Consulting report

Air emissions and water pollution discharges from ships with scrubbers

The number of ships using exhaust gas cleaning systems, better known as “scrubbers,” has grown from just three ships in 2008 to more than 4,300 in 2020. Although scrubbers are effective at reducing air emissions of sulfur dioxide (SO2), the sulfur and other contaminants removed from the exhaust gas—including carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals—are dumped overboard in the form of washwater, also called discharge water. Meanwhile, the International Maritime Organization’s (IMO) guidelines for scrubber discharges have not been strengthened since 2008, despite being reviewed in 2009, 2015, and 2020, and the guidelines ignore the cumulative effects of many ships operating and discharging in heavily trafficked areas. Such cumulative effects are to be expected given the rapid increase in the number of ships with scrubbers. 

This study estimates air and water emission factors for ships using heavy fuel oil (HFO) with scrubbers based on the available literature and the methods of the Fourth IMO Greenhouse Gas Study. Additionally, the authors compare the emissions associated with ships using scrubbers to ships without scrubbers using marine gas oil (MGO). 

Regarding air emissions, results show: 

  • SO2 emissions from ships using 2.6% sulfur HFO with a scrubber are on average 31% lower than ships using 0.07% sulfur MGO. 
  • Particulate matter emissions are nearly 70% higher using HFO with a scrubber compared with MGO. 
  • Black carbon emissions are 81% higher using HFO with a scrubber than using MGO in a medium-speed diesel engine and more than 4.5 times higher than using MGO in a slow-speed diesel engine. 
  • Scrubbers are therefore not equivalently effective at reducing total air pollution emissions compared to using MGO. Additionally, direct carbon dioxide (CO2) emissions are 4% higher using HFO with a scrubber compared to MGO, and even though HFO has lower upstream emissions than MGO, the extra fuel consumption associated with powering the scrubber results in 1.1% higher CO2 emissions on a life-cycle basis when using HFO. 

Regarding water emissions, the study finds:

  • Scrubber discharges typically comply with IMO guidelines, but all scrubbers—open-loop, closed-loop, and hybrid—discharge water that is more acidic and turbid than the surrounding water. This contributes to ocean acidification and worsens water quality. 
  • All scrubbers emit nitrates, PAHs, and heavy metals that accumulate in the environment and food web and can negatively affect both water quality and marine life. 
  • PAHs and heavy metals have been linked to cancers and reproductive disorders in threatened and endangered marine mammals, including Southern Resident killer whales and beluga whales. 

Given these findings, the ICCT recommends that individual governments continue to take unilateral action to restrict or prohibit scrubber discharges from both open-loop and closed-loop systems. This could include an immediate prohibition on scrubber discharges in ports, internal waters, and territorial seas. Internationally, the IMO should consider prohibiting the use of scrubbers on newbuild ships and phasing out scrubbers on existing ships, because scrubbers are not equivalently effective at reducing air pollution compared to using lower sulfur fuels.


This report was updated on 3 December, 2020 to correct SFOC values in Table 6. Additionally, Table 8 was replaced to correct errors in criteria grading and some text above the table was amended for consistency. The original paper can be found here.