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Exhaust Gas Scrubber

The IMO 2020 Sulphur Cap requires the reduction of SOx emissions either by using compliant fuel oil or installing an exhaust gas scrubber. SOx reduction deadline was set to January 2020 and the ban on the carriage of non-compliant fuel is set to March 2020. There are still ongoing debates about the safety of ship scrubber washwater. In this article, we aim to shed some light on the environmental impact of sulfur scrubber washwater.

The Affirmative Speech: Exhaust Gas Scrubbing Is Economical

Exhaust gas cleaning or scrubbing is recognised to be more economical than using low sulfur fuel with payback time as low as less than a year for large ships especially with the current fuel price spread of over $300/MT between HFO and VLSFO (pricing of 6th January 2020). With Kamelia Cleantech scrubbers the payback period is even shorter as the scrubbers are designed to be commercial, compact, and economical to install. Nevertheless, there have been debates concerning scrubbers as they are said to increase the CO2 footprint of the ship in the long run.¹

The well to wake emissions of using exhaust gas scrubber with HFO can be lower than those of using MDO if the same speed is used for the vessels at sea. The increase in energy usage at the refinery for further refining to MDO / MGO is 5-10 %. 7 The well to wake emissions depends on how the ship and scrubbers are operated. Also, the lower combustion temperatures of HFO result in less NOx production at sea.5

The Argument: Is the Scrubber Washwater Safe?

Water-soluble components of exhaust gases are removed by dissolution into washwater. This washwater is then discharged by the ship into the vast oceans and seas. The question has been whether this discharge is safe or not and if it contains harmful substances. Chemical Oxygen Demand (COD) of washwater is also thought to have a potentially adverse impact on aquatic systems.

Tests made in 2011 have also shown that washwater contains arsenic, copper, lead, nickel, zinc, cadmium, chromium, vanadium, and selenium – all metals that are known to be toxic to aquatic life and humans, in higher concentrations.² This could potentially raise an issue regarding the wash water’s safety to the environment. Are these arguments substantial enough to opt out of exhaust gas scrubbers? Let’s take a look at the science behind scrubbers.

Sulphur Emissions in Washwater

Shipping exhausts sulfur emissions typically consist of 95 percent sulfur dioxide (SO2), and the remaining 5 percent is sulfur trioxide (SO3). In a wet scrubber, sulfur oxides are dissolved whereby the sulfur dioxide is ionised to bisulfite (HSO32). and sulfite (SO32-). These are then further oxidised to sulfate (SO42-) in seawater containing oxygen. ³ This produces acidity that is neutralised by the alkalinity of the seawater due to its natural bicarbonate content. The pH of washwater is reduced significantly to around a pH of 3 after the initial buffering capacity is consumed, which is equivalent to that of orange juice.

With respect to the discharge of washwater to the marine ecosystem, seawater naturally consists of a large amount of sulfate; therefore, SOx deposited and dissolved into the surrounding seawater is negligible. 9 The acidic water from the scrubber will be neutralized by the pH of natural seawater, therefore the change of pH is not detectable. Independent studies conducted in Japan have shown that the time to reach the dilution rate of 500 folds dilution is estimated at around 3 seconds and that either any short- or long-term effects on marine organisms cannot be caused by the use of open-loop scrubbers. 6

COD, Nitrates and Heavy Metal Emissions from Scrubbers

Various other particulates also get removed from the gases during the scrubbing process. It has been debated if these particulates are harmful to the environment in the levels that they are present in washwater. The studies conducted in Japan show that if only 5 % of ships are installed with scrubbers, the accumulated concentration of COD and nitrates is less by order of 2000 compared to land-based emission standards.

It has also been demonstrated that the actual amount of heavy metals in the discharge water from scrubber were substantially less than the emission standard for land-based sources in Japan, by order of 100. 6 The average washwater emissions from early adopter ships have also been compared to stricter water quality standards such as the EU surface water standards for inland waters, German Wastewater Ordinance, Annex 6 part 5 of 2010/75/EU Industrial Emissions Directive, Annex II part A of 2013/39/EU, WHO Drinking Water Guidelines, 4th Edition incorporating 1st addendum.

The studies show that the emission levels were well below the requirements set out in the standards. 8 Based on these studies it can be safe to say that while there are harmful emissions from ships, the concentrations of washwater emissions are not significant. 

MARPOL Annexure – Compliance for Washwater Discharge

Marine Exhaust Gas Scrubber manufacturer systems strictly follow the MARPOL Annexure by performing measures and monitoring of washwater before discharging to the sea. IMO requests shipowners to provide a sample and analyse washwater. The regulation requires SOx scrubbers to be certified by various authorities to make sure that they are capable of following strict limits on washwater emissions. Operational audits and in-service tests or continuous emission monitoring are also requested under the regulation to enforce these standards.


Scrubber washwater is safe and has a negligible environmental effect, even in the long run. In a new independent study conducted by Netherlands-based CE Delft, the environmental impact from accumulated washwater in ports are at very low levels and well below applicable regulatory limits.4

The clear guidelines set by MARPOL & IMO require exhaust gas scrubber systems to be accurately monitored for emissions. With the right scrubber manufacturer, exhaust scrubbers remain to be a strong choice for complying with the IMO regulation. In a nutshell, scrubbers enable to mitigate potential risks to human health by reducing harmful substances; not only SOx and PM but also PAHs and other materials that would otherwise be emitted into the atmosphere. 6

For your ship scrubber systems requirements, Kamelia Cleantech provides you with a fully integrated solution. Our 360 Approach combined with our ability to install on-voyage gives our clients confidence in their compliant choice. Send us an email at info@kameliacleantech.com to enquire.


1 https://www.forbes.com/sites/flexport/2019/04/08/imo-2020-what-shippers-need-to-know-now/#601ae2e44812

2 https://www3.epa.gov › npdes › pubs › vgp_exhaust_gas_scrubber

3 EGCSA 2018, originally from Hasselöv and Turner, 2007

4 https://www.seatrade-maritime.com/

5 https://www.maritime-executive.com/article/ten-scrubber-questions-answered

6 Presentation on 19/2/2019 Washwater discharge from open-looped SOx scrubber system Ministry of Land, Infrastructure, Transport and Tourism (MLIT), JAPAN

7 Cleaner Fuels in shipping or are Climate and Cost better off with HFO & Scrubbers, Dr. Elizabeth Lindstad SINTEF Ocean AS (MARINTEK)

8 Compilation and Assessment of Lab Samples from EGCS Washwater Discharge on Carnival ships, Clean Shipping Alliance 2020 Technical Conference February 21st, 2019

9 Karle, I. and Turner, D., Seawater Scrubbing – reduction of SOx emissions from ship exhausts, The Alliance for Global Sustainability (2007) 26p.

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