were absent in all samples, both 50 μm and 300 μm filter
samples. Fragments, films and filaments were below the LOD, with filaments and films being too few in numbers and fragments being too high in the blank giving a high LOD value. However, spheres were found in P1 and P2 and were absent in all procedure blanks. Using the average diameter of four spheres found on the 50 μm filter of P1, 298.308 μm, and the density of the most common plastic polymer polyeten, 0.91 g/cm3, the total load of spheres released with the effluent
from Skebäck during 2019 was calculated to 3.7 kg in Örebro.
Figure 6. The graphs depict the sample compositions of sample P1 and P2 of the 50 μm filters in quantity as well as percentage after blank correction. The lower bound represents the minimum amount of plastic in the sample. The upper bound represents the highest possible amount of plastic in the sample. The values below LOD are represented as 0 for the lower bound, and the LOD value for the upper bound.
3.3 Fibers
The high amount of fibers found in the procedure blanks, average of 92 fibers, and the negative values obtained after blank correction made it impossible to use any data from the 50 μm filters regarding fibers. The total amount of fibers on the 50 μm filters was therefore <LOD. The fibers in the 300 μm filter were also fewer than the corresponding blank and had to be labeled <LOD. The fact that merely 6 fibers were left after the hydrogen peroxide treatment from an initial 49 fibers indicated that only 10% of all fibers that were counted were synthetic fibers. However this value is only based on one sample.
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3.4 FTIR analyses of microplastic particles
From the 9 analyzed particles with ATR-FTIR, 6 spectra had a high match with a spectrum from the reference library. Although the other 3 particles did not have a high match, they still had
characteristic peaks at 2 800 cm-1 and 1 400-1 500 cm-1 to be identified with high certainty as 2 PP
polymers and 1 PE polymer. As seen in figure 7, there was too much interference in the fingerprint region of the spectrum around 1 000 cm-1 to obtain a good library match, however the characteristic
peaks for PP polymers at 2 800 cm-1 and 1 400-1 500 cm-1 were deemed enough to identify the
particle as a PP plastic polymer. The spectral interference could be due to leftover organic matter (~1 000 cm-1) and water (~3 500 cm-1) as well as the small size of the particles which results in a
weak signal. The result was that 7 of the 9 particles (78%) were PE polymers and 2 were identified as PP polymers. All analyses from the FTIR are summarized in table 3.
Figure 7. FTIR spectrum for a transparent film found in the 300 µm filter of sample P3. 90% correlation with PP.
Table 3. Results of the ATR-FTIR analysis.
Description of the particle Sample & filter Matching percentage with spectra in the reference library
Transparent fragment P1, 50 µm filter 96% PE
Grey fragment P1, 50 µm filter 74% PP
Transparent film P1, 50 µm filter 95% PE
Transparent particle P1, 50 µm filter 96% PE Transparent fragment P1, 50 µm filter 96% PE Transparent fragment P1, 50 µm filter 96% PE
Black particle P2, 50 µm filter 65% PE
Transparent film P3, 300 µm filter 90% PP
4 Discussion
4.1 General discussion
The presence of microplastics in the effluent from Skebäcks WWTP could be confirmed. However, the concentrations for the 50 μm samples were in a wide range between 0 MP/m3 to 291MP/m3 for
the lower bound, and 72 MP/m3 to 435 MP/m3 for the upper bound. This is in part due to the high
LOD value for fragments as well as the low amount of microplastics found in sample P2 and P3, resulting in most of the counted categories of microplastics to be below LOD. A possible reason for the difference in microplastic concentrations in the samples could be the water flow, which was 12 m3/min, 14 m3/min and 25 m3/min respectively for samples 1-3. Another difference is the time of
sampling, which for sample 1 and 2 was 8:30 in the morning (25/9 and 3/10-2019) but 10:00 for sample 3 (22/4-2019). After consulting an environmental engineer at Skebäck29 it was found that
the average time needed for water to go through the WWTP is approximately 24 hours. Depending on the water flow it can be a little faster, which makes it difficult to draw conclusions based only on the three sampling occasions. Previous studies also reported a variation of microplastic discharges in tertiary wastewater effluent taken from the same sample sites, from 1000 MP/m3 to 250 MP/m3
or 250 (±40) MP/m3 using 20 μm filters30, 24. However it is difficult to directly compare the results
of these studies because of the different methods used, including mesh size, sample volume and sample preparation. In addition sample 3 was taken during the corona pandemic which could have resulted in a lower microplastic concentration in the wastewater because of the lowered business activity from laundries, hotels and restaurants.
The 300 μm filter had considerably less microplastics than the 50 μm samples with only 3 fragments and 1 film, giving a concentration of 1.8 MP/m3. A previous study done with 300 μm
filters in Sweden found that the WWTPs, Henriksdal in Stockholm had 15-80 MP/m3 in the effluent
water, Ryaverket in Göteborg 0-10 MP/m3 and Långevik in Lysekil 10-40 MP/m330. However, a
different study which was done on the WWTP Långevik in Lysekil, showed that the further away from the outlet the water was sampled, the less microplastics were caught11. 200 m away from the
outlet no particles were detected, while fibers were still found. This could be a reason for the differences in results from this study compared to others as the samples were taken 220 m away from the outlet.
Another study that sampled using 300 μm filters, taken on the same occasion as the 50 μm filters of the P1 and P2 samples in the fall of 2019, had concentrations of 32 MP/m3 and 78 MP/m3
respectively31. This gives a rather large range of 1.8 MP/m3 to 78 MP/m3, with the only obvious
differences being the time and date of sampling as well as the variation in water flow. However because there were limited samples and no data on how much microplastic was coming in with the influent water at the sampling occasions, no firm conclusions could be made.
Although not many particles were analyzed with FTIR, PE was the predominant type of plastic found in the samples, which also is in agreement with the results of other studies11, 32.
16 The amount of wastewater flowing through Skebäck in 2019 was 17 818 935 m3 which means that
according to the concentration of 63 MP/m3 from the lower bound count in the 50 μm filters 1.1
billion microplastic particles were released into Svartån during that year. In comparison the highest value of the upper bound count in 50 μm filters gives a release of 7.7 billion microplastic particles from Skebäck during 2019.
Using the concentration of the 300 μm filter 1.8 MP/m3, 32 million microplastics/year or 3 664
MP/hour are released from Skebäck. Compared to other WWTPs in Sweden such as Hendriksdal in Stockholm (879 725 MP/hour), Ryaverket in Göteborg (103 142 MP/hour) and Långviken in Lysekil (13 650 MP/hour), the hourly release of microplastics from Skebäck was much lower30.
However as mentioned before, the 300 μm filters analysed in another study but taken on the same occasion as the 50 μm filters of the P1 and P2 samples in the fall of 2019, had concentrations of 32 MP/m3 and 78 MP/m3 which would give an hourly release of 65 092 MP/hour and 158 662
MP/hour. The value of the 300 μm filter from sample P3 is therefore considered to be on the lower end of the range. The variations between different WWTPs are to be expected taking into
consideration the different factors mentioned above as well as the variability in treatment processes and populations connected to the WWTPs.
The major entrance for microplastic spheres from personal care products to the marine environment is via wastewater from households and facilities where people wash themselves. However, it is difficult to get a complete overview on what personal care products contain microplastics spheres, the amount of plastic these products contain and what volumes that are being consumed. Since 1 january 2019, Sweden passed a ban against microplastics in cosmetic products that are rinsed off or spat out. The definition covers, for example, body scrubs, shower soap, shampoo/conditioner and toothpaste, but not for example, make-up, skin cream and suntan lotion. A study done in Stockholm with data from 2015 estimated that the amount of microplastics released to the wastewater, from products which are not covered by the ban from 2019, is 0.99 – 2.05 ton per year33. This number is
based on the scenario in which all of the microplastic contained in the products end up in the wastewater. According to the data from this study an estimate of 3.7 kg microplastic spheres were released from Skebäck in 2019. If 99% of the spheres were retained in the WWTP it is estimated that a total of 370 kg (0.41 tonnes) of plastic from cosmetic products was present in the incoming wastewater from Örebro. This shows that microplastic spheres are still present in the wastewater after the ban on microplastics in personal care products.
4.2 Reflections on the applied method
4.2.1 Sampling
The filter size of 300 μm was chosen because the first findings of microplastics in the sea that received attention were in zooplankton samples collected with trawl nets with a mesh size of 330 μm. To use 300 µm filters when sampling microplastics in sea water has become a common
be able to compare the results with other studies34. As microplastics are defined as plastic particles
<5 mm a 50 μm filter was used because it is still big enough to allow for visual identification through an optic microscope, which has a lower limit of 20 μm30.
4.2.2 Analysis
There are many ways in which surface water samples could be handled depending on what kind of microplastics are of interest. In this project NaCl (1.2 g/cm3) was used for the density separation as
it is recommended by the Marine Strategy Framework Directive (MSFD) Technical Subgroup on Marine Litter and widely used among scientists35-38. Although its recovery is rather low compared to
other brine solutions, being between 70 - 90%, it is an inexpensive and non hazardous solution39, 40.
It is adequate for extracting low density plastics, however it doesn't lift denser plastics such as PVC (1.3-1.45 g/cm3) and PET (1.38 g/cm3), commonly used in textiles and to produce plastic bottles,
which could result in an underestimation of the abundance of plastics found, particularly high density plastics14, 41. However lower density polymers, such as PP and PE, dominate most surface
water samples (25% and 42%, respectively)42, which is why it was deemed sufficient to use NaCl
instead of other brine solutions in this project where only surface water was sampled.
The use of density separation and hydrogen peroxide treatment lowers the uncertainty of the method by removing non plastic particles from the sample. However, it may contribute to an
underestimation of microplastics as some loss throughout the method could increase. As no recovery test was done it is not known how many particles were lost during sample preparation. A possible loss could have occurred during the removal of the upper layer of the liquid after the density separation, as the sides of the funnel were not rinsed with water. On the other hand
microplastic particles may have been introduced through the use of a white membrane filter (PP, 10 µm pore size) for filtering the samples as it may have contributed to the presence of white
fragments in the procedure blanks and samples. The blanks were subtracted from the samples - this should reduce the risk of overestimating the number of microplastics. However, because of high contaminations of fragments and fibers in the procedure blanks it increased the LOD and lowered the sensitivity of the method. It is therefore recommended to use metal filters or other non plastic membrane filters in future studies to decrease additional contamination. In addition visual
characterisation may lead to either an overestimation or underestimation of microplastic particles depending on how accurately microplastic particles were identified. The smaller the particles are, the higher the uncertainty with visual characterization because it becomes hard to perform tactile inspection of particles, leaving sight as the primary identification tool. Although visual
identification is a common tool for characterising microplastics it appeared insufficient for telling synthetic fibers and textile/organic fibers apart, as merely 6 of the 49 suspected fibers found in the 300 μm filter were left after the peroxide treatment. This indicates that only 10% of all fibers that were counted were synthetic fibers. However, this number is only based on the results of one filter. These factors add an uncertainty to the results of the 50 μm samples.
18
5 Conclusions and recommendations for the future
In this study the presence of microplastic was confirmed in the effluent from the WWTP Skebäck in Örebro, which indicates that the WWTP is contributing to the content of microplastics in surface water. However, it has to be remembered that WWTPs act not only as sources but also as sinks for microplastics, as they retain >90% of the microplastics contained in the wastewater through the cleaning processes. It is also not yet possible to estimate the relative importance of WWTPs compared to other sources of microplastics into the water as few studies have been done on the quantitative input of microplastics from other sources. In the report from IVL from 2016 the contribution of microplastics from WWTPs was estimated to be 4.7-42 tonnes per year in Sweden. However depending on which amount is closer to the true value, the lower amount (4.7 tonnes per year) or the larger amount (42 tonnes per year), the amount of microplastics from WWTPs is minor, respectively major compared to other Swedish sources13.
This study is very limited in sample size (3 samples), and although the samples were taken in different time periods, fall and spring, they are too limited in quantity to be representative of these periods. In addition the low concentration of microplastic in the samples and high LOD values suggest that bigger sample volumes should be taken to raise the amount of microplastics in the samples or optimising the method to lower the blank contamination while raising the sensitivity. This study can only be a guideline for further research, and these results should be validated and repeated by replicates taken at different times and seasons. Only then can reliable concentrations and yearly microplastic discharges of this WWTP be presented.
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7 Appendix
Appendix 1. Visual characterization of fragments, films, particles, filaments, foam and spheres in all
procedure blanks for 50 μm samples with lower and upper bound. The upper bound is represented in parentheses. Average, standard deviation (STD) and limit of detection (LOD) are presented for each category.
Sample Fragment Film Particles Filament Sphere Total
Blank 1 15 (29) 1 2 - - 18 (50) Blank 2 22 (36) - 1 - - 23 (37) Blank 3 9 (19) 1 1 (2) 3 (4) - 14 (25) Average 15 (28) 0.6 1.3 (0.6) 1 (1.3) - 18 (37) STD 6.5 0.6 0.6 1.7 4.5 (13) LOD 35 2.3 3.0 6.2 32 (56)
