Ships with scrubbers will emit at least 10 gigatonnes (Gt) of scrubber washwater each year absent additional regulations, the International Council on Clean Transportation (ICCT) said in a report.

As disclosed, the entire shipping sector carries about 11 Gt of cargo each year.

Nearly all of the washwater will be emitted by open-loop scrubbers
and hybrid scrubbers working in open-loop mode, with 0.3 million tonnes (Mt) estimated to be emitted from closed-loop scrubbers.

Together, container ships, bulk carriers, and oil tankers are responsible for about 70% of the discharges. Cruise ships are expected to account for 15% of scrubber discharges.

The report used 2019 ship traffic, as a pre-COVID-19 baseline, and considered approximately 3,600 ships that had scrubbers installed by the end of 2020.

The council estimates that real-world discharges might be higher, as the authors used conservative estimates for washwater flow rates and the scrubber-equipped fleet now stands at more than 4,300 ships.

Of the scrubbers installed on the 3,628 ships, open-loop scrubbers are the most common type (85% of all scrubbers installed) and hybrids are second most popular (14%); only 1% of the scrubbers are closed loop.

Bulk carriers are the most common ship type outfitted with scrubbers—1,246 ships, or 34% of all outfitted ships. However, within each ship
type, cruise ships have the largest share of their fleets outfitted with scrubbers: 34% of all cruise ships, the report shows.

 Scrubbers remove sulfur from ship exhaust by spraying a buffer solution, usually seawater, over it and then discharging the washwater overboard.

Numerous organizations have voiced environmental concerns over the contents of the waswater, which is believed to be more acidic than the surrounding seawater.

ICCT said that since it contains polycyclic aromatic hydrocarbons, particulate matter, nitrates, nitrites, and heavy metals scrubber wash water is toxic to some marine organisms and can worsen water quality.

Approximately 80% of scrubber discharges occur within 200 nautical miles of shore.

Global discharge water from scrubbers is distributed unevenly and the report found three spatial hot spots: the Caribbean Sea; the Baltic Sea,
English Channel, and Mediterranean Sea in Europe; and a route through
the Strait of Malacca along the South and East China Seas.

Scrubber discharges also occur in IMO-designated Particularly Sensitive Sea Areas (PSSAs), including the Great Barrier Reef, where about 32 million tonnes (Mt) of scrubber washwater is expected, according to ICCT.

But that represents only 5% of the 665 Mt expected to be discharged in PSSAs around the world. The Baltic Sea PSSA, for example, is projected to receive 295 Mt of discharges, the report shows.

Although several governments have taken preventative measures and banned the use of scrubbers in their ports, internal waters, and territorial seas, many have not.

A total of 16 countries had banned scrubbers either in their ports or
territorial seas as of June 2020.

“We estimate that approximately 421 Mt of discharge water will not be released globally due to these national regulations, and that means that about 4% of potential scrubber discharges are avoided by these restrictions,” the report says.

To further limit scrubber washwater discharges, the report recommends several courses of action, including an IMO regulation to call on ships to voluntarily stop dumping scrubber washwater in PSSAs.

Another potential scenario is mandating a ban on the use of scrubbers by the IMO as a means of compliance with fuel sulfur standards and require that ships use cleaner fuels at all times.

Finally, more countries and ports could ban scrubber discharges in their waters, and flag states could agree to phase out the use of scrubbers on ships flying their flag.

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