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All about scrubbing


Due to implementation of the IMO202 Sulphur cap in the shipping industry there is an increase interested in the use of marine scrubbers, otherwise known as Exhaust gas Cleaning Systems. This article give you answers to most questions you can have about marine scrubbing, what scrubbers are, what types exist and how scrubbers work.


A scrubber is a device installed in the exhaust system after the engine or boiler, with the goal of removing most of the SOx from the exhaust and reducing particulate matter (PM). It treats exhaust gas with a variety of substances, which may include seawater, chemically treated fresh water or dry substances. After passing through the scrubber system, the compliant exhaust is released to the atmosphere. Scrubbers are also called EGCS, which stands for Exhaust Gas Cleaning System. All scrubber technologies create a waste stream containing the substance used for the cleaning process plus the SOx and PM removed from the exhaust.


There are different types of scrubber systems available for the marine industry. We distinct between:

  • wet scrubbers
  • dry scrubbers
  • membrane scrubbers


The exhaust gas passes through a liquid medium in order to remove the SOx compounds from the exhaust gas by chemically reacting with certain compounds in the wash liquid. The most commonly used liquids are untreated seawater and chemically treated freshwater. The wash liquid, also called washwater, generally must have a high sodium chloride content in order for the SOx to bind to the salts and thus be removed from the exhaust gas.  

Systems are identified as either an open loop or closed loop system. Hybrid systems offer both methods of scrubbing. In open loop operation, water is sourced and passed through the tower to remove SOx from the exhaust gas. In a closed loop scrubber, the water is generally treated freshwater with the exception of a few closed loop systems that use seawater. Additives are used to react with the washwater after each pass through the scrubbing tower to maintain high, available salt levels in the washwater, which is then recycled back into the scrubber in a continuous closed cycle. Additional additives and freshwater/seawater (as designed) are added as needed to maintain effective water levels and correct chemical composition.

Due to washwater discharge limitations set by the IMO (international maritime organization) and various regional and U.S. Federal regulations, the pH of the washwater discharge must be measured prior to overboard discharge. Monitoring of turbidity and PAH are also mandatory.


A dry scrubber does not use water or any liquid to carry out the scrubbing process, but instead exposes the exhaust gas to hydrated lime-treated granulates to create a chemical reaction, which removes the SOx emission compounds. Dry scrubbers are commonly used on land-based EGC installations.

Dry scrubbers use granules of caustic lime (Ca(OH)2) that react with sulfur dioxide (SO2) to form calcium sulfite:

SO2 + Ca(OH)2 → CaSO3 + H2O

Calcium sulfite is then air-oxidized to form calcium sulfate dehydrate:

CaSO3 + .O2 → CaSO4

which with water forms:

CaSO4 • 2H2O (Gypsum)


Membrane Scrubbers are wet scrubber types that do not directly mix the liquid scrubbing solution with the exhaust gases, but instead rely upon a membrane to contact, capture and remove SOx. Membrane gas separation technologies are commonly used on land-based applications, as well as limited marine applications such as inert gas nitrogen generators.

The membrane scrubber consists of an array of ceramic tube membranes suspended in the exhaust stream. A manifold system circulates the absorbent solution through the membrane tubes. Exhaust gases pass over the porous membranes, and SOx is absorbed by the absorbent solution. The spent caustic solution containing sulfates in the absorbent solution tank must be periodically replaced with fresh caustic solution. The spent caustic solution can be discharged at ashore when fresh caustic soda is taken on, or regenerated onboard via an electrolyzer.

It has been reported that the key advantage of membrane scrubbers over wet scrubbers for shipowners is the elimination of washwater generated by wet scrubbers. Overboard discharge PAH, turbidity and pH issues that require attention in wet scrubber installations are not concerns with membrane scrubbers. The key disadvantage of the membrane scrubbers over wet open loop scrubbers is the additional operating cost of the caustic soda.


While there can be significant differences in the detailed design of EGCS and the liquid medium used to carry out the scrubbing process, all wet scrubbers operate using the same basic chemical processes. The objective is to dissolve the water-soluble gases contained in the exhaust gas by mixing the exhaust gas with the scrubbing liquid using some combination of water spray or cascading liquid system.

Some scrubbers employ a packed bed of various shapes and materials through which the water flows downward, cascading over a maze-like packing as the exhaust gas travels through the liquid, promoting mixing of the two streams. Other scrubbers may have a tower-like structure with spray nozzles and baffles to create a turbulent environment and mix the streams. In all wet scrubbers, the intent is to maximize the surface area of liquid in contact with the exhaust gas to promote SOx absorption in the liquid while still allowing sufficient exhaust flow and complying with the exhaust backpressure limits of the engine or boiler. Once the SOx mixes with the liquid, various chemical reactions can take place depending on the chemistry of the liquid. In all cases, alkaline liquid (i.e. a liquid with a high pH) must be provided to neutralize the acidic SOx-based constituents.

SOx (SO2 plus SO3) gases are water-soluble. Once dissolved, these gases form strong acids that react with the natural alkalinity of the seawater, or the alkalinity derived from the added substances (normally sodium hydroxide), forming soluble sodium sulfate salt, which is found naturally in the ocean. In addition, the PM in the exhaust will become entrapped in the washwater, adding to the sludge generated by a scrubber. The waste stream and generated sludge has to be processed as per the 2015 IMO Guidelines before overboard discharge (where allowed), or stored and discharged to a shore reception facility as a waste substance.

Engine Exhaust Gas Chemistry:

S + O2 → SO2 ~ 95%

SO2 + .O2 → SO3 ~ 5%

SOx Reactions in a Scrubber:

SO2 + H2O → H2SO3 (Sulfurous Acid)

SO3 + H2O → H2SO4 (Sulfuric Acid)

Sulfurous gases in water are in a state of rapid oxidation; sulfur dioxide (SO2) oxidizes to sulfur trioxide (SO3), which dissolves in water to form sulfuric acid (H2SO4). Also, upon dissolution in water, SO2 forms the hydrate SO2+ H2O or sulfurous acid H2SO3, which dissociates rapidly to form the bisulfate ion HSO3, which in turn is oxidized to sulfate.


In an open loop scrubber using seawater, the washwater will react with SOx to produce mainly sodium, but also some calcium sulfate and sulfites. When in alkaline (hard) river or estuary water, which contains calcium-based and other salts, calcium sulfate or other sulfites may form in the washwater. As there is always free oxygen present in the exhaust, SOx will form sulfates (SO4) from the SO3 portion of the SOx. When SO2 is further oxidized, the SOx gas can also produce acid sulfate. Since the natural alkaline buffer salts are used and depleted in the reactions, the pH of the washwater mixture in the scrubber will be lowered. In addition, the drop in temperature of the exhaust gas during the scrubbing can cause unburned hydrocarbons to condense. Also, the momentum effects of changes in direction will cause larger particles to fall out of the gas stream. These two combine and mix in the scrubber to form larger particles in the scrubber effluent.

In marine closed loop type scrubbers, the washwater is treated with an alkaline substance, usually sodium hydroxide (NaOH) or  caustic soda, to create the desired level of alkalinity in the washwater. Some effluent, containing sodium sulfate salt, is periodically removed and some freshwater/seawater is added to maintain the proper chemistry.


Since the primary goal of scrubbers is the removal of SOx from the exhaust stream to achieve SOx emission levels equivalent to ships consuming low-sulfur fuel, the effectiveness of scrubbers in SOx removal is of great importance and the key measure of their performance. To this end, the effectiveness of the scrubbing process and other key parameters are continuously monitored and logged when the scrubber is in operation. When effectiveness is insufficient, corrective actions can be performed.

One key element of wet scrubber performance, particularly in an open loop operation, is the need for alkaline substances in the water. Closed loop alkalinity is directly controlled by the dosing process that injects an alkaline material into the washwater, so the performance of the scrubber can generally be controlled. For open loop scrubbers, the alkalinity of the washwater depends on the characteristics of the source water that the ship is traveling through. Therefore, the effectiveness of an open loop scrubber is significantly reduced if the vessel operates in brackish or soft freshwater with a lower pH than normal seawater. Washwater throughput volume is another parameter that impacts scrubber effectiveness and can partially counteract low pH values. Even when using washwater with a lower alkaline pH, an SOx removal rate to the required levels can possibly be achieved if a sufficient volume of washwater is used.

Scrubber Performance Factor Rate % Remark
SOx Removal Required. 97.10. Makes 3.5% S fuel equivalent to 0.1% S fuel.
Expected SOx Removal Rate. >96. Depends on alkalinity of the water.
Tupical Particulate Removal Rate. 30 – 60. When using heavy fuel, particulates emissions are higher than for 0.1% S distillate diesel fuel.

Value Maritime scrubbers were originally developed to remove particulate matter only. Later the product was remodeled to remove sulfur as well, resulting in removal rates of 99% sulfur, 99% PM10 and 90% PM2.5 from exhaust gases. This discriminates the VM Scrubber from other competing products as we offer unique, high filtering rates of particulate matter as well. It is well established that  PM is very harmful to human health and the environment and it is expected that regulations will be enforced restricting PM emission in marine industry. As such, Value Maritimes offers our customers a scrubber system, which not only complies with current regulations on sulfur emission but also complies with expected future regulations on PM emission.


An open loop scrubber uses seawater as the medium for cleaning or scrubbing the exhaust, as shown in

Figure 1. Seawater is normally supplied by a dedicated pump.

CO2 dissolves in seawater forming carbonic acid, bicarbonate or carbonate ions, depending on the pH. The positive ion to which it will bind can be calcium (Ca2+) or sodium (Na+). Here the sodium carbonate salt is used as an example.

When the carbonate/bicarbonate ion reacts with an acid, CO2 is released.

Chemical steps in open loop system:

Na2CO3 + H2SO3 → Na2SO3 + H2O + CO2 (Sodium Sulfite)

Na2SO3 + .O2 → Na2SO4 (Sodium Sulfate)

Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2 (Sodium Sulfate)

marine scrubbing - open loop

Each EGCS manufacturer has their own scrubbing technique to mix the exhaust gas with the washwater. As previously mentioned, an open loop scrubber is only effective if the source water is alkaline. Some river water is ‘hard’ water and thus has significant alkalinity, which sometimes is even higher than seawater, so open loop scrubbers can also work effectively in some port and river areas.

The effectiveness of an open loop scrubber depends on the alkalinity and the chemistry of the water that the vessel is operating in. Hence, a vessel’s intended operational area should be taken into account at the design and selection stage, and prior to deploying a vessel to new operational areas. If the water is not sufficiently alkaline, the scrubber will not meet the required performance level and the operator will have to use low-sulfur fuel to be in compliance with the applicable SOx emission regulations.

As required by the 2015 IMO Guidelines, scrubber manufacturers must state the operational limits in terms of maximum fuel sulfur content to be in compliance with MARPOL Annex VI Regulation 14 requirements. Open loop scrubbers have larger water flow rates than closed loop scrubbers, because there is less control over water alkalinity and more water is needed to make the scrubbing process effective when lower alkalinity water is used. After the basic scrubbing process takes place in the main scrubber tower, the exhaust mixture may pass through a demister or water droplet separator to remove water particles from the gas, which reduces the potential for steam generation as the exhaust is emitted into the atmosphere. This is advantageous because a steam plume, albeit  harmless, gives a false impression of unfiltered exhaust smoke being emitted.


marine scrubbing - closed loop

In a closed loop type scrubber, treated water is circulated through the scrubber to keep the scrubbing process independent of the chemistry of the waters through which the vessel is sailing. An alkaline chemical, usually sodium hydroxide (NaOH) or rarely magnesium oxide (MgO) is used in marine EGCS to control the water alkalinity, which can also be produced by electrolysis of seawater (see Figure 2).

The closed loop scrubber internals are similar to those of an open loop scrubber, and the chemical processes to remove the SOx emissions are similar. The major difference between the two systems is that rather than going overboard, most of the circulating washwater is processed after it leaves the scrubber tower to make it suitable for recirculation as the scrubber washwater medium. The washwater can be fresh or salt water depending on the scrubber design. In the treatment process, the residues are removed from the washwater, and the water is dosed with alkaline chemical to restore its alkalinity prior to returning to the scrubber tower.

Manufacturers claim a closed loop marine scrubber requires about half or less of the washwater flow than an open loop scrubber to achieve the same scrubbing efficiency. The rationale behind this is that higher levels of alkalinity are ensured by the direct control of the pH level using the alkaline chemical injection process. In fresh water scrubbers, SO2 combines with an injected salt and consequently does not react with the natural bicarbonate of seawater. Subsequently, there is no release of CO2.

Chemical steps in closed loop system:

2NaOH + SO2 → Na2SO3 + H2O (Sodium Sulfite)

Na2SO3 +SO2 +H2O → 2NaHSO3 (Sodium Hydrogen Sulfite)

NaOH + H2SO4 → NaHSO4 + H2O (Sodium Hydrogen Sulfate)

2NaOH + H2SO4 → Na2SO4 + 2H2O (Sodium Sulfate)

In a closed loop system, the dirty washwater exiting the scrubber goes to a process or circulating tank providing the required water quantity and any losses in water level and alkalinity is replenished. A limited quantity of washwater from the bottom of the process tank, where the residues have collected, is extracted using low suction. It then moves on to a hydro-cyclone or separator, where the residues are removed, or for some systems the extracted water passes through a bleed-off treatment unit (BOTU). During these processes, the cleaned bleed-off water is discharged either overboard or to a holding tank, depending on the ships location and local regulations. Residual sludge removed from the washwater goes to a residue/sludge tank for disposal ashore. Make-up water is added to the process tank to replace the washwater lost in the treatment process and bleed off, and evaporation during the scrubbing process. A dosing unit adds caustic soda back to the washwater, with the amount varied depending on the alkalinity requirements for the water. The washwater passes through a cooler before re-injection into the scrubber. A pump circulates the washwater from the process tank back to the scrubber.


Open and closed loop have the following advantages:

Open loop systems:

  • do not require caustic soda,
  • do not need to process washwater, reducing costs and number of actions to be performed and monitored by personnel

Closed loop systems:

  • work with the same efficiency independent of where the vessel is operating
  • there is little or no water discharge making it best suited for coastal, port and inland waters

In order to utilize the advantages of both systems, some manufacturers have developed hybrid scrubbing systems. These can be operated as an open loop system when in the open ocean and as a closed loop system when in a sensitive sea or in a low alkalinity water area (see Figure 3). The changeover from open to closed loop is done by changing over the circulating pump suction from seawater to the circulating tank, and by changing the washwater discharge from the overboard discharge to the circulating tank.

marine scrubbing - hybrid



The lower portions of a wet scrubber (especially the open loop type) may have a high concentration of acid and chlorides. Accordingly, they must be designed to incorporate acid-resistant materials. Scrubbers suitable for dry operation and wet scrubbers without a facility for bypassing exhaust gas when the washwater system is not in operation will experience higher operating temperatures which therefore limits the materials selection to high temperature acid- and chloride-resistant alloys; typically nickel alloys. Ferritic-austenitic stainless steels (duplex stainless steels) perform well in corrosive environments. Above the lower portions of the scrubber unit, a less corrosion resistant stainless steel may be used. However, the selection of the appropriate stainless steel grades should be considered in detail and should be based on conservative assumptions.

Value Maritime Scrubbers are constructed out of the highest quality material there is available.


When the scrubber is not needed, such as when sulfur limit-compliant fuel is used, the exhaust bypass can be used and the scrubber is shut down, reducing EGCS power consumption. An exhaust gas bypass allows the exhaust gas to bypass the scrubber and go directly to the atmosphere. Unless made with suitable materials able to withstand the high exhaust temperatures, wet scrubbers are not normally recommended to be operated dry, i.e. operated with exhaust gas passing through them without washwater flowing. For scrubbers that are suitable for dry operation, a separate bypass may not be required. For most scrubbers, fitting a bypass is a requirement if there is a need to operate the equipment connected to the scrubber when the scrubber is non-operational. This would apply to engines and boilers considered as essential services for a vessel.

Bypass exhaust pipes are as large as the original exhaust pipe, therefore the required space in the engine exhaust system casing for the bypass pipe and valve can be large. The bypass pipe normally passes alongside the scrubber and requires a separate exhaust outlet at the top of the funnel in addition to the scrubber outlet. The bypass valve, which may be a metal-to-metal seated butterfly valve, controls the direction of the exhaust flow between the scrubber and atmosphere.

Where the valve is a two-damper design, an interlock would be required to prevent both dampers from being closed at the same time. Exhaust bypass valves may require frequent maintenance because of the hot gas environment and soot accumulation that occurs. For main engines, an exhaust gas economizer (EGE) is also frequently provided before the scrubber. For operations wherein the scrubber is bypassed, it is recommended that a silencer be fitted to the exhaust system.


Scrubbers may have an impact on the operation of any engine/boiler to which they are added if they cause excessive exhaust system backpressure. Continual compliance with IMO MARPOL Annex VI Regulation 13 requirements on NOx emissions may be affected if the engine is operated at an exhaust backpressure outside of the approved limits detailed in the Technical File. For this reason, before a scrubber is installed on an exhaust system, it is important to verify that the certified design and operational exhaust backpressure limits will not be exceeded.

If necessary, a fan may be provided on the scrubber outlet to the exhaust pipe to lower the pressure in the scrubber and thereby prevent excessive backpressure in the system. A fan may not normally be required for a scrubber attached to a single engine, but it is more commonly seen in scrubbers connected to multiple engines and boilers to prevent higher backpressure from one engine or boiler affecting the other interconnected fuel burning units, either stationary or in operation.

Due to the potential impact of excessive backpressure on engine operation and safety concerns of exhaust backflow to idle units, the impact of a scrubber fan failure on the safe operation of the fuel burning units should be carefully considered. The scrubber manufacturer should submit complete details related to the anticipated backpressure across the full load range of operation, and this should be verified as being compatible with the engine or boiler manufacturer backpressure limits to determine that the backpressure will not create problems for the safe and continued operation of the equipment.

The VM EGCS has low back pressures that do not affect engine operations or require a fan.


Wet scrubbers and their associated equipment are large units. They are required to be installed in the exhaust system after any waste heat recovery equipment, such as an EGE, since the scrubber cools the exhaust. Figure 4 shows the schematic side view of the VM scrubber and conventional design, based on the engine power rating for scrubbers. In general, it is anticipated that a scrubber will typically be about two to three times the size of a typical EGE or composite boiler. It is estimated that the expanded engine exhaust system casing structure to support a wet scrubber will weigh about 50 percent of the scrubber weight, so the weight impact on vessel deadweight and stability will be about 150 percent of the scrubber operational weight.

Figure 4: side view of VM Scrubber & control room
Conventional design for scrubbers

Value Maritime Scrubbers dimensions are much smaller than the traditional wet scrubber systems, making them appealing for the relatively small sea vessels. For example, the 3.0MW VM scrubber fits in a 20ft casing, which already includes the control room with all the analyzing equipment making the system plug & play.


One concern with wet scrubber operations is scrubber flooding, which occurs if the washwater drainage from the sump, either by pump or gravity drain, stops or is blocked. This will quickly flood the scrubber and overflow of water down the exhaust pipe, and subsequent damage to the attached engine/boiler may occur. The scrubber automation system should prevent such critical situation; this may be achieved with a high water level alarm, an automatic cutoff of the water supply to the scrubber and opening of the exhaust bypass (if fitted), and simultaneous appropriate functions for maintaining the associated fuel burning systems in a safe status.

Value Maritime is proud to emphasize the inherently safe design of her EGCS unit in this regard. When the exhaust inlet remains above the scrubber outlet, as designed, it will be impossible for washwater to backflush into the ships exhaust lines when using the VM EGCS Unit. Moreover, the below timeline with corresponding alarms, events and actions demonstrates the superior protection of the Value Maritime EGCS Unit.


Scrubbers require several piping systems to be installed, each with different material requirements. Considerations for the key piping systems are as follows:


For open loop scrubbers, seawater is supplied to the scrubber for the scrubbing process; standard seawater piping material can be used. Typical materials are steel pipe with polyethylene or rubber lining, galvanized piping or glass reinforced epoxy (GRE) pipe, which must be an approved type for use in machinery spaces. For closed loop scrubbers, seawater is used for cooling purposes; the same pipe material requirements apply for those pipes.


For closed loop scrubbers that use treated freshwater for scrubbing, the piping should be of appropriate material suitable for the particular closed loop chemistry.


The water draining out of the scrubber is acidic and corrosive, and thus requires special piping materials. Similar to inert gas scrubbers on tankers, steel pipe with polyethylene or rubber lining can be used. Alternatively, approved GRE piping has been known to perform satisfactorily. Valves should be rubberlined butterfly type or of suitable stainless steel grade. In closed loop systems, the washwater will be considered corrosive until the point where the water is dosed with the alkaline material and the pH is raised.


Exhaust piping before the scrubber would typically match that for standard exhaust systems; however, the exhaust gas exiting the scrubber will tend to have a high relative humidity and, therefore, highly corrosion-resistant materials such as stainless steel are preferable.


In addition to the scrubber itself, there is a need to consider the space and power requirements of the associated scrubber auxiliary equipment, such as pumps, process tanks, particulate separators and coolers, which are similar in size to other engine room auxiliary equipment of the same type. The auxiliary equipment can be located lower down in the machinery space than the engine exhaust system casing since it does not have to be directly adjacent.

There are three main reasons that more power is needed:

• Raise the water up from the lower engine room to the scrubber in the upper engine exhaust system casing

• Overcome pressure losses in the piping and to supply water at the required pressure to the spray nozzles (about 2 bar)

• Increase scrubbing water flow

Multiple water supply, circulating pumps or variable speed pumps are needed so that the water supply to the scrubber can be varied with fuel oil combustion equipment load, otherwise excessive water supply will occur at low loads. There is significant difference in the types of auxiliary equipment in use and flow rates dependent on whether a scrubber is an open loop type or closed loop type and for hybrid scrubbers in which mode they are operating in.

Scrubber systems require electrical power and a control and monitoring system. For wet scrubbers, the electric load is primarily for pumping the washwater. The electrical load for pumping can be substantial; several hundred kW for open loop scrubbers for large engines. There are also other additional electric loads to consider such as sludge removal, alkaline dosing, seawater cooling, induced draft fans and process control. It is expected that the total electric load will be about 115 to 125 percent of the scrubber pumps electric load. These loads can be more than the surplus electric capacity available in an existing vessels electric power system and may require the addition of a separate generator.

Besides electric power to the scrubber, an automation and control system must be installed. Control panels can be local to the scrubber, but basic scrubber control should be available from the engine control room with a tie-in for the scrubber alarms to the ships central alarm and monitoring system.

EGCS require automation and monitoring of their systems, operation and effectiveness to ensure that the scrubber provides the required level of exhaust gas and washwater discharge cleaning. The monitoring and data logging system should be tamper proof and in compliance with MARPOL and any additional national regulations that may be applicable to the scrubber

Value Maritime Scrubbers have been granted approval from class agencies Bureau Veritas, DNV-GL and Loyds Register and comply with all regulations regarding electrical and monitoring systems.