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An Exhaust Gas Cleaning System (EGCS) — commonly called a scrubber — is an alternative compliance method for the MARPOL Annex VI sulphur limits. Instead of burning fuel with a sulphur content at or below the required limit (0.50% globally since 1 January 2020; 0.10% in ECAs since 1 January 2015), a scrubber removes SO₂ from exhaust gas after combustion, allowing the ship to burn high-sulphur fuel oil (HSFO, typically 2.5–3.5% S). The legal basis is MARPOL Annex VI Regulation 4 (equivalents) and the MEPC.184(59) Guidelines for Exhaust Gas Cleaning Systems, now updated by MEPC.259(68) in 2014 and further guidance in MEPC.1/Circ.883.
Scrubbers became commercially significant when the global 0.50% sulphur cap was confirmed for 2020. Owners who invested in scrubbers could continue burning cheap HSFO while competitors paid a premium for very low-sulphur fuel oil (VLSFO) or marine gas oil (MGO). As of early 2025, approximately 5,000 ships operate scrubbers — predominantly large tankers, bulk carriers, and cruise ships where installation cost is justified by fuel savings over multi-year payback periods.
Seawater drawn continuously through the scrubber tower, reacting with SO₂ in the exhaust gas to form sulphite/sulphate ions. Treated washwater discharged overboard after dilution.
Pros: Lowest capital cost; no washwater holding tank required; simplest operation at sea.
Cons: Cannot operate where discharge of washwater is banned (ports, coastal areas, ECAs for some states). Unsuitable in low-alkalinity water (Baltic, some freshwater approaches).
A buffer solution (typically seawater treated with sodium hydroxide/NaOH) circulates in a closed circuit. Washwater is treated, the sludge is collected, and only a small bleed stream is discharged overboard after treatment.
Pros: Can operate in ports with open-loop bans; can use freshwater with chemical additive.
Cons: Higher capital cost; requires chemical storage (NaOH); produces sludge requiring disposal at port reception facilities.
Switches between open-loop and closed-loop modes depending on location. Operates open-loop at sea and closed-loop (or zero-discharge mode) in restricted areas.
Pros: Maximum operational flexibility; one system covers all trading areas.
Cons: Most complex and expensive of the three. Mode-switching procedure must be practised; incorrect operation in a ban area is a serious violation.
The environmental acceptability of open-loop scrubber washwater is contested. Critics argue the discharge transfers sulphur pollution from the atmosphere to the ocean and introduces PAHs and heavy metals into port waters. Several jurisdictions have responded with discharge bans:
Open-loop washwater discharge banned in all Singapore port waters (since 1 January 2020).
Applies within the port limits. Ships must switch to compliant fuel or operate in closed-loop/hybrid zero-discharge mode.
Open-loop discharge banned in Chinese inland waters, rivers, and ports (Emission Control Area since 2019, full ECA from 2020). Coastal ECA applies to mainland ports.
Enforcement has been vigorous; BWRB-equivalent scrubber log books inspected by MSA officers. Some provinces applied tighter restrictions before the national rule.
Open-loop discharge banned in territorial waters. Germany extended the ban to the entire German North Sea EEZ for environmental protection reasons.
The North Sea ban area means ships transiting German waters must close-loop or use compliant fuel well before port entry.
Discharge of open-loop washwater banned in Norwegian fjords and UNESCO World Heritage fjord areas.
Applies to scenic fjord routes popular with cruise ships. Ships on Norway/Svalbard routes need hybrid or closed-loop capability.
California state law bans open-loop scrubber washwater discharge in state waters (within 3 nm of the California baseline).
California's Ocean Protection Act provisions are enforced by the State Water Resources Control Board. Violations can result in significant fines.
Open-loop washwater discharge banned within Fujairah port limits.
Significant bunkering hub; many ships bunker at Fujairah and must comply regardless of whether they are entering port for cargo.
IMO is reviewing open-loop washwater standards under its Comprehensive Review of MARPOL Annex VI. Additional restrictions or a global standard are expected from MEPC deliberations in 2025–2026. Masters of scrubber-fitted ships must maintain a current list of banned areas and ensure the EGCS Log Book correctly records mode changes.
The economic driver is the HSFO/VLSFO spread — the price difference between high-sulphur fuel and compliant low-sulphur fuel. When the 2020 cap was implemented, this spread reached USD 200–300/mt in some markets, making scrubber payback periods of 2–4 years feasible on high-consumption vessels. By 2024 the spread compressed to USD 80–150/mt in many ports as VLSFO supply matured, extending payback to 4–7 years for new installations. On a 30,000 mt/year consumption VLCC, even a USD 100/mt spread yields USD 3M/year — justifying a USD 5–10M scrubber investment. Smaller ships with lower fuel consumption may find the economics marginal. Carbon intensity (CII) calculations treat HSFO with a higher CO₂ emission factor than VLSFO, which slightly reduces the scrubber advantage under EU ETS and IMO CII regimes.
Monitor washwater pH (typically must be > 6.5 at the overboard discharge point). Continuous pH monitoring is mandatory for compliance monitoring.
Failure mode: Probe fouling, calibration drift, or membrane degradation gives false readings. Ships have been detained for pH probe failures with no valid backup calibration records.
Monitor polycyclic aromatic hydrocarbons (PAH) proxy — turbidity is a surrogate for organic content. Required under MEPC.259(68) guidelines to remain below 25 FNU (formazin nephelometric units) increase over intake.
Failure mode: Sensor blockage or window fouling causes alarm suppression. Falsified or missing turbidity records are a common PSC deficiency.
Some jurisdictions require direct PAH measurement (polycyclic aromatic hydrocarbons — carcinogenic compounds absorbed from fuel combustion). PAH in washwater is an unresolved controversy.
Failure mode: PAH sensor calibration is complex; reagent supply chain issues can leave ships without valid monitoring. Ongoing IMO debate may eventually tighten PAH limits.
Spray washwater throughout the scrubber tower for SO₂ absorption. Nozzle blockage reduces contact surface area and scrubbing efficiency.
Failure mode: Scale deposition in hard-water areas; corrosion from sulphuric acid formed in the scrubber. Regular inspection and chemical cleaning required.
From 2024 onwards, several technology companies have been developing onboard carbon capture and storage (OCCS) systems that use scrubber-like towers to absorb CO₂ from exhaust gas into a liquid solvent (typically amine-based). The CO₂-rich solvent is then regenerated and the CO₂ liquefied and stored onboard for offloading at port. At least two pilot installations were operational in 2024 on bulk carriers. The IMO is developing a life-cycle GHG methodology to account for onboard carbon capture under the GHG intensity framework, but as of 2025 no formal regulatory credit mechanism exists for OCCS under CII or EU ETS.
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