Ida Maja Hassellöv, Chalmers University of Technology, presenting how scientists can use modeling to assess the various pressures of shipping. Photo: Jenny Rosen

Shipping pollution caused by accidents, such as oil spills from tankers, usually gain quite a lot of attention, but even during normal operation, shipping causes a range of pressures on the marine environment. Ship traffic produces underwater noise, exhausts and combustion particles are being released to the air, and chemicals and nutrients reach the water through liquid waste streams such as discharges of wastewater, scrubber water and residues from tank cleaning, tells Ida-Maja Hassellöv, Professor in Maritime Environmental Science at Chalmers University of Technology, and also part of the shipping group at the Swedish Institute for the Marine Environment (Havsmiljöinstitutet).

“From a marine environmental management perspective, we need to consider the combined pressures from all these different onboard systems from all ships that are operating in an area”, says Ida-Maja Hassellöv. “Despite this, most holistic assessments fail to consider all these different types of impacts.” 

As of today, good environmental status is not reached in the Baltic Sea, neither with regards to eutrophication nor contaminants.

“Every ship that is operating in the Baltic Sea contributes to this. Therefore, we can see that here is great potential in reducing the pressures, and by reducing the emissions and discharges from ships, we can contribute to a better environment in the Baltic Sea”, says Ida-Maja Hassellöv.

Today, almost all ships are equipped with an AIS transponder showing where they are operating and at what speed. This information enables scientists to model for example the total volumes of discharges and emissions to the air. The results have shown that for cupper, as one example, shipping and boat traffic accounts for almost 40 percent of the load to the Baltic Sea.

Tank cleaning and its regulations

A pressure that is not yet included in these models is the pollution caused by tank cleaning. A ship transporting liquids usually carries different types of cargo – a large ship can have up to 60 separate holding tanks. After unloading at port, the tanks need to be cleaned before loading a new type of liquid. This tank cleaning is often done at sea, as is the discharge of the cleaning water.

Depending on the type of liquid that has been transported in the tank, the regulations differ when it comes to procedure before cleaning. For products regarded as the least toxic ones, so-called category Z which includes liquid fertilisers, it is allowed to have 75 litres of the product left in the tank when it’s being washed at sea. For some of the more toxic category Y products, and for all the most toxic category X products it is, however, mandatory to do a prewash at port before the tank cleaning at sea.

“There is no complete ban on discharging, you can always have some products left when you do the operational tank cleaning at sea and the discharge”, says Anna Lunde Hermansson, PhD student at Chalmers University of Technology specialised on marine environmental effects from shipping.

The tank cleaning and following discharge to the sea is allowed as long as the ship is en route, at a speed of more than 7 knots, at least 12 nautical miles from land and at a depth more than 25 metres. The Swedish definition of ‘enroute’ means that a ship can unload its cargo in a port, go out at sea and do a tank cleaning and then return to the same port for loading, something that is not allowed in all countries.

Anna Lunde Hermansson, Chalmers University of Technology, explaining the tank cleaning procedure and how it is regulated. Photo: Lisa Bergqvist

Lack of data

Data is sparse on where and when tank cleaning is performed, but one indication of the frequency is the unknown substances or confirmed chemicals detected by the Swedish Coast Guard, although this only includes spills or discharges that are on the surface and change the surface structure. Between 2014 and 2021 in total 371 such detections were made, some of which in marine protected areas or in the potential spawning areas for the Baltic cod.

On behalf of the Swedish Institute for the marine environment Anna Lunde Hermansson and her colleagues have compiled the knowledge on tank cleaning and its impact on the marine environment in a report published 2022. The task was a tricky one, due to the lack of data, but the scientists finally estimated the total volumes of tank cleaning emissions to about five million tonnes in Swedish waters and at least 12 million tonnes in the whole Baltic Sea.

“If we assume that the ships operate under the legal framework, we can say that what has been discharged is mainly Y and Z products such as sulphuric acid, sodium hydroxide, fatty acids, methyl esters and a number of other things”, says Anna Lunde Hermansson.

The last couple of years, media has on several occasions reported spotted discharges, such as yellow goo on the Swedish west coast, and different chemical spills detected by satellites. In April this year, a large discharge of palm oil was found outside Gotland. These finding can often be the results of tank cleaning, and in some cases completely legal. 

The International Convention for the Prevention of Pollution from Ships, MARPOL, is the main international convention covering prevention of pollution of the marine environment by ships. pollution caused by tank cleaning is covered in its Annex 1 & 2.  Anna Lunde Hermansson, Chalmers University of Technology presenting. Photo: Jenny Rosen

Effects in the marine environment

The different chemicals and oils that are discharged following tank cleaning have different effects on the marine environment, explains Anna Lunde Hermansson. Discharges of oil – mineral or vegetable ones – can, even in small amounts, affect birds such as the long-tailed duck, causing immobility or even death. 

If oil solidified products are eaten by ducks they might suffer from starvation or intoxication, because some of these products can absorb and accumulate other toxins, such as metals or polycyclic aromatic hydrocarbons. Contaminants that sink to the bottom can form slicks on the sediment surface that suffocate the benthic environment, and liquid fertilisers can enhance eutrophication.

“Also, some compounds have acute toxic effects. That means mortality essentially, or more long-term effects of chronic toxicity that leads to sub-lethal effect like impairment of growth or development in different species”, says Anna Lunde Hermansson.

However, the discharges from tank cleaning often contain a mixture of different products, including the wash detergents used. This can cause effects that are yet not known, as for example acid or alkaline products can change the mobility and bioavailability of other compounds added or already existing in the marine environment.

As always, the Baltic Breakfast seminar was livestreamed and recorded. Photo: Jenny Rosen

Proposals from the scientists

To reduce the environmental impact of tank cleaning, the discharged loads need to be reduced, according to the scientists. In a recently published policy brief they therefore propose Sweden to act for a mandatory prewash on all category Y products, including fatty acids, methyl esters and tall oils which are solidifying but are not under the prewash demand today.

“We also want to prohibit discharge of tank cleaning effluents in marine protected areas”, says Anna Lunde Hermansson. “We think it’s important to enforce strict reporting of liquid bulk products and tank cleaning operations for traceability and transparency, so if we have a discharge, we are able to trace it back to a ship. This is also important for the Swedish Coast Guard, because if an accident happens and the Coast Guard doesn’t know what is on the ship, it can turn into something very dangerous.”

Finally, the scientists want to change the Swedish definition of 'en route', so that it is no longer allowed to go out from a port to perform a tank cleaning and then return to the same port. Couldn’t this cause more transport, as ships need to go to another port in order to do a tank cleaning, asks moderator Ellen Bruno from Stockholm University Baltic Sea Centre.

“We don’t want anything to be discharged, but this Swedish interpretation of 'en route' makes it really hard to control what ships are doing; are they just doing this detour or are they actually going somewhere?” explains Anna Lunde Hermansson. “If you want to unload and load in the same port, you could do the tank cleaning in the port. It requires infrastructure, of course, but it could be done. Then at least you don’t get these extra pressures, of going out and taking this unnecessary detour.”

“Shippings is an international business and many people are calling for IMO to implement stricter restrictions, how come you recommend Swedish and HELCOM policy actions?”, asks moderator Ellen Bruno both of the scientists.

“I think you need to work from both perspectives”, says Ida-Maja Hassellöv. “It’s important for the industry to have global rules, but the Baltic Sea is already pointed out as a special area so there is some flexibility, and in coastal waters the countries have possibilities to regulate themselves. I think it’s important to move faster with things we can regulate ourselves in Sweden and in HELCOM, but at the same time strive for a long-term change also globally, but that has historically taken a long time.”

Text: Lisa Bergqvist

Further reading

Report: Tank cleaning and its impact on the marine environment

Policy brief: Minska kemikalieutsläpp till havs från tankrengöring

See a recording of the seminar: