Published 24 April 2026 · last updated 28 April 2026

Exhaust gas cleaning system (scrubber)

An exhaust gas cleaning system (EGCS), commonly called a scrubber, is a shipboard device that removes sulphur dioxide (SO2) from engine and boiler exhaust gases by absorbing it into an alkaline liquid medium. The technology allows ships burning high-sulphur heavy fuel oil (HFO) to meet the sulphur emission limits set by MARPOL Annex VI - a global cap of 0.50% mass/mass from 1 January 2020 and a 0.10% m/m limit inside designated Emission Control Areas (ECAs) - without switching to a more expensive compliant fuel. Regulatory acceptance rests on the equivalence principle in MARPOL Annex VI Regulation 4, which permits an approved alternative arrangement that delivers the same emission outcome as burning low-sulphur fuel. The IMO Guidelines for EGCS, first adopted as MEPC.184(59) in 2009 and substantially revised as MEPC.340(77) in 2021, specify the performance standards and monitoring requirements that a system must meet to qualify. Approximately 5,000 vessels had been fitted with scrubbers by 2024, representing roughly 15% of world fleet gross tonnage - a concentration among the largest container ships, bulk carriers, and cruise vessels whose high fuel consumption makes the capital investment economically attractive. ShipCalculators.com provides a suite of calculators for EGCS compliance, including the EGCS SOx scrubber NaOH dosing calculator, the scrubber SO2/CO2 ratio check, and the wash water quality checker.

Contents

Background and regulatory context

The sulphur problem in marine combustion

Heavy fuel oil - the residual fuel burned by most large ocean-going ships - contains sulphur as an inherent product of crude oil refining. When sulphur-bearing fuel undergoes combustion, the sulphur is oxidised to sulphur dioxide (SO2), a gas that causes acidic deposition on land, contributes to the formation of fine particulate matter (PM2.5), and is a recognised respiratory hazard at high concentrations near ports and shipping lanes. A large container ship burning 150 to 200 tonnes per day of 3.5% sulphur HFO emits on the order of 10 to 14 tonnes of SO2 per day. The North Sea and Baltic alone historically carried enough ship traffic to make shipping a major SO2 source in north-west Europe.

International regulation of ship-source sulphur emissions developed progressively under MARPOL Annex VI, which was adopted in 1997 and entered force in 2005. The initial limits were modest, but MEPC.176(58) in 2008 established a tightening schedule that culminated in the rules now in force: a global cap of 0.50% m/m and a 0.10% m/m limit inside ECAs. The IMO 2020 sulphur cap is the regulatory event against which most EGCS investments have been judged.

Emission control areas

ECAs are geographically defined sea areas in which stricter emission standards apply under MARPOL Annex VI Regulation 14. Four sulphur ECAs were designated before 2020:

North American ECA (including the US and Canadian coasts to 200 nautical miles offshore) - effective 1 August 2012 for SOx
United States Caribbean Sea ECA - effective 1 January 2014
North Sea ECA - effective 19 May 2006
Baltic Sea ECA - effective 19 May 2006

A fifth ECA, covering the Mediterranean Sea, entered force for SOx and particulate matter on 1 May 2025, following adoption at MEPC 82. The 0.10% m/m SOx limit applies in all these zones. Ships transiting ECAs without a compliant fuel on board must instead operate an approved EGCS that achieves equivalent SOx reduction.

MARPOL equivalence principle

MARPOL Annex VI Regulation 4 permits a ship to use any technology, fuel, or operational method that achieves at least the same emission reduction as the prescribed fuel standard, provided the flag state approves it as an equivalent arrangement. For EGCS, equivalence is demonstrated by showing that exhaust gas SO2 concentration, expressed as a ratio to CO2 concentration in the exhaust, does not exceed the thresholds that would result from burning compliant fuel.

The ratio metric normalises SO2 against CO2 to account for variable engine load and exhaust dilution. For the 0.50% global cap, MEPC.340(77) specifies that the SO2 (ppm) to CO2 (% v/v) ratio must not exceed 4.3. For the 0.10% ECA limit, the threshold is 0.86. Use the scrubber SO2/CO2 ratio compliance tool to verify these thresholds against measured gas concentrations. The SOx emissions from fuel sulphur calculator converts fuel sulphur fraction directly to mass SO2 emitted.

Commercial development

Practical commercial deployment of marine scrubbers was negligible before the 2008 revision confirmed the 2015 ECA timetable for 0.10% limits. The first generation of production open-loop towers appeared around 2010 to 2012 on large cruise ships and bulk carriers operating intensive Baltic and North Sea schedules. Investment accelerated sharply from 2017 to 2019 once the 2020 global cap date was fixed, as the anticipated spread between HFO and very low sulphur fuel oil (VLSFO) provided a compelling payback calculation. Retrofit orders peaked in 2018 and 2019, and the installed base crossed 4,000 units before the cap took effect in January 2020. Cruise ships were early adopters because their intensive port schedules in Nordic and North American ECAs made low-sulphur fuel mandatory but expensive, and their large generator loads provided the scale needed to justify capital costs. By end 2024 the total exceeded 5,000 vessels, with bulk carriers and tankers accounting for roughly 70% of the fleet.

The formal recognition of EGCS as equivalent compliance was established by MARPOL Annex VI Regulation 4 and formalised by IMO Resolution MEPC.184(59) in July 2009. The guidelines underwent substantive revision at MEPC 77, producing Resolution MEPC.340(77), adopted on 26 November 2021 and taking effect in 2022. MEPC.340(77) tightened wash water monitoring parameters for open-loop systems, clarified requirements for hybrid mode switching, strengthened EGCS Record Book provisions, and introduced updated guidance on CO2 co-monitoring as part of the SO2/CO2 ratio methodology.

Chemistry of wet scrubbing

Sulphur dioxide absorption

The core chemistry of a wet EGCS is acid-base neutralisation. SO2 dissolves in water to form sulphurous acid (H2SO3), which is a weak acid. In an open-loop system, the natural alkalinity of seawater - primarily bicarbonate (HCO3-) and carbonate (CO32-) ions - neutralises the acid. The overall reaction sequence is: SO2 absorbs into water forming H2SO3, which dissociates to bisulphite (HSO3-) and hydrogen ions. The bisulphite is neutralised by seawater bicarbonate to yield sulphate ions, water, and carbon dioxide. Dissolved oxygen further oxidises sulphite to sulphate (SO42-), the final stable species.

Global seawater has a typical total alkalinity of two to three milliequivalents per litre, derived mainly from bicarbonate. This is sufficient to neutralise the SOx load at sulphur concentrations of 2.5 to 3.5% without pH depression below the regulatory discharge floor, provided flow rates are matched to the SO2 load. At 3.5% fuel sulphur the wash-water requirement for open-loop systems is typically 30 to 70 m3 per megawatt-hour of shaft power.

In a closed-loop system, the scrubbing medium is fresh water dosed with sodium hydroxide (NaOH). The dominant reaction is: SO2 + 2 NaOH yields sodium sulphite (Na2SO3) + H2O. Further oxidation converts sodium sulphite to sodium sulphate (Na2SO4). NaOH is consumed in stoichiometric proportion to SO2 absorbed; the EGCS NaOH dosing calculator computes the daily NaOH mass demand from fuel consumption and sulphur content.

Mass balance for a large ship

For a large two-stroke main engine burning 150 t/day of 3.5% sulphur HFO, the sulphur mass flow is 5.25 t/day. Since the molecular mass of SO2 is 64 g/mol versus 32 g/mol for sulphur, the stoichiometric factor is two, giving a SO2 production rate of approximately 10.5 t/day. At 97% removal efficiency, the absorbed SO2 is approximately 10.2 t/day. The NaOH demand at two moles NaOH per mole SO2 (molecular mass ratio 80/64) works out to approximately 12.7 t/day of pure NaOH, typically supplied as a 50% aqueous solution and therefore requiring approximately 25 to 27 t/day of 50% NaOH solution. Global supply prices for 50% NaOH solution ranged from approximately US$350 to US$450 per tonne in the period 2020 to 2024.

For SO2 in the exhaust, the SOx from fuel sulphur formula page expresses the mass relationship as: mSO2 = mfuel × 2 × S% / 100, where S% is the sulphur content as a percentage by mass and the factor two is the stoichiometric ratio of SO2 molecular mass to sulphur atomic mass.

Removal efficiency and CO2 interaction

EGCS do not absorb CO2 in meaningful quantities under normal operating conditions. CO2 is a much weaker acid than SO2 and at the rapid gas-liquid contact times in a scrubber tower, CO2 uptake is minor. This is why the SO2/CO2 ratio metric works: the CO2 signal in the exhaust is a proxy for fuel mass burned, and the SO2 signal represents unremoved sulphur. The scrubber SO2/CO2 equivalence formula shows that the ratio R = SO2 [ppm] divided by (10,000 × CO2 [%]) must be at or below 4.3 for the global 0.50% cap and at or below 0.86 for the 0.10% ECA limit.

Absorption efficiency of a properly designed and operated wet EGCS for SO2 is typically above 95%, with many systems achieving 97 to 99%. The efficiency depends on liquid-to-gas ratio, contactor design, contact time, and the alkalinity of the incoming liquid.

System types

Open-loop systems

An open-loop EGCS draws seawater continuously from the sea chest, pumps it through the scrubbing tower in contact with exhaust gas, and discharges the resulting wash water overboard. No chemical addition is required because the natural alkalinity of seawater provides the neutralising capacity. The system is simple and has a low consumable cost, but it generates a continuous stream of acidic, turbid, and chemically loaded wash water.

Typical seawater demand is 30 to 45 m3 per megawatt-hour of thermal input, equivalent to 200 to 300 m3 per hour for a large main engine. After passing through the scrubber, the wash water temperature rises by several degrees Celsius, its pH falls from the ambient 7.8 to 8.2 toward values as low as 3.0 to 4.0, and it contains dissolved sulphate, polycyclic aromatic hydrocarbons (PAH) leached from the fuel, turbidity-contributing particles, and traces of heavy metals and nitrates.

MEPC.340(77) sets continuous monitoring and discharge limits for open-loop effluent:

Parameter	Discharge limit
pH at 4 m from discharge point	Not less than 6.5 (instantaneous); 7.0 (15-minute average)
PAH (as phenanthrene equivalent)	Not more than 50 µg/litre above inlet
Turbidity	Not more than 25 NTU above inlet
Nitrate	Not more than 60 mg/litre above inlet

The wash water quality compliance calculator checks measured values against these thresholds, implementing the criteria set out in the scrubber wash water quality formula page. In ports with low-alkalinity water or restricted tidal flushing, the pH limit can be difficult to satisfy, which is one practical reason some ships switch to closed-loop mode in port even where no formal ban exists.

Closed-loop systems

A closed-loop EGCS recirculates a fixed inventory of scrubbing liquid - typically fresh water with NaOH dosing - through the tower in a closed circuit. Exhaust heat is removed by a dedicated heat exchanger before or within the circuit. A bleed stream continuously removes a fraction of the circulating liquid to prevent build-up of dissolved salts and sludge; the bleed is collected in a hold tank and retained on board for discharge at a port reception facility.

The hold tank must be sized for the anticipated voyage between discharge opportunities. For a ship with a main engine consuming 150 t/day of HFO at 3.5% sulphur, the bleed volume is roughly 2 to 5 m3 per hour. A voyage of several days can produce tens to hundreds of cubic metres of sludge-bearing effluent that must be offloaded ashore.

The closed-loop scrubber fresh-water consumption calculator estimates daily make-up water demand from fuel consumption rate and bleed factor, following the methodology in the closed-loop FW consumption formula page. The fresh-water supply system must be sized accordingly, and on vessels without a water-maker of sufficient capacity, fresh water may need to be bunkered.

Closed-loop systems satisfy open-loop discharge bans because no wash water enters the sea during operation, but they add complexity, operating cost, and tank space requirements.

Hybrid systems

A hybrid EGCS incorporates switching capability between open-loop and closed-loop modes. In open-loop mode, seawater is used; in closed-loop mode, the system draws on the fresh water/NaOH circuit and stores bleed-off in the hold tank. Hybrid systems are the dominant type installed on new-builds and many retrofits, because they preserve maximum operational flexibility: open-loop mode is used on the open ocean where discharge is permitted and operating costs are lower, while closed-loop mode is engaged in ECAs, restricted waters, and ports with discharge bans.

The switching sequence typically involves opening the sea chest bypass to the closed-loop make-up tank, activating NaOH dosing pumps, opening the bleed valve to the hold tank, and confirming stable pH and liquid level in the circuit before closing the sea-to-overboard path. Manufacturers typically design the switchover to complete within 10 to 15 minutes. MEPC.340(77) requires that every mode change in a hybrid system be logged in the EGCS Record Book with the vessel’s position, time, and reason for the change.

Regulatory certification: Scheme A and Scheme B

MEPC.340(77) describes two certification pathways:

Scheme A is a performance-based pre-certification approach. Before installation, the system undergoes a formal test at an approved test bed or on board under supervised conditions. Emission measurements at defined test points demonstrate compliance with the SO2/CO2 ratio thresholds. Once certified, the system is deemed to be compliant when operated within its certified load envelope and with the monitoring system functioning. Scheme A is analogous to the type-approval model used for NOx under the NOx Technical Code.

Scheme B is a continuous emission monitoring (CEM) approach. The EGCS is equipped with permanent inline gas analysers that measure SO2 and CO2 concentrations in the exhaust continuously, feeding a data logger that records and stores the SO2/CO2 ratio at defined intervals - typically one minute. Compliance is demonstrated in real time by the continuous monitoring record rather than by a prior type test. Scheme B is more common in practice because it provides an auditable record and is more flexible for engines that operate across wide load ranges.

Both schemes require that wash water quality monitoring be continuously active for open-loop operation, recording pH, PAH, turbidity, and nitrate at sampling points defined in MEPC.340(77). The data must be retained in a tamper-evident EGCS Monitoring System log for inspection by port state control officers. Three years of data retention is required.

A Scheme B system must include redundancy in gas sampling lines and analysers, with automatic alarms if a sensor fails, and documented fallback procedures if continuous monitoring is lost. When a monitoring failure occurs, the vessel must either switch to a compliant fuel or report the failure to the flag state and follow the contingency plan approved as part of the EGCS approval documentation.

Physical design and engineering

Tower design and sizing

The absorption tower is the core process vessel of an EGCS. Exhaust gas from the main engine turbocharger outlet - or from auxiliary engines, boilers, or all combined in a multi-stream configuration - enters the tower at the base, flows upward, and contacts a downward-flowing or cross-flow spray of scrubbing liquid. The gas velocity, liquid-to-gas ratio, and contact surface area determine absorption efficiency.

Three contactor geometries are in commercial use. A spray tower is the most common: exhaust gas rises through a vertical cylindrical vessel while alkaline wash liquid is sprayed downward from nozzles distributed across multiple levels. Droplet impingement and surface absorption achieve SO2 removal efficiencies above 97%. A packed-bed contactor fills the scrubber body with structured packing - typically polypropylene or corrosion-resistant metallic elements - providing high specific interfacial area at the cost of greater pressure drop. A venturi scrubber uses the kinetic energy of the gas stream to atomise the wash liquid in a short throat section; these are compact and handle high-particulate exhaust but are less energy-efficient as primary SOx absorbers.

For a large two-stroke main engine with a maximum continuous rating of 50 to 80 MW, the scrubber tower typically has an internal diameter of 4 to 6 m and a height of 12 to 18 m. Retrofitting a scrubber requires structural modifications to the funnel or casing, rerouting of exhaust piping, and installation of sea water pumps, heat exchangers, sludge tanks, monitoring equipment, and control systems. The total footprint - tower plus associated equipment - may occupy 100 to 200 m2 of deck or casing space.

Two principal physical configurations govern retrofit feasibility. A vertical in-line scrubber is mounted directly above the engine exhaust outlet, replacing the silencer and upper exhaust trunk. This is preferred for new-builds and for tankers and bulk carriers whose relatively open funnel casings have headroom. A U-type or side-stream scrubber routes the exhaust horizontally into a separate scrubber module positioned beside the existing stack, with cleaned gas rejoining the main stack above. U-type designs are common on container ships where air-draught constraints prevent adding height to the stack.

Materials of construction

The combination of wet sulphuric acid, seawater chlorides, and elevated temperatures - exhaust gas enters the scrubber at 250 to 350°C and exits at 60 to 80°C after the wash quench - creates one of the most challenging corrosion environments in shipboard plant. Three material families are used. Glass-reinforced plastic (FRP) with vinyl-ester or epoxy resin is the most cost-effective choice for the outer scrubber body and piping exposed to diluted wash water, and its lower density reduces topside weight relative to steel. Duplex stainless steels - most commonly 2205 (UNS S31803/S32205) and super-duplex grades such as 2507 (UNS S32750) - are used for the gas inlet section, spray headers, and components exposed to hot gas or high-chloride wash water; super-duplex grades offer pitting resistance equivalent (PREW) values above 40. Rubber-lined carbon steel is used in certain proprietary designs where a combination of mechanical fabrication cost and corrosion resistance is preferred.

Seawater pump and back-pressure considerations

The seawater pump must deliver the full liquid flow required at rated scrubber throughput. At 30 to 45 m3 per MWh and a main engine thermal input of 70 MW, the pump duty is 2,100 to 3,150 m3/h - a flow comparable to a medium-sized ballast pump. Two-pump configurations with one duty and one standby are standard.

A wet scrubber installed in the exhaust gas path introduces a pressure drop that must be overcome by the engine’s turbocharger. Most scrubber designs specify a maximum permissible back-pressure of 200 to 400 Pa at maximum engine load, verified as part of the commissioning test. The seawater or recirculation pumps, chemical dosing systems, monitoring instruments, and fans collectively consume 200 to 400 kW of auxiliary electrical power on a large installation, representing 0.5 to 1.5% of the vessel’s installed auxiliary generation capacity.

Wash water treatment train

Open-loop and hybrid systems include a treatment train between the contactor and the overboard valve. A hydrocyclone exploits centrifugal force to separate coarse particles - fly ash, heavy metal compounds, unburnt carbon - into a concentrated underflow. The overflow passes to a settling tank or sludge separator where finer solids settle out. The accumulated sludge is transferred to MARPOL sludge tanks and handled as oily waste for reception ashore. EGCS sludge is classified under MARPOL Annex V as ship-generated waste and must not be discharged overboard.

Auxiliary engine and boiler integration

MARPOL Annex VI Regulation 14 applies to all fuel combustion equipment on board, not only to the main engine. Auxiliary diesel generators and oil-fired boilers burning HFO must also comply. Several installations use a combined scrubber that treats exhaust from both the main engine and auxiliary engines via a common EGCS and a single SO2/CO2 analyser on the combined clean-gas outlet. This simplifies monitoring and record-keeping but requires larger scrubber sizing for the combined flow. Some vessels use marine gas oil exclusively in the auxiliary boiler to avoid EGCS coverage of that source entirely.

Discharge monitoring and effluent quality

Open-loop discharge limits under MEPC.340(77)

MEPC.340(77), adopted at MEPC 77 on 26 November 2021, replaced MEPC.184(59) from 2009. The revision tightened monitoring, data recording, and wash water quality requirements:

pH measurement is required at a point 4 m from the discharge outlet and must be logged continuously.
PAH (measured as phenanthrene equivalent, PAHphe) must not exceed 50 µg/litre above the inlet measurement.
Turbidity must not exceed 25 NTU above inlet.
Nitrate (as NO3-) must not exceed 60 mg/litre above inlet.
Wash water must not contain chemicals added to enhance scrubbing performance that would not otherwise be permitted.
Annual verification measurements by an approved body are required to confirm that continuous sensors remain calibrated.

The EGCS Monitoring System log must record all parameters at intervals of no more than one minute and must be retained for three years.

The pH in the effluent stream can be as low as 2.5 to 3.5 immediately downstream of the scrubber discharge point. The regulation requires that the pH at 4 m downstream does not fall below 6.5 instantaneously or below 7.0 on a 15-minute average. This relies on mixing with the surrounding seawater and is more easily satisfied in high-flow open-water conditions than in confined port basins with low current. This is the technical reason why several port authorities have imposed open-loop discharge bans beyond MARPOL: they have concluded that local hydrodynamic conditions cannot guarantee adequate dilution.

Sludge and residue management

Both open-loop and closed-loop systems generate sludge: a mixture of collected particulate matter, heavy metals, combustion products, and absorbed chemical compounds that settle from the wash water in separators or hold tanks. Open-loop systems produce a smaller sludge volume because the bulk of the wash water is discharged overboard; closed-loop sludge contains concentrated residues from the recirculating circuit blowdown. The EGCS Record Book must log all sludge quantities generated and disposed of.

Discharge bans and port restrictions

The MARPOL Annex VI framework permits open-loop discharge subject to the MEPC.340(77) limits. However, Regulation 3.1 of MARPOL Annex VI allows ports and coastal states to impose stricter standards within their waters. A significant number of jurisdictions have exercised this right:

Singapore: banned open-loop EGCS discharge in port waters from 1 January 2020.
Malaysia: banned open-loop discharge in Malaysian ports from 1 January 2020.
China: banned open-loop discharge in its coastal emission control area from 1 January 2019, with subsequent extensions to additional ports.
United States - California: prohibited discharges within three nautical miles of the coast under state law, effectively requiring closed-loop operation or compliant fuel in California waters.
Baltic and North Sea ports: several individual ports in Germany, Denmark, Belgium, and Norway have adopted local discharge bans or restrictions in ecologically sensitive fjord zones.
European Union: no bloc-wide prohibition on open-loop discharge exists as of early 2026, but regulatory pressure is sustained and the trajectory is toward tighter controls, with multiple North Sea Commission member states having advocated for an ECA-wide ban.

Ships operating in jurisdictions with discharge bans must either use compliant fuel in those waters or operate in closed-loop mode with a hold tank of sufficient capacity. On large vessels operating primarily in European waters, hold tanks of 1,000 to 2,000 m3 have been installed.

Economics and fleet adoption

Capital expenditure

EGCS installation costs vary substantially depending on engine power, system type, whether the installation is a new-build or a retrofit, and market conditions. Indicative retrofit CAPEX ranges span US$3 million to US$7 million for a single main engine installation, with multi-engine or large cruise ship installations reaching US$10 million or more. New-build installations are generally less expensive per unit because structural work is integrated into the shipbuilding programme. A closed-loop or hybrid system costs approximately 20 to 40% more than a comparable open-loop unit, reflecting additional tanks, coolers, NaOH storage, and sludge handling equipment.

Installation also entails indirect costs: dry-docking time (typically 15 to 25 days for a major retrofit), lost revenue, and project management. Vessels that combine the scrubber installation with a scheduled dry-docking for class renewal incur little or no additional off-hire cost.

Fuel price spread and payback

The economic case for an EGCS hinges on the price differential between HFO and compliant fuel. This spread fluctuated markedly between 2019 and 2025:

Before the 2020 cap, anticipation of large HFO-VLSFO price differentials drove a surge in scrubber orders from 2017 onward.
In 2020 to 2021, the spread reached US$100 to US$200 per tonne and the payback period for a high-consuming container ship was two to five years on the CAPEX.
In 2022 to 2024, energy market volatility compressed and then widened the spread; at spreads above US$150 per tonne, a large container ship consuming 200 t/day saves US$30,000 per day in fuel cost, giving a simple payback of under one year on a US$6 million installation.
At lower daily consumption (50 t/day) and a US$100 spread, the payback extends to four to five years.

The calculation must account for OPEX additions (NaOH, electricity for pumps, maintenance, sludge disposal) of roughly US$500,000 to US$1.5 million per year for a large system. The EGCS NaOH dosing formula gives the stoichiometric basis for estimating NaOH operating cost.

Use the ShipCalculators.com calculator catalogue to explore fuel cost and emission calculations relevant to scrubber economics.

Fleet adoption and supplier landscape

By 2024, approximately 5,000 vessels had scrubbers fitted or on order, representing roughly 15% of world fleet gross tonnage. The concentration is heavily weighted toward the largest vessel types: large container ships, capesize bulk carriers, and cruise vessels account for the majority of installations. The scrubber fleet expanded rapidly between 2018 and 2021, with annual installation rates peaking at over 1,500 vessels in 2019.

Dominant EGCS suppliers include Wärtsilä (Puregas and Hamworthy brands), Alfa Laval (PureSOx), Yara Marine Technologies, Ecospray Technologies, Mitsubishi Heavy Industries, and CR Ocean Engineering. The market has broadly converged on hybrid as the standard configuration for new orders. Every EGCS installed on a classed vessel must carry type approval from a recognised classification society - Lloyd’s Register, DNV, Bureau Veritas, ClassNK, American Bureau of Shipping, and RINA all maintain schemes aligned with MEPC.340(77). The general certification framework is described in the classification society article.

Effect on EEDI and CII

An EGCS does not directly change a vessel’s fuel consumption and therefore does not alter the attained EEDI value. The CII Carbon Intensity Indicator under MARPOL applies to CO2 emissions per transport work; HFO has a slightly lower CO2 emission factor per unit calorific value than VLSFO (3.114 g CO2/g fuel versus 3.151 for VLSFO under MARPOL accounting), so a scrubber-fitted vessel burning HFO shows a marginally lower CO2 intensity than if it burned VLSFO at the same consumption rate - a minor difference that does not reverse the broader economics. The CII interaction is discussed further in the what is CII and slow steaming and CII articles.

Environmental debate and wash water science

The wash water discharge controversy

Open-loop EGCS have been the subject of sustained scientific and regulatory debate regarding the environmental effects of wash water discharge. The central concerns are:

Acidification: discharge of pH 3 to 4 wash water in ports or near coastlines could locally lower seawater pH, adversely affecting calcifying organisms and benthic communities.
PAH loading: PAH compounds are persistent and carcinogenic. Although the MEPC.340(77) limit of 50 µg/litre PAHphe is calibrated as protective under open-ocean conditions, the cumulative load from a large fleet may be significant in high-traffic areas.
Heavy metals: wash water contains trace concentrations of vanadium, nickel, and other metals present in HFO combustion products. These concentrate in the sludge fraction but some fraction remains dissolved in the discharged stream.
Turbidity: elevated turbidity reduces light penetration and may affect primary productivity in shallow coastal waters.

The EGCS Correspondence Group (EGCS CG), operating under the MEPC, reviewed scientific evidence through several sessions and commissioned studies including the CE Delft 2021 report on the effects of scrubber discharge water. Findings indicated that, while the discharge is chemically complex, modelling and monitoring data did not demonstrate clear harm at the open-ocean scale - though the report noted significant data gaps and recommended tighter monitoring. MEPC.340(77) responded with updated limits and more rigorous recording requirements. MEPC 79 (December 2022) continued the review on submissions by Denmark, Finland, the Netherlands, and Sweden, but no global prohibition on open-loop discharge resulted.

The environmental criticism that open-loop scrubbers transfer pollution from the atmosphere to the marine environment rather than destroying it has been characterised as a substitution argument. The industry response is that sulphate is a naturally occurring seawater constituent and that dilution in open-sea conditions renders concentrations well below ecotoxicological thresholds; critics respond that near-coastal and ECA conditions are not equivalent to open ocean.

Comparison with fuel-switching alternatives

The comparison between scrubber use and switching to compliant fuel is not straightforward. VLSFO composition varies widely among refineries, and some blends have caused compatibility and stability problems as well as machinery damage incidents documented under fuel quality alert cycles. From an air quality standpoint, both pathways deliver similar SO2 reductions when the scrubber achieves designed efficiency. The CO2 profile is marginally different: HFO has a slightly lower carbon-to-energy ratio than distillate fuels, so a scrubber-fitted HFO-burning ship produces slightly less CO2 per unit of energy than a VLSFO-burning ship.

The selective catalytic reduction system often installed alongside a scrubber addresses NOx emissions, which are not affected by the fuel sulphur content or the scrubbing process.

Integration with other emission control technologies

SCR and EGR

MARPOL Annex VI Regulation 13 sets NOx emission limits independent of the SOx controls. Inside NOx ECAs, Tier III NOx limits apply to engines installed on or after 1 January 2016, requiring approximately 80% NOx reduction relative to Tier II - limits that cannot be met by combustion optimisation alone. The two main Tier III technologies are selective catalytic reduction (SCR) and exhaust gas recirculation (EGR). The NOx tier limit calculator computes the applicable Tier I, II, and III NOx limits from engine speed under MARPOL Annex VI Regulation 13.

SCR injects urea (as aqueous urea solution, typically 32.5% as AdBlue) upstream of a catalyst bed, where NOx reacts with ammonia derived from urea hydrolysis to form nitrogen and water. SCR can coexist with an EGCS on the same exhaust path, but the relative position matters: an SCR upstream of the scrubber operates in clean, hot gas conditions; an SCR downstream may be exposed to acid mist and lower temperatures that degrade catalyst performance. Most installations place the SCR upstream - directly after the turbocharger, before the scrubber - to exploit residual exhaust heat.

EGR recirculates a fraction of the exhaust gas back into the engine charge air, reducing combustion temperature and thus NOx formation. EGR-scrubber combinations are less common, partly because the recirculated gas may contain SO2 that causes corrosion in the charge air path when HFO is the fuel.

Waste heat recovery

Exhaust gas cooling in a scrubber reduces the temperature available for waste heat recovery. Ships equipped with an exhaust gas boiler or power turbine upstream of the scrubber retain the benefit of those devices, since heat recovery precedes the scrubbing stage. Installations in which the scrubber is placed between the turbocharger and the boiler will find that the boiler gas inlet temperature is reduced, lowering steam generation. This design trade-off must be evaluated against the fuel-cost savings from the scrubber. The waste heat recovery system article covers the broader context of shipboard heat integration.

Operational procedures and record-keeping

EGCS Record Book requirements

MEPC.340(77) requires that every EGCS-fitted vessel maintain an EGCS Record Book recording:

Dates, times, and positions of open-loop and closed-loop operation
Any failures or malfunctions of the monitoring system, with duration and corrective action
Sludge tank quantities, discharge to reception facility dates and volumes
Maintenance activities on sensors and the scrubbing system
Entries on switchover between open-loop and closed-loop modes, including vessel position and reason for the change

The record book may be maintained electronically provided the system meets tamper-evident storage requirements. Port state control officers routinely inspect EGCS record books under inspections within the Paris MOU, Tokyo MOU, and equivalent regional agreements. Deficiencies in the record book, missing or implausible monitoring data, or evidence of operation in restricted waters without closed-mode activation are grounds for detention.

IAPP Certificate and surveys

A vessel’s International Air Pollution Prevention Certificate records the approved means of compliance with Regulation 14. When an EGCS is installed, the certificate must be amended by the flag Administration or its recognised organisation to include the EGCS type, serial number, applicable approved guidelines (MEPC.340(77)), and the operational modes approved. Annual surveys include a record review and spot-checks on monitoring system calibration. The initial commissioning programme covers functional testing of all monitoring instruments, verification of the control and automation system at calibrated reference points, measurement of SO2/CO2 ratio at representative engine loads (typically 25%, 50%, 75%, and 100% of maximum continuous rating), and open-loop wash water analysis at an accredited laboratory against the full MEPC.340(77) parameter suite.

FONAR procedure

If an EGCS fails while the vessel is operating in an ECA and compliant fuel is not available on board, the master must file a Fuel Oil Non-Availability Report (FONAR) under MARPOL Annex VI Regulation 18.2. The FONAR sulphur compliance calculator assists in verifying whether FONAR conditions are met and in preparing the notification. The FONAR does not grant automatic exemption; it notifies the flag state and the port state of the circumstances and requires demonstration of best efforts to obtain compliant fuel.

Crew training and operational responsibilities

MEPC.340(77) mandates that crew responsible for EGCS operation are trained in the system’s principles, operating procedures, alarm responses, and record-keeping requirements. Manufacturer training is typically provided as part of the commissioning contract, supplemented by an EGCS Operating Manual and, for vessels subject to SMS requirements under the ISM Code, a set of shipboard procedures covering start-up, shutdown, mode change, system fault response, and port ban compliance.

The officer of the watch has a direct role in EGCS management because open-loop to closed-mode switching is a real-time operational decision that interacts with voyage planning, fuel tank management, and port state control compliance. Many operators use electronic chart systems overlaid with proprietary geo-fence databases identifying port ban areas, enabling an automatic alert when the vessel approaches a zone requiring closed-mode operation. Poorly managed mode changes - failing to switch to closed mode before entering a banned zone, or operating in closed mode with insufficient NaOH dosing - are among the deficiencies most commonly cited by port state control inspectors during MARPOL Annex VI concentrated inspection campaigns.

Interaction with DCS and MRV reporting

EGCS-fitted vessels burning HFO participate in the same emissions data collection and reporting regime as all other ships above 5,000 gross tons. The IMO Data Collection System (DCS) under MARPOL Annex VI Regulation 22A requires annual fuel consumption reporting; the EU MRV Regulation requires voyage-level CO2 data for ships operating to and from EU ports. Neither system distinguishes between HFO consumed through an EGCS and HFO consumed without one - the CO2 emission factor applied to HFO is the same in both cases. A vessel must present both the EGCS Record Book (for SOx compliance) and the MRV/DCS records (for CO2 reporting) as separate but concurrent documentation. The IMO DCS versus EU MRV article explains the two frameworks and their interaction.

Modern practice and regulatory trajectory

MEPC.340(77) and subsequent developments

The 2021 revision of the EGCS Guidelines under MEPC.340(77) represented a significant tightening of monitoring, recording, and data management requirements. Key changes included: alignment of the SO2/CO2 ratio thresholds with actual emission equivalence calculations for the 0.50% global cap (replacing the 21.7 ratio that applied under the superseded 3.50% global cap with the operative 4.3 limit), updated wash water limits, mandatory electronic data logging with defined retention periods, and clearer requirements for EGCS approval documentation submitted to flag states.

MEPC 79 (December 2022) and MEPC 80 (2023) continued to discuss the wash water science under the EGCS Correspondence Group agenda. Several delegations proposed that a global ban on open-loop discharge in ECAs be adopted; as of early 2026, this has not achieved the consensus required for adoption, but the deliberations continue and further scientific review is expected.

FuelEU Maritime and CII interactions

The FuelEU Maritime regulation, which applies from 1 January 2025 to vessels above 5,000 GT calling at EU ports, assesses fleet greenhouse gas intensity using a well-to-wake methodology. HFO carries a higher well-to-wake carbon intensity than LNG or certain alternative fuels, and its use in a scrubber-fitted vessel does not change the well-to-wake GHG accounting for FuelEU. Similarly, the CII Carbon Intensity Indicator under MARPOL applies to CO2 emissions per transport work; a scrubber affects neither the CO2 emitted from HFO combustion nor the ship’s speed or cargo capacity. The scrubber is therefore neutral in the FuelEU and CII frameworks - it solves the SOx compliance problem but provides no GHG benefit.

For EU ETS context, see EU ETS for shipping. EU ETS allowance prices were above EUR 60 to EUR 70 per tonne CO2 for much of 2023 to 2024, imposing a real carbon cost on HFO combustion that the scrubber does not reduce. As GHG regulations tighten toward the IMO 2030 and 2050 targets, the long-run economics of HFO-plus-scrubber face increasing pressure. Vessels investing in scrubbers today commit to HFO combustion as their fuel strategy for 10 to 15 years. If carbon pricing, FuelEU penalties, or alternative fuel availability shifts the economics against HFO, the scrubber asset may become stranded before full payback.

Alternative fuels and EGCS obsolescence

Ships converting to LNG as a marine fuel, methanol as a marine fuel, or ammonia as a marine fuel do not require an EGCS because these fuels are inherently near-zero or zero sulphur. A vessel that converts from HFO-plus-scrubber to a low-sulphur alternative fuel would either decommission the scrubber or retain it as a standby for dual-fuel operation. Biofuels in shipping present a more nuanced case: some bio-HFO blends retain a sulphur content requiring either scrubber use or blending down. The regulatory status of bio-based fuels in the context of MARPOL Annex VI equivalence is still being developed at MEPC.

Emerging technologies

Several engineering groups have investigated integrating CO2 capture into existing scrubber infrastructure. An EGCS already cools and wets the exhaust gas - conditions conducive to amine scrubbing for CO2 absorption. Pilot installations combining SOx scrubbing with CO2 capture have been reported on coastal vessels, but none had reached commercially proven large-scale status as of early 2026. The potential regulatory incentive from the EU ETS for shipping could improve the CO2 capture business case if verified onboard sequestration were credited against ETS obligations.

Oxidative scrubbing using hydrogen peroxide (H2O2) has been studied at laboratory and pilot scale as an alternative to alkaline wet scrubbing, with claimed simultaneous SOx and NOx removal capability. Commercial marine deployment remained at an early stage as of early 2026. Dry sorbent injection - spraying dry sodium bicarbonate or lime into the exhaust duct upstream of a fabric filter - has been applied in small marine installations, particularly on Ro-Pax ferries; removal efficiencies are typically 85 to 92%, below wet-scrubber performance, but potentially sufficient to achieve 0.50% compliance when combined with a moderately low-sulphur input fuel.

Fleet statistics and market outlook

Scrubber uptake has been concentrated in specific vessel types and trade routes where the economics of HFO retention are most favourable. Bulk carriers - particularly Capesize and Newcastlemax vessels engaged in Pacific iron ore and coal trades - have the highest penetration rate, reflecting the combination of high daily fuel consumption, relatively long ocean passages on which open-loop operation is uninterrupted, and short port approaches amenable to closed-mode compliance. VLCC and Suezmax crude tankers show similar uptake patterns. Container ships have adopted scrubbers at a lower rate proportionally, partly because the ship type is more exposed to public and cargo-owner sustainability scrutiny and partly because tight funnel profiles make U-type retrofits complex and expensive.

By end 2024, Clarksons Research and DNV fleet databases indicated that more than 5,000 vessels had an EGCS installed or on firm order - roughly 10 to 12% of the world fleet by number but a higher proportion by gross tonnage and fuel consumption, since large bulk carriers and tankers account for a disproportionate share of global HFO consumption.

Projections for further growth are sensitive to the HFO/VLSFO price spread and to the trajectory of open-loop port bans. If a North Sea ECA-wide ban on open-loop discharge were implemented, roughly 30 to 40% of current scrubber installations would require hybrid or closed-loop conversion to maintain compliance on that trade route without fuel change-over, creating a secondary market for conversion services. A widening ban landscape would simultaneously reduce the economic attractiveness of new open-loop-only investments.

References

IMO Resolution MEPC.340(77) - 2022 Guidelines for Exhaust Gas Cleaning Systems. International Maritime Organization, London, 26 November 2021.
IMO Resolution MEPC.184(59) - 2009 Guidelines for Exhaust Gas Cleaning Systems. International Maritime Organization, London, July 2009.
MARPOL Annex VI - Regulations for the Prevention of Air Pollution from Ships. International Maritime Organization (consolidated text, 2021 edition).
IMO Resolution MEPC.176(58) - Revised MARPOL Annex VI including tightened Regulation 14 sulphur limits. October 2008.
CE Delft (2021) - Scrubbers: An Economic and Ecological Assessment. Report commissioned by North Sea Commission member states. CE Delft, Delft, Netherlands.
MEPC.1/Circ.881 - Guidance on the Provision of Bunker Delivery Notes when a Ship Uses an Exhaust Gas Cleaning System. International Maritime Organization, London, 2018.
Singapore Maritime Port Authority Circular No. 6 of 2019 - Prohibition of Open-Loop Scrubber Discharge in Port Waters. MPA, Singapore.
Clarksons Research - World Fleet Monitor, selected issues 2022 to 2024.
IMO NOx Technical Code 2008, as amended by MEPC.177(58). International Maritime Organization, London.

External links

IMO MARPOL Annex VI text and amendments - official IMO air pollution prevention resource
EGCS Association (EGCSA) - industry body representing scrubber manufacturers and operators
MEPC.340(77) guidelines - downloadable IMO resolution