Exhaust gas cleaning system (EGCS): marine SOx scrubbers

An exhaust gas cleaning system (EGCS), or scrubber, is shipboard equipment that washes sulphur dioxide out of engine and boiler exhaust into an alkaline liquid, and MARPOL Annex VI Regulation 4 accepts an approved unit as an equivalent to burning compliant fuel provided the exhaust holds an SO₂ to CO₂ Emission Ratio at or below 21.7 against the 0.50% m/m global cap and 4.3 against the 0.10% m/m limit inside an emission control area .

That single sentence carries the whole regime, and almost every practical dispute about scrubbers turns on one of its clauses. The equipment is accepted under Regulation 4, not exempted from Regulation 14 . The approval belongs to the flag Administration, not to a classification society. The measure of compliance is a ratio in the exhaust, not a sulphur figure on a bunker delivery note . And the standards the ship is held to live in resolution MEPC.340(77) , the 2021 Guidelines for Exhaust Gas Cleaning Systems, which are themselves recommendatory.

What an exhaust gas cleaning system does

A scrubber removes sulphur dioxide from exhaust gas by dissolving it into a liquid that can neutralise the resulting acid. It does nothing else of regulatory consequence. It does not reduce nitrogen oxides, it does not reduce carbon dioxide, and it does not improve any efficiency metric. What it does is break the link between the sulphur content of the fuel in the tank and the sulphur oxides leaving the funnel, which is precisely the link Regulation 14 relies on.

The commercial logic follows from that break. Residual fuel with 3.50% sulphur, the grade the world fleet burned before 2020, is cheaper than the 0.50% blends that replaced it. A ship that can burn the cheaper fuel and still comply captures the price difference, and pays for the equipment, the space, the auxiliary power and the operating attention it takes. The fuels involved are covered at heavy fuel oil , very low sulphur fuel oil and marine gas oil , and the switch itself at fuel switching operations .

The unit sits in the uptake between the engine or boiler and the funnel, downstream of the turbocharger and, on most ships, downstream of the waste heat recovery economiser. Exhaust enters hot and leaves cooled and saturated. A single unit may serve one engine or several, and a ship commonly runs a scrubbed main engine alongside auxiliary engines and a boiler burning compliant fuel, which is one reason the compliance documentation is written per combustion unit rather than per ship.

Three terms are used interchangeably in the trade and should not be. EGCS is the term in the Guidelines; EGC unit is the term MEPC.1/Circ.883 uses for the individual cleaning device; scrubber is the shipyard and engine room word. In compliance documents, use the term the document uses.

The legal basis: Regulation 4 equivalence, not a Regulation 14 exemption

The whole arrangement rests on MARPOL Annex VI Regulation 4 , which permits an Administration to allow a fitting, material, appliance, apparatus or alternative procedure in place of one the Annex requires, provided the substitute is at least as effective in terms of emission reductions. That is a substitution of method, not a relaxation of duty.

The distinction is not academic. A ship operating an approved equivalent is still subject to Regulation 14 in full, which is why it demonstrates compliance continuously through monitored emissions rather than through retained bunker delivery notes. It is also why the carriage prohibition introduced by resolution MEPC.305(73), in force 1 March 2020, does not bite: Regulation 14.6 prohibits carriage of non-compliant fuel oil for combustion purposes unless the ship has an approved equivalent arrangement, and section 2.3.3 of the certificate supplement is the checkbox for a ship that does not.

Regulation 3, which is sometimes cited as the source of local scrubber bans, does no such thing. It is headed Exceptions and exemptions: 3.1 disapplies the Annex to emissions necessary for securing the safety of a ship or saving life at sea, and to emissions resulting from damage; 3.2 provides a research exemption. There is no provision anywhere in Annex VI authorising a port State to impose a stricter discharge standard on a ship exercising Regulation 4 equivalence, which is why local restrictions rest on a different legal foundation entirely.

How the equivalence is recorded

Following the installation survey, paragraphs 4.2.3.4 and 5.3.2 of MEPC.340(77) require sections 2.3 and 2.6 of the Supplement to the International Air Pollution Prevention Certificate to be completed. Section 2.3, as replaced by resolution MEPC.305(73) on 26 October 2018, carries a checkbox for an equivalent arrangement approved in accordance with Regulation 4.1 that is at least as effective in terms of SOx emission reductions as using a fuel oil at the applicable limit, with 2.3.1 covering operation outside an emission control area at 0.50% and 2.3.2 mirroring it inside one at 0.10%.

Section 2.6, headed Equivalents (regulation 4), is a three-column table of system or equipment, equivalent used, and approval reference. That table is where the scrubber is actually entered, and it is the first line a port State control officer reads. IACS Unified Interpretation MPC101 Rev.1 Corr.1, issued September 2020, reads section 2.3 as permitting an entry in all relevant boxes, on the basis that the bunker delivery notes retained for three years provide the means to check what the ship actually did.

Approval sits with the Administration

Paragraphs 4.2.1.3, 5.2.1 and 10.3.1 place certification of the unit, of the Scheme B monitoring system and of the discharge water monitoring arrangement with the Administration. Regulation 5.3.1 permits delegation to a recognized organization , and in practice most flags delegate to a classification society . What does not follow is that class type approval is itself the legal requirement. It is a route the Administration may accept, and describing it as mandatory misstates who carries the duty. The same point applies to class notations , which are contractual between owner and society.

The Emission Ratio: the number the regime turns on

Table 1 of MEPC.340(77) sets two limits, and they are the two numbers most often reported wrongly.

The ratio is defined at paragraph 8.1 as sulphur dioxide in parts per million over carbon dioxide in per cent by volume, with no scaling divisor. The limits apply to exhaust from combustion of petroleum-derived distillate or residual fuel oils; appendix 2 carries the assumptions behind them.

Carbon dioxide is in the denominator because it is a proxy for fuel burned. Ratioing against it removes the effect of excess air, of dilution, and of any leakage into the sample, so the number reflects what entered the cylinder rather than the concentration of whatever gas is in the duct. That is why the quantity is dimensionally odd and operationally robust.

Where 21.7 and 4.3 come from

Appendix 2 Table 2 gives the sulphur to carbon molar ratio of fuel oil at 1.50% sulphur as 0.00661 mol/mol for distillate and 0.00646 for residual, corresponding to exhaust ratios of 66.12 and 64.60 ppm per per cent, rounded to 65. The relationship is linear in sulphur, so 65 multiplied by S/1.50 reproduces every published value: 43.3 at 1.00%, 21.7 at 0.50%, 4.3 at 0.10%. The unit algebra behind it is that SO₂(ppm)/CO₂(%) equals the sulphur to carbon molar ratio multiplied by 10⁴. That factor of ten thousand belongs to the derivation and not to the operative formula, which is the origin of a persistent error in secondary sources that divide the measured ratio by it.

Which rows are historical

The 2015 Guidelines, resolution MEPC.259(68), carried six rows: 195.0 at 4.50% sulphur, 151.7 at 3.50%, 65.0 at 1.50%, 43.3 at 1.00%, 21.7 at 0.50% and 4.3 at 0.10%. MEPC.340(77) reduced the operative table to the two rows matching limits still in force, keeping 1.50% only in the appendix 2 derivation. The 2021 revision changed no value. It dropped the rows above 1.50% because the limits they corresponded to had gone.

So 43.3 and 151.7 remain correct arithmetic and are useful for calculating the removal a given fuel requires. They are not current limits, and they should be labelled as values from the superseded instrument whenever they are quoted.

Removal efficiency is derived, not a system property

The habit of quoting a scrubber as “97 to 99% efficient” inverts the logic. Efficiency is whatever the fuel and the applicable limit require.

On 3.50% sulphur fuel, whose uncontrolled ratio is 151.7, meeting the ECA limit of 4.3 requires 4.3 divided by 151.7, that is 2.8% survival, so 97.2% removal. Meeting the global limit of 21.7 on the same fuel requires 21.7 divided by 151.7, 14.3% survival, so 85.7% removal. On 2.70% fuel the same two targets need less. Publish the derivation, and state the fuel it assumes.

Wet scrubbing chemistry and stoichiometry

Sulphur in the fuel burns to sulphur dioxide almost quantitatively.

The molar masses make the trade’s rule of thumb exact: sulphur dioxide is 64.06 against sulphur at 32.06, so each kilogram of sulphur burned produces 1.998 kg of sulphur dioxide, and doubling is correct to within 0.1%. One tonne of fuel therefore yields 2.00 kg of sulphur dioxide at 0.10% sulphur, 9.99 kg at 0.50%, 29.97 kg at 1.50%, 53.95 kg at 2.70% and 69.93 kg at 3.50%.

Open loop: the carbonate buffer does the work

Seawater absorbs sulphur dioxide because it is buffered by carbonate alkalinity, not because it is wet. The sequence is dissolution to sulphurous acid, dissociation, and consumption of the proton by the carbonate system:

SO₂ + H₂O gives H₂SO₃, which dissociates to H⁺ and HSO₃^-. The bicarbonate present in seawater takes the proton: HCO₃^- + H⁺ gives H₂O + CO₂. The bisulphite then oxidises to sulphate, HSO₃^- + half O₂ gives SO₄^2- + H⁺.

The end product is sulphate, a natural constituent of seawater at about 2.7 g/kg, which is the environmental argument for the open loop. The operational consequence is less comfortable: every mole of sulphur dioxide absorbed consumes roughly a mole of bicarbonate and releases a mole of carbon dioxide, so the buffer is destroyed as it works. Low-alkalinity water cannot sustain the same absorption rate, which is why open-loop performance falls in brackish and estuarine water. Solubility also falls as temperature rises, so warm water absorbs less at a given flow, but salinity rather than warmth is the operational problem.

There is no universal regulatory floor on inlet alkalinity. Two anchors exist in the instrument. Paragraph 10.1.2.1.2.4 sets reference seawater at alkalinity 2.2 mmol/L and pH 8.2 for demonstrating the 4 metre pH criterion, with an example titration curve at appendix 4. Paragraphs 4.2.2.1.2.6 and 5.6.1.2.4 require the manufacturer to declare the minimum inlet water alkalinity, referenced to ISO 9963-1:1994 and ISO 9963-2:1994, as an approved operating limit of the particular unit. The unit’s declared limit is the operative number, and figures circulating in micromoles per kilogram cannot be traced to any primary source.

One designation defect belongs to the instrument rather than to the reader. MEPC.340(77) writes “ISO 9963-1-2:1994”, which is not a real designation. The two published parts are ISO 9963-1:1994, total and composite alkalinity, and ISO 9963-2:1994, carbonate alkalinity, both dated 10 November 1994.

Closed loop: caustic soda, and the number that follows from it

A closed loop recirculates fresh water dosed with alkali, so it does not depend on the receiving water at all. With sodium hydroxide the reaction is SO₂ + 2NaOH giving Na₂SO₃ + H₂O, followed by oxidation to sodium sulphate. With soda ash it is SO₂ + Na₂CO₃ giving Na₂SO₃ + CO₂. Paragraph 10.1.6.1 names exactly these two as neutralisation agents requiring no further discharge assessment, provided discharge pH does not exceed 8.0.

Two moles of sodium hydroxide at 39.997 g/mol per mole of sulphur dioxide at 64.06 g/mol gives 1.249 kg of pure NaOH per kilogram of sulphur dioxide. Working that through one tonne of 3.50% sulphur fuel at complete removal: 35 kg of sulphur, 69.93 kg of sulphur dioxide, 87.3 kg of pure NaOH, which is about 175 kg or 114 litres of the 50% w/w solution ships actually bunker, at a solution density near 1.53 kg/L at 15 degrees C.

Consumption per megawatt hour requires a specific fuel oil consumption basis and is meaningless without one. At 165 g/kWh the figure is 14.4 kg of pure NaOH per MWh, that is 28.8 kg or 18.8 litres of 50% solution. At 175 g/kWh it is 15.3 kg; at 190 g/kWh, 16.6 kg; at 210 g/kWh, typical of a four-stroke auxiliary, 18.3 kg. Quote the SFOC alongside the consumption or the number cannot be checked.

A worked case for a large two-stroke: 150 tonnes per day of 3.50% sulphur fuel is 5.25 t/day of sulphur and 10.49 t/day of sulphur dioxide. At 97% removal the system absorbs 10.18 t/day, needing 12.71 t/day of pure sodium hydroxide, which is 25.4 t/day of 50% solution or about 16.6 m³/day of storage and handling. That is the reason a closed-loop ship carries a caustic tank measured in tens of cubic metres and a bunkering routine to match.

Discharge water flow: use the regulator’s basis

Flow figures quoted per megawatt of thermal input and per megawatt of shaft power are not comparable, and both appear in the trade literature. The instrument settles it. Paragraphs 10.1.3.3 and 10.1.5.1 normalise the discharge criteria to a specific discharge water flow of 45 tonnes per megawatt hour, where the megawatt figure is the aggregated maximum continuous rating of the combustion units monitored at that discharge point. Table 5 scales the criteria across 0 to 1, 2.5, 5, 11.25, 22.5, 45 and 90 t/MWh, which is the regulator’s own statement of the flow envelope an open loop occupies.

Recirculation and bleed rates for closed-loop systems are maker-specific and no primary source states them, so this article states none.

System types

Four arrangements are in service, and the choice between them is a trading pattern decision more than an engineering one.

Open loop

The open loop is the simplest arrangement: seawater is pumped to the tower, contacts the exhaust, and goes overboard after monitoring and any treatment. It has no consumables beyond pump power, no reagent storage and no holding tank. Its two weaknesses are the ones already stated, low-alkalinity water and local discharge restrictions, and the second has proved far more consequential than the first.

Closed loop

The closed loop recirculates fresh water dosed with alkali through a process tank and a cooler, bleeding off a small stream to control dissolved solids and topping up to replace it. It is indifferent to the receiving water, so it works in the Baltic and in port. It needs alkali bunkering, a process tank, a cooler, and either a bleed discharge that meets the same section 10 criteria or a holding tank if the ship is to discharge nothing at all.

Hybrid

A hybrid unit can run in either mode and switch between them, which is what most retrofits since 2019 have specified. It carries the capital and space cost of both arrangements, and it buys the ability to enter a discharge-restricted port without shutting the system down or changing fuel. The zero-discharge endurance is set by holding tank capacity, which is a naval architecture decision made at design.

Dry

A dry system passes the exhaust through calcium hydroxide granulate rather than a liquid, producing gypsum. There is no discharge water at all, so the entire section 10 criteria set and the whole discharge restriction question fall away. The trade is consumable logistics and volume. Note that the Guidelines are written for wet systems: paragraph 2.2.2 states that EGCS means a wet system generally, and 2.2.3 allows the Guidelines to be applied to others as appropriate, which leaves a dry installation dependent on how its Administration exercises that discretion.

Approval: Scheme A and Scheme B

Paragraph 2.1.2 defines the two routes. Scheme A is system certification with in-service continuous operational parameter monitoring and periodic emission checks. Scheme B is continuous emission monitoring by an approved monitoring system together with periodic operational parameter checks. Under Scheme A the Administration approves the cleaning unit; under Scheme B it approves the monitoring system.

MEPC.340(77) applies to systems on ships keel-laid on or after 1 June 2022, to systems whose contractual delivery to the ship, or in the absence of a contract actual delivery, falls on or after that date, and to amendments undertaken on or after that date. Paragraph 6 of the resolution records that it supersedes MEPC.259(68), the 2015 Guidelines, not the 2009 Guidelines in MEPC.184(59). Older installations continue under the instrument they were approved against, which is why two document sets circulate and why a surveyor asks which applies before asking anything else.

The document set

Table 4 fixes what each scheme requires.

The SECP is required of any ship using a system in part or in total as an approved equivalent, under paragraphs 2.1.6 and 9.1.1. Paragraphs 9.1.2 and 9.1.3 require it to list each combustion unit that may burn Regulation 18 compliant fuel and each that may operate under Scheme A, Scheme B or both, with the demonstration provisions at 9.2.1 and 9.2.2. It is the document that answers the first question on a mixed installation: which engine complies by which route.

The Onboard Monitoring Manual covers each system with its identified combustion unit and specifies the installation, operation, maintenance, servicing and calibration of every monitoring chain. It is the document that fixes calibration intervals and drift handling, which makes it more load-bearing than its title suggests.

Scheme A in detail

A Scheme A unit is certified against a Certified Value: the Emission Ratio the manufacturer specifies the system achieves continuously at its specified maximum fuel sulphur content within a declared set of operating parameters, per paragraph 4.1.2.1 and Table 3. Each certified unit is issued a SOx Emissions Compliance Certificate by the Administration under paragraph 4.2.1.3, on the form at appendix 1, and paragraph 4.2.3.3 requires it before the unit is used. Application may be made by the manufacturer, the owner or another party. Sister units of the same design and rating may be issued a certificate without retesting; different ratings fall under the product range approval provisions at 4.1.4, and partial-flow systems receive special consideration under 4.2.1.7.

Emission testing for certification uses a minimum of four load points defined by exhaust gas mass flow rate, not by percentage of maximum continuous rating. Paragraph 4.3.2 requires one point at 95% to 100% of the maximum certified flow, one within plus or minus 5% of the minimum, and two equally spaced between, with more where a discontinuity or an emission peak exists.

In service, paragraph 4.4.7 requires automatic recording of washwater pressure and flow at the unit inlet, exhaust pressure before the unit and the pressure drop across each unit, combustion unit load, and exhaust temperature before and after the unit, each against its approved range. Where there is no continuous emission monitoring, paragraph 4.4.8 adds a daily Emission Ratio spot check of not less than five minutes at a minimum recording frequency of 0.1 Hz, taken at normal working condition for each outlet to atmosphere with readings allowed to stabilise.

Scheme B in detail

Scheme B measures the Emission Ratio continuously. Paragraph 5.4.1 requires measurement after the cleaning unit and before any mixing with ambient air. Paragraph 5.4.2 requires sulphur dioxide in ppm, carbon dioxide in per cent, and the ratio to not less than one decimal place, continuously monitored and recorded at not less than 0.0035 Hz whenever the system is in operation, which is about one reading every 286 seconds. The same operating parameters listed at 4.4.7 are checked daily under 5.5.2.

Data recording, and the two retention periods

Section 7 requires the recording system to be robust, tamper-proof and read-only capable. It records the paragraph 4.4.7, 5.4.2 and 10.3 data whenever the system is operating, including overboard discharges from any associated tanks, against UTC, GNSS position, and whether the ship was inside or outside a Regulation 14.3 emission control area. It must be pre-settable with the applicable Emission Ratio limit, the overboard pH limit, the PAH limit and the turbidity limit, and it must record aggregated time above 15 minutes in any rolling 12-hour period where differential PAH exceeds its limit by more than 100%, and where rolling average differential turbidity exceeds by more than 20%. A downloadable copy must clearly indicate periods of non-compliance.

Two retention periods apply and they are routinely confused. Recorded data is retained for not less than 18 months under paragraph 7.4, and stays on board if the recording device is changed. EGC Record Book entries are maintained for a minimum of three years after the last entry, under 4.4.9 and 5.7.1.

Installation, safety and IACS UR M86

Until November 2024 there was no class Unified Requirement dedicated to exhaust gas cleaning systems, and MEPC.340(77) says almost nothing about the safety of the installation because it is an emissions and discharge instrument. IACS UR M86 , Monitoring and Safety Functions for Exhaust Gas Cleaning (SOx) Systems, closed that gap. It is uniformly implemented on ships contracted for construction on or after 1 January 2026, with “contracted for construction” taking its meaning from IACS Procedural Requirement No. 29, and it remains the only IACS Unified Requirement on the subject.

Section 2 requires a bypass arrangement so that the engines can continue to operate, working automatically on the conditions in the requirement’s Table 1. It may be omitted only where the system is designed for dry operation. Where bypass and uptake dampers are individually controlled, an interlock is required to prevent both being closed at once, which may be implemented as an upstream pressure sensor that opens the bypass on high back pressure. Paragraph 3.2 of MEPC.340(77) separately requires measures against leakage of exhaust gas from the damper to bypass lines.

Section 3 requires single-fault tolerance in the control and monitoring system, so that a single fault of a component will not lead to a potentially dangerous situation for human safety or for the vessel, and requires an FMEA or equivalent to be submitted to class where the control system is connected to an integrated control system. Unmanned machinery space integration is permitted. Section 4 requires a safety shutdown independent of control and alarm as far as practicable, with local and remote visual and audible alarms indicating the causing parameter, manual emergency shutdown at both positions, and no automatic restart without manual reset.

Table 1 of UR M86 sets the alarm and shutdown points: exhaust temperature after the unit with high alarm and high-high shutdown; pressure before and differential pressure across the unit on the same basis; water level in a wet unit; fan or blower running and stop alarm; position indication for bypass, isolation and mixing valves; washwater pump running or low supply pressure; chemical treatment fluid tank temperature and level, cross-referring UR M81.2.5; and chemical leakage detection in the drip tray or drain tank, cross-referring UR M81.2.13. A shutdown stops all system pumps, and automatic bypass is required where the unit cannot run dry.

Back pressure

Back pressure is a real constraint and is routinely quoted with a wrong number. The correct framing is not a generic figure but the limits declared in the approved technical manual under paragraph 5.6.1.2.6, monitored with a high alarm and a high-high bypass or shutdown under UR M86. The engine builder’s total allowable back pressure for the whole exhaust system is a separate figure and belongs to the engine project guide, not to the scrubber. Do not add the two or confuse them.

Materials and the safety note

Section 3 of MEPC.340(77) is a safety note in its own right. Paragraph 3.1 covers storage of pressurised calibration and pure gas cylinders, safe sampling positions with permanent access platforms, positioning of the discharge outlet with regard to existing seawater inlets, and the balance between low-pH discharge and the anti-corrosive resistance of wetted surfaces: to avoid premature failure of sea chests, discharge pipework and hull penetration finishes, due care should be taken in the preparation of surfaces and the correct selection and application of protective coatings.

Wetted parts are typically duplex or super duplex stainless steel , glass reinforced plastic, or coated carbon steel. Duplex selection is indexed on the pitting resistance equivalent number, PREN, calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen, with 2205 as EN 1.4462 and 2507 as EN 1.4410 and super duplex conventionally at PREN 40 or above. For tungsten-bearing grades such as UNS S32760 the tungsten-inclusive PREW index applies instead. Take the alloy from the maker’s approved manual; the generic index tells you how to read a specification, not what to specify. The interaction with hull coatings and cathodic protection is covered at marine cathodic protection and hull coatings .

Entry into a scrubber tower is confined space work with a distinctive hazard set: residual acidic liquor, oxygen deficiency, sulphite and sulphate sludge, hot surfaces, and exhaust ingress if the damper interlock has been defeated. Work it under the ship’s enclosed space entry procedures and the safety management system, not from this page.

Discharge water criteria under section 10

Section 10 of MEPC.340(77) is the part of the regime under the most active review, and it is the part most often misquoted. Four parameters carry numeric limits, and three of the four are normalised or referenced rather than absolute.

Which of the two pH alternatives applies to a given ship is recorded in its technical manual. The 4 metre alternative is established by direct measurement or by an agreed computational or empirical method against reference seawater of alkalinity 2.2 mmol/L and pH 8.2.

The PAH limit scales with flow because it is a concentration standing in for a mass rate. Table 5 gives 2,250 ug/L at 0 to 1 t/MWh, 900 at 2.5, 450 at 5, 200 at 11.25, 100 at 22.5, 50 at 45 and 25 at 90. A low-flow discharge is held to a much higher concentration because it carries the same mass in less water. Measurement is taken downstream of the water treatment equipment including any reactant dosing unit, and upstream of any dilution used for pH control, so that the reading reflects treatment rather than dilution.

Nitrate is the parameter most often misdescribed as a differential. It is not. It is a cap expressed as the greater of two normalised figures, and it exists because a wet scrubber incidentally absorbs a little nitrogen dioxide. It is not continuously monitored: paragraph 10.1.5.2 requires a laboratory sample within the first three months of operation after the installation or initial survey and again three months before each renewal survey, analysed and made available to the Administration, with the certificate retained in the EGC Record Book. There is no annual approved-body verification of discharge water.

Instrumentation and calibration

Paragraph 10.2.2 caps instrument deviation at 0.2 pH units, 5% of the nominal standard test concentration for PAH with that nominal being not less than 80% of the scale range, and 2 FNU or NTU for turbidity. The pH instrument must have a resolution of 0.1 pH units with temperature compensation, electrode performance at least to BS 2586 or ASTM D1293-18, and a meter meeting or exceeding IEC 60746-2:2003 or an internationally accepted equivalent.

The PAH instrument must have a range at least twice the applicable limit and a deviation of not more than 5% across the working turbidity range, with ultraviolet techniques at lower flows and, per Table 5 note 3, fluorescence technology at any flow rate above 2.5 t/MWh. Turbidity measurement is to meet ISO 7027, and the turbidimeter must identify when turbidity cannot be reliably quantified.

That last reference carries a designation trap. ISO 7027 no longer exists as a single standard: it split into ISO 7027-1:2016 for quantitative methods, published 9 June 2016, and ISO 7027-2:2019 for semi-quantitative methods. The requirement maps to ISO 7027-1:2016, and the IMO text carries the undated pre-split reference.

Discharge water parameters are recorded continuously at not less than 0.0111 Hz under paragraph 10.4.1, with calibration and drift recorded by the system or entered in the Record Book. Continuous monitoring of pH, PAH, turbidity and temperature is required whenever the system is operated in ports, harbours or estuaries and during any discharge from temporary storage; elsewhere the equipment runs whenever the system does, except for short maintenance periods defined in the Onboard Monitoring Manual, and no maintenance or cleaning is permitted during any overboard discharge from temporary storage.

Emission analysers are covered separately in section 6, which applies the NOx Technical Code 2008 except as otherwise provided. Carbon dioxide is measured by non-dispersive infrared, sulphur dioxide by non-dispersive infrared or ultraviolet, with other principles subject to Administration approval. Samples are held above condensation temperature to avoid sulphur dioxide loss, drying must not lose sulphur dioxide, and the two species are compared on the same residual water content. Ingress leakage is verified at initial start-up and at the manual’s intervals, and recorded in the Record Book.

Additives and residues

Paragraph 10.1.6.1 requires no further discharge assessment where only a neutralisation agent, meaning sodium hydroxide or sodium carbonate, plus flocculants approved for oily-water separating equipment are used and discharge pH does not exceed 8.0. Anything else calls for an assessment along the lines of the ballast water G9 procedure.

Residues are unambiguous. Paragraph 10.5.1 states they should be delivered ashore to adequate reception facilities and should not be discharged to the sea or incinerated on board. The incineration prohibition is the provision most often missed: a ship with an incinerator certified under Regulation 16 may not use it on this stream, and the waste goes ashore through the reception facility route. Storage and disposal are recorded with date, time and location under 10.5.2. The separation equipment itself is a different animal from the oily water separator and should not be plumbed to it.

Port and coastal State discharge restrictions

The single largest operational constraint on an open-loop ship is not in MARPOL at all. Discharge restrictions imposed by port and coastal States rest on their own jurisdiction over ports, internal waters and the territorial sea, not on any provision of Annex VI. That is why the register changes without an IMO amendment, why the scope differs from a berth-only prohibition to a territorial-sea one, and why it has to be checked voyage by voyage against the issuing authority’s own circular rather than learned once from a summary table.

The IMO’s response is resolution MEPC.1/Circ.899, the 2022 Guidelines for risk and impact assessments of the discharge water from exhaust gas cleaning systems, approved at MEPC 78 on 10 June 2022. It gives a State a common method for assessing local impact before it acts and gives owners a common framework to argue with, without displacing the State’s jurisdiction. The mechanics are covered at EGCS discharge water risk and impact assessment and the criteria themselves at scrubber discharge water criteria .

This article publishes no restriction register. A register is only useful if every row carries the issuing authority’s own instrument, and one built from secondary summaries goes stale silently, which is the worst failure mode for a document a master relies on before entering port.

Malfunction, survey and record keeping

The one-hour rule

MEPC.1/Circ.883, dated 21 May 2019 and approved at MEPC 74, is the guidance that answers the question a chief engineer actually asks. Its paragraph 1 defines a malfunction as any condition that leads to an emission exceedance, excluding the short-term exceedances at paragraphs 7 and 8 and the sensor failures at 9 to 11.

On evidence of a malfunction, paragraphs 2 to 4 require the troubleshooting checklist and remedial action list in the approved technical manual to be followed, and paragraph 5 requires the event to be logged in the EGC Record Book with start date and time, resolution, actions and follow-up. Paragraph 6 is the operative rule: a system malfunction that cannot be rectified is regarded as an accidental breakdown, and the ship should then change over to compliant fuel oil if the system cannot be put back into a compliant condition within one hour. Where the ship does not have compliant fuel, or not enough of it, a proposed course of action to bunker compliant fuel or carry out repairs should be communicated to the relevant authorities including the ship’s Administration for their agreement. Paragraph 12 requires any malfunction lasting more than one hour, or repetitive malfunctions, to be reported to the flag and port State with an explanation of the steps being taken.

Paragraphs 7 and 8 protect normal behaviour: a sudden change in exhaust mass flow produces a dynamic response, and transitory periods and isolated spikes in the recorded output do not necessarily mean an exceedance and should therefore not be considered a breach, on conditions specified in the approved manual. Paragraphs 9 to 11 deal with a single failed sensor, where the interrelation between the Emission Ratio, the washwater pH and the other parameters indicates an instrument rather than a system fault: keep interim records, record the sulphur content of the fuel in use from the start of the malfunction, log it, and repair as soon as practicable.

One caveat has to be stated rather than quietly re-badged. Circ.883 is drafted against the 2015 Guidelines, MEPC.259(68), and its title says so. It has not been re-issued against MEPC.340(77), so its procedural guidance is applied to a system approved under the 2021 Guidelines by analogy rather than by direct reference.

This is not a fuel oil non-availability report situation. Regulation 18.2 addresses a ship that cannot purchase compliant fuel in accordance with its voyage plan, with notification under 18.2.4, which is a supply problem rather than an equipment breakdown. The two are governed by different provisions and mixing them produces the wrong notification to the wrong party. The documentation side is covered at FONAR and BDN documentation .

Survey

Paragraphs 4.2.3.1 and 5.3.1 require the system under Scheme A, and the monitoring system under Scheme B, to be surveyed on installation and at the initial, annual, intermediate and renewal surveys. Under 5.3.1 the installation or initial survey includes operation of the system to demonstrate the functionality of the monitoring system.

Regulation 5 of the Revised Annex VI sets the underlying regime for ships of 400 gross tonnage and above: an initial survey before the ship is put in service or the certificate first issued; a renewal survey at intervals not exceeding five years; an intermediate survey within three months of the second or third anniversary date, replacing one annual survey; an annual survey within three months of each anniversary date; and an additional survey after important repairs or renewals, which a scrubber retrofit triggers. Regulation 5.3.3 requires a recognized organization to ensure corrective action is taken, to notify the Administration, and to notify the port State where the ship is in another Party’s port. Below 400 gross tonnage the Administration may set appropriate measures under Regulation 5.2.

What the surveyor does is set by paragraphs 4.4.1 to 4.4.6. The ETM-A carries an approved verification procedure written so that it needs no specialised equipment and no in-depth system knowledge, covering a documentation check and a physical check. The surveyor verifies that installation matches the manual and that a valid compliance certificate is held. Where more than one unit is fitted the survey may be abbreviated, but the entire survey is completed for at least one unit of each type on board. The governing principle at 4.4.1 is that if all relevant components and settings are within their approved ranges, performance is assumed compliant without continuous emission measurement.

The record book

The EGC Record Book carries more than maintenance. It records service including like-for-like replacement of components, residue storage and disposal with date, time and location, monitoring system maintenance, sample system ingress leakage checks, the nitrate analysis certificate, and any malfunction under Circ.883. It may be kept electronically, and entries are held on board for a minimum of three years after the last entry. Mode changes are recorded automatically against UTC, position and emission control area status under paragraph 7.2. Nothing in the instrument requires a reason for a mode change to be logged, and a procedure that demands one is a company requirement rather than a MARPOL one.

Port State control

The legal basis is MARPOL Annex VI Regulation 10, port State control on operational requirements. Regulation 10.1 permits inspection where there are clear grounds for believing that the master or crew are not familiar with essential shipboard procedures relating to the prevention of air pollution from ships. Regulation 10.2 requires the Party to take steps to ensure the ship does not sail until the situation has been brought to order. Paragraphs 4.2.3.2 and 5.5.1 of MEPC.340(77) confirm that the system and the Scheme B data may be inspected under it.

The only broad dataset on how that plays out is the joint Paris MoU and Tokyo MoU concentrated inspection campaign on MARPOL Annex VI, which ran from 1 September to 30 November 2018. In the Tokyo MoU region the campaign covered 8,270 inspections on 7,657 ships, of which 6,604 used the campaign questionnaire, at 0.13 campaign deficiencies per inspection. Of 198 ships detained in the period, five were detained on campaign topics, a rate of 0.07%.

Question 4 was the scrubber question: are alternative arrangements, for example scrubbers, installed on board according to regulation 4.1 approved by the flag State. The answers were 217 yes, one no at 0.46% of applicable responses, and 6,386 not applicable at 96.70%, which is the pre-2020 fitted population showing through. It was one of only three questions where a negative answer could lead to detention, alongside the bunker delivery note sulphur question and the VOC Management Plan familiarity question. One negative answer was received on each, and none of those ships was detained.

The deficiency codes in the Annex VI series are worth knowing: 14601 for technical files and the monitoring manual, 14602 for the record of engine parameters, 14604 for bunker delivery notes, 14608 for the incinerator and 14611 for ozone depleting substances. Two of the three campaign-related detentions recorded in the Tokyo MoU questionnaire fell under 14601 and 14602, which is to say the documentation rather than the machinery.

No concentrated inspection campaign on MARPOL Annex VI has run since 2018. That is a gap worth noting when someone cites campaign data as evidence of the current fitted fleet’s compliance: the population inspected then was overwhelmingly not fitted with a scrubber.

Where a scrubber sits in the carbon regime

A scrubber scores on Regulation 14 and on nothing else, and this is the structural fact that has changed the investment case since 2020.

The reason is the same in each of the last four rows. Carbon accounting is driven by fuel consumed multiplied by a carbon conversion factor, and the factors in resolution MEPC.364(79) section 2.2.1 are keyed to ISO 8217 grades: 3.206 for DMX through DMB, 3.151 for RMA through RMD, and 3.114 for RME through RMK. Residual fuel behind a scrubber carries a lower factor per tonne than distillate but a lower calorific value with it, and the system’s own auxiliary power draw adds consumption. Nothing in that arithmetic responds to sulphur abatement.

The same holds under FuelEU Maritime, Regulation (EU) 2023/1805, in force 12 October 2023 and applying from 1 January 2025 at a limit of 89.34 g CO₂eq/MJ. It prices well-to-wake greenhouse gas intensity per unit of energy, and a scrubber changes neither the numerator nor the denominator.

So the investment case rests entirely on the price spread between high-sulphur residual fuel and the compliant alternative, over the remaining life of the ship, weighted by the fraction of that life spent in waters where the system’s discharge mode is permitted. This article states no spread, no capital cost and no payback period, because none of those has a citable primary source and a made-up figure is worse than none. The bunker price indices article covers where the spread is published, and the charter party allocation of the benefit is covered at bunkers on delivery and redelivery and in the time charter party . Per fixture, the spread enters the bunker line of a voyage estimate and is worth the tonnes burned outside an emission control area multiplied by the spread on the day.

Interaction with NOx abatement and heat recovery

An emission control area designated since 2022 is a SOx and particulate matter area, and several are also NOx areas, so a ship trading in one may need both a scrubber and a Tier III arrangement. The two coexist in the same uptake but not in any order.

Selective catalytic reduction needs a minimum inlet temperature that rises with fuel sulphur, because ammonium bisulphate forms and fouls the catalyst below it. Resolution MEPC.291(71) makes that explicit: paragraph 3.2.5 requires the allowable maximum and minimum exhaust gas temperature at the chamber inlet to be documented, 3.2.6 the allowable pressure loss across chamber and ducting, and 3.2.7.1 the maximum allowable sulphur content of fuel oil which can be combusted while maintaining compliance. Vanadium-based catalysts are comparatively sulphur-tolerant, which is why high-sulphur fuel with SCR is viable and in service, but only where the reactor is dimensioned and its minimum temperature set for the declared maximum fuel sulphur. The reactor may sit downstream of the turbocharger in a low-pressure arrangement or upstream of it in a high-pressure one, chosen precisely to reach activation temperature; the retrofit case is at SCR retrofit on two-stroke engines .

Exhaust gas recirculation carries a point that surprises people arriving from the sulphur side: a marine two-stroke EGR system contains its own water-based scrubber inside the recirculation loop, stripping sulphur dioxide and particulate before the gas re-enters the charge air. An EGR ship on residual fuel is already operating a scrubber, and the design question is whether its water treatment and bleed handling are shared with the uptake unit or separate. The retrofit case is at EGR retrofit on two-stroke engines .

Waste heat recovery interacts through temperature. The arrangement runs turbocharger, then economiser, then scrubber, so the economiser exit temperature is the scrubber inlet temperature, and deeper heat recovery reduces the quench duty and cooling water flow the scrubber needs. That is the opposite of the common framing that a scrubber costs you steam. The limit is the acid dew point , which rises with fuel sulphur, so on high-sulphur fuel a deep recovery risks acid corrosion and soot fires in the economiser. The same mechanism governs cold corrosion in two-stroke engines and the cylinder oil base number an engine needs on high-sulphur fuel.

What a scrubber does not abate

Three claims are made routinely for scrubbers and none of them is supported by the instrument.

Nitrogen oxides. A scrubber is not a NOx control. The only NOx figure anywhere in MEPC.340(77) is the discharge water nitrate cap at paragraph 10.1.5.1, expressed as the nitrate associated with a 12% removal of NOx from the exhaust. That number exists because a wet scrubber incidentally absorbs a little nitrogen dioxide; nitric oxide, the dominant species in engine exhaust, is nearly insoluble. Read it as a discharge water limit expressed in NOx-equivalent terms, never as an abatement claim. Regulation 13 compliance requires a different technology.

Particulate matter. Paragraph 10.1.4.1 requires the treatment system to minimise suspended particulate matter, including heavy metals and ash, which describes the mechanism precisely: particulate is transferred to the water, not destroyed. No primary removal percentage exists. There is a regulatory point worth stating, though: the Mediterranean designation and the 2027 and 2028 designations are SOx and particulate matter areas whose operative limit is a fuel sulphur limit, so a system accepted as equivalent on sulphur satisfies the particulate element by the same route.

Black carbon. No primary source states a figure, and the work at the Sub-Committee on Pollution Prevention and Response is unfinished. This article states none.

Alternatives and the comparison

Four routes to Regulation 14 compliance are in commercial use, and they are not equivalent in what else they buy.

The comparison that matters commercially is the first two rows, because the last two are newbuilding or major conversion decisions rather than compliance choices. Between distillate and a scrubber, the trade is a certain capital cost and a set of operational restrictions against an uncertain fuel price spread, over a hull with a known remaining life. Neither the spread nor the restriction register is stable, which is why the decision is revisited rather than made once. The wider technology landscape is at decarbonization technologies and alternative marine fuels , and the reporting obligations at emissions monitoring and reporting and the EU MRV Regulation . In port, shore power removes the emission at source and is increasingly the regulated alternative for the cruise and ferry sectors that adopted scrubbers earliest.

Limitations

This article states no fitted-fleet count, no market share, no capital cost, no payback period and no bunker price spread. None of those has a citable primary source, and figures for all of them circulate widely with no traceable origin. A reader needing them should take them from a source that names its methodology and its date.

MEPC.340(77) is recommendatory. Paragraph 1.4 says so in terms. What binds a ship is the flag Administration’s approval of the arrangement under Regulation 4, and Administrations approve against the Guidelines, so the practical effect is close to mandatory. The distinction matters where an Administration accepts a departure from a guideline provision, which it may do, and where a port State asserts a guideline provision as though it were treaty text.

The EU criteria set is related to the IMO one but not identical. Directive (EU) 2016/802 Article 8 permits emission abatement methods, and its Annex II criteria still reference MEPC.184(59), the 2009 Guidelines, rather than MEPC.340(77). Commission Implementing Decision (EU) 2015/253 governs sampling and reporting. Treat compliance in EU waters as a separate check rather than an inference from the IMO position.

MEPC.1/Circ.883 is drafted against MEPC.259(68) and has not been re-issued against the 2021 Guidelines, so its procedures apply to a modern installation by analogy. The paragraph numbering it cites belongs to the superseded instrument.

Discharge water science is contested and under active review. Section 10 criteria have already changed once between guideline generations, MEPC.1/Circ.899 exists precisely because coastal States were reaching different conclusions from the same evidence, and further change is more likely than not.

Figures specific to dry and hybrid systems, to tower dimensions, to pump power, to pressure drop and to alloy specification are maker-dependent and belong to the approved technical manual for the individual unit. No OEM project guide figure is published here, because none was obtainable from a primary document. Class notations for exhaust gas cleaning installations are similarly omitted: the societies serve their rule documents behind access controls, and a notation string quoted from memory is worse than no notation string. Take both from the society’s own rule finder.

The discharge restriction register is deliberately absent for the reason given above. It changes without any IMO instrument, and a stale register is more dangerous than none.

Related Articles

• MARPOL Annex VI Regulation 14 : the sulphur cap itself, its paragraphs, its thresholds and its enforcement
• MARPOL Annex VI Regulation 4 : the equivalence clause that makes a scrubber lawful
• Emission control areas : the eight designated areas, their instruments and their entry dates
• Scrubber discharge water criteria : the section 10 parameter set in full
• Open-loop scrubber discharge restrictions : the port and coastal State register and where it comes from
• SOx Emissions Compliance Certificate : the Scheme A unit certificate and the Certified Value
• EGC Record Book : what is recorded, by whom, and for how long
• IACS UR M86 : monitoring and safety functions for exhaust gas cleaning systems
• International Air Pollution Prevention Certificate : the certificate and the Supplement sections a scrubber is entered in
• Heavy fuel oil : the residual fuel the equipment exists to make usable
• Marine gas oil : the distillate alternative and its ISO 8217 grades
• Selective catalytic reduction : the NOx route that shares the uptake and dislikes sulphur
• Exhaust gas recirculation : the other Tier III route, with a scrubber of its own inside the loop
• Waste heat recovery system : the economiser upstream, and the temperature it hands over
• Scrubber sludge and residue management : what comes out of the treatment plant and where it goes
• Particulate matter from ship exhaust : the pollutant named in every ECA designation
• Port state control : the inspection regime and the Regulation 10 basis
• What is CII : the carbon metric a scrubber does not move