Potential impact of wash water effluents from scrubbers on water acidification in the southern North Sea

Sulphur Oxides (SOx) in atmospheric ship emissions resulting from the burning of fuel with high sulphur content are known to be harmful to human and ecosystem health. Since January 1st 2020, the International Maritime Organisation (IMO) lowered the previous limit for sulphur content in ship fuel from 3.5% m/m (mass by mass) to 0.50%. In the emission control areas (SECAs), the limit for the sulphur content had been set to 1.0% in 2010 and is kept below 0.1% since 2015. To comply with these limits, ship operators and owners can switch to fuel oil with lower sulphur content (LSFO). Alternatively, they can continue to burn fuel with high sulphur content by using technical means such as exhaust gas cleaning systems (or scrubbers) that reduce the atmospheric SOx emissions to a level equivalent to the required fuel oil sulphur limit. Scrubbers use sea water as cleaning media to remove SOx from the air emissions. There are three main categories of scrubbers: (1) the open-loop scrubbers that continuously discharge their wash water effluent, (2) the closed-loop scrubbers that treat the wash water before it is discharged, and (3) the hybrid scrubbers that can switch from open to closed modes. Scrubbers transform the air pollution into direct marine discharge. As hybrid scrubbers are more likely to discharge their sulphur waste into sea water rather than using land infrastructures, they are hereafter taken as open-loop ones. The effect of SOx contribution from ship on sea water pH is assessed for the English Channel and the southern North Sea by means of a marine biogeochemical model that includes a detailed description of the carbonate chemistry. This model allows testing different scenarios of SOx contribution resulting from the maritime traffic. To this end, realistic scenarios with ship traffic density estimated for the years 2019, 2020 and 2030, assuming a year-to-year ship traffic increase of 3.5% and several SOx pollution reduction strategies have been tested. An additional model simulation with null SOx contribution from the shipping sector is used as a reference level to comparatively assess the impact of each scenario on the sea water pH. Model results show a pH decrease of 0.004 units over the whole domain in case of a 2019-like ship traffic density with 15% of the fleet (in Gross Tonnage) using open-loop and hybrid scrubber systems. For future scenarios, assuming that 35% of the fleet is equipped with open-loop and hybrid scrubbers, the pH is estimated to decrease by 0.008 to 0.010 units in average over the whole domain. The magnitude of pH changes is not evenly distributed through space. According to the model results, the largest pH changes would occur in areas of high traffic density, such as along the Belgian and Dutch coasts and in the vicinity of large harbours such as Rotterdam. Ocean acidification rate attributed to climate change is estimated at 0.0017-0.0027 pH units per year. In comparison, the total pH decrease owing to the use of open-loop scrubbers would be equivalent to 2 to 4 years of climate change acidification on average over the whole domain, and to 10 to 50 years, in more local areas. The cumulative impact of ocean acidification due to climate change and to maritime traffic should therefore be considered in ecosystem assessment studies.

Report, RBINS Publication(s), PDF available