SOx Scrubbers: Open-Loop vs. Closed-Loop Systems for Ships

The maritime sector faces a pressing need to cut sulfur oxide (SOx) emissions. Shipowners are now embracing exhaust gas cleaning systems (also called scrubbers), to meet International Maritime Organization (IMO) standards. These systems are vital for reducing sulfur emissions to 0.50% globally and 0.10% in Emission Control Areas (ECAs). Marine scrubbers can effectively remove up to 98% of SOx emissions, making them essential in the battle against ship-borne pollution.

The debate over open-loop versus closed-loop scrubbers is intense. Open-loop systems use seawater, raising environmental concerns about discharging wash water overboard. On the other hand, closed-loop systems employ freshwater and additives, offering a greener option but at a higher cost. Shipowners must consider regulatory demands, economic factors, and environmental responsibility when making their choice.

Key Takeaways

• Scrubbers can remove up to 98% of SOx emissions from ship exhaust gases

• Open-loop scrubbers use seawater, while closed-loop systems rely on freshwater and chemical additives

• Environmental concerns surround open-loop scrubbers due to wash water discharge

• Closed-loop scrubbers are more eco-friendly but have higher installation and operating costs

• The choice between open-loop and closed-loop depends on regulations, economics, and environmental factors

Understanding SOx Scrubbers for Ships

The maritime industry is under pressure to reduce its environmental footprint and adhere to stricter regulations. SOx scrubbers have become essential for ships to cut sulfur oxide emissions. These systems help vessels meet the International Maritime Organization's (IMO) 2020 standards, which limit marine fuel sulfur content to 0.5% globally and 0.1% in Emission Control Areas (ECAs).

Due to these regulations, some ship owners favour scrubber systems over costly low-sulfur fuels. SOx scrubbers eliminate sulfur oxides from exhaust gases by utilizing seawater or freshwater combined with alkaline additives, neutralizing acidic pollutants before they are emitted.

Role of Scrubbers in Reducing Sulfur Oxide Emissions

Scrubbers are essential in controlling maritime pollution and play a pivotal role in the efforts to mitigate the environmental impact of the shipping industry. Before the implementation of stringent regulatory changes, the shipping sector was responsible for contributing approximately 10% of global sulfur oxide (SOx) emissions, which posed significant health risks and contributed to environmental degradation.

The introduction of scrubber technology has transformed the landscape of maritime operations by enabling vessels to effectively reduce their SOx emissions by as much as 77%. This substantial decrease in emissions is not merely a statistic; it translates into tangible health benefits, including the prevention of an estimated 150,000 premature deaths and millions of childhood asthma cases each year. The operation of scrubber systems involves a sophisticated process that requires careful monitoring to ensure compliance with international regulations.

Specifically, these systems must meet the standards set forth by the Marine Environment Protection Committee (MEPC) under resolution MEPC 184(59). This resolution outlines the criteria for wash water discharge, ensuring that the water released back into the sea does not harm marine ecosystems. The rigorous monitoring of scrubber performance and wash water quality is crucial, as it helps maintain the balance between maritime operations and environmental protection.

IMO Regulations Driving the Adoption of Scrubbers

The IMO 2020 regulations have been a major driver for scrubber adoption in the shipping industry. These rules mandate a sulfur content reduction in marine fuels from 3.5% to 0.5% globally & down to 0.1% within SECA areas. Shipowners are turning to scrubbers as a compliance solution, allowing them to use high-sulfur fuels while meeting emission standards.

The marine environmental compliance landscape is rapidly evolving. An estimated 4,800 vessels are equipped with scrubbers (as of 2025). Scrubbers offer a temporary solution until cleaner alternatives (alternative fuels) are fully developed & readily available or ship bunker operations.

Types of SOx Scrubbers

There are three main types of SOx scrubbers: open-loop, closed-loop, and hybrid systems. Each has its own set of characteristics and benefits.

Open-Loop Scrubbers

Open-loop scrubbers use seawater as the scrubbing medium, relying on its natural alkalinity to neutralize SOx emissions. These systems are simple and cost-effective, making them appealing to many shipowners. They can eliminate up to 98% of SOx emissions, significantly reducing air pollution and ensuring compliance with IMO regulations.

Closed-Loop Scrubbers

Closed-loop scrubbers, on the other hand, use freshwater mixed with alkaline additives, such as caustic soda, in a closed system with minimal discharge. This method offers greater control over the scrubbing process and reduces environmental impact. Closed-loop scrubber advantages include the ability to operate in areas with strict discharge regulations and the generation of less wastewater compared to open-loop systems.

Scrubber Type	Scrubbing Medium	SOx Removal Efficiency
Open-Loop	Seawater	Up to 98%
Closed-Loop	Freshwater with Alkaline Additives	Up to 98%

Hybrid Scrubbers

Hybrid scrubbers for ships combine the benefits of open-loop and closed-loop systems. These exhaust gas cleaning solutions can operate in both modes. Shipowners can adapt to varying environmental regulations and operational requirements. Hybrid scrubbers use open-loop mode while at sea and switch to closed-loop mode in port or in areas with strict discharge regulations.

In 2021, vessels equipped with marine scrubbers earned between $14,000 and $20,000 increased charter revenue per day during voyages, effectively doubling their profits compared to vessels without scrubbers.

The shipping industry's focus on sustainability and compliance with environmental regulations is driving the adoption of SOx scrubbers, notably hybrid systems. This trend is expected to grow in the coming years.

Open-Loop Scrubbers: Functionality and Characteristics

Open-loop scrubbers are an essential marine pollution solution for ship exhaust treatment. They enable vessels to comply with the sulfur cap regulations established by the International Maritime Organization (IMO). These systems utilize seawater to neutralize sulfur oxides (SOx) in exhaust emissions, making them a more economical alternative to low-sulfur fuels.

Seawater as the Scrubbing Medium

The efficiency of open-loop scrubbers relies on the natural alkalinity of seawater, which neutralizes acidic SOx emissions. The scrubber system uses significant volumes of seawater to effectively eliminate SOx pollutants. The processed wash water is subsequently discharged back into the ocean, raising environmental concerns.

The scrubbing efficiency can vary based on seawater's alkalinity and salinity. In areas with low salinity or high temperatures, efficiency may drop. This makes open-loop systems less ideal for these regions.

Wash Water Discharge and Environmental Concerns

The release of treated scrubber wash water into the sea raises environmental worries. Concerns include ocean acidification and the release of pollutants like heavy metals and PAHs (Polycyclic Aromatic Hydrocarbons).

According to studies, marine ecotoxicity damage costs from scrubber water discharge in the Baltic Sea Area from 2014–2022 exceed €680 million euros.

Closed-Loop Scrubbers: Functionality and Characteristics

Closed-loop scrubbers have emerged as an essential ship emissions reduction technology, vital for complying with IMO standards. They utilize freshwater combined with alkaline substances, such as sodium hydroxide (NaOH), to neutralize sulfur oxides (SOx) in exhaust gases. The wash-water is then recirculated, with a portion treated to eliminate pollutants before being discharged.

These scrubbers demonstrate high efficiency, cutting sulfate emissions by approximately 95%. This impressive performance assists ships in adhering to IMO's stringent regulations.

Freshwater and Alkaline Additives as the Scrubbing Medium

The use of freshwater and alkaline additives, such as caustic soda, is a defining feature of closed-loop scrubbers. The amount of these additives varies from 0.5% to 5%, based on the wash-water's acidity. This method effectively neutralizes SOx emissions, ensuring compliance with discharge standards that can exceed 90% when monitored properly.

Parameter	Open-Loop Scrubbers	Closed-Loop Scrubbers
Scrubbing Medium	Seawater	Freshwater with Alkaline Additives
SOx Reduction Efficiency	90-95%	Up to 99%
Operational Cost	Higher	Up to 25% Lower
Environmental Impact	Higher (Washwater Discharge)	Lower (Controlled Discharge)

Washwater Treatment and Sludge Disposal

Closed-loop scrubbers are advanced systems specifically designed to treat washwater generated during the exhaust gas cleaning process on ships, with the primary goal of minimizing environmental impacts associated with maritime operations. These systems operate by circulating washwater through a series of filtration and treatment processes, ensuring that harmful pollutants are effectively removed before any discharge occurs. Remarkably, approximately 70-80% of the washwater is processed and subsequently reused within the system, which significantly reduces the volume of water that needs to be discharged into the marine environment.

This reuse not only conserves water resources but also mitigates the potential for harmful substances entering the ocean, thereby protecting marine ecosystems. Moreover, the remaining washwater, after treatment, is carefully monitored to ensure it meets stringent environmental regulations before any release takes place. In cases where the water cannot be discharged, the sludge that accumulates during the filtration process is collected.

This sludge is a concentrated mixture of pollutants, including heavy metals, hydrocarbons, and other hazardous materials that have been removed from the washwater. To prevent environmental contamination, this sludge is securely stored onboard the vessel until it reaches designated port facilities equipped to handle such waste.

Closed-loop systems typically require approximately 70-80% of the wash water to be treated and reused, significantly minimizing environmental impacts.

The use of closed-loop scrubbers is growing, with over 50% more installations from 2018 to 2020. This trend is expected to continue, with a 15% annual growth in installations over the next five years, as global sulfur emission regulations tighten.

Environmental Impact of Open-Loop and Closed-Loop Scrubbers

The maritime industry has turned to scrubber technology to meet stricter marine environmental regulations, mainly to cut down on ship sulfur emissions. Yet, the environmental effects of open-loop and closed-loop scrubbers are being closely examined. Concerns about their role in ocean acidification and their potential harm to marine life are growing.

Ocean Acidification and Aquatic Life

Open-loop scrubbers, which use seawater to clean pollutants, face criticism for their acidic wash water discharge. This water contains pollutants like sulfur oxides, heavy metals, and PAHs. The release of this water can lead to ocean acidification, threatening marine ecosystems and aquatic life.

Closed-loop scrubbers, on the other hand, aim to reduce these impacts by treating wash water and keeping the sludge for land disposal. By recycling the wash water and neutralizing pollutants, closed-loop systems are seen as a more eco-friendly option. This is crucial in areas with strict discharge rules.

Regulatory Landscape and Discharge Bans

The rules on scrubber wash water discharge are undergoing significant changes, reflecting a growing global concern for environmental protection and the need to mitigate the adverse effects of maritime air pollution. As international regulatory bodies and local governments increasingly recognize the detrimental impact that scrubber wash water can have on marine ecosystems, some regions have begun to implement bans or impose strict limitations on its use. These regulatory steps are designed to protect marine life and preserve water quality in oceans and coastal areas, which are vital for both biodiversity and human activities such as fishing and tourism.

As these regulations evolve, ship operators and owners must stay informed about the specific requirements in the regions where they operate. Compliance with these rules not only helps protect the environment but also ensures that vessels remain in good standing with regulatory authorities, thereby avoiding potential fines and penalties. The maritime industry is at a pivotal moment, where the adoption of cleaner technologies and adherence to stringent environmental standards will play a crucial role in shaping its future sustainability.

Region	Open-Loop Scrubbers	Closed-Loop Scrubbers
European Union	Discharge allowed in designated areas	Discharge allowed ashore with treatment
China	Discharge banned in inland waters and ports	Discharge allowed ashore with treatment
Singapore	Discharge banned in port waters	Discharge allowed ashore with treatment
California, USA	Discharge banned within 24 nautical miles	Discharge allowed ashore with treatment

The maritime industry faces challenges in reducing sulfur emissions and following environmental rules. The choice between open-loop and closed-loop scrubbers will depend on several factors. These include operational needs, costs, and the specific rules of the area where the vessel operates.

Economic Considerations for Shipowners

The choice to install marine scrubber systems is heavily influenced by economic factors. Shipowners must weigh the costs and benefits of these systems. They aim to comply with sulfur cap regulations while keeping their operations profitable.

The initial cost for installing scrubbers is high, with prices varying by vessel size and type. For instance, retrofitting an existing vessel can cost anywhere between $1 million and $8 million, depending on the ship's size & the choice made on scrubber system (closed, open or hybrid system). Yet, this would allow the use of high-sulfur fuels (which come at a lower cost) which could then lead to significant operational savings over the long term.

Installation and Operating Costs

The decision between open-loop and closed-loop scrubbers affects costs. Open-loop systems are cheaper upfront and easier to operate, using seawater and discharging it back into the sea. Closed-loop systems, though more expensive to install, require additional equipment for washwater treatment and sludge storage.

The table below outlines the average costs for different scrubber types:

Scrubber Type	Installation Cost (USD)	Operating Cost (USD/year)
Open-loop	1-4 million	100,000-200,000
Closed-loop	3-6 million	150,000-300,000
Hybrid	4-8 million	200,000-400,000

Fuel Cost Savings and Compliance Flexibility

Installing marine scrubbers enables shipowners to utilize more affordable high-sulfur fuels while complying with emissions regulations. The price gap between standard and ultra-low sulfur blends may range from $30 to $150 per ton, varying by port. Consequently, shipowners can reduce fuel expenses over time.

Scrubber systems also offer greater compliance flexibility. Hybrid scrubbers can switch between open-loop and closed-loop modes. This adaptability is crucial for ships navigating areas with strict environmental regulations or sensitive ecosystems.

The economic viability of scrubber technology hinges on fuel prices, vessel operations, and future regulations. Shipowners must evaluate the investment's return and consider other emission reduction strategies. This ensures their decisions align with business goals and sustainability objectives.

Emerging Trends and Future Developments

The maritime industry faces a future with stricter emissions standards. To meet these, marine scrubber technology is becoming a key solution. The global marine scrubber market is expected to grow at a 14.0% annual growth rate until 2030.

Alternative Fuels and Emission Reduction Technologies

Marine scrubbers are crucial for reducing sulfur oxide emissions. Yet, the industry is also exploring alternative fuels and technologies for sustainability. Liquefied natural gas (LNG) is gaining traction as a cleaner fuel, with the Asia Pacific leading in adoption.

Technologies like Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR) are being developed to tackle nitrogen oxide emissions. These technologies, along with low-sulfur fuels and scrubbers, aim to reduce the environmental impact of shipping.

The maritime industry's future depends on collaboration between shipowners, technology providers, and regulators. By adopting innovative solutions like hybrid scrubbers, alternative fuels, and emission reduction technologies, the industry can move towards cleaner shipping practices.

Conclusion

The maritime industry faces significant challenges in reducing sulfur oxide emissions. Sox scrubbers for ships have become a crucial solution. There are two main types: open-loop and closed-loop scrubbers. Each offers a different way to meet IMO 2020 regulations.

Open-loop systems use seawater, making them cost-effective but raise concerns about washwater discharge. Closed-loop scrubbers, using freshwater and additives, ensure consistent performance and lower environmental impact. They come with higher costs, though.

The choice between these systems depends on several factors. These include a ship's routes, fuel use, and sustainability goals. Shipowners must weigh the economic and environmental impacts to choose the best option for their fleet.

The industry must stay ahead of regulations and environmental concerns.

Future developments, like alternative fuels, will influence the maritime sector. Research and innovation in cleaning systems are key to balancing environmental care with economic needs. As regulations tighten, sox scrubbers will play a vital role in making shipping cleaner and more responsible.

FAQ

What are the main types of SOx scrubbers used on ships?

Ships employ three primary SOx scrubber types: open-loop, closed-loop, and hybrid. Open-loop systems use seawater, while closed-loop systems mix freshwater with alkaline additives. Hybrid scrubbers can switch between these modes, adapting to operational needs and regulations.

How do open-loop scrubbers work to reduce sulfur oxide emissions?

Open-loop scrubbers inject large seawater volumes to neutralize SOx in exhaust gases. The treated water is then released back into the sea. Their effectiveness hinges on seawater's alkalinity and salinity.

What are the environmental concerns associated with open-loop scrubbers?

Open-loop scrubbers release acidic wash water, containing pollutants like heavy metals and PAHs, into the sea. This can lead to ocean acidification and harm marine life, raising environmental concerns.

How do closed-loop scrubbers differ from open-loop systems?

Closed-loop scrubbers employ freshwater and alkaline additives to neutralize SOx. The wash water is recirculated, with treated water and sludge managed onboard. This system minimizes environmental impact by retaining sludge for proper disposal.

What are the advantages of closed-loop scrubbers compared to open-loop systems?

Closed-loop scrubbers reduce environmental harm by treating wash water and retaining sludge. They use less water and perform consistently, regardless of seawater quality. This system also offers greater compliance flexibility, crucial in areas with discharge bans.

What factors should shipowners consider when choosing between open-loop and closed-loop scrubbers?

Shipowners must evaluate economic factors like installation and operating costs, fuel savings, and compliance flexibility. They should also consider environmental impact, regulatory requirements, and operational limitations in discharge ban areas.

What are the emerging trends and future developments in SOx scrubber technology?

Hybrid scrubbers, capable of open-loop and closed-loop operation, enhance flexibility for shipowners. Retrofitting vessels with scrubbers is gaining traction as a cost-effective compliance measure. Exploration of alternative fuels and emission reduction technologies is underway, aiming to complement or replace scrubbers in the future.