Soot Pollution from Inert Gas Systems and Open-Loop Scrubbers
An Escalating Environmental, Regulatory, and Financial Risk
The maritime industry is confronting an increasingly serious environmental challenge -soot pollution arising from inert gas (IG) systems and open-loop exhaust gas scrubber operations. What were once considered isolated or incidental events have now evolved into a systemic issue, exposing shipowners and operators worldwide to potential significant operational disruption, financial loss, and regulatory scrutiny.
An Escalating Environmental, Regulatory, and Financial Risk
The maritime industry is confronting an increasingly serious environmental challenge - soot pollution arising from inert gas (IG) systems and open-loop exhaust gas scrubber operations. What were once considered isolated or incidental events have now evolved into a systemic issue, exposing shipowners and operators worldwide to potential significant operational disruption, financial loss, and regulatory scrutiny.
To date, the Club has recorded over 60 documented incidents, although the true scale of the problem is believed to be substantially greater. These events are no longer regarded as minor environmental infractions. Instead, they frequently carry severe legal and financial consequences, including vessel detentions, delays, clean-up obligations, and substantial fines. As environmental enforcement intensifies across multiple jurisdictions, ship operators are facing heightened scrutiny and increasingly punitive responses from port and coastal State authorities.
Escalating Enforcement and Penalties
Growing environmental awareness has led many ports and coastal States to prohibit open-loop scrubber operations entirely. Elsewhere, inspections have become more intrusive and penalties more severe. Jurisdictions such as Turkey and the UAE are notable for their strict and highly visible enforcement approaches.
Depending on the jurisdiction the fines can be in the range of a few thousand dollars to millions of dollars. The Club has experience where vessels have been subject to fines exceeding USD 1 million for serious pollution incidents.
The absence of uniform international standards - particularly for Inert Gas Systems (IGS) discharges - further complicates compliance and exposes ship operators to regulatory uncertainty, financial risk, and reputational damage.
Preventive Measures and Best Practices
Most soot pollution incidents are entirely preventable through disciplined operations, effective training, and proactive maintenance. Shipowners and operators are strongly encouraged to adopt the following best practices:
- Strict Adherence to Manufacturer Guidance
Follow manufacturer-recommended operating procedures and maintenance instructions for IG systems and scrubber systems without deviation. - Comprehensive Pre-Operation Inspections
Conduct thorough system checks before activation, ensuring all critical components and alarms are fully functional. - Crew Training and Competency Development
Ensure crew members are adequately trained in normal operation, fault recognition, troubleshooting, and emergency response. - Adoption of Cleaner Technologies
Consider transitioning to hybrid or closed-loop scrubber systems, particularly when trading in regions with strict environmental controls. - Robust Record-Keeping
Maintain detailed logs of system operation, maintenance activities, sensor readings, and visual observations to demonstrate compliance and support incident investigation. - Enhanced Preventive Maintenance Regimes
Implement maintenance programmes that exceed minimum regulatory requirements, including scheduled inspections and timely replacement of critical components. A vessel-specific operational checklist is strongly recommended. - Real-Time Monitoring and Early Detection
Utilise continuous monitoring systems to identify deviations in performance at an early stage, allowing corrective action before a discharge occurs.
Understanding the Sources: Inert Gas Systems and Scrubber Systems
Inert Gas Generators
Inert gas generators (IGG) play a critical role in tanker safety by supplying oxygen-depleted gas to cargo tanks, thereby preventing the formation of flammable or explosive atmospheres. However, poorly maintained or improperly operated IGG systems can produce excessive soot - fine carbon particulates generated by incomplete combustion - which may be discharged to sea together with wash water.
Such discharges most commonly arise during:
System start-up or shutdown,
Maintenance activities,
Combustion inefficiencies, including poor fuel atomisation, fouled burners, or incorrect air–fuel ratios.
When released, this soot can cause visible pollution, attracting immediate attention from port authorities and environmental regulators
Inert Gas Systems Using Boiler Flue Gas
An alternative to dedicated Inert Gas Generators is the use of inert gas systems that utilise boiler flue gas. These systems extract exhaust gases from the vessel's main or auxiliary boilers, cool and clean them, and then deliver the oxygen-depleted gas to cargo tanks. This approach offers several operational and environmental advantages:
Decreased risk of soot discharge: Since the flue gas is already a by-product of controlled combustion in the main boilers, the risk of incomplete combustion and resulting soot formation is significantly reduced compared to standalone IGSs.
However, boiler flue gas systems are not without challenges. They require effective cooling, scrubbing, and filtration to remove particulates and ensure the gas meets oxygen content requirements.
Rapid boiler load changes, inadequate maintenance of scrubbers, demisters, or cooling systems can still result in carryover of soot particles or water droplets into cargo tanks or, if vented overboard, into the marine environment.
Open-Loop Scrubber Systems
Open-loop scrubbers, widely adopted following the introduction of the IMO 2020 global sulphur cap, allow vessels to continue burning high-sulphur fuel oil while reducing sulphur oxide (SOx) emissions. However, these systems present their own environmental risks.
Equipment failures - such as malfunctioning spray nozzles, blocked lines, or defective circulation pumps - as well as incorrect operational practices can result in the discharge of scrubber wash water contaminated with black carbon and other particulate matter directly into the marine environment. Such discharges are increasingly viewed by authorities as unacceptable, particularly in environmentally sensitive or heavily regulated ports.
Regulatory Standards and Gaps
To address the environmental impact of scrubber wash water, the IMO adopted Resolution MEPC.340(77), which establishes strict criteria governing discharge quality. These include:
A minimum pH of 6.5 at the discharge point (measured 4 metres from the outlet),
Defined limits on Polycyclic Aromatic Hydrocarbons (PAHs),
Controls on turbidity and nitrate concentrations to protect marine ecosystems.
Regulatory and Liability Treatment of IGS Soot Discharges
In contrast, there are no specific IMO-level numerical discharge standards applicable to wash water or soot discharges from inert gas system operations. Unlike scrubber systems, inert gas system discharges are not subject to harmonised international criteria for parameters such as pH, turbidity, or PAHs.
Notwithstanding this regulatory gap, IGS-related soot discharges are routinely treated as pollution incidents by port and coastal States. Enforcement action is commonly pursued under MARPOL and National and local environmental legislation, many of which impose strict or near-strict liability for visible pollution.
The Club’s experience confirms that these general provisions are increasingly relied upon by authorities when responding to IGS soot incidents.
Enforcement is typically reactive and discretionary, driven by:
- Visible pollution, such as blackened water or soot deposits on the sea surface,
- Alleged non-compliance with local water quality requirements, and
- The broad discretionary powers vested in port State authorities.
As a consequence, similar incidents may result in markedly different outcomes depending on jurisdiction - ranging from no formal action to substantial fines, operational delays, or vessel detention
Soot pollution associated with inert gas systems and scrubber operations represents a growing environmental, regulatory, and operational challenge for the maritime industry. The potential financial, operational, and reputational consequences are significant. However, these risks are largely controllable and, in most cases, preventable.
Through diligent maintenance, informed and disciplined operation, and appropriate technological choices, shipowners and operators can substantially reduce exposure, ensure regulatory compliance, and contribute meaningfully to the protection of the marine environment.