The Dutch boy mopping up a sea of plastic

Boyan Slat is a 20-year-old on a mission - to rid the world's oceans of floating plastic. He has dedicated his teenage years to finding a way of collecting it. But can the system really work - and is there any point when so much new plastic waste is still flowing into the sea every day?

"I don't understand why 'obsessive' has a negative connotation, I'm an obsessive and I like it," says Boyan Slat. "I get an idea and I stick to it."

This idea came to him at the age of 16, in the summer of 2011, when diving in Greece. "I saw more plastic bags than fish," says Slat. He was shocked, and even more shocked that there was no apparent solution. "Everyone said to me: 'Oh there's nothing you can do about plastic once it gets into the oceans,' and I wondered whether that was true."

Over the last 30 to 40 years, millions of tonnes of plastic have entered the oceans. Global production of plastic now stands at 288 million tonnes per year, of which 10% ends up in the ocean in time. Most of that - 80% - comes from land-based sources. Litter gets swept into drains, and ends up in rivers - so that plastic straw or cup lid you dropped, the cigarette butt you threw on the road… they could all end up in the sea.

The plastic is carried by currents and congregates in five revolving water systems, called gyres, in the major oceans, the most infamous being the huge Pacific Garbage Patch, half way between Hawaii and California.

Although the concentration of plastic in these areas is high - it's sometimes described as a plastic soup - it's still spread out over an area twice the size of Texas. What's more, the plastic does not stay in one spot, it rotates. These factors make a clean-up incredibly challenging.

"Most people have this image of an island of trash that you can almost walk on, but that's not what it's like," says Slat. "It stretches for millions of square kilometres - if you went there to try and clean up by ship it would take thousands of years." Not only that, it would be very costly in terms of both money and energy, and fish would be accidentally caught in the nets.

Slat had always enjoyed working out solutions to puzzles, and while pondering this one, it came to him - rather than chase plastic, why not harness the currents and wait for it to come to you?

At school, Slat developed his idea further as part of a science project. An array of floating barriers, anchored to the sea bed, would first catch and concentrate the floating debris. The plastic would move along the barriers towards a platform, where it could then be efficiently extracted. The ocean current would pass underneath the barriers, taking all buoyant sea life with it. There would be no emissions, and no nets for marine life to get entangled in. The collected ocean plastic would be recycled and made into products - or oil.

The high school science project was awarded Best Technical Design at Delft University of Technology. For most teenagers, it would probably have ended there, but Slat was different. He had been interested in engineering from a very young age. "First I built tree houses, then zip-wires, then it evolved towards bigger things," he says. "By the time I was 13, I was very interested in rocketry." This led him to set a Guinness World Record for the most water rockets launched at the same time: 213, from a sports field in his native Delft. "The experience taught me how to get people crazy enough to do things you want, and how to approach sponsors." Useful skills, as it turned out.

When Slat began studying aerospace engineering at Delft University, the idea of cleaning up the oceans just wouldn't let him go - he says it niggled at him like "an asymmetrically positioned label" on a pair of boxer shorts. He set up a foundation, The Ocean Cleanup, and explained his concept in a TedX Talk: How the Oceans can Clean Themselves. Then, six months into his course, he made the decision to pause both university and social life to try make it a reality.

His entire budget consisted of 200 euros (£160) of saved-up pocket money, so he spent a few desolate months trying to get sponsorship. "It was so disheartening, because no-one was interested," he says. "I remember one day contacting 300 companies for sponsorship - only one replied, and that, too, resulted in a dead end."

Boyan Slat with some of the ocean plastic his team have collected

But then something happened. On 26 March 2013, months after it had gone online, Slat's TedX talk went viral. "It was unbelievable," he says. "Suddenly we got hundreds of thousands of people clicking on our site every day. I received about 1,500 emails per day in my personal mailbox from people volunteering to help." He set up a crowd-funding platform that made $80,000 in 15 days.

Slat still doesn't know what made his idea take off like that, but he describes it as a great relief. "A year ago I wasn't sure it would succeed," he says. "But considering the size of the problem it was important to at least try."

The amount of plastic being discarded into the marine environment is such that we could eventually see an ocean where the amount of plastics is roughly one third the total biomass of fish - 1lb of plastic for every 2lbs of fish, according to Nicholas Mallos from Ocean Conservancy, which organises coastal clean-ups.

According to the UN Environment Programme there are on average 13,000 pieces of floating plastic per square kilometre of ocean - but that goes up to millions of pieces in the gyres. Many of these particles end up being accidentally ingested by marine animals, which can die of starvation because of the plastic filling their stomachs.

A decayed albatross carcass shows the amount of plastic it had ingested

Albatrosses are particularly vulnerable because they feed on the eggs of flying fish, which are attached to floating objects - now most likely a piece of plastic. Dr Jan Andries van Franeker from the Institute for Marine Resources and Ecosystem Studies (IMARES) in the Netherlands has some of these objects in a pot in his office: a toothbrush, cigarette lighters, floaters from fish nets, a golf ball, a tampon applicator - all found in albatross chick's carcasses. "The plastics may not directly kill the bird, but it will have less energy reserves and it will have a higher load of chemicals so if things get problematic at sea, or if you have to raise a chick, those are the ones that die first," he told the BBC.

Plastic objects found in albatross chick carcasses including fishing tackle and items of personal hygiene

Turtles tend to be the victims of plastic bags, which when immersed in water look just like jellyfish. Evolutionary adaptations make it impossible for turtles to reject bags once they've started to eat. "Because jellyfish are so slippery, turtles have a system in their throat that stops their prey from slipping out, so even if you find out it's a plastic bag, it has to go in all the way," says van Franeker.

A turtle entangled in netting and other debris

The amount of industrial plastic pellets van Franeker finds in the birds has halved since the 1980s - it seems the industry has at least partially cleaned up its act. "It's an economic loss if the factory loses raw product," he says. "Unfortunately with consumer plastic, there is little profit in taking back waste. It doesn't cost us anything to throw it away."

But the cost to us could be very high, in the long term.

Plastics can act as a sponge and soak up chemicals in the water. "There are a lot of pollutants in the oceans now, things like DDT," Nancy Wallace, director of the US National Oceanic and Atmospheric Administration's Marine Debris Program, told the BBC. "Those chemicals adsorb on to the plastic and we know birds and fish are eating those pieces of plastic - so the question is, how does that transfer up the food chain and what is the impact?"

Find out more

• Boyan Slat spoke to Outlook, BBC World Service - Listen via the iPlayer

• Jan Andries van Franeker and Nancy Wallace spoke to Shared Planet, BBC Radio 4 - Listen via the iPlayer

• Professor Richard Thompson and Dr Kerry Howell spoke to Costing the Earth, BBC Radio 4 - Listen via the iPlayer

It is a grave situation - so when Slat came along with a seemingly simple solution, he began making headlines across the world. Could a teenager save the world's oceans? His enthusiasm fired up millions of people, but along with the offers of help and donations, came criticism. It wouldn't work, some said. Others argued that it would be better to collect litter from beaches, where it gets deposited by waves.

"It's in my nature that when people say something is impossible I like to prove them wrong," Slat says. Having caught the world's attention, the first thing he did was to disappear from sight. He needed scientific evidence to back up his theory and answer his critics.

He assembled a team of 100 people, mostly volunteers, who were spread out across the world - the lead oceanographer was based in Australia. To manage so many people at such a young age was "interesting", says Slat. "We had an intern who was 24 and we weren't too satisfied with him - I remember some colleagues saying: 'Oh, he's so young, he'll learn,' not realising that they were talking to a 19-year-old."

During the feasibility study Slat visited the gyre known as the North Atlantic Garbage Patch, where the platform is destined to be built. "I was pretty seasick the first three days. There were winds of 25-30 knots and waves 3m high. It was quite an experience," he says.

Boyan Slat testing out equipment at sea

In June, a month before his 20th birthday, Slat re-emerged with a 530-page feasibility report, the cover of which was made out of recycled ocean plastic. The report, based on extensive testing and computer simulations and authored by 70 scientists and engineers, answered many of the questions which had been levelled at him by his critics. It was followed by another crowd-funding campaign which swiftly reached its target of $2m. This will fund a larger pilot next year and Slat hopes the North Atlantic platform could be a reality in 2020.

But if Slat expected all experts in the field of ocean plastics to welcome his concept this time, he was wrong.

Plastic soup on your plate?

• Ocean plastic breaks down into tiny fragments called microplastics, a term first coined by Prof Richard Thompson.

• Thompson found small quantities of microplastics in a range of fish species in the English Channel. "This in my opinion doesn't constitute a risk in terms of human consumption, but I am concerned about the longer-term," he told the BBC.

• A study from Ghent University in Belgium found microplastics in mussels and oysters.

• Particles smaller than 2mm wouldn't get caught by Slat's system, but removing larger pieces would help to prevent the quantity of microplastics from increasing.

One problem is that plastic isn't just floating on the surface, but found throughout the water column, even in sediment at the bottom of the ocean. Dr Kerry Howell, a deep-sea researcher at the University of Plymouth, told the BBC that she has found rubbish in the deepest parts of the ocean. "You're going to a place no-one's ever been to before, you're going to the last frontier on earth, exploring new places, and you find that our litter's got there first," she says. "It's like going to the moon and finding a crisp packet."

Another issue is the potential effect on wildlife. "In terms of biological damage the concept is flawed," says van Franeker. "They say anything alive will be able to swim under the curtain, but some, like the fish eggs, will be trapped with the plastic which means they will still be there to be eaten by albatrosses - and in 10 years' time you will take away all the fish eggs along with the plastic."

Aside from the question whether the Ocean Cleanup technology could work, there is also the question whether it should be a priority.

"It seems a foolish strategy to focus on approaches to take litter out of the oceans, when we could prevent it from getting there in the first place," says Prof Richard Thompson of Plymouth University.

"If I had a sum of money to invest in the problem then I would spend 95% of it on approaches to stop the plastic from entering the oceans. Of course we want to find ways to remove litter but we shouldn't delude ourselves. It's like trying to mop up the bathroom floor while leaving the bath overflowing and the taps turned on full."

The mop analogy is one that Slat has heard often, and it really gets him fired up. "First of all, the 'mop' hasn't been invented yet so it certainly can't do any harm to try," he says. As for focusing solely on prevention, he feels it is an "uninspiring and demotivating message to say 'The best we can do is not make it worse'."

He adds: "Of course it shouldn't be an excuse to pollute, but I think it's a motivating message that it's not a hole that's too deep to climb out of."

A sea lion in Lima, Peru, struggles to return to the water because of all the plastic on the beach

Nicholas Mallos disagrees. "The risk is that people think there is one device that will solve our problem in a few years - this grossly over-simplifies the problem," he says.

"He has bright ideas about how to get plastic of various sizes out of the water, but it would be better if he directed his efforts to smaller-scale projects in river outlets," says van Franeker. "Only a fraction of the money would be necessary and it would be more effective."

Several other companies are now emerging with clean-up technology designed to capture plastic in rivers and streams, like the Plastic Visser ('plastic fisher') which is being trialled in the Netherlands, or the Trash Wheel - a solar-and water-powered barrier being used in Baltimore harbour.

Slat, too, is looking to develop spin-off technologies for use in rivers. "It is difficult to adapt something that works in rivers to the sea, whereas it's actually quite easy to adapt something that was developed for the worst conditions in the world - the sea - to work in rivers," he says. "That is why we're approaching it in this order."

At this point he is planning to stay in the Netherlands. "A lot of big names are here that I have to work with so it's a very suitable place to be - it's like the Silicon Valley of the off-shore industry," he says. "Perhaps [the Dutch] are under the impression that through engineering everything can be solved, and we're pretty good at mastering the ocean."

Meanwhile, as his friends lead normal carefree lives, Slat still works 15-hour days. "I haven't seen my friends for ages, they try to annoy me by telling me how fun university is," he says.

Slat doesn't think his youth has held him back, if anything it may have been an advantage: "Not only does it make the story more appealing, but I think I'm very enthusiastic about my concept and that really helped," he says.

Besides, he had everything to play for. "I had nothing to lose except my study income, so it was not a worry," he says. "If you want to do something, do it as soon as possible." A rallying cry to teenage inventors everywhere.

Boyan Slat spoke to Outlook on the BBC World Service. Listen again on iPlayer or get the Outlook podcast.

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