Shipping’s dirty secret: how ‘scrubbers’ clean the air – while contaminating the sea

Told to reduce air pollution, the shipping industry could have switched to cleaner fuels – instead, many vessels turned to special devices that simply dump the toxins into the water

In 2019, Alan Ladd, a marine engineer, was on a cruise ship that was slowing down to give passengers a better view of the Hubbard Glacier – the largest tidewater glacier in North America. Briefly looking away from the harbour seals and orcas, Ladd noticed a stream of black grease, with a rainbow sheen, bubbling to the surface of the water.

“The only reason I saw it was because the vessel had stopped. All of a sudden I could see this pollutant and this soot,” says Ladd, who works with Alaska’s Ocean Ranger programme as one of several independent observers of effluent from shipping. “The thing that really disturbed me more than anything is they didn’t do anything about it.”

What Ladd saw was the result of a decision by the shipping industry to reduce air pollution at the expense of the ocean.

After the International Maritime Organization (IMO) set out to lower sulphur emissions in the atmosphere – which regulators say is harmful to human health – the shipping industry was faced with the choice of switching to cleaner but pricier fuel or installing a system to clean exhaust gases – known as “scrubbers” – that dump the chemicals removed from the exhaust directly into the sea instead.

Scrubbers are dirty and dirt cheap, but as of 2020 more than 4,300 ships globally had installed them – up from 732 ships in 2018.

An aerial view of a huge container ship guided into port by tugs
A container ship at Tsing Yi port, Hong Kong. About 10,000,000,000 tonnes of scrubber washwater discharge enters the oceans a year. Photograph: Suriyapong Thongsawang/Getty

It is a trade-off: clear the skies but contaminate the waters.

“The writing has been on the wall for many years with scrubbers and their environmental implications,” says Andrew Dumbrille, adviser for the Clean Arctic Alliance, a coalition of environmental organisations working to protect the polar region from the impact of shipping.

“The issue is that more ships are going to be installing scrubbers, and so the problems are predicted to get worse.”

The race to install scrubbers only began recently. In January 2020, the IMO – the United Nations body overseeing shipping – announced a new global sulphur cap of 0.5%, reduced from 3.5%. To meet the target, it urged the global shipping fleet to switch to low-sulphur fuel.

But it also allowed for “equivalent” compliance measures, as long as ships reduced their emissions.

Scrubbers have proved to be the cheapest way to do so. The cost of buying and fitting a scrubber is £1.5m to £5m, whereas cleaner fuel is £250-£400 a tonne. The scrubber pays for itself within a year.

“It’s been a loophole for industry to continue burning the cheapest, dirtiest fuels,” says Lucy Gilliam, of Seas at Risk, an association of European environmental organisations.

Scrubbers, which sit in the funnels, or exhaust stacks, of ships, use seawater to spray or “scrub” the sulphur dioxide pollutants from the engine’s exhaust.

Microscopic image of crustaceans
Washwater from ships in the North Sea was found to have ‘severe toxic effects’ on zooplankton, the tiny crustaceans that are food for cod, herring and other fish. Photograph: tonaquatic/Getty/iStockphoto

Most vessels use an open-loop system, meaning that instead of holding waste in a tank to be disposed of at dedicated port facilities, the ships directly dump the acidic wash – up to 100,000 times more acidic than seawater – overboard, says Eelco Leemans, an Arctic marine researcher.

Roughly 10 gigatonnes – 10,000,000,000 tonnes – of scrubber washwater are discharged into oceans annually, according to an International Council on Clean Transportation (ICCT) report on global discharge waste – just less than the total weight of all the cargo transported by ships in a year.

The toxins do not just disappear. Aside from being acidic, scrubbers contain heavy metals that accumulate in marine food chains. The Swedish Environmental Research Institute found that washwater from North Sea ships has “severe toxic effects” on zooplankton, which cod, herring and other species feed on. Meanwhile, a Belgian study found that scrubber discharges contain high concentrations of metals such as nickel, copper and chromium, which all devastate marine ecosystems.

What most concern experts, though, are polycyclic aromatic hydrocarbons (PAHs). These have been linked to several types of cancers and reproductive dysfunction in marine mammals, including southern resident orca in the north Pacific and beluga whales.

“A lot of the discharge is toxic and contains all these nasty substances,” says Leemans, adding: “It’s the whole cocktail together that makes it even worse.”

An IMO spokesperson, Natasha Brown, says scrubbers were developed as an “equivalent” to comply with air pollution limits and the IMO is now looking at the wider issue in response to concerns.

Approximately 80% of scrubber discharges occur within 200 nautical miles of shore, with global hotspots along major shipping routes, including the Baltic Sea, North Sea, the strait of Malacca and the Caribbean Sea, according to the ICCT.

The US has the highest amounts of scrubber washwater discharge, with the UK second, mainly due to its 14 overseas territories, particularly the Cayman Islands.

Cruise ships, such as the one Ladd was on, were early adopters of scrubbers. They account for 15% of scrubber discharge in ports, despite making up only 4% of scrubber-installed ships.

The cruise line routes up the Canadian Pacific coast to Alaska are particularly prone to dumped water pollution. An estimated 200m litres of toxin-laden scrubber washwater are generated on a one-week trip from the north-west US to Alaska and back along the Canadian coast, according to a report by the environmental organisations and West Coast Environmental Law.

For Ladd, the solution is simple: stop using scrubbers. A few nations have done so, restricting or banning the use of open-loop scrubbers in their waters; one of the most recent restrictions came in March in Vancouver, the world’s fourth most polluted port from scrubber washwater.

In 2021, the IMO updated its guidelines for scrubbers, setting stricter limits for open-loop scrubbers on acidity and discharge of polycyclic aromatic hydrocarbons and nitrates. In June of this year, the IMO’s Marine Environment Protection Committee approved additional guidelines on scrubbers. Ultimately, however, UN member states must enforce any action.

And whatever the stricter measures, experts agree it was a misstep by regulators to allow scrubbers at all.

“It’s a huge mistake,” says Gilliam. “We could solve the problem of sulphur pollution by switching to cleaner fuels. But instead we’re just transferring the problem from one place to the other. And that’s really frustrating.”


Richa Syal

The GuardianTramp

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