Efficiency of River Plastic Waste Interception and Implications for Ocean Plastic Mass

Abstract

Plastic pollution is a growing threat to marine ecosystems and human welfare. A recent fundraising campaign, #TeamSeas, aims to remove 30 million pounds of plastic from the ocean. One of the beneficiaries of the campaign, The Ocean Cleanup, has developed river Interceptors to capture plastic waste in rivers before it reaches the ocean. By approximating the ocean as a well-mixed volume and assuming optimal positioning of Interceptors in the 1000 most polluted rivers, this paper estimates that the Interceptors will have a net efficiency of 35%. Based on multiple studies of ocean plastic sources and sinks, this could result in a drop of the steady state ocean plastic mass by 1.5 – 1500 million pounds, with the majority of models indicating that the Interceptors will meet the #TeamSeas goal. However, due to discrepancies in the magnitude of ocean plastic sources and sinks, the conservation equation used in this paper to represent total ocean plastic is likely incomplete. Future research on plastic sinks is necessary to develop more accurate models. Furthermore, this paper recommends investigating mechanisms for extracting microplastics and long term recycling of recovered materials for future cleanup efforts.

1 Introduction

The accumulation of plastic debris in the ocean is a growing threat to marine ecosystems and human welfare. Over 344 species have been entangled and over 244 species have ingested plastics. These include multiple species of turtles, seals, whales, seabirds, fish and invertebrates (Ritchie and Roser, 2018). Plastic pollution can also negatively affect fisheries, aquaculture, tourism and commercial shipping, resulting in an annual economic loss of 19 billion USD (Slat, 2021). Despite these adverse effects, plastic production is predicted to grow exponentially over the next several decades (Ritchie and Roser, 2018).

In light of these concerns, on October 29, 2021, YouTube personalities Jimmy Donaldson (“Mr. Beast”) and Mark Rober launched a fundraising campaign, dubbed #TeamSeas, to remove plastic debris from the oceans (#TeamSeas, 2021). The campaign aims to raise 30 million USD and promises to remove 30 million pounds (13.61 million kg) of waste. Donations will be split evenly between two nonprofits: The Ocean Conservancy and The Ocean Cleanup. The Ocean Conservancy organizes the International Coastal Cleanup (ICC), which conducts beach and underwater cleanup efforts with volunteers and professional divers (Ocean Conservancy, 2021). The Ocean Cleanup is currently constructing and deploying river Interceptors to capture plastic waste in rivers before it enters the ocean (The Ocean Cleanup, 2021).

1

The permanent removal of 30 million pounds of plastic waste from the ocean requires not only the direct removal of waste, but a permanent reduction in the steady state mass of plastic in the ocean. For this reason, The Ocean Cleanup Interceptors are of particular interest as they have the potential to “close the tap” on a major source of ocean plastic thus reducing the steady state mass of ocean plastic (The Ocean Cleanup, 2021). This paper intends to investigate the theoretical magnitude of this reduction and the extent to which it aligns with the goal of #TeamSeas to remove 30 million pounds of plastic.

In order to determine the impact that the Interceptors might have, it is necessary to (1) characterize the efficiency of the Interceptors, (2) develop a conservation equation for ocean plastic, and (3) apply the percent reduction in river plastic due to the Interceptors to the source term in the conservation equation. Below I attempt to summarize the current state of research on the profile of plastic debris in rivers and the magnitude of river and ocean plastic concentrations.

1.1 River Plastic Profiles

The distribution of plastic debris in the cross section of a river – henceforth referred to as the “plastic profile” of a river – varies significantly due to climatic, geographic, and hydrologic factors (van Emmerik et al., 2020). For example, heavy rainfall increases river discharge and results in the remobilization of beached plastics and high riverine plastic emissions (Rober, 2021; Roebroek et al., 2020; van Emmerik et al., 2020). Additionally, the size of plastic debris exerts a large influence over transport mechanisms, which can further alter the plastic profile (Cowger et al., 2021; van Emmerik et al., 2020). The literature distinguishes between several distinct size regimes for plastics. The most commonly cited categories are macroplastics (> 0.5 cm) and microplastics (< 0.5 cm) (Lebreton et al., 2018; van Emmerik et al., 2020). These definitions will be used throughout this paper.

Flow velocity is the most important factor for the horizontal transport of debris (van Emmerik et al., 2020). A 2019 study on plastic emissions from Jakarta identifies a positive correlation between flow velocity and plastic concentration in river cross sections (van Emmerik et al., 2019). Existing Interceptors are deployed along smooth, straight stretches of river (Rober, 2021). Approximating these conditions with laminar flow, one obtains a parabolic velocity cross section along both the width and depth of the river (Fowler, 2012).

Turbulent mixing and plastic density are the most important factors for vertical transport. Most field studies quantifying plastic transport by rivers assume that macroplastics are transported primarily at the river surface (van Emmerik et al., 2018). However, several recent investigations have found significant quantities of plastic at depths > 1 m (van Emmerik et al., 2019). In contrast, microplastics are typically uniformly distributed throughout the water column and tend to accumulate in river sediments (Gallitelli et al., 2020; van Emmerik et al., 2020). However, the exact depth concentration profile of microplastics depends on the manner in which they are transported (i.e. immobile, bed load, setting-suspended load, wash load, rising- suspended load, or surface load) (Cowger et al., 2021).

1.2 Ocean Plastic Mass

Measuring the amount of plastic waste in the oceans has proven remarkably difficult. The first estimates for the amount of plastic entering the ocean via coastal regions were developed in 2015 by Jambeck et al.. Their model, which estimates the fraction of plastic entering the ocean locally based on the amount of Mismanaged Plastic Waste (MPW) in each country, serves as the basis for later estimates of riverine plastic transport to the ocean (Lebreton et al., 2017; Schmidt et al., 2017; Roebroek et al., 2020; Meijer et al., 2021). These models yield riverine plastic emissions on the order of millions of metric tons (t) per year. However, estimates for the total amount of plastic in the ocean are typically 1- 2 orders of magnitude lower. This has led to a search for a “missing sink” of ocean plastic and debate over the validity of MPW models (Ritchie and Roser, 2018; Weiss et al., 2021).

1.2.1 Current Ocean Plastic Mass

Current estimates for the total amount of plastic in the ocean range significantly (See Supporting Table S1). Lebreton et al., 2018 offers a first attempt at quantifying the concentration of microplastics in the Great Pacific Garbage Patch (GPGP) over time using data from existing literature. Their results reveal that the concentration of microplastics is growing exponentially and growing fastest in ocean gyres, such as the GPGP.

1.2.2 Plastic Sources

Recent studies have attempted to characterize the amount of plastic entering the ocean through rivers. Most are MPW models (Lebreton et al., 2017; Schmidt et al., 2017; Roebroek et al., 2020; Meijer et al., 2021). However, others have used the Human Development Index (HDI) (Mei et al., 2020) or population density (sPop) and drainage intensity (sQ) (Weiss et al., 2021) as model parameters (See Supporting Table S2).

In addition to coastlines and rivers, marine debris likely constitutes a significant source of ocean plastic. A 2017 study by Geyer et al. on the production and fate of plastics estimates that 80% of marine plastics come from land-based sources while 20% come from marine sources. Similarly, Lebreton et al., 2018 estimates that 28.1% of plastics are from marine sources. Based on sampling, marine debris is overrepresented in the GPGP compared to these global models (Lebreton et al., 2018).

1.2.3 Plastic Sinks

Numerous sink mechanisms have been proposed to account for the apparent mass imbalance in the ocean. Among these are (1) biodegradation, (2) photochemical decay, (3) fragmentation into microplastics, (4) ingestion by marine life, (5) biofouling and sinking of non- buoyant plastics, and (6) coastal beaching (Weiss et al., 2021).

Recent studies indicate that biodegradation and photochemical decay of plastics are negligible in seawater (LI, W. C et al., 2016). Additionally, polyethylene (PE) and polypropylene (PP), which form the majority of plastics in the GPGP water column (Egger et al., 2020), have

fragmentation rates of < 1% per year, which means it can take over a century for these plastics to fully degrade into microplastics (Gerritse et al., 2020). While bioaccumulation of microplastics in marine organisms may be significant, microplastics only constitute 8 – 20% of the plastic mass of the GPGP (Egger et al., 2020; Lebreton et al., 2018). It has been speculated that the high mass fraction of PE and PP and low mass fraction of microplastics may be the result of chemistry or size selective sinks. Further research is necessary to verify these theories and hence this paper considers these sinks negligible.

Only 60% of all plastics ever produced are buoyant (Jambeck et al., 2015; Lebreton et al., 2018). Buoyant plastics may also lose buoyancy and sink as a result of biofouling: the colonization of plastic surfaces by marine microorganisms, plants or algae (Gerritse et al., 2020). Biofouling is most significant for debris with high surface to volume ratios (Gerritse et al., 2020; van Emmerik et al., 2020). Microplastics have also been observed to sink below the sea surface (Egger et al., 2020). The sinking of microplastics in the top 2000 m of the ocean may account for 10% of the plastic mass in the GPGP (Egger et al., 2020).

Coastal beaching is predicted to be the largest sink. A 2019 model estimates that 122 million metric tons of plastic have been washed up along shorelines while only 1.75 million metric tons reside in the ocean proper (Ritchie and Roser, 2018). However, beach cleanup efforts by the ICC give rise to macroplastic sink estimates 1- 2 orders of magnitude lower than expected.

2 Methods
2.1 Interceptor Efficiency

Figure 1: Assuming laminar flow, the surface velocity and plastic profiles are parabolic across the width of the river (a). Multiplication of the surface velocity and plastic profiles yields a surface flux profile (b). The amount of surface plastic collected is obtained by integrating over the surface flux and normalizing (c). The maximum value of the ratio between the surface plastic collected and the river width traversed occurs at 72.4% of the river width (d). This is the optimal Interceptor location.

To determine the efficiency of The Ocean Cleanup Interceptors, the plastic flux profile of the river must be known (See Supporting Fig. S1). The plastic flux profile (?) can be obtained by multiplying the velocity (?#$%&#) and plastic (?()*+,$-) profiles as per:

? = ?#$%&# ∙ ?()*+,$-

Integrating over the cross sectional area of the flux distribution that the Interceptor can reasonably intercept and dividing by the integral of total flux we acquire the percentage of the plastic mass1 that an Interceptor can capture. The Interceptor is attached to land by a floatation device that serves to concentrate plastic into the aperture of the Interceptor. This device extends 0.5 m deep into the water and stretches from the shore to the Interceptor itself (Rober, 2021). Thus the depth of the river must be known to determine the percentage of depth that the Interceptor covers.

1 Technically, this gives the percentage of plastic mass per time, but if the time interval is constant then the percentage of plastic mass collected is the same.

Since the depth varies by river we shall assume, based on the findings of Emmerik et al., 2019, that 58.4% of riverine plastic mass resides in the upper 0.5 m of the water column. The surface velocity and plastic profiles can be converted to surface flux which can be integrated and normalized to give the percentage of surface plastic collected with respect to the percentage of the river width covered (See Fig. 2 (a-c)). Dividing the percentage of surface plastic collected by the percentage of the river width covered and normalizing, we can determine the optimal location of the Interceptor (See Fig. 2 (d)).

Note that the Interceptor is only built to intercept macroplastics so assuming that 14% (8% – 20%) of the total mass is microplastics, the efficiency determined above must be reduced by a factor of 0.86 (Egger et al., 2020; Lebreton et al., 2018). Additionally, The Ocean Cleanup only plans to deploy Interceptors on the 1000 most polluted rivers, which account for 80% of river plastic inputs to the ocean (Meijer et al., 2021). Thus the efficiency must be reduced by an additional factor of 0.80.

2.2 Ocean Plastic Mass Conservation

The conservation equation for the mass of ocean plastic (?) takes the following form where ?2 has units of million metric tons per year and ?4 has units of years-1. ?5, the initial mass is obtained from the values of ocean plastic mass in Supporting Table S1:

??(?)=Σ??????? −Σ(?????) ??

? ? = ?5?DEF, + ?++(1 − ?DEF,)

?++ = ?2 ?4

2.2.1 Plastic Sources

Coastal and river inputs were approximated using the median or midpoint values from the studies in Supporting Table S2. Marine inputs were estimated by assuming that they account for 24% of net inputs (20% – 28.1%) (Egger et al., 2020; Lebreton et al., 2018). Since the only estimate for direct coastal inputs comes from Jambeck et al., 2015, the only dynamic variable in the source term is the estimate for river inputs.

?2 = Σ ??????? = ?-I*+,*) + ?#$%&# + ?J*#$K& ?J*#$K& = 0.24(?-I*+,*) + ?#$%&#)

2.2.2 Plastic Sinks

Since 40% of plastic in the waste stream is non-buoyant (?KIKDQRIS*K,) it is likely that at least 40% of ocean plastic mass sinks per year, assuming that ocean plastic is continually renewed by river, coastal and marine inputs (Lebreton et al., 2018). An additional 10% of ocean

plastic mass probably sinks in the form of microplastics (?+$KE$KT) based on observations of microplastic concentrations under depths of 5 m in the GPGP (Egger et al., 2020).

Finally, to determine the amount of plastic beached each year (?-I*+,)$K&), the annual international mass totals for the ICC (?#&-I%&#&U) were divided by the total distance covered (?-I%&#&U) and multiplied by the length of the global coastline (?-I*+,)$K&). The results were of the same order of magnitude and displayed no trend over time from 1991 to 2020 thus the average was taken. Finally, the results were divided by the current amount of plastic in the ocean (?5) to obtain units of years-1, and divided by the percentage of macroplastics (?J*-#I()*+,$-) to account for uncollected microplastics.

?4M = Σ ????? = ?KIKDQRIS*K,? + ?+$KE$KT? + ?-I*+,)$K& ? ?J*-#I()*+,$- ?5

?-I*+,)$K& = ?#&-I%&#&U ?-I*+,)$K& ?-I%&#&U

2.3 Interceptor Application to Ocean

The percent reduction in ocean plastic inputs due to the Interceptors (?WK,&#-&(,I#) would result in a reduction in the source term, ?2. This would result in a new steady state mass (?′++) and a percent change in the amount of steady state plastic (?++).

?′++ = ?WK,&#-&(,I#?2 ?4

?++ =1−?′++ ?++

3 Results
3.1 Interceptor Efficiency

For optimal performance, Interceptors should span 72.4% of the width of the river. At this location, an Interceptor can recover 86.7% of the surface plastic in the river. Accounting for plastic missed in the lower water column, the inability to collect microplastics, and deployment on rivers accounting for only 80% of river plastic inputs, the net efficiency of Interceptors is approximately 35%.

3.2 Ocean Plastic Mass Concentration

Figure 2: The conservation equation developed in the Methods section is plotted for each of the six models of riverine plastic emissions in Supporting Table S2. Each subplot (a-d) corresponds to a different initial ocean plastic mass estimate given in Table 1. Note the high steady state mass and low residence time for each of the models.

Based on the information in Supporting Table S1 and Supporting Table S2, Fig. 2 projects the total mass of plastic in the ocean into the future. Note that the MPW models consistently estimate steady state masses 25 – 30% higher than the non-MPW models. Since no non-MPW estimates of coastline plastic emission have been made, it is unknown how dramatically a fully non-MPW model of ocean plastic sources would differ.

The residence times of these models also vary from a few weeks to several months. This is inconsistent with observations of decades old plastics found in the GPGP but is consistent with previous attempts to characterize residence times by modeling inputs and outputs (Lebreton et al., 2018; Weiss et al., 2021). Furthermore, the exponentially growing concentration of microplastics in the GPGP is consistent with the introduction of an exponential into the source term of the conservation equation but this behavior is not captured in the simplified model presented in Fig. 2. Nevertheless, Fig. 2 allows for an order of magnitude approach to determine the impact that the Interceptors will have on the steady state mass of ocean plastic.

3.3 Interceptor Application to Ocean

The results of applying the reduction in steady state mass by the Interceptors to each of the models in Fig. 2 is summarized in Supporting Table S3. Reductions range between 1.5 and 1500 million pounds. Of the 24 models studied, 20 indicate that the Interceptors will meet the #TeamSeas goal of removing 30 million pounds of plastic. The 4 results where the Interceptors fail to meet this goal occur exclusively in the non-MPW models of river plastic. In addition, the non-MPW models predict that the percentage of ocean plastic eliminated is about an order of magnitude lower than in the MPW models (0.5% vs 5.9%). This is due to the fact that river inputs are an order of magnitude lower in these models, while coastal inputs are constant among all of them. Although more research is necessary to verify which model is the most accurate, the majority of cases indicate that the Interceptors will meet the goal of removing 30 million pounds of plastic.

4 Discussion

The Ocean Cleanup Interceptors offer a viable means to reduce the amount of plastic entering the ocean annually. In the future, Interceptors could be used to validate models of river emissions and track upstream waste management progress. Making this data available to the public will be beneficial to future research efforts and will help inform future policy decisions.

Once plastic waste is removed from rivers, it must be recycled or disposed of in a sanitary manner. Otherwise, the upstream concentration of mismanaged plastic waste will simply increase and the Interceptors, despite collecting the same percentage of plastic, will lose more total plastic to the ocean. To this end, I recommend a potential collaboration with TerraCycle. TerraCycle is a Trenton based recycling company dedicated to making all waste recyclable. As The Ocean Cleanup acquires larger hauls, partnerships of this sort will help to ensure that the recovered plastic stays out of landfills for as long as possible.

Additionally, microplastics remain a significant concern. Neither the Interceptor or International Coastal Cleanup have a mechanism to remove microplastics from the environment. To this end, recent research by new Andlinger Center Distinguished Postdoctoral Fellow, Fernando Temprano-Coleto, on membrane-less filtration of microplastics using diffusiophoresis may provide a scalable solution (Seltzer, 2021). Temprano-Coleto’s research will be especially useful for wastewater treatment plants, which are a significant source of microplastic fibers in the environment (van Emmerik et al., 2020). Another approach to removing microplastics from water using ferrofluid was proposed by 2019 Google Science Fair winner Fionn Ferreira (Nace, 2019). Developing these technologies commercially would be a first step towards removing microplastics from the environment permanently.

Acknowledgements

I would like to thank Professor Bourg for his generous extension and feedback on the idea for this paper. I also want to thank Thomas Underwood for his insights on writing a conservation equation for ocean plastic and Avery Agles for his assistance brainstorming topics in office hours.

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Supporting Information

Figure S1: Assuming laminar flow, the velocity profile is parabolic along both the width and depth of the river (a). Assuming that plastic is carried as a rising-suspended load and is proportional to the velocity along the width of the river, the plastic profile is parabolic along the width of the river and decays exponentially with depth (b). The plastic flux profile is obtained from the multiplication of the velocity and plastic profiles (c).

Table S1: Estimates of Ocean Surface Plastic.

Table S2: Estimates of Ocean Plastic Sources.

*Obtained by subtracting relevant river sources from the coastline estimates in Jambeck et al., 2015.

Table S3: Reduction in Steady State Ocean Plastic Mass (millions of pounds).

*See Table S1 and S2 for the estimates associated with each study.