Ships’ scrubber discharge accounts for over 90 pct of marine contaminants, study reveals

Ships’ emissions of metals and hazardous substances pose a significant threat to the marine environment, warns a new study conducted by Chalmers University of Technology in Sweden.

Illustration; Scrubber; Image credit: Wartsila

The research found that water discharged from ships’ scrubbers, designed to clean exhaust gases, accounted for over 90% of the contaminants when assessing the contaminant load in four ports.

Scrubbers are exhaust gas cleaning systems that enable ships to comply with regulations set by the International Maritime Organization (IMO). However, while these scrubbers prevent airborne sulphur emissions, they introduce other contaminants such as heavy metals and toxic organic compounds into the water, which is often discharged directly into the sea without intermediate treatment, the study adds.

More than 90 per cent of the environmentally hazardous metals and PAHs (polycyclic aromatic hydrocarbons) came from scrubber discharge water, while antifouling paints accounted for the biggest load of copper and zinc,” the study shows.

“The results speak for themselves. Stricter regulation of discharge water from scrubbers is crucial to reduce the deterioration of the marine environment,” says Anna Lunde Hermansson, a doctoral student at the Department of Mechanics and Maritime Sciences at Chalmers.

Hermansson with colleagues Ida-Maja Hassellöv and Erik Ytreberg are the authors of the new study that looked at emissions from shipping from a cumulative perspective.

Previous environmental risk assessments focused on individual emission sources, such as antifouling paints. However, shipping involves multiple sources of emissions, making a cumulative assessment necessary.

The emissions from a single ship encompass various types, including greywater, blackwater, antifouling paint, and scrubber discharge water. To address this, the study examined emissions from shipping ports cumulatively.

 Actual data from Copenhagen and Gdynia were used for two of the ports. They were selected due to high volumes of shipping traffic, and a substantial proportion of these ships having scrubbers.

The findings revealed that three out of four port environments presented unacceptable risks due to emissions from antifouling paint and scrubber discharge water, which were the primary sources of hazardous substances.

In addition, the results showed that the cumulative risk levels in the ports were, respectively, five and thirteen times higher than the limit that defines acceptable risk.

Only the port environment with the highest water exchange, facilitating significant water turnover due to tidal movement, showed an acceptable risk level. The researchers stressed that contaminated water does not vanish but is transported elsewhere, potentially reaching pristine sea areas and causing even greater consequences.

The study demonstrates that a single source assessment of emissions can underestimate the risk level for environmental damage. When combining multiple emissions sources, the cumulative risk becomes unacceptable. The marine organisms affected by contaminants do not differentiate their source, emphasizing the importance of considering the total load rather than individual emissions.

“It’s important to remember that the contaminated water doesn’t just disappear – it is transported elsewhere. In the port environments studied, there might be a kind of acceptance of environmental damage – that in this particular environment we have decided that we will have an industry and that it will result in pollution. However, when the contaminated water is washed out to sea, it can end up in pristine sea areas and have even greater consequences. This is something we address in our research. We look at the total load, how much is actually discharged into the environment,” says Lunde Hermansson. 

Ships with high fuel consumption primarily install scrubbers due to their economic viability. Although the use of heavy fuel oil contradicts the IMO’s emission reduction commitments, the number of ships equipped with scrubbers has been increasing.

The Swedish Agency for Marine and Water Management, together with the Swedish Transport Agency, has proposed prohibiting the discharge of scrubber water into internal waters. While this is a step in the right direction, researchers advocate for a more extensive ban across larger marine areas to regulate international shipping effectively.

The risk assessment method

The research conducted by Chalmers University of Technology employed a bottom-up approach to assess environmental risk in ports, combining emissions data from various sources. The resulting cumulative risk assessment provides a comprehensive evaluation of the environmental impact within a specific area.

Image credit Chalmers University

In Step 1 (at the bottom), the loads from various emission sources from shipping were calculated. The volumes were estimated using STEAM, a model that assesses emissions from ship traffic. The volumes were combined with specific concentrations of the substances within each emissions source to calculate the load of the different substances. 

In Step 2, the daily load was used to estimate the resulting concentrations in the environment, called PEC or predicted environmental concentration, using the MAMPEC model. The model calculates PEC for a defined environment (in this case a port), using the properties of the substances (in this case of 9 metals and 16 polycyclic hydrocarbons) and the daily load of the substances (from Step 1). MAMPEC calculates PEC for one substance at a time at a constant load.

In Step 3, the results were then combined to enable the inclusion of more substances as well as loads from different emissions sources simultaneously. To calculate the environmental risk, the PEC is compared with the limit values that represent the concentration that can be considered safe, in other words, where no negative effect on the marine environment is found. This is also known as the PNEC (Predicted No Effect Concentration). If the PEC is higher than the PNEC, it is said that there is an unacceptable risk.

In Step 4, the risk characterisation ratios (RCRs) from multiple substances were added together, which means you can calculate the cumulative risk and present a more comprehensive environmental risk assessment within an area.

The study, published in the Marine Pollution Bulletin, underscores the urgent need for stricter regulations and a holistic approach to address the cumulative impact of ship emissions on the marine environment. By implementing effective measures, it is possible to safeguard marine ecosystems and minimize the risks associated with contaminant discharge from ships.