Abstract:

A rapidly growing number of ships are being fitted with exhaust gas cleaning systems, or “scrubbers,” as a way to comply with the International Maritime Organization’s (IMO) 2020 global fuel sulfur limit. Scrubbers remove sulfur from ship exhaust by spraying a buffer solution, usually seawater, over it and then discharging the washwater overboard, often without treatment. The washwater is more acidic than the surrounding seawater and contains polycyclic aromatic hydrocarbons, particulate matter, nitrates, nitrites, and heavy metals including nickel, lead, copper, and mercury. Scrubber washwater is toxic to some marine organisms, harms others, and can worsen water quality. This report is the first global assessment of the mass of washwater discharges expected from ships using scrubbers. The authors used 2019 ship traffic, as a pre-COVID-19 baseline, and considered approximately 3,600 ships that had scrubbers installed by the end of 2020. Results show that absent additional regulations, ships with scrubbers will emit at least 10 gigatonnes (Gt) of scrubber washwater each year. For context, the entire shipping sector carries about 11 Gt of cargo each year. Real-world discharges might be higher, as the authors used conservative estimates for washwater flow rates and the scrubber-equipped fleet now stands at more than 4,300 ships.