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Authors of a study published by the International Council on Clean Transportation (ICCT) say the International Maritime Organization (IMO) should consider prohibiting the use of scrubbers on newbuild ships and phasing out scrubbers on existing ships.

The study, Air emissions and water pollution discharges from ships with scrubbers, argues that scrubbers are ‘not equivalently effective’ at reducing air pollution compared to using lower sulphur fuels.

It noted that the number of ships using exhaust gas cleaning systems (EGCS) has grown from three ships in 2008 to more than 4,300 in 2020.

While acknowledging their effectiveness at reducing air emissions of sulphur dioxide (SO2), the authors of the report say the sulphur and other contaminants removed from the exhaust gas are dumped overboard in the form of washwater, also called discharge water.

Moreover, the report notes that the IMO’s guidelines for scrubber discharges have not been strengthened since 2008, ‘despite being reviewed in 2009, 2015, and 2020’, and that the guidelines ignore the cumulative effects of many ships operating and discharging in heavily trafficked areas.

The study estimates the 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). 

For air emissions, the results show SO2 emissions from ships using 2.6% sulphur HFO with a scrubber are on average 31% lower than ships using 0.07% sulphur 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,’ ICCT said. ‘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.’

For 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, says ICCT, contributes to ocean acidification and worsens water quality. 

It also found that 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.

‘Given these findings, the authors recommend that individual governments continue to take unilateral action to restrict or prohibit scrubber discharges from both open-loop and closed-loop systems,’ said ICCT. ‘This could include an immediate prohibition on scrubber discharges in ports, internal waters, and territorial seas.’

ICCT continued: ‘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 sulphur fuels.’

The study’s findings have already gained traction with North American environmental organisation Stand.earth which has accused ships that use scrubbers of ‘cheating the global fuel standard’.

Citing the findings of the study, Anna Barford, Canadian Shipping Campaigner, Stand.earth, said: ‘Cruise ships release the majority of this highly contaminated scrubber washwater in sensitive ecosystems. We must bring in rules to protect our oceans from scrubbers, as other countries have already done.’

Barford continued: ‘Now is the time to ban scrubbers, before these polluting floating resorts come back into southern resident killer whale and beluga habitat.’

To read the Air emissions and water pollution discharges from ships with scrubbers study in full, click here.

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