The fate of different types of microfibers in Amsterdam surface waters

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Bachelor Project Future Planet Studies

The fate of different types of microfibers in

Amsterdam surface waters

Author:

Emma Kok

Primary Supervisor:

Dr. A. Praetorius

Daily Supervisor : Dr. R.P.J. Hoondert

Word Count : 3923 words

Date: May 30, 2021

Place: Amsterdam

Future Planet Studies

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Abstract

This study uses a multimedia fate model to investigate how different types of microfibers (cotton, rayon and polyester) behave in the surface waters of Amsterdam. This is done by adjusting an existing model, suitable for microplastics (which have a more spherical shape than microfibers does). There is studied how fast these fibers settle, in which part of the river they settle and how far from the source they are found. Results show that the sinking behaviour of a small fibre can be complicated and that a different approach is needed than for other microplastics, besides that rayon has the highest settling velocity in comparison with cotton and polyester. Each type of these three microfibers tend to settle in the compartment of the flowing water. However, because of the larger size and the higher density of rayon, a small part settles in the sediments. These microfibers can flow approximately 12.5 km before the major part is settled. More research should be done in order to find out which microfiber is most harmful for the environment, nonetheless this study contributes to more accurate modelling of the fate of microfibers in surface waters.

Keywords

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1 Introduction

Textile is made over the entire globe, having different applications such as clothing, upholstery and carpeting (Napper et al., 2020). The demand of textile has increased rapidly over the past 60 years, due to the changing consumption patterns of clothing (Dahlbo et al., 2017; Saratale et al., 2020). In order to keep up with the fast changing fashion, the global textile fibre productions has expanded from 8.5 Megaton in 2013 to an estimated 130 Megaton in 2025. (Dahlbo et al., 2017; Napper et al., 2020).

Textile products release microplastics, which are plastic particles, fragment or fibers smaller than 5mm (Frias and Nash, 2019). According to Belzagui et al. (2020) the sources of microplastics are distinguished in two groups: primary microplastics and secondary microplastics. Primary microplastics are those emitted in the environment in microplastic size range. Secondary microplastics are microplastics originating from the degradation or fragmentation of larger plastic items. Primary microplastics include a wide range of sources, where microfibers is the most abundant type of microplastics found in the environment (Belzagui et al., 2020; Yang et al., 2019). Microfibers are defined as: “a threadlike piece of plastic with a length between 100 µm and 5 mm and a width of at least 1.5 orders of magnitude shorter than the length (Xu et al., 2018)”. The main distinction between microfibers and microplastics is the shape. Most types of microplastics have a more spherical shape, whereas microfibers have a more elongated shape (Wu et al., 2018). According to Boucher and Friot (2017) and Napper et al. (2020), 35 % of all primary microplastics are derived from the laundry of synthetic textiles. Napper et al. (2020) & De Falco et al. (2019) found that one average laundry of 6kg could produce over 700,000 microfibers. These microfibers can be transported out of the washing machines as wastewater. This wastewater can either flow directly to surface waters or it enters the surface waters through wastewater treatment plants (WWTPs) (Napper et al., 2020; Xu et al., 2018). Due to the small size of the fibers in the wastewater, part of these fibers can pass through the WWTPs (Belzagui et al., 2020; Napper et al., 2020).

Several studies registered the consequences of microfibers in aquatic environments (Almroth et al., 2018; Auta et al., 2017; Avio et al., 2020; Belzagui et al., 2020). The ingestion of microfibers by marine biota can cause both mechanical and physical harm (Auta et al., 2017). Besides that, research has shown microplastics are also found in products for human consumption, such as table salt, sea food and bottled water (Belzagui et al., 2020; Cauwenberghe and Janssen, 2014). However the exact risks still remain unclear and will need more research in the future.

Currently, few studies investigated the behaviour of microfibers in the aquatic environment and most of those studies investigate the behaviour of microfibers in seawater (Gago et al., 2018; Pedrotti et al., 2021). The behaviour of a microfibers in rivers and canals is still a subject which needs more research. Besides that, most modelling is done using microplastics and these models do not take the differences into account. As said before, the shape of microplastics and microfibers differ, therefore the behaviour of these particles in aquatic environments differ as well. For example, the speed at which a fiber and a plastic settle is a subject which needs more attention. This, in combination with the expected grow of emission of microfibers as a result of the growing proportion of synthetic fibers in our apparel (Almroth et al., 2018), shows that this subject needs more attention. In order to map the current situation with microfibers flowing through surface waters, it is important to investigate the fate of microfibers in surface waters. There will be studied at which speed the microfibers settle, where the fibers accumulate and how far the fibers travel from the source.

In this research three different types of microfibers will be compared: natural, synthetic and regenerated cellulosic microfibers (Liu et al., 2019). Table 1 shows the different types of microfibers and examples of that type.

Table 1: The different types of microfibers and their examples which will be compared in this research.

Type of microfiber

Explanation Examples Reference

Natural fibers plant and animal microfibers wool, cotton, flax and silk Browne et al. (2011); Liu et al.

(2019)

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1.1 Research questions

In order to come up with a good distinction to show how each type of microfiber behaves the surface waters, it is important to investigate how these different types of microfibers behave, in terms of how fast they settle and where they settle or accumulate. This will be done by answering the research question: How do different kind of microfibers released from textiles behave in the surface waters of Amsterdam? The sub questions which will be answered are:

1. How do the settling velocity from each type of microfiber differ from microplastics and from each other? 2. What is the most abundant type of microfiber in the North Sea Canal?

3. In which compartment of the North Sea Canal are microfibers likely to settle? 4. What is the travelling distance of different kind of microfibers from the source?

The city of Amsterdam was chosen because of the growing number of inhabitants and therefore the increasing washing events in a city (Amsterdam, 2021). Besides that, not a lot of research is done in the Netherlands for this topic. In order to contribute to a more integrated model for microfibers, it is important to study how these microfibers travel, where these microfibers end up and how far from the source they are found. There are several ways in which textile microfibers can enter the marine environment. Such a diffuse source can be people swimming in surface waters and wearing swim wear which is made of textile products (Carr, 2017). There is chosen to disregard these microfibers, because the microfibers derived from doing laundry is causing most pollution in surface waters (Carr, 2017). Therefore, the focus lays on microfibers derived from doing laundries.

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2 Methods

2.1 Location

This study modelled a simplified version of how different types of microfibers moved through the surface waters of Amsterdam. For this research one WWTP was chosen as a place where the different microfibers entered the surface water of Amsterdam. The WWTP where the fibers entered the water is: Waternet -RWZI Westpoort (Waternet, 2019). This is located in the West side of Amsterdam (see the red square in figure 2). The river which will be analysed is the North Sea Canal (outlined in light blue in figure 2).

Figure 1: The wastewater treatment plant which was chosen in this research is indicated by a red square. At this point the microfibers enter the environment. The fibers move through the North Sea canal which is

outlined in blue.

This research used an existing multimedia fate model, developed in the Nano2Plast project (Praeto-rius et al., 2019). Modelling was done in Python, using Spyder as editor. Until now the model is suitable for microplastics, which are, as said before, slightly different particles than microfibers. Therefore, several adjustments had to be made.

2.2 Model parametrization

The model is characterised as a multimedia model, meaning that in this case the North Sea Canal is divided into different boxes. All these boxes consists of four compartments: the surface of the river, the part where the water flows, the part where the water stagnates, and the sediments. The model consists of 21 boxes, each representing a horizontal section of the river with a length of 1000m. This corresponds with the length of the North Sea Canal, which is 21km (Rijkswaterstaat, 2021). Figure 2 shows the description of the multimedia fate and transport model. RS0, RS1 and RS2 are the names of the first three boxes. The red arrows represents the settling of the fibers and the blue arrows represents the rising of the fibers.

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Figure 2: A simplified figure of the model which is used in this study. The microfibers enter the environment at box RS0 at the water surface. One box is divided into four compartments: the water surface, the flowing water, the stagnant water and the sediments. In total the model consists of 21 boxes. The microfibers can settle (indicated by a red arrow), they can flow further to the next box or they can rise up again (indicated by

a blue arrow).

Each compartment has different characteristics. Table 2 shows the different characteristics of the first river section. These values were found using data from Praetorius et al. (2019) & Rijkswaterstaat (2021). This box was copied twenty times to represent the entire river, only the name differs (RS0 is the first box, RS1 the second, RS2 the third and so on). The G parameter is the shear rate of the water and stands for the turbulenty of the water (Praetorius et al., 2012). The T is the temperature in Kelvin, Vflow is the flow velocity in the water in m/s. SPM is the suspended matter content in the water.

Table 2: The different characteristics of the first box of the North Sea Canal. This box was copied twenty times to represent the entire river, only the name differs (RS0 is the first box, RS1 the second, RS2 the third

and so on) The G parameter is the shear rate of the water and stands for the turbulenty of the water (Praetorius et al., 2012). The T is the temperature in Kelvin, Vflow is the flow velocity in the water in m/s.

SPM is the suspended matter content in the water.

Name Compartment type Depth Length Volume Width G T Vflow SPM

RS0 Surface 0.1 1000 13500 270 10 284.45 0.45 30

RS0 Flowing water 0.1 1000 1834650 270 10 284.45 0.45 30

RS0 Stagnant Water 0.1 1000 203850 270 10 284.45 0 30

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Besides the characteristics of the water, the different properties of each type of microfiber were needed. Table 3 shows the different properties of a the different types of microfibers. The values were retrieved using data from Zambrano et al. (2019). The values that were changed for input values were the composition, density, the diameter and the length of the fibers.

Table 3: The different types of microfibers which were investigated in this study. Values are retrieved from (Zambrano et al., 2019).

Name Composition Density (kg/m3₎ _MPshape _{Diameter (µm)} _{Length ( µm)}

MF1 COT 1540 fiber 0.20 16.5 MF1 COT 1540 fiber 0.21 17.2 MF1 COT 1540 fiber 0.22 19.5 MF1 COT 1540 fiber 0.23 20.1 MF1 COT 1540 fiber 0.25 23.9 MF2 PE 980 fiber 0.20 13.5 MF2 PE 980 fiber 0.21 13.6 MF2 PE 980 fiber 0.23 13.9 MF2 PE 980 fiber 0.24 14.2 MF2 PE 980 fiber 0.25 14.9 MF3 RAY 1600 fiber 0.8 16.5 MF3 RAY 1600 fiber 0.81 16.7 MF3 RAY 1600 fiber 0.82 16.9 MF3 RAY 1600 fiber 0.84 17.2 MF3 RAY 1600 fiber 0.85 17.9

In order to see where microfibers settle, the model was run for a period of two years. This time span was chosen because microfibers travel with a low velocity. Besides that, the amount of fibers entering the North Sea Canal is 100 per minute. For follow up research there can be studied how much fibers enter the environment per time step. In this study the exact amounts are not necessary to see where the microfibers settle and how fast they travel.

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2.3 Settling velocity

This model is originally made for microplastics, which have a more spherical shape than microfibers have (Wu et al., 2018). Therefore the speed by which an elongated microfiber settles, differ. The initial formula, which is used for microplastics, is called the Stokes law and is seen in equation one.

wx = 2 9 · ρs− ρ v · g · radius 2 ₍₁₎ where:

wx = settling velocity of a microplastic [m/s2_]

v = kinematic viscosity of the water [m2_/s]

ρs = particles density [kg/m3]

ρ = density of the water [kg/m3]

g = gravitational acceleration on earth [m/s2]

The adjusted formula will be based on Bagaev et al. (2017) & Waldschl¨ager and Sch¨uttrumpf (2019). The formula which will be used is an expansion of the Stokes Law. The main difference between the two formulas is that the Stokes law uses the diameter of a microplastic, whereas the adjusted method uses both the length and the diameter of the microfiber.

Equation five shows the settling velocity of the fibers. Equation two, three and four show which variables are needed. shows which formula’s are used in order to give a more accurate approximation of the settling velocity of the fibers.

Re = w · d v (2) CSF = √ radius length · radius (3) Cd = (√ 4.7 Re +√CSF (4) wx = s 4 3· d Cd| ρs− ρ ρ |g (5) where:

Re = dimensionless quantity to determine if the particle flows laminar or turbulent w = settling velocity based on Bagaev et al. (2017) [mm/s]

d = shortest particle side (fiber diameter) [m] v = kinmatic viscosity of the water [m2/s]

CSF = dimensionless shape factor to determine the relative flatness of the particle radius = radius of the particle [m]

length = length of the particle [m]

Cd = dimensionless drag coefficient for microfibers ρs = particles density [kg/m3]

ρ = density of the water [kg/m3_]

g = gravitational acceleration on earth [m/s2_]

wx = settling velocity of a microfiber [m/s]

In order to study the behaviour of a microfiber, different ways how a microfiber can settle were considered. Firstly, there is looked at how the particle settles using the Stokes method, using the smallest side as the diameter. Secondly, there is studied how a fiber settles using the Stokes method, using the longest side as the diameter. After that, there is looked at how a fiber settles using the adjusted method, when it is horizontally orientated. This is done by using the longer side as the length and the smaller side as the radius. Lastly, there is studied at how a fiber settles vertically, using the longer side as the radius and the smaller side as the length. Every velocity is calculated with Python, using the smallest and the lowest diameter and length and 50 steps in between that. This is done in order to come up with a average velocity among the different sizes of fibers. The results show the mean velocities of each fiber. Besides that, a statistical test was done to see if there were significant differences between the two methods.

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3 Results

3.1 Settling velocities

First of all, there is studied how fast the fibers settle, using the Stokes method. As said before, the Stokes method is suitable for calculating the velocity of spherical microplastics, due to the fact only the diameter is needed. Figure 3 shows the settling velocity using the this method. The blue bars represent the velocity using the smallest side as the radius and the orange bars represent the velocity using the longest side as the radius.

Figure 3: The settling velocities per type of microfiber using the Stokes method. The blue bars show the Stokes method using the radius of the smallest side and the orange bars represent the settling velocity using

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After that, with the help of the adjusted method, the velocities were calculated. With this method, both the length and the width is taken into account. This is shown in figure 4, where the blue bars represent the velocity of a fiber when it is horizontally orientated and the orange bars represent the velocity when the fiber is vertically orientated. The difference between these two alignments in the formula is the horizontally orientated fiber uses the diameter as the smallest side and the length as the largest side. Therefore the model treats it as if it flows horizontally. The vertically orientated fiber uses the length as the smallest side and the diameter as the longest side.

Figure 4: The settling velocities per type of microfiber using the adjusted method. The blue bars represent the velocity of a fiber when it is horizontally orientated and the orange bars represent the velocity when the fiber is vertically orientated. When the fiber is horizontally orientated, the model uses the diameter as the smallest side and the length as the largest side. When the fiber is vertically orientated, the model uses the length as the

smallest side and the diameter as the longest side.

The figure which shows the comparison between the four different ways is shown in Appendix 6.2, figure 11. There is a significant difference found between the Stokes method using the smallest side as a radius and the adjusted method when it is horizontally orientated. Because the data was not normally distributed, a Mann-Whitney U test was done. The p value showed a value of 0.040, meaning there is a significant difference using the Stokes Method and the adjusted method horizontally orientated. For the rest of the research the horizontally orientated velocity is used, because this velocity is in line with other studies (Bagaev et al., 2017).

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3.2 Most abundant type of microfiber in the North Sea Canal

Figure 5 shows how the different types of microfibers move through the water bodies. The light blue line and the orange line represent the cotton microfibers with a diameter respectively of 0.22 µm and 0.23 µm and a length of respectively 17.2 µ and 19.5 µm. Furthermore the darker blue and the purple lines represent the polyester microfibers with a diameter of respectively 0.22 µm and 0.23 µm and a length of respectively 13.9 µ and 14.2 µm. Lastly, the green line shows the rayon with a diameter of 0.83 µm and a length of 16.9 µm. The simulation is run over a time period of two years and per minute 100 microfibers entered the environment. The y-axis represent the concentration of particles in number/m3 and the x-axis represent the days. The upper three graphs represent the compartment of the surface water, the second row shows the compartment of the flowing water, the third row represents the stagnant water compartment and the last row represents the compartment of the sediments. The distance of the source increase from the left to the right column. The entire graph can be found in Appendix 6.1, figure 10. The results of the simulation show that rayon is the most abundant type of microfiber in every compartment, regardless how far away from the source they are found.

Figure 5: The different types of microfibers in the different compartments. The light blue line and the orange line represents the cotton microfibers with a diameter respectively of 0.22 µm and 0.23 µm and a length of respectively of 17.2 µ and 19.5 µm. Furthermore the darker blue and the purple lines represent the polyester

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3.3 Different water compartments

Figure 6 shows how different types of microfibers are divided among the different compartments after a time period of two years. The orange bar represents the cotton microfibers, the yellow bar represents the polyester microfibers and the green bar represents the rayon microfibers.

Figure 6: The distribution of the different type of microfibers in the compartments after a time period of two years. The y-axis represent the percentage of the total abundance of the microfibers. The x-axis is divided into four compartments; surface water, the flowing water, the stagnant water and the sediment. The orange

bar represents the cotton microfibers, the yellow bar represents the polyester microfibers and the green bar represents the rayon microfibers. Most microfibers settle in the flowing water and a small part settles in the

compartment of the stagnant water. A small part of the microfibers of rayon settle in the sediments.

There is shown that cotton and polyester mainly settles in the compartment where the water is flowing. Besides that, a small part settles in compartment of the stagnant water. Rayon is the only one of these three type of microfibers that settles in the sediments1.

1_{It is important to mention that the zero implies that there are no microfibers found in the surface waters for example,}

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3.4 Distance to the source

Figure 7 shows how far cotton microfibers travel in the North Sea Canal after a time period of two years. On the y-axis the concentration of microfibers in kg/m3_{is shown. This concentration is the sum of all the cotton}

fibers that are found in every compartment (surface water, flowing water, stagnant water and in sediments). Figure 7 shows that a large part of the microfibers tends to settle in the first 12.5km from the source. The distance travelled by polyester and rayon microfibers was equal to the distance travelled by cotton microfibers. These figures can be seen in Appendix 6.3, figure 12 and 13.

0.0

2.5

5.0

7.5 10.0 12.5 15.0 17.5 20.0

Distance to the source (in km)

0.00

0.01

0.02

0.03

0.04

0.05

0.06 Concentration of microfibers (in kg/m^3)

Distance travelled by cotton microfibers after a time period of two years

Figure 7: This graph shows the concentration of cotton microfibers in the North sea canal after two years of being released by the wastewater treatment plant in Amsterdam West. This concentration is the sum of every concentration of cotton microfibers in every compartment (surface water, flowing water, stagnant water and

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4 Discussion

4.1 Analyzing results

This study adjusted the multimedia fate model of Praetorius et al. (2019). This model was made for microplastics, which have a different shape than microfibers. This study adjusted the settling velocity of the fibers and therefore contributes to a wider use of this environmental fate model.

First of all, the settling velocity of the microfibers is studied. When looking at the results of this study, the Stokes method fails to give a proper approximation of this velocity. The Stokes method is suitable for microplastics with a spherical shape. When calculating the settling velocity, only the diameter is needed. However, when interpreting a microfiber (with a more cylindrical shape) as a microplastic, it is seen as either a very small particle or a very large particle. Therefore, the adjusted method is needed, which takes both the length and the diameter into account. When using the adjusted method, the fiber settles faster when it is vertically orientated. This is also proven by Bagaev et al. (2017). Figure 7 shows sketches of forms of an 8mm long fibre in its free fall in a still water tank. The corresponding sinking velocity is in mm/s. Due to the hydrodynamic resistance of the flow, a vertically oriented fibre sinks faster than a horizontal orientated fibre (Bagaev et al., 2017).

Figure 8: Sketches of forms of an 8-mm-long fibre in different runs during its free fall in a still water tank. The corresponding mean sinking velocity is shown in mm/s. It is important to mention that these speeds are not representative for this study, because this study is not done in a still water tank, but in the North Sea

canal, where other factors (like shear rate and suspended matter rate) also play a role. This image is retrieved from the study of (Bagaev et al., 2017).

According to the study of Bagaev et al. (2017), the mean settling velocity of a microfiber is 6·10−6m/s. This research shows that the values of the settling velocity, calculated using the adjusted method, are more in line with this speed. For this study the horizontal alignment is most in line with the results. Looking at those results, there is shown that rayon has the highest settling velocity. This can be explained that rayon has the highest density and it is the largest of the three. Due to the higher density and the larger size, the microfibers are likely to settle faster (Dietrich, 1982). After that, cotton, which is four times as small as the rayon microfibers, has the fastest settling velocity. Lastly, polyester has the slowest settling velocity. This is in line with literature, because polyester has the lowest density and is the smallest, therefore, it settles slowest (Zhiyao et al., 2008).

Second of all, there is studied which type of microfiber is the most abundant type in the North Sea Canal. The results show that rayon is found to be the most abundant type in the North Sea Canal, regardless in which compartment it is found. This is also found in the study of Suaria et al. (2020), where of all synthetic fibers in oceanic waters 79.5% was classified as regenerated cellulosic microfibers.

Third of all, the place where the microfibers settle was studied. Most percent of all types of fibers settle in the compartment of the flowing water. For every microfiber a small part sinks further to stagnant water and an even smaller part of the microfibers of rayon also settle in the sediments. This can be explained by the higher density of the rayon, therefore it sinks easier to the sediments. Figure 6 shows that there is no percentage of microfibers in the surface waters, however by looking at figure 5, there can be seen there is still a small amount of microfibers in the surface waters. This number is close to zero, therefore figure 6 treats this number as 0.

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Lastly, there is looked at how far the microfibers travel from their source. There was no large difference found between the three types of microfibers and the distance to the source. Higher concentrations of microfibers are found close to the source until approximately 12.5 km from the input of these fibers to the water. Not a lot of research is carried out to investigate the travelling distance of the microfibers and the effect on the environment. This is a topic which will need further research in the future.

A possible solution to decrease the load of microfibers released to the environment could be placing devices to capture these microfibers during the washing cycle. Several research is done and is planned to study the most useful way of integrating this in every washing machine (McIlwraith et al., 2019; Napper et al., 2020). This method could help declining the input of microfibers in surface waters.

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4.2 Future improvements

The values for the diameter and the length of the microfibers are retrieved from literature. Zambrano et al. (2019) investigated which sizes of each type of microfiber were mostly released during washing events. For this study the diameter and the length of the microfiber is based on the most common type. However, one fiber could have slightly different sizes or could be longer or thicker than the ones which are used in this study. For this study, the most common shapes of microfibers were enough to give a realistic view how these types of microfibers behave in the North Sea Canal. For further research, a wider range of sizes and shapes can be implemented in the model.

Besides that the values of the sheer rate (G) and the organic matter content (SPM) are based on values of the study of Praetorius et al. (2019). These values are based on another the river: the Rhine. For this study there was chosen to use uniform conditions throughout the model, According to Praetorius et al. (2019) more detailed parametrization does not lead to fundamental differences in the results, however for

future modelling it can make sense to define the river parameters in more detail. This can help to predict realistic concentrations for a specific river and can help to make a detailed risk assessment.

Additionally, the settling velocity used for the Reynolds number was found in the article of Bagaev et al. (2017). In that study the settling velocity was investigated in a laboratory using a plastic fibre with a length of 8mm and a density of 1.04 g/cm3. Because the sizes are in the same order of magnitude, there is chosen to use that as a speed to calculate Reynolds numbers. For future modelling it will make sense to study the Reynolds number in more detail, in order to come up with a more accurate settling velocity. However, because the velocities are in the same order of magnitude it won’t affect the results majorly.

Lastly, more research should be done in order to investigate the harmfulness of the types of microfibers. One factor that should be taken into account is the biodegradability. The research of Zambrano et al. (2019) shows that the biodegradability of polyester is way less than from cotton and rayon, meaning that it stays in the water for a longer period of time. Figure 6 shows the percentage of biodegradation over a period of time. In this figure the pink line shows the polyester, the purple line the cotton and the green line the rayon.

Figure 9: Biodegradation curves of the textile yarns used to knit the fabrics used for the laundering experiments. This figure is retrieved from the research of Zambrano et al. (2019).

In conclusion, for future modelling it will be necessary to integrate more detailed parametrization for the river and for the microfibers. Next to that, it is necessary to take more factors into account when coming up with a detailed risk assessment. Factors like biodegradation are necessary to say which type of microfibers pollutes the surface water the most.

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5 Conclusion

This study investigated the fate of different types of microfibers in the North Sea Canal in the Netherlands. This was done adjusting a multimedia fate model of Praetorius et al. (2019), which was originally suitable for microplastics. Microplastics have a more spherical shape, whereas microfibers have a more elongated shape. Therefore calculating the settling velocity was adjusted. Results show that rayon has the highest settling velocity in comparison with cotton and polyester. Besides that most of the microfibers of each type tend to settle in the compartment of flowing water. However, because of the larger size and the higher density of rayon, a small part settles in the sediments. More research should be done on the biodegradability of the different types of microfibers in combination with the findings of this study, to find out which microfibers is most harmful for the environment.

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7 Appendix

7.1 Graph of the concentration of particles

Figure 10: The different types of microfibers in the different compartments. The green line show the rayon, the blue and orange lines show the cotton fibers and the blue and purple lines show the polyester fibers. On the y-axis the concentrations of the particles are given in num/m3_{. On the x-axis the time is given in days.}

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7.2 Comparison of the four ways

Figure 11: The differences in settling velocities. The red bar represents the cotton microfibers, the orange bar represents the polyester microfibers and the pink bar represents the settling speed of the rayon microfibers. The first settling velocities is calculated with the Stocks method. The second method is when the microfiber

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7.3 Distance travelled by polyester and rayon microfibers

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 Distance to the source (in km)

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Concentration of microfibers (in kg/m^3)

Distance travelled by polyester microfibers after a time period of two years

Figure 12: This graph shows the concentration of rayon microfibers in the North sea canal after two years of being released by the wastewater treatment plant in Amsterdam West. This concentration is the sum of every concentration of cotton microfibers in every compartment (surface water, flowing water, stagnant water and sediments).

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 Distance to the source (in km)

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Concentration of microfibers (in kg/m^3)

Distance travelled by rayon microfibers after a time period of two years

Figure 13: This graph shows the concentration of polyester microfibers in the North sea canal after two years of being released by the wastewater treatment plant in Amsterdam West. This concentration is the sum of every concentration of cotton microfibers in every compartment (surface water, flowing water, stagnant water and sediments).

The fate of different types of microfibers in Amsterdam surface waters

Bachelor Project Future Planet Studies