An exhaust gas cleaning system, also referred to as a scrubber, can take three forms; open-loop scrubber, closed-loop scrubber, and hybrid scrubber. Today, approximately 80 % of exhaust gas scrubber systems installed on vessels are open-loop scrubbers due to their low installation and operation costs. About 17 % of all scrubbers are hybrid scrubbers as they deliver some insurance in terms of compliance. With merely 2 %, the least installed scrubber is closed-loop scrubbers despite it being the best solution to comply with the tightening environmental regulations. There are significant differences between the scrubbers and their impact on our ecosystems, wildlife, and human health.


Open-Loop Scrubber

An open-loop scrubber utilizes seawater to scrub the exhaust gases. Thus, this method exploits the natural chemical composition of seawater. The seawater is pumped into and fed to the exhaust gas cleaning system, and afterward, it is discharged. Still, the discharged water must abide by the scrubber wash water discharge limits set by the IMO. Learn more about the scrubber water discharge limits here. 

In order to obtain this water quality, huge quantities of seawater are pumped into the system. This ultimately delivers low turbidity, low pH, low temperature, as well as a low concentration of polycyclic aromatic hydrocarbons (PAHs). As this process demands large quantities of seawater, much wash water is generated, which is both more acidic and turbid than natural seawater. This is discharged overboard without any wash water treatment, which harms ecosystems and wildlife as the water can contain gaseous and particulate emissions, particulate matter (PM), PAHs, heavy metals, nitrate, and unwanted particulates such as soot and ash. Moreover, as this solution requires much water, it requires a large pumping capacity. 

Simply put, open-loop scrubbers move the pollution from air to sea, causing ocean acidification. To combat ocean acidification, several countries have adopted an open-loop scrubber ban. See the complete list here. If a vessel operating an open-loop scrubber wants to enter these countries or the ECA, it must convert to a hybrid scrubber or a closed-loop scrubber. Alternatively, it can also switch to a more expensive compliant fuel containing a lower amount of sulfur or an alternative fuel with no sulfur. To accommodate this, an increasing number of open-loop scrubbers are designed as hybrid-ready, making it easy for shipowners to upgrade to a hybrid scrubber.

Additionally, a paper conducted by Ship & Offshore on Open-loop scrubber discharge bans identifies that open-loop scrubbers are not effective if salinity levels are low. If salinity is low, more water needs to be pumped into the system. Both freshwater, brackish water, and seawater are not adequate for an open-loop scrubber at high ambient temperature. Therefore, an open-loop scrubber is highly dependent on its operating conditions. This makes it an inadequate technology for areas where the salinity levels are low.Open-Loop Scrubber Illustration - LiqTech Water


Closed-Loop Scrubber

Closed-loop scrubbers typically utilize alkaline-dosed seawater to scrub the exhaust gas.  A closed-loop scrubber only requires half the amount of water to clean the exhaust gas streams than that of its open-loop scrubber counterpart, making it more energy-efficient. 

Seawater is pumped into the scrubber’s process tank, where the pH is adjusted by adding alkalis such as sodium hydroxide (NaOH), also called caustic soda or magnesium hydroxide (MgOH). Dosage can also be inline before the scrubber. Besides this alkaline, no other chemicals are needed to neutralize the scrubber wash-water. From the process tank, the water is pumped through the scrubber tower and back to the process tank.

The bleed flow from the scrubber’s process tank is continuously filtered by a water treatment unit (WTU) to remove particulate matter and PAHs. Additionally, the water quality is reached by adjusting the pH value and maintaining a low temperature by cooling the scrubber water with seawater in a heat exchanger. Thus, the discharged water always abides by IMO’s scrubber water discharge limits.

Closed-loop scrubbers retain the scrubber sludge for safe onshore disposal. Therefore, it is essential to have adequate sludge handling facilities onboard a ship. The sludge can be dewatered with a filter press for further solid and liquid separation before onshore disposal to lower the handling fees. By utilizing a closed-loop scrubber, the wash-water and sludge can be handled correctly and sustainably. The treated water can be discharged without any damages done to our ecosystems and wildlife.

Closed-Loop Scrubber Illustration - LiqTech Water


Hybrid Scrubber

A hybrid scrubber combines the open-loop scrubber and the closed-loop scrubber, which can operate with both seawater and freshwater. Hybrid scrubbers typically operate in open-loop mode at sea and in closed-loop mode in regulated waters, such as in the ECA as well as other areas, which have prohibited the use of open-loop scrubbers and wash water discharge. This means that you can discharge your scrubber wash water at sea where it is allowed while treating it in the seas and oceans where discharge is prohibited. Thus, this solution secures compliance with global and local restrictions. Still, most ships with a hybrid scrubber operate it in open-loop mode whenever possible to avoid scrubber sludge discharge fees. Although this method delivers low OPEX, sulfuric wash water discharged into the oceans harms the environment and aquatic species by causing ocean acidification.


The Conversion to Compliance and a Greener Future

As the open-loop scrubber is highly pollutant and prohibited by many countries, converting it into a closed-loop scrubber or a hybrid scrubber is possible. By doing so, shipowners can continue to utilize the scrubbers they have invested in, and at the same time, they can significantly reduce the amount of discharged sulfuric wash water causing ocean acidification. Learn more about open-loop to closed-loop conversion.

A study conducted by the International Council on Clean Transportation (ICCT) reveals that in 2017, 30 open-loop scrubber vessels dumped 35 million tons of polluted wash water effluent near British Columbia, including 3.3 million tons within the defined critical habitat for endangered and threatened killer whales. Cruise ships accounted for approximately 90 % of these discharges.

Although a hybrid scrubber is compliant with current local regulations as this system can switch between operating in open-loop and closed-loop mode, it is insufficient to avoid harming aquatic species. To fully prepare for a sustainable future and comply with future strict restrictions, the solution is to convert your open-loop scrubber to a closed-loop scrubber. 

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Visit our Marine Knowledge site to learn everything worth knowing about marine scrubbers, wash water treatment, and how to comply with the IMO 2020 sulfur cap.


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01-06-2021

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An exhaust gas cleaning system, also referred to as a scrubber, can take three forms; open-loop scrubber, closed-loop sc...