Abstract
BackgroundThe International Maritime Organization (IMO) has set limits on sulphur content in fuels for marine transport. However, vessels continue to use these residual high-sulphur fuels in combination with exhaust gas cleaning systems (EGCS or scrubbers). Next to high sulphur, combustion of these fuels also results in higher emissions of contaminants including metals and PAHs. In scrubbers, exhaust gases are sprayed with water in order to remove SOx, resulting in acidic washwater with elevated contaminant concentrations discharged in the aquatic ecosystem. The number of vessels with scrubbers is increasing rapidly, but knowledge on washwater quality and impact are limited.ResultsThe scrubber washwater is found to be acidic with elevated concentrations of, e.g. zinc, vanadium, copper, nickel, phenanthrene, naphthalene, fluorene and fluoranthene. Model calculations on the effects of scrubber discharge under scenario HIGH (20% of vessels, 90th percentile concentrations) on the water quality in harbour docks showed a decrease in pH of 0.015 units and an increase in surface water concentrations for e.g. naphthalene (189% increase) and vanadium (46% increase).ConclusionsThe IMO established sulphur regulations to mitigate the impact of high sulphur emissions of the maritime sector. However, the use of open-loop scrubbers as an abatement technology will not reduce their contribution to ocean acidification. In addition, different types of scrubbers discharge washwater that is acutely toxic for aquatic organisms. However, washwater is diluted and the compounds for which a large increase in surface water concentrations was calculated in the Antwerp (Belgium) harbour docks (naphthalene > phenanthrene > fluorene > acenaphthene > vanadium) were not the compounds that already exceed their respective Water Quality Standards (WQS). Nevertheless, the WQS of several ‘priority hazardous substances’ (Water Framework Directive) are already exceeded in the docks and the Scheldt estuary. Since these hazardous substances are also identified in the washwater, scrubber washwater discharge should be discouraged in coastal waters and estuaries with large ecological value.
Highlights
The International Maritime Organization (IMO) has set limits on sulphur content in fuels for marine transport
The washwater in closed-loop scrubbers circulates within the system, contaminants accumulate over time, resulting in higher concentrations of metals and Polycyclic aromatic hydrocarbons (PAH) in the discharged water compared to open-loop mode (2-way ANOVA; metals: F1,323 = 26,7; p < 0.001, PAH: F1,475 = 7.27; p = 0.007) (Fig. 1)
Several pollutants that were measured in elevated concentrations and discharged with the scrubber washwater are identified as ‘priority substances’ (Fluoran, Naph, Ni) or ‘priority hazardous substances’ (Ant, B(a)P, Cd) by the European Water Framework Directive (WFD) and as such are of major concern for European Waters
Summary
The International Maritime Organization (IMO) has set limits on sulphur content in fuels for marine transport. Vessels continue to use these residual high-sulphur fuels in combination with exhaust gas cleaning systems (EGCS or scrubbers). International shipping has a significant effect on air quality, with a contribution to global emissions of more than 20% for nitrogen oxides (NOx), around 10% for sulphur oxides (SOx) and almost 8% for particulate matter (PM) [1, 2] This has a number of environmental consequences [3], such as ocean acidification [4] and disturbance of Teuchies et al Environ Sci Eur (2020) 32:103. Open-loop systems, referred to as seawater scrubbing technology, dominate the market In these systems, the exhaust gases are sprayed with seawater at a high flow rate, and the SOx in the exhaust gas is trapped and converted to sulphurous acid (SO32−) and sulphuric acid (SO42−). ‘hybrid systems’ exist, whereby vessels can shift the scrubber operation between open- or closed-loop mode
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