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The study, the first to assess the global impact of scrubber washwater, estimates 80% is discharged within 200 nautical miles of shore, with hotspots including the seas around Europe, the Strait of Malacca and the Caribbean Sea.

The report bases its findings on pre-pandemic traffic patterns for ships that were fitted with scrubbers by the end of 2020. According to the International Council for Clean Transportation (ICCT), approximately 3,600 scrubber-equipped vessels will emit at least 10 billion tonnes of washwater every year for the next several years. To put this into context, the global shipping industry transports around 11 billion tonnes of cargo each year.

The ICCT says it has used conservative estimates for washwater flow rates, while the scrubber-equipped fleet currently stands at over 4,300 vessels.

According to the ICCT’s research, containerships, bull carriers and oil tankers account for around 70% of scrubber discharges. Cruise ships represent only 4% of the scrubber equipped fleet but account for 15% of washwater discharge.

Close to 300 million tonnes of washwater is discharged in ports, and the ICCT lists the top five as Georgetown in the Cayman Islands, Southampton in the UK, Freeport in the Bahamas, Vancouver in Canada and Nassau in the Bahamas.

Cruise ships account for at least 96% of discharges in seven of the 10 ports with the highest total washwater discharges, although this is based on pre-pandemic vessel traffic and operations in the global cruise sector are currently on hold.

The ICCT also notes that ships discharge 665 million tonnes of washwater in IMO-designated Particularly Sensitive Sea Areas (PSSAs) every year. The Council highlights the Great Barrier Reef, ‘where we expect 32 million tonnes of scrubber washwater to be discharged.’

Ships registered to Panama, the Marshall Islands and Liberia account of about 40% of scrubber discharges, with Panamanian-flagged vessels accounting for 18% of total global discharges.

Although scrubbers are effective at reducing sulfur dioxide from ship exhaust, previous ICCT research has asserted that the use of scrubbers results in high amounts of carbon dioxide, particulate matter and black carbon compared with using marine gasoil (MGO).

The most popular type of scrubber is an open loop system, and ICCT says that this constantly discharges large amounts of acidic washwater and contains polycyclic aromatic hydrocarbons (PAHs), particulate matter, nitrates, nitrites, and heavy metals including nickel, lead, copper, and mercury.  The Council also notes that closed loop scrubbers emit the same pollutants in lower volumes, but at higher concentrations.

In its study, the ICCT also suggests some actions that could be instigated by policymakers and IMO and local level to mitigate the impact of washwater discharge.

It notes that the Marine Environmental Protection Committee at the IMO could pass a resolution calling on vessels to stop discharging in protected areas, such as PSSAs.

Another option would be for the IMO to prohibit the use of scrubbers as an equivalent fuel sulphur compliance option for new ships under the International Convention for the Prevention of Pollution from Ships (MARPOL) and establish a timeline for phasing out scrubbers already installed on existing ships.

At a regional or national level, the authorities could prohibit all scrubber discharges in waters under their jurisdiction. Flag states could also decide to phase out the use of scrubbers on ships flying their flags and ports could also ban the use of scrubbers in waters under their jurisdiction.

Alongside its study, the ICCT has also published an interactive map detailing washwater discharge activity.



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