Background : The 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.
Results : The 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 (20% of vessels) discharge on aquatic systems showed a decrease in pH of 0.015 units and an increase in surface water concentrations for e.g. naphthalene (110% increase) and vanadium (17% increase).
Conclusions : The 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 the acidification. In addition, different types of scrubbers discharge washwater that is acute toxic for aquatic organisms. However, washwater is diluted and the compounds for which a large increase in surface water concentrations was calculated (Naphthalene > Phenanthrene > Fluorene > Acenaphthene > Vanadium) were not the compounds that already exceed their respective Water Quality Standards (WQS). Nevertheless, existing WQS exceedances of ‘priority hazardous substances’ (Water Framework Directive) that are also identified in the washwater indicate that coastal waters and estuaries, often with large ecological value, are already under pressure. In these areas the discharge of scrubber washwater should be discouraged.