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Exhaust Gas Cleaning Systems (Scrubbers) on Ships: Clearing the Air on the Controversy

6 minute read

Scrubbers effectively remove sulphur oxide pollution from ship exhaust – yet the environmental impact of the discharge water they produce, and how to manage and regulate it, is sparking controversy.

Exhaust gas cleaning systems are a relatively simple technology designed to remove the polluting sulphur oxides from a ship’s engine exhaust. They’re called scrubbers for short.

But despite that modest moniker, scrubbers have generated a giant size controversy that accuses the devices — and the companies that use them — of threatening marine life, contributing to ocean acidification and being complicit in shipping’s “dirty secret.

As part of its mandate to provide the facts around responsible marine shipping and to demystify complex issues, Clear Seas conducted a study on the levels of pollution in the discharge water from scrubbers. Our team of researchers explored the main concerns relating to scrubber discharge water, including levels of heavy metals, polycyclic aromatic hydrocarbons (PAHs), which result from burning oil and gas, the potential local impact of acidic scrubber discharges, and sources of uncertainty in the assessment of scrubber discharge waters.

The evidence from the Clear Seas study supports the conclusions of policy makers and local regulatory bodies who are restricting the discharge from scrubbers in confined waters like estuaries, harbours, and anchorages. Because the subject is controversial, we wanted to tackle some of the questions that come up and answer them in an honest and forthright way. The full report and study highlights are available here.

What are scrubbers and why are they raising concerns?

Since 2005, the global shipping industry has worked to reduce sulphur emissions from commercial ships to improve air quality and reduce the health effects associated with pollution such as heart and lung disease. The main goal has been to limit the amount of sulphur permitted in ship fuel. The latest sulphur regulation, established by the International Maritime Organization (IMO) in 2020, allows for a sulphur content of no more than 0.5% in marine fuel, for ships operating worldwide. To meet this requirement, ships can either switch to a low sulphur type of fuel – which costs up to 50% more than conventional heavy fuel oil1 – or they can install scrubbers.

Scrubbers are a solution approved and regulated by the IMO. They use either sea or fresh water to ‘wash’ engine exhaust gas, removing the sulphur oxides (SOx) from the exhaust gas. Unfortunately, the exhaust gas from ship engines burning fossil fuels also contain heavy metals and PAHs, and these also end up being picked up by the scrubbers along with the sulphur. So, what happens to scrubber washwater and the pollutants it contains? That is where the concerns lie. Depending on the type of scrubbers2 ships have on board and how they are operated, washwater will be discharged into the marine environment, therefore moving pollution from the air to the water.

Environmental advocacy groups have raised concerns that open-loop scrubbers, which continuously discharge the ‘spent’ washwater and its pollutants back into the sea, will reduce water quality, and have called for a ban on the use of scrubbers within Canada’s exclusive economic zone, which extends 200 nautical miles from the shore.3,4 On the flipside, independent studies commissioned by the cruise and ferry sectors have concluded that the impact of open-loop scrubber discharges on port waters are small in relation to environmental quality standards,5 and that their use is a safe and effective means of compliance with the IMO sulphur cap requirements.6

How are scrubbers regulated?

To both mitigate and monitor the environmental impacts of exhaust gas cleaning systems, the IMO has established guidelines that specify requirements for the testing, surveying, certification and verification of scrubbers, for ships operating these system [MEPC.259(68)].7 The IMO sulphur regulations specify that any installed scrubbers (or equivalent means), has to be at least as effective in reducing the amount of sulphur emitted by ships as would be achieved in using low sulphur fuel. The scrubber guidelines also set requirements on the quality of the water discharged overboard from the ships operating scrubbers.8

Some countries have banned the use of scrubbers — or at least the discharge of washwater in their territorial waters (extending 12 nautical miles from the shore) — due to the uncertainty around the impact on the marine environment. A more common approach seems to be to restrict the discharge in ports, harbours, and confined waters. A growing list of ports around the world restrict the use of open-loop scrubbers or the discharge of scrubber washwater within their jurisdictions. The Canadian ports of Vancouver9 and Sept-Îles10 are among them.

How do scrubbers affect the environment?

Either using a low sulphur fuel or scrubbers – any reduction in emissions seems like a win for the environment. But the increasing use of scrubbers has raised a debate on the net benefits achieved in reducing SOx emissions by discharging washwater into the marine environment, a trade-off between air and water quality. A complete assessment of the impact of scrubbers can’t only focus on air emissions and pollution. It must also consider the effects associated with the discharge of scrubber washwater into the marine environment.

When it comes to achieving their primary goal of reducing air pollution from sulphur, scrubbers have been very effective at removing SOx air pollution from ship emissions – even more effective than burning the fuel with the lowest sulphur content.11 However, while the goal of ‘scrubbing’ is to remove SOx, other things come along for the ride. Scrubber washwater picks up other potentially polluting substances contained in the exhaust gas, either non-combusted components of fuel or combustion by-products. Of particular concern is the discharge of water containing heavy metals and PAHs in confined or near-shore waters like ports and anchorages. Discharge water from scrubbers is also acidic, which can negatively affect ocean chemistry (pH level, notably) as well as marine life.

What will happen to scrubber use in the future?

By investing in scrubbers that enable ships to continue to burn heavy fuel oil instead of more expensive low-sulphur fuels, shipping companies can make big savings. But is the true cost of discharging potentially polluted scrubber water into the marine environment correctly factored into the business case? Restrictions on scrubber use close to shore will begin to eat into the savings, but only by a small amount. With uncertainty in global energy supply driving up the price of ship fuel, the use of scrubbers to keep the cost of global shipping low while combatting air pollution looks set to continue.

Got a question? Be a part of the conversation on scrubbers

Do you have a question about scrubbers and their effect on the ocean and the environment. We invite you to submit it to and we’ll do our best to answer it.


1 The price difference between high-sulphur heavy fuel oil and low-sulphur fuels fluctuates daily with global oil prices. More information available here.
2 There are three types of scrubbers: open-loop, closed-loop, and hybrid which can operate in either open-loop or closed-loop mode. Open-loop scrubbers continuously take seawater in from the waters the ship is sailing through and continuously discharge the ‘spent’ washwater back to sea as discharge effluent. Unlike open-loop systems, closed-loop scrubbers recirculate the washwater and only a small volume of is bled-off and treated to remove suspended solids. This bleed-off water can be retained on board or discharged as effluent water.
3 The International Council on Clean Transportation. (2019). Consulting Report: A whale of a problem? Heavy Fuel Oil, Exhaust Gas Cleaning Systems, and Bristish Columbia Resident Killer Whales.
4 WWF. (2020). The Trouble with Scrubbers: Shipping’s Emissions “Solutions” Creates New Pollution.
5 IMO Sub-Committee on Pollution Prevention and Response. (2019). PPR 7/INF.18. EGCS washwater discharges and accumulation levels in port water and sediment. Submitted by CLIA and Interferry.
6 Carnival Corporation & PLC and DNV-GL. (2019). Compilation and Assessment of Lab Samples from EGCS Washwater discharge on Carnival ships.
7 MEPC.259(68). (2015). Guidelines for Exhaust Gas Cleaning System.
8 The current IMO guidelines specify discharge criteria for four parameters: pH, total PAHs, turbidity, and nitrates, yet do not stipulate discharge limits for heavy metals and individual PAHs.
9 Port of Vancouver. (2022). Restrictions on discharge of scrubber wash water take effect.
10 The International Council on Clean Transportation. (2021). Will we soon B.C.-ing a Scrubber Discharge Ban in Vancouver?
11 Comer, B., Elise Georgeff, & Liudmila Osipova. (2020). Air emissions and water pollution discharges from ships with scrubbers. International Council on Clean Transportation (ICCT).