Risks associated with ships' scrubber discharge water

Air pollution from maritime transportation is a major environmental pressure. Sulph​ur oxides released from the burning of marine fuels react with water in the air to form acid rain and are responsible for a variety of health and environmental impacts. In 2008, the International Maritime Organization (IMO) adopted regulations to control air pollution from ships, stipulating a progressive reduction in the emissions of sulphur oxide until 1 January 2020, when a maximum 0.50% of emissions could comprise sulphur oxides.To comply with these limits, ships should switch to a fuel with lower sulphur content or install an exhaust gas cleaning system, also known as a scrubber.

The price difference between heavy fuel oil and low sulphur fuels has meant that an increasing number of ships have opted to install scrubbers. The installation of a scrubber allows for the continued use of lower cost fuels that have higher sulphur content. Within the scrubber, the exhaust gas passes through a fine spray of alkaline water which readily dissolves sulphur oxides so that levels are sufficiently reduced in air emissions. Nitrogen oxides, and other contaminants are also washed out. However, the resulting scrubber discharge water is a chemical cocktail of acidifying, eutrophying, and contaminating substances and elements. The impacts of scrubber discharge water can be completely avoided through the use of alternative fuels, such as distilled low sulphur fuels. Distilled fuels have the added benefit that they remove the threat of heavy fuel oil spills from shipping activities.

Discussions had taken place within the IMO to emphasize that air pollution was not just transferred to the marine environment. However, while the number of ships with installed scrubber systems is increasing, scrubber discharge water remains poorly regulated.


A background report details the consequences and impacts of scrubber discharge water, including immediate mortality in plankton, negative synergistic effects, and further impacts through bioaccumulation, acidification and eutrophication in the marine environment.


ICES recommends that if the use of alternative fuels is not adopted, and scrubbers continue to be considered an equivalent method to meet the sulphur emissions limits, then there is urgent need for significant investment in technological advances and port reception facilities to allow zero discharge closed loop scrubber systems, improved protocols and standards for measuring, monitoring and reporting on scrubber discharge water acidity and pollutants, and evidence-based regulations on scrubber water discharge limits that consider the full suite of contaminants.


The full article can be found at ices.dk