At any given moment, huge tankers, vast cruise ships and enormous bulk carriers are sailing the oceans powered by one of the dirtiest fuels on Earth.
Heavy fuel oil (HFO) dominates shipping. To clean up the resulting air pollution, the industry mostly uses exhaust gas cleaning systems (EGCS), commonly referred to as “scrubbers”.
As of mid-2024, 28.3% of the global shipping fleet’s total tonnage (approximately 5,838 ships) was covered by an EGCS. Hundreds more are being installed on new vessels or retrofitted every year.
But these scrubbers have created an environmental disaster. According to a growing group of scientists and campaigners, they have also delayed action on the underlying problem: dirty shipping fuel.
First introduced in the early 2000s, scrubbers can efficiently reduce the levels of sulphur oxides and nitrogen oxides in ship exhaust fumes. These gases are linked with heart and lung diseases, asthma and high mortality rates.
The International Maritime Organization (IMO), the UN body responsible for regulating international shipping, has expressed growing concern over the amount of sulphur oxides that shipping is releasing into the atmosphere. In 2020, it brought in a tough cap on the amount of sulphur permitted in a ship’s fuel oil, cutting it from 3.5% to 0.5%.
To comply, shipping companies could switch to more expensive, low-sulphur fuel. Or they could install scrubbers. The use of scrubbers in the shipping industry duly skyrocketed.
“At the time, shipping companies were genuinely interested in doing better,” says Erik Nøklebye, CEO of the Swedish shipping company Wallenius Lines. “Some truly believed that using EGCS was helping to reduce air pollution while saving them money.”
But the way scrubbers work means the harmful pollutants taken out of the air are often dumped into the water instead.
“There was a lot of ignorance about the real consequences of using scrubbers … it just wasn’t a well-enough-educated decision,” Nøklebye adds. He is now among many in the industry calling for a global ban.
Ships with scrubbers are thought to discharge some 10 billion cubic metres of toxic water into the ocean per year, causing significant environmental damage, according to a 2021 analysis by the International Council on Clean Transportation, a nonprofit research organisation. The cost of this harm to the Baltic Sea area, for example, was estimated to be over EUR 680 million (USD 770 million) between 2014 and 2022.
While a global agreement is yet to be found, countries across the world are bringing in their own restrictions aiming at limiting the long-term damages of this technology. Earlier this year, the IMO discussed a ban on scrubbers in sensitive ecosystems like the Arctic. Campaigners hope ship operators running scrubbers may be forced to adopt a different pollution solution before it’s too late.
Cleaning the air by dirtying the water
The cheapest and most widely used scrubbers are open-loop systems. They continually suck in seawater, spray it into engine exhaust gases, then discharge the resulting mixture into the ocean. An alternative is the closed-loop system, which works in a similar way, but stores the wastewater on the ship until it can be discharged. The third option is a hybrid which allows for switching between an open or closed loop.
A large body of scientific work shows scrubber wastewater contains pollutants including heavy metals, polycyclic aromatic hydrocarbons (PAHs), nitrates, nitrites and sulphates. These substances accumulate in organisms at the base of the ocean food chain, such as marine invertebrates and microplankton. Consequently, they build up in animals that eat them. PAH and heavy metals have been linked to cancer and reproductive dysfunction in marine mammals. Discharges of wastewater in shallow coastal regions lead to these substances accumulating in sediments.
“The harm is undoubted,” says Anna Lunde Hermansson, a marine chemist at the Chalmers University of Technology in Gothenburg, Sweden. Hermansson’s work focuses on the environmental effects of shipping. “Even at concentrations of heavy metals and PAHs as low as 0.0001%, there are adverse effects on marine environments and wildlife,” she adds.
Hermansson acknowledges scrubbers are a valid way to comply with regulations. Her research suggests a scrubber pays off the cost of installation in just a few years by enabling the use of cheaper shipping fuel. She believes these industry gains are comparatively small when set against the severe damage scrubbers cause to the environment, and says there is broad consensus on this point.
Liudmila Osipova, a senior researcher at the International Council on Clean Transportation, says scrubber proponents argue the levels of pollutants in wastewater are too low to cause significant harm once diluted in the sea. “But they don’t consider the over-time accumulation factors, especially when ingested by marine animals,” she counters.
Osipova is also sceptical of the closed-loop systems that are sometimes deemed a greener option. She says facilities able to properly treat the resulting sludge are rare and expensive. Therefore, “you are only going in a circle with the problem – transferring the toxic discharge from the air to either the water [with the open loop] or the landfill [with the closed loop]”. Furthermore, the hybrid systems just raise the risk that “concentrated toxic sludge is dumped in the sea as soon as [a ship is] outside a controlled zone”.
Many shipping industry representatives did not respond to requests for comment for this story. One who was willing to talk was Nicholas Confuorto, a senior advisor at the scrubber company CR Ocean Engineering. In his opinion, ocean-based scrubber discharge is much cleaner than land-based discharge of this waste from ships, and cleaner than regulations demand.
“Clean fuels are not yet available in the quality and quantity that are required,” Confuorto tells Dialogue Earth. “Until such time, fossil fuels will remain the fuel for many shipping companies. The industry cannot sit around and wait – ships must travel reliably with an available fuel. Cleaning the emissions with scrubbers is the best choice now.”
A patchy regulatory map
Shipping is sometimes touted as being at the forefront of decarbonisation and sustainability. The industry hails advances in fuel types, engine efficiency and other technologies. In the Arctic, some stricter regulations are already in place, such as a 2024 IMO ban on the burning of heavy fuel oil. But there are often waivers to new regulations, including using scrubbers.
“We see more and more passenger vessels venturing to the most pristine and delicate areas,” says Kay Brown, Arctic policy director at the Pacific Environment NGO. “They burn gallons of HFO, using scrubbers to comply with current regulations.”
Brown works with local communities and other environmental groups to push for regulations that limit scrubber discharge in delicate Arctic waters. They faced opposition at a January IMO meeting where the issue was discussed. Ultimately it was only agreed that discussions on a scrubber ban for the Arctic would continue.
The IMO declined to comment, instead pointing to a statement on the outcome of the meeting. The IMO has asked countries and international organisations to submit evidence and proposals for a meeting in early 2026.
While progress at the IMO is slow, more and more countries are acting independently. “Worldwide, at least 50 places have a ban or restriction,” says Brown. “The more local bans we manage to implement, the easier it will be to convince the IMO to legislate on a global ban.”
China was among the first to ban the discharge of open-loop scrubbers into some rivers, ports and parts of the Bohai Sea. The ban, which has been in place since 2019, restricts wastewater discharges in China’s emission control areas.
This approach does not stop scrubber wastewater from being discharged in the South and East China seas, but estimates indicate it will avoid up to 89% of discharges close to coastal areas. The Chinese government is working on limiting the environmental impact of shipping more broadly. Inland and river-to-sea ships are increasingly operating on batteries or alternative fuels. 2024 policy aims at financially subsidising the industry to phase out fossil fuel vessels in favour of new, clean-energy ships.
Similar efforts are ongoing in Europe. Discussions at the European Commission, the Convention for the Protection of the Marine Environment of the North-East Atlantic (Ospar) and the Baltic Marine Environment Protection Commission (Helcom) could lead to an amalgamation of individual countries’ bans. The result could be an effective regional ban covering much of the North, Baltic and Mediterranean seas.
While campaigners continue to urge an end to the use of scrubbers, more and more ships continue to adopt this controversial technology, with damage potentially locked in for years to come.
“At the beginning, we were more ignorant on this,” says Nøklebye. “But now, we know enough to understand how bad it is. We must continue to push for a change, and at some point, we’ll win the opposition over.”
