UPDATE: The Ocean Cleanup released a feasibility study in June 2014 that attempted to address many of the concerns we had below. You can read our updated technical review here.
I’m just going to come out and say it, any project that touts itself as the “World’s first realistic Ocean Clean-up Concept” is just asking to be torn apart.
“The Ocean Cleanup” is the brainchild of a 19-year old Boyan Slat. He proposes using the oceans themselves to clean up plastic. By setting up a line of giant sifting booms across the major ocean gyres, ocean currents will push plastic into these giant traps to be collected and reused for profit. He plans to set up an array of 24 of these sifters and calculates they will clean the ocean in 5 years.
Before I add my two cents, here’s what Miriam had to say about the idea at the Marine Debris Listserv:
I’ve tried to stop fact-checking to every cleanup scheme, but I guess it’s an addiction at this point. Also, I feel that as a community we cannot move forward with practical solutions to marine debris until we lay some of these common misconceptions to rest. These points respond Boyan Slat’s TEDx talk, but you can also see photos of his proposal here: http://www.boyanslat.com/plastic5/ and http://www.boyanslat.com/in-depth/.
- Most zooplankton don’t survive being caught in a standard manta net, never mind being spun in a centrifuge. They might still be twitching, but they have lost a lot of their important parts, like antennae and feeding apparatus. When we want to capture live zooplankton, we use special live-collection nets and are very, very careful. For gelatinous zooplankton like salps, the only way to bring them up in good condition is to individually capture them in glass jars on SCUBA. I am highly skeptical that any significant proportion of zooplankton are viable after caught in a net and spun at 50 RPM. (though I realize that he’s not proposing to do this on a large scale.)
- Mooring fixed “ships” in the open ocean (avg depth 4000 meters) is highly improbable for a lot of reasons. Just to pick one: I could not find data on the absolute deepest mooring in the world, but this implies that it is approximately 2,000 meters. http://www.offshore-technology.com/projects/atlantisplatform/. So these ships would have to be moored at twice the depth of one of the deepest moorings that existed ~2007.
- Having seen no data, I can’t really speak to the efficacy of floating booms in removing microplastic. However, Giora Proskurowski & colleagues have shown that microplastic get mixed down below the surface in fairly moderate winds. These booms would be unlikely to function in any significant wind and wave action. And the mixed layer in the open ocean can get quite deep, around 100-150 meters in the winter with storms.
- Speaking of wind and wave actions, ships on fixed moorings and thousands of miles of booms (because the scale of this is also improbable) have the potential to create a lot more marine debris, and seem particularly hazardous to entanglement-prone marine life.
- This isn’t even getting into issues of scale (the California Current alone is ~300 miles across), maintenance and fouling…
I realize that Mr. Slat is a student, and have no doubt that he, and the inventors of countless other plastic cleanup schemes, have only the best of intentions. I am hoping we can work together as marine debris professionals to channel their energies into more productive directions.
While I can’t speak to what these booms will and will not pick up, I completely agree with her I am highly skeptical whether the design is even feasible from an ocean engineering standpoint. Here are some of the major unanswered technical questions:
1) How does the sifter work? To be honest, I am not completely sure. The website and TED talk are completely devoid of technical details. But from what I can gather from the concept art and the talk, I think the booms
have large nets underneath them that gather plastic into what I think is a oversized swimming pool leaf trap shaped like a manta ray. UPDATE: I misinterpreted the images on the website. The design as it stands now has no nets, only the initial tests had nets. Now I have to ask, what is that sheet hanging down from the booms?
2) The booms. The claim is that only 24 sifters are need to clean the ocean and span the gyre radius, which means the booms have to be huge. Possibly 100’s of kilometer wide. Are they rigid or flexible? Are they the manta rays? How will they be kept in formation?
3) Anchoring something that large. I am going to assume that the booms need to stay relatively taut to retain their shape and the most obvious way to do this will be with multiple anchor lines. The water depths are deep (>3000 m), horizontal surface motions needs to be small and then there is all that water pushing on what is essentially a giant paddle. That means a fairly sophisticated plan for anchoring the array will have to be developed. Having seen how large anchors are for low-tension subsurface moorings (>1000 kg), I can’t even begin to imagine what they are going to use or how that is going to be set up.
4) Biofouling. I forsee two major biofouling issues. The first is biological growth, which can be particularly bad because all the major mechanical parts are near the surface. There is going to be growth on the mesh, on the booms, on everything submerged which can make the booms
and nets heavy, dragging them underwater. The second is fishbite. Did you know that fish attack underwater moorings like crazed rabid zombie munchers? Now I don’t know if fish would actually chew on the mesh, but previous experience indicates they are not picky about what pieces of underwater line they snack on. So what will happen if fish gnaw holes through the collection nets?
5) The assumption of low current speeds. This is a bad assumption. While the array may not be placed in the most energetic current regime, storms and eddies can briefly induce large currents which could place a lot of stress and shear on such a large array.
6) Zero bycatch by net avoidance . Sorry, I couldn’t resist.
The Ocean Cleanup project is still in the planning stage, so all these problems have the potential to be solved. But I think it is highly unlikely that an array of this size and magnitude will ever be feasible.
UPDATE: Another good roundup by Micheal Cote of potential issues with the Ocean Cleanup design can be found here. http://climateadaptation.tumblr.com/post/46515698066/this-invention-keeps-popping-up-in-my-daily
UPDATE 2: Some more critiques brought to my attention in the comments