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Study: Open-Loop Scrubbers Contribute to PAH Pollution in the Baltic

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Published Oct 20, 2022 5:55 PM by The Maritime Executive

A new study by researchers at Chalmers University in Sweden suggests that washwater from open-loop scrubbers is responsible for as much as nine percent of certain pollutants in the Baltic Sea. The authors noted that the study's dataset was collected four years ago, and the number of scrubber-equipped vessels in the Baltic has since tripled. 

The study, published in Marine Pollution Bulletin, looked at all sources of polycyclic aromatic hydrocarbons (PAHs) and heavy metals entering the Baltic Sea. This includes releases from shoreside industries, vessels, river runoff, atmospheric sources, recreational boats and merchant ships. Using AIS to track ships known to be equipped with scrubbers, the researchers modeled washwater release from 180 vessels passing through the Baltic. 

Based on this data, they estimated that scrubber-equipped vessels collectively discharged about 50 billion gallons of washwater into the Baltic over the span of 2018. The estimate is conservative, the researchers said: they relied on an outdated IMO paper to estimate average washwater flow rate, and an updated flow rate would suggest a tripling of the release of pollutants. In addition, the number of scrubber-equipped vessels transiting the Baltic has tripled since the dataset was collected. Both factors suggest that the release quantities could be higher today than estimated in the publication. 

The share of PAHs entering the water from scrubber-equipped ships ranged up to nearly nine percent for key compounds, according to the authors. The other 8,700 ships operating in the region lack scrubbers and operated on MGO, which contributes much less to this pollutant category. Total PAH emissions from shipping in the Baltic measured in the hundreds of kilograms per year, but were dwarfed by multi-tonne depositions of PAHs from pollution in the atmosphere (rainwater). Still, for certain compounds, open-loop scrubbers contributed a measurable share: about 8.5 percent of the PAH anthracene released in the Baltic that year came from scrubber washwater. The authors called for international regulations to ensure the use of closed-loop scrubber operation or MGO throughout the Baltic Sea, and noted that Sweden is currently working on a more limited set of restrictions on scrubber discharge. 

Separately, vessel antifouling paints were found to be a leading source of copper, a heavy metal with biocidal properties. Shipping-related releases of copper were estimated in the range of 500 tonnes per year, substantially less than contributions from river inflows, which were estimated to be about 850 tonnes per year.