According to the Exhaust Gas Cleaning Association, it is estimated that approximately 10 % of sulfur oxide (SOx) emissions stemming from human activities are caused by the global shipping industry. To lower this significantly, the International Maritime Organization (IMO) has implemented an international sulfur cap.

With the new global IMO sulfur cap, the exhaust must be lower or equivalent to using fuel with 0.1-0.5 % sulfur. 

The limit for sulfur in ships’ exhaust gas is lowered significantly from 3.5 % m/m (mass by mass) to 0.5 %. The IMO sulfur cap does not affect the Emission Control Areas (ECA). ECA is defined by the Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL) Protocol and represents peculiar sensitive waters, such as the Baltic Sea, the North Sea, most of the US, including the Caribbean, and Canada. Here, the upper limit for sulfur in the exhaust gas is as low as 0.1 % m/m. With the new IMO 2020 regulation, the amount of harmful sulfur oxide (SOx) emissions emitted from ships and vessels is, thereby, reduced by as much as 77 %, which delivers an annual drop of approximately 8.5 million metric tons of SOx. This reduction benefits the environment and human health, especially people living in port cities and coastal areas of Asia-Pacific, Africa, and Latin America.


Marine Scrubbers

A scrubber is an exhaust gas cleaning system, which removes particulate matter and harmful substances, such as sulfur oxide (SOx) and nitrogen oxide (NOx) from exhaust gas streams emitted by ships.

There are three kinds of scrubbers, being open-loop scrubbers, hybrid scrubbers, and closed-loop scrubbers. Open-loop scrubbers have previously been preferred due to their low installation and operational costs. Still, this method does not deliver full compliance as many countries have already prohibited open-loop scrubbers and the discharge of scrubber wash water to prevent ocean acidification, and many ports around the world are soon expected to join the ban. Learn more about which countries have adopted a ban on open-loop scrubbers and scrubber wash water discharge. Therefore, our focus is on closed-loop scrubbers and hybrid scrubbers as these are the methods for compliance and a sustainable future.

How Does a Closed-Loop Scrubber System Work

A closed-loop scrubber typically has three basic components:

  1. A scrubber tower to treat exhaust gas
  2. A scrubber wash water treatment unit (WTU)
  3. A sludge handling facility

1. The Scrubber Tower

As a vessel operates, its engine generates exhaust gas streams due to combustion processes in the engine, which, among others, contains harmful sulfur oxides (SOx). Before the shipping industry started to adopt exhaust gas cleaning systems, all the exhaust went directly into the atmosphere, causing great harm to our planet and human health. But by utilizing a scrubber, up to 98 % of the SOx emissions are eliminated as the exhaust will move through a scrubber tower, where the gases are scrubbed with water to eliminate pollutants. A closed-loop scrubber typically employs seawater with added alkalies such as sodium hydroxide (NaOH), also known as caustic soda, or magnesium hydroxide (MgOH) to scrub the exhaust gases. Yet, a closed-loop scrubber can also utilize seawater. The scrubbing process will turn the SOx from the exhaust gas into sulfates such as Na2SO4, which is harmless to the environment. A scrubber tower is typically placed high up in the ship or close to the funnel area due to available space and easy access.

2. The Scrubber Water Treatment Unit (WTU)

Although exhaust gas cleaning systems bring about clean exhaust, they generate dirty scrubber wash water. In order to treat this, closed-loop scrubbers require a water treatment unit (WTU). A water treatment unit treats the wash water bleed-flow so it complies with present scrubber water discharge limits on turbidity, pH, temperature, and PAH. Read more about the discharge limits for scrubber wash water here.

Marine scrubber water treatment can remove harmful particulate matter, such as unburned fuel oil, soot particles, ash, PAH, and heavy metals from the scrubber wash water so these elements do not end in the oceans. Closed-loop scrubbers recirculate the wash water between the scrubber tower and a process tank. A continuous bleed flow from the process tank is treated and filtered. This process ensures that no polluted scrubber wash water is discharged into the oceans.

A water treatment unit can be installed on new scrubbers or be retrofitted onto existing scrubbers.

Marine Scrubber Wastewater System

Based on more than 20 years in the filtration industry, we have developed an innovative and efficient water treatment unit based on patented membrane technology. The LiqTech marine scrubber water treatment unit (WTU) is compact, which means that it solely takes up a small footprint of your vessel. Add to this, the design is modular to secure a flexible installation that matches your needs. The WTU is highly efficient, which provides efficient and high-quality scrubber water treatment in compliance with current and future discharge limits set by the IMO and local authorities.

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3. The Sludge Handling Facility

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 of concentrated pollutants is stored separately, where it can be further dewatered into a dry cake of up to 80 % dry matter with a filter press before it is discharged in port. A filter press is acknowledged as one of the most efficient and cost-effective tools to dewater sludge. When sludge is dewatered, its weight is reduced significantly, which can ultimately lead to lowered handling costs. Learn more about a filter press here.


Want to learn more?

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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