03 Aug 2023
by Amy McLellan

The trouble with scrubbers - The unintended marine pollution caused by air pollution legislation

The law of unintended consequences has dogged many environmental initiatives, from electric vehicles and their toxic battery components to low emission zones that encourage vehicle owners to take longer routes, thereby burning more fuel and displacing pollution elsewhere.

These unforeseen consequences often hit when regulation is poorly thought out, or denuded as a result of protracted horse-trading during the consultation process. The maritime environment is no stranger to such issues, and the IMO’s 2020 cap on atmospheric sulphur oxide (SO₂) emissions is now facing its own array of apparently unforeseen consequences.

The rules, which lowered the maximum percentage of sulphur from 3.5% to 0.5% for all ships operating worldwide, have been very effective in reducing a pollutant that is well known to be a threat to public health and the environment. Indeed, the regulation’s impact has been visibly noticeable, with the USA space agency NASA drawing on two decades of satellite imagery to show a substantial reduction in artificial “ship track” clouds – the polluted clouds that trail ocean-going vessels.

So far, so good. Yet the law of unintended consequences is dirtying the regulation’s reputation. As early as 2020, for example, the Clean Arctic Alliance claimed that some of the blended very low sulphur fuels introduced to comply with the sulphur cap may contribute to black carbon emissions and thus accelerate Arctic sea ice melt. Climate experts have also pointed out that the rapid reduction in SO₂, which has a strong cooling effect on the climate, is likely to have contributed to this year’s spiking temperatures. The impact of reduced SO₂ had been factored into many climate models, but the sudden drop in emissions from January 2020 was more dramatic than had been anticipated.

Now, it seems, there’s the potential for further unintended consequences – and potentially another brace of regulation to deal with them. Most ships complied with the sulphur cap by switching to cleaner fuels but others opted for the “alternative mechanisms” allowed under the regulations, which allowed them to continue burning heavy fuel oil as long as the SO₂ was captured using an exhaust cleaning system, typically known as a scrubber.

According to data from classification society DNV, scrubber installations ramped considerably in 2019, in preparation for the IMO’s 2020 deadline. Then, between 2020 and 2023, 644 additional scrubbers were installed, and more are coming with the scrubber fleet expected to tally at least 5,061 scrubbers by 2025. Of these, 81% are predicted to be open-loop (4,097), which are the least expensive to install and operate but also the most damaging to the marine environment. According to Bryan Comer of the International Council on Clean Transportation (ICCT), these open-loop scrubbers “suck in seawater, spray it into the exhaust, and discharge it overboard, often without treatment”.

Seventeen per cent of the scrubber fleet are hybrid systems, but will typically mainly operate in “open” mode, and around 1% are closed-loop, where an onboard tank of freshwater is filtered and recirculated with only a small amount of bleed-off water discharged overboard. With closed-loop scrubbers, the scrubber sludge must be stored onboard until it can be safely processed onshore.

Inevitably, the increase in the scrubber fleet is giving rise to concerns about the impact of all that washwater on the marine environment. Many of the contaminants found in scrubber discharge water are highly hazardous substances including toxic metals, particulate matter, nitrates, nitrites and polycyclic aromatic hydrocarbons. These pollutants have been linked to cancer and reproductive dysfunction in marine mammals.

Many are also concerned about the complexity of the contamination in ports and coastal areas, where scrubber washwater may interact with pollutants from biofouling coatings and bilge water. When the IMO’s Group of Experts on Scientific Aspects of Marine Environmental Protection (GESAMP) reviewed the scientific literature, it concluded there was insufficient scientific information to enable evidence-based decision-making but did caution that “large-scale use may lead to the deterioration of environmental status, especially in the ecologically vulnerable and sensitive areas such as coastal waters, semi-enclosed seas and also in ports and harbours.”

Some are now worried that updated guidelines issued last year from the IMO’s Marine Environment Protection Committee to assess the environmental impact of discharge water are inadequate to deal with the increased risks. In the resulting regulatory vacuum, many maritime authorities are putting in place their own rules around scrubber washwater, mainly targeting the use of open-loop scrubbers, and typically going further than the IMO requirements. According to the ICCT, there are around 93 measures in 45 countries, with 86 per cent being bans rather than more limited restrictions. 

“Untreated open loop and hybrid scrubber effluent discharge is becoming more and more unpopular,” says Sahan Abeysekara, BWMS Principal Specialist at Lloyd’s Register. “Some administrations consider the waste to be subjected to the 'London convention', whilst some propose that effluent discharge should be referred to GESAMP before approval is issued.”

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Sahan Abeysekara, Lloyd’s Register

This is challenging for vessel operators. Varying local requirements create confusion, add costs and could potentially provoke non-compliance, giving rise for calls for some harmonisation of regulatory approach.

Against the painstakingly slow grind of international regulation, technology continues to advance. “Many scrubber manufacturers claim their latest systems can clean washwater more thoroughly than earlier generations as the technology advances and operational experience is gained from active installations,” notes an explainer from IMarEST.

While there’s no doubt that close-loop scrubbers are currently the most environmentally-friendly option, ship operators will inevitably weigh this against other considerations such as higher capex and opex and footprint challenges.

But as Sahan Abeysekara of Lloyd’s Register points out, this is an issue that is not going away. “There is currently external pressure on the shipping industry to act on ship discharges and the recently adapted UN High Seas treaty (BBNJ) is a good example of positive change and further clarity for the industry,” he says.

Highlighting the founding principles of the UN High Seas treaty - There is a duty not to transfer damage or hazards or transform one type of pollution into another - Abeysekara adds: “Pollution cannot simply be ignored and pushed ‘out of mind and out of sight.”

It may be an unintended consequence but it’s one that the industry cannot afford to ignore.

Interested in learning more about sustainability initiatives in shipping? Read our recent article on green shipping corridors, or join the IMarEST’s Ship, Energy & Environment SIG