The water in the English Channel and the Southern North Sea is becoming more acidic due to SOx discharges from open loop scrubers. This follows from a study conducted by the Royal Belgian Institute of Natural Sciences on behalf of the Belgian Federal Public Service Mobility and Transport.

The IMO 2020 Sulphur Cap has lowered the limit for sulphur content in ship fuel to 0.5 per cent. For the North Sea Sulphur Emission Control Area (SECA), this limit is even more strict at 0.1 per cent. To comply with these regulations, ships can either choose to use low sulphur fuels or opt for the use of scrubbers.

The latter reduce the sulphur content in air emissions. When a closed loop scrubber is selected, the resulting wash water is collected on board and disposed of in port, but in the case of (cheaper) open-loop scrubbers, the SOx is discharged directly into the water. Here they contribute to ocean acidification and potentially create problems for a range of marine organisms, according to the Belgian Institute.

The Royal Belgian Institute of Natural Sciences used a biogeochemical model to quantify the potential impact in the southern North Sea. The results showed that the largest changes occur in areas of high traffic density, such as along the Belgian and Dutch coasts and in the vicinity of large harbours, where the changes are said to be sufficiently big to contribute to environmental degradation and a loss of economic potential.

Ocean acidification

When the wash water from open-loop scrubbers is discharged into the sea, the SOx are neutralised by the sea water. This however lowers the pH of the sea water (a lower pH means more acidic water) and thus contributes to the acidification of the ocean. This process adds to the ongoing climate-change driven ocean acidification resulting from the uptake of atmospheric CO2.

Negative effects of ocean acidification have already been observed affecting marine organisms such as clams, oysters, prawns and even fish. More acidic waters compromise the creation of shells and skeletons and can lead to the dissolution of existing structures. Furthermore, some studies have shown effects on the ability of fish to smell, hear and see, and their general cognitive functioning. More acidic waters may also have an economic impact for fisheries and aquaculture, as quality loss has been shown for certain species of prawns and clams, in terms of taste, texture, appearance and nutritious properties.

Situation in the Southern North Sea

The study conducted sought to quantify the potential impact of SOx discharges from maritime traffic on water acidification in the southern North Sea. ‘In the English Channel and the Southern North Sea, the results show a pH decrease between 0.004 and 0.010 pH units (on a scale of only 14 units) among different maritime traffic scenarios,’ says Valérie Dulière, lead author of the study. ‘In areas of high traffic density, such as the shipping lanes along the Belgian and Dutch coasts and in the vicinity of large harbours, the pH changes can be 5 to 12 times larger than average. The modelled changes indicate a potential negative impact on the water quality in ports, estuaries and coastal waters.’

The estimated pH decrease attributed to the shipping sector is also significant when compared to the ongoing acidification resulting from climate change (0.0017-0.0027 pH units per year), according to the researchers. The pH change in response to SOx pollution due to shipping with open-loop scrubbers is 2 to 4 times bigger than the contribution of climate change when averaged over the whole study area, and up to 10 to 50 times bigger in more local areas. That is why the researchers state the impact of ocean acidification due to maritime traffic should be considered in ecosystem assessment studies, together with climate change.

Corona may have a positive effect on acidification

After this study was completed, the COVID-19 crisis started to reshape the year 2020 in an unforeseen way. It is observed that the ship traffic density estimation for the year 2020 on which the calculations were based to estimate the quantity of SOx in emissions and wash water discharges is lower than expected. Therefore, acidification will not go as fast either. Nevertheless, this study still brings some very useful information on how the use of open-loop and hybrid (set in open-mode) scrubbers can contribute to the acidification of the southern North Sea, in a business as usual situation.

It is also noted that the current unfavorable economic climate led to the cancellation of many orders for scrubber installations, and the researchers hope that the companies concerned will consider switching to the use of low sulphur fuel when resuming normal operations.

Picture by Karl Baron.