The Ocean Cleanup: Barking Up the Wrong Tree
The Ocean Cleanup has pivoted, or rather expanded its remit from a technically challenging mid-ocean plastic removal system to a riverine-based one, and while not quite yet at the source of the plastic problem, (that would be households) they are mopping closer to the taps. Such a move is to be commended as rivers are a significant source of ocean-bound plastic.
Global models of river inputs have yielded estimates of 1.15–2.41 million tonnes per year (Mt/y) (Lebreton et al, 2017), 0.47–2.75 Mt/y (Schmidt et al, 2017, corrected version 2018), and 0.8–2.7 Mt/y (Ocean Cleanup website). This probably represents around 20% of total land inputs into the marine environment, thought to be 4.8–12.7 Mt/y in 2010 (Jambeck et al, 2015) for coastal areas. Many readers may be surprised at that 20% figure, having been told by numerous reputable news sources that as much as 90–95% of all ocean plastic comes from just 10 rivers. This is false, and appears to be based on a misreading of a press release from the research institution where the work was carried out. (Interested readers can find several other myths regarding marine plastic waste here.)
Still, 20% is a significant chunk, and any technology that can put a dent in river inputs is to be encouraged. That said, I have some thoughts on the current incarnation of The Ocean Cleanup’s Interceptor river plastic waste collector, pictured below. (source: Ocean Cleanup)
RIVER INPUTS
According to the OC (Ocean Cleanup) website, Interceptors will be put into the top 1000 polluting rivers, accounting for 80% of riverine input, by 2025. However, Schmidt et al (2017, 2018) suggest that there is a non-linear relationship between river catchment size and plastic waste inputs. That is, large rivers with large population bases deliver a disproportionately higher level of plastic to oceans. Their top 10 sources account for 88–95% of global riverine inputs (macro and microplastics) into the sea. Lebreton et al (2017) estimate that the top 20 sources discharge 67% of ocean-bound plastic from rivers. The OC model itself has the top 10 sources accounting for about 12%, as far as I can tell through visual inspection of the website. The non-linearity is apparent from the following table. (Note that the Schmidt data is for macroplastics only).
Thus a focus on a few dozen of the top polluters (recommended by Schmidt et al) might seem prudent. But there is a problem.
Many of the top polluting rivers are really big.
RIVER WIDTH
The top 3 polluters according to Lebreton et al (2017) are the Yangtze (330.000 t/y), the Ganges (115,000 t/y), and the Xi (73,900 t/y). Their widths have been color-coded here (source: MERIT).
For much of their lengths, these rivers are between 0.5 and 2 km wide.
There is another problem with wide and/or urban rivers.
BOATS
The narrowest point of the Yangtze within 300 km of the coast, pictured below (near Nanjing, 1 km width) is a 4-lane super-highway for ocean-going freighters and numerous small craft. (Google Maps)
The Pasig River in Manila, the single biggest global plastic source (96,543 t/y) according to the OC model is congested with boat traffic.
The Interceptor system as presently envisioned (long, floating barrier guiding floating plastic to a 1–2 meter-wide conveyor belt on a small, stationary craft) is not appropriate on behemoths like the Yangtze, or on waterways with considerable boat traffic flowing through urban zones.
TURBULENCE
Fast-flowing rivers, especially after storm and flood events experience significant turbulence which can suspend buoyant and non-buoyant plastics throughout the water column (Nizzetto et al, 2016; Gonzalez et al, 2016). A collection system that functions at the top 30 or so centimeters (as the Interceptor seems to do) may be missing a significant fraction of waste. Hohenblum et al (2015) found that 66–79% of plastic waste was present below 1.5 meters depth in the Danube. Van Emmerik et al (2019) assume that 30% of plastic waste in Jakarta’s waterways is transported at depths greater than 1 meter. There is likely great variability in plastic distribution within water columns of different rivers, with significant fractions transported below the functional reach of the Interceptor as presently designed. (A superb discussion of riverine plastic pollution in general, including turbulence, sedimentation, and vertical / horizontal transport can be found in van Emmerik and Schwarz (2019)).
THE OC MODEL
There may be a problem with the model used to estimate global riverine inputs of plastic waste. If the model is being used to allocate resources, including deployment of Interceptors, then care is warranted. If it is merely used to produce numbers for a graphically pleasing web-based marketing tool, then I have no problem with it; the site is quite beautiful. (Although I’d recommend putting river names on it). While examining it the other day I zoomed in on Java, Indonesia. And noticed an anomaly.
This is Sidoarjo Regency, a mostly residential area of 2.16 million people just south of Surabaya, the second largest city in Indonesia. Clicking on the red dot (indicating a top 1000 polluting river) gives us this:
The model estimates that the regional population of 840,633 emits about 303 tonnes(t) of plastic into the ocean per year. However, an extensive study carried out in 2017 in this same region (although with a broader population base of 2.4 million) suggests that 7,616 t were emitted (Renaud et al 2018); an 8 fold difference (after accounting for population difference).
Lebreton et al (2017) have 4 Indonesian rivers among their top 20 global plastic polluters: Brantas (38,900 t/y), Solo (32,500), Serayu (17,100), and Progo (12,800). The OC website for these same rivers has vastly lower estimates: Brantas (1,008), Solo (298), Serayu (919), and Progo (449 t/y). The World Bank’s recent Marine Debris Hotspot Rapid Assessment for Indonesia (Shuker & Cadman, 2018) estimates that Jakarta releases 55,000 t of plastic a year into the ocean. The OC website suggests around 7,000 t. At least for Indonesia, the OC model seems to be significantly underestimating river inputs.
One potential problem may be reliance on Waste Atlas, a crowd-sourced platform collecting global municipal solid waste data. I don’t know if the OC model uses Waste Atlas data, but their researchers are certainly familiar with it, as indicated by their scientific publications.
Waste Atlas provides self-reported data at the country level; in other words, a single value is used for the entire country for each indicator. For example, municipal solid waste collection for Indonesia as a whole is estimated at 69% by Waste Atlas, masking large regional variability, from 0% in many rural areas to 98% in West Jakarta (Shuker & Cadman, 2018). For models predicting global estimates this usually isn’t a problem, but any particular country-level error will throw off more granular estimates within a particular country. And there is almost certainly an error in the Indonesian data. Waste Atlas has the rate of mismanaged plastic waste at 25% for Indonesia, which seems impossibly low when compared to the Philippines (85%) and Thailand (60%). When we compare Waste Atlas with World Bank data (Hoornweg & Bhada-Tata, 2012, as reported in Jambeck, 2015) for various countries in the region for this same indicator, there is generally good agreement. Except for Indonesia.
According to the OC website, a section of the Manila Bay area receives 138,193 t/y (population 16 million) while Jakarta Bay receives 14,111 t/y (36 million); a 22-fold difference (after accounting for population difference) where we would expect relatively equal per capita levels.
Using country-level data is a blunt instrument, and caution is warranted when estimating at higher (i.e. more local) resolutions. To be clear, I have no idea if the OC model uses Waste Atlas or any other country-level data. I haven’t seen the model, which as far as I know, hasn’t been published.
RECOMMENDATIONS
If The Ocean Cleanup decides to pursue their current strategy, there are plenty of suitable rivers (high plastic load, relatively narrow, few boats) to choose from. For example, most of the length of the Brantas River in Java (Lebreton et al’s 6th biggest global source) is less than 100 meters wide.
However I believe a bigger impact can be made if the focus were on a different technology altogether.
Five of the top eight national sources of marine-bound plastic waste are in South-East Asia. A focus on the waterways of the major coastal urban areas there, especially drainage and irrigation canals, would be the most efficient use of resources. Urban canals are dumping grounds for household waste due to insufficient municipal waste collection, inability or unwillingness to pay, lack of awareness of the negative environmental implications of dumping, and tradition (Shuker & Cadman, 2018). But instead of a high-tech Interceptor-type system, (you would need many thousands of units), how about expanding on what many cities already have? Waste traps. A wide range of waste trap technologies should be developed and custom-fit to local conditions. Waterway debris is idiosyncratic. Waste composition will vary greatly over time and space, and clever engineering can accommodate this.
Jakarta’s low-tech waste traps remove 165 tonnes of debris from waterways every single day, 25% of which is plastic. That’s 15,000 tonnes of plastic per year (Shuker & Cadman, 2018). But if the World Bank’s estimate for Jakarta is accurate (55,000 tonnes per year of plastic emitted into the ocean), then there is room for considerable improvement. Surely a more efficient, economical, and extensive waste trap system (or a range of technologies) can quickly be developed and implemented on Asia’s urban waterways. The Ocean Cleanup has experience, appropriate engineering expertise, and a 5.3 million dollar payroll (2018 annual report). Perhaps they would find it more productive to pivot once again, but this time going low tech.
REFERENCES
González, D., Hanke,G., Tweehuysen, G., Bellert, B., Holzhauer, M., Palatinus, A., Hohenblum, P., and Oosterbaan, L. (2016). Riverine Litter Monitoring — Options and Recommendations. MSFD GES TG Marine Litter Thematic Report; JRC Technical Report; EUR 28307; doi:10.2788/461233
Hohenblum P, Frischenschlager H, Reisinger H, Konecny R, Uhl M and Mühlegger S. (2015). Plastik in der Donau: Untersuchung zum Vorkommen von Kunststoffen in der Donau in Österreich Report 0547 Ämter der Landesregierung Oberösterreich, Niederösterreich und Wien Erstellt
Hoornweg, D. & Bhada-Tata, P. (2012). What a Waste : A Global Review of Solid Waste Management. World Bank. http://hdl.handle.net/10986/17388.
Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R., (2015). Plastic waste inputs from land into the ocean. Science 347, 768–771.
Lebreton, L. and Andrady, A. (2019). Future scenarios of global plastic waste generation and disposal. Palgrave Communications 5:6 https://doi.org/10.1057/s41599-018-0212-7
Lebreton, L. C. M. et al. (2017). River plastic emissions to the world’s oceans. Nat. Commun. 8, 15611 doi: 10.1038/ncomms15611.
Nizzetto, L. et al. (2016). A theoretical assessment of microplastic transport in river catchments and their retention by soils and river sediments. Environmental Science: Processes and Impacts. 18:8. DOI: 10.1039/c6em00206d
Renaud, P. et al. (2018). Marine Litter Prevention. Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH. https://www.giz.de/en/downloads/giz2018_marine-litter-prevention_web.pdf
Schmidt, C., Krauth, T., and Wagner, S. (2017). Export of Plastic Debris by Rivers into the Sea. Environ. Sci. Technol. , 51, 12246–12253
Schmidt, C., Krauth, T., and Wagner, S. (2018). Correction to export of plastic debris by rivers into the sea. Environ. Sci. Technol. 52:927. doi: 10.1021/acs.est. 7b06377
Shuker, Iain G.; Cadman, Cary Anne. (2018). Indonesia — Marine debris hotspot rapid assessment : synthesis report. Washington, D.C. : World Bank Group. http://documents.worldbank.org/curated/en/983771527663689822/Indonesia-Marine-debris-hotspot-rapid-assessment-synthesis-report
van Emmerik, T. et al. (2019). Riverine plastic emission from Jakarta into the ocean. Environ Res Lett. 14 084033
van Emmerik T, Schwarz A. Plastic debris in rivers. WIREs Water. (2019); e1398. https://doi.org/10.1002/wat2.1398
FURTHER READING
For those relatively new to the scientific literature on plastic pollution, there are several excellent review articles that I highly recommend but weren’t cited directly in the article above.
Forrest A, Giacovazzi L, Dunlop S, Reisser J, Tickler D, Jamieson A and Meeuwig JJ (2019). Eliminating Plastic Pollution: How a Voluntary Contribution From Industry Will Drive the Circular Plastics Economy. Front. Mar. Sci. 6:627 doi: 10.3389/fmars.2019.00627
GESAMP (2019). Guidelines for the monitoring & assessment of plastic litter in the ocean Reports & Studies 99 (editors Kershaw, P.J., Turra, A. and Galgani, F.)
R. Geyer, J. R. Jambeck, K. L. Law. (2017) Production, use, and fate of all plastics ever made. Sci. Adv. 3, e1700782
Rhodes, C.J. (2019). Solving the plastic problem: From cradle to grave, to reincarnation. Science Progress 1–31.
Worm, B., et al. (2017). Plastic as a persistent marine pollutant. Annual Review of Environment and Resources. 42:1–26.