Exhaust Gas Recirculation systems used to reduce nitrogen oxide emissions from marine diesel engines produce a water bleed-off that must be monitored before it can be discharged to sea or ashore. Rivertrace oil-in-water monitors are used to confirm that the oil content of this bleed-off water is within the limits set by the applicable discharge standard before any discharge takes place.
| Key thing to remember
EGR water bleed-off discharge is subject to the same oil content monitoring requirement as bilge water discharge under MARPOL Annex I and MEPC.107(49). The bleed-off water must pass through an oil content monitor that meets MEPC.107(49) and must not be discharged if oil content exceeds the permitted limit. The monitoring and recording requirements are the same as for oily water separator discharge. |
What Is an EGR System and Why Does It Produce Water?
Exhaust Gas Recirculation (EGR) is a technology used on marine diesel engines to reduce the formation of nitrogen oxides (NOx) during combustion. NOx is produced when nitrogen and oxygen in the air react at the high temperatures found inside a diesel engine cylinder. EGR reduces NOx formation by recirculating a portion of the exhaust gas back into the engine air intake, which lowers combustion temperatures and reduces the oxygen available for NOx formation.
When exhaust gas is recirculated, it carries particulate matter, sulphur compounds and acidic components that must be removed before the gas re-enters the engine. A water scrubber integrated into the EGR system washes the recirculated exhaust gas, removing these contaminants. This washing process produces a water bleed-off that contains the removed contaminants, including residual oil, particulates and acidic compounds from the exhaust stream.
This bleed-off water must be managed and monitored before it can be discharged. Depending on fuel oil sulphur content, the specific handling requirements may vary, but the fundamental requirement to monitor oil content before discharge applies in all cases.
What NOx Regulations Apply to Marine Diesel Engines?
NOx emissions from marine diesel engines are regulated under MARPOL Annex VI, through the NOx Technical Code (NTC) adopted alongside Annex VI. Three NOx emission tiers apply to different engine build dates and operating areas:
| NOx Tier |
Engine build date |
Emission limit (n < 130 rpm) |
What it means in practice |
| Tier I |
Before 2000 |
17.0 g/kWh at n < 130 rpm |
Baseline standard for older engines |
| Tier II |
2000 to 2010 |
14.4 g/kWh at n < 130 rpm |
Applies to ships built between 2000 and 2010 |
| Tier III |
2016 onwards in ECAs |
3.4 g/kWh at n < 130 rpm |
Applies in Emission Control Areas — requires EGR, SCR or alternative NOx reduction technology |
Tier III requires a reduction in NOx emissions of approximately 80% compared to Tier II. This cannot be achieved through engine design alone and requires dedicated NOx reduction technology. The two primary technologies used on marine vessels to achieve Tier III compliance are Exhaust Gas Recirculation and Selective Catalytic Reduction (SCR). EGR is particularly common on two-stroke main engines, while SCR is more widely used on four-stroke auxiliary engines.
What Emission Control Areas Require Tier III Compliance?
The Tier III NOx limit applies only when a vessel is operating within a designated Emission Control Area (ECA) where stricter emissions standards are enforced. The current NOx ECAs are:
- North American ECA — covering waters within 200 nautical miles of the North American coastline, including the US and Canadian coasts and Hawaii
- United States Caribbean Sea ECA — covering waters around Puerto Rico and the US Virgin Islands
Additional NOx ECAs are under consideration in various regions and may be designated in the future. A vessel fitted with an EGR system for Tier III compliance in a current NOx ECA must operate the EGR system when in the ECA and may switch to non-EGR operation outside it, depending on the engine management system configuration.
How Does the EGR System Produce Water Bleed-Off?
The EGR water bleed-off is produced by the EGR scrubber unit integrated into the system. The scrubber washes the recirculated exhaust gas with water to remove particulate matter, sulphur compounds, soot and other combustion residues before the cleaned exhaust gas re-enters the engine air intake.
The wash water becomes contaminated with the extracted residues during this process and is collected as bleed-off water. The composition of the bleed-off depends on:
- The sulphur content of the fuel being burned — higher sulphur fuel produces more acidic bleed-off with higher sulphur compound loading
- The engine load and operating conditions — higher engine loads generate more exhaust gas and more bleed-off
- The efficiency of the EGR scrubber — a well-maintained scrubber removes more contaminants from the exhaust gas, concentrating them in the bleed-off
- The recirculation rate — the proportion of exhaust gas recirculated affects the volume and composition of bleed-off produced
The bleed-off water typically contains oil, particulate matter, soot, sulphur compounds and acidic components. It must be monitored for oil content before discharge and must comply with the applicable discharge standard.
What Discharge Standards Apply to EGR Water Bleed-Off?
EGR water bleed-off is treated as an oily mixture under MARPOL Annex I and is subject to the same discharge requirements as bilge water from the machinery space. Under MEPC.107(49), the bleed-off may only be discharged overboard if the oil content does not exceed 15 parts per million, the vessel is more than 12 nautical miles from the nearest land, the vessel is proceeding en route, and the discharge is monitored by a type-approved 15ppm oil content monitor.
The handling requirements for EGR bleed-off may vary depending on fuel oil sulphur content, as the acidic content of the bleed-off changes with the sulphur level of the fuel. Where high-sulphur fuel oil is used, additional precautions may be required for shore disposal of the bleed-off.
In all cases, the oil content of the bleed-off must be measured before discharge, and discharge must be automatically stopped if oil content exceeds 15ppm.
| EGR bleed-off is not exempt from MARPOL monitoring requirements
Some operators assume that EGR water bleed-off, because it originates from an emissions compliance system rather than the machinery space, may be exempt from MARPOL bilge water discharge requirements. It is not. EGR bleed-off is classified as an oily mixture under MARPOL Annex I and is subject to the same 15ppm limit and type-approved monitoring requirement as oily water separator discharge. |
Which Rivertrace Product Is Used to Monitor EGR Water Bleed-Off?
The oil content monitor used for EGR bleed-off discharge must be type-approved to MEPC.107(49). Rivertrace offers two products suitable for this application, depending on the characteristics of the bleed-off water stream and the installation requirements.
| Product |
Application |
Key capability |
| SMART PFM 107 |
EGR water bleed-off discharge monitoring where the bleed-off has a variable or complex composition |
MEPC.107(49) approved; uses optical microscopy to differentiate oil, gas bubbles and solids; suitable for complex EGR bleed-off water streams |
| SMART BILGE+ |
EGR water bleed-off discharge monitoring where the bleed-off characteristics are similar to standard bilge water |
MEPC.107(49) type-approved 15ppm oil content monitor; automatic discharge shut-off; Oil Record Book logging |
The SMART PFM 107’s optical microscopy measurement technology is particularly relevant for EGR bleed-off applications where the bleed-off contains gas bubbles, particulates or soot that could cause false high readings in a standard light scattering monitor. Its ability to differentiate oil particles from gas and solid particles makes it more robust against false alarms in the variable composition environment of EGR bleed-off water.
How Is EGR Water Bleed-Off Monitoring Different From Scrubber Wash Water Monitoring?
EGR water bleed-off monitoring and exhaust scrubber wash water monitoring are related but distinct applications with different regulatory bases. Exhaust scrubbers used for SOx compliance (not NOx reduction) are regulated under MEPC.259(68), which requires monitoring of PAH, turbidity, pH and temperature — not just oil content. EGR systems used for NOx reduction produce a different water stream that is regulated under MARPOL Annex I and MEPC.107(49) for oil content. For a full explanation of exhaust scrubber wash water monitoring requirements, see the exhaust scrubber wash water discharge application page.
Where a vessel is fitted with both an EGR system for NOx reduction and an exhaust scrubber for SOx compliance, both monitoring requirements apply simultaneously to their respective water discharge streams.
What Should Operators Check to Maintain EGR Bleed-Off Monitoring Compliance?
For ship operators and technical superintendents responsible for EGR systems, maintaining oil content monitoring compliance for bleed-off discharge requires attention to the same elements as bilge water monitoring compliance:
- Confirm the oil content monitor is type-approved to MEPC.107(49) and that the type approval certificate is held onboard
- Confirm the calibration certificate is current and will not expire before the next IOPP survey
- Verify that the automatic shut-off valve on the bleed-off discharge line is connected to and activated by the oil content monitor
- Confirm that all bleed-off discharge operations are recorded in the Oil Record Book with the correct entries
- Ensure that crew responsible for the EGR system and bleed-off discharge are trained in the monitoring and recording requirements
- Confirm that the sample conditioning system is correctly maintained and delivering sample to the monitor within the specified operating range
Frequently Asked Questions
What is EGR water bleed-off?
EGR water bleed-off is the water produced by the wash scrubber integrated into an Exhaust Gas Recirculation system on a marine diesel engine. The scrubber washes recirculated exhaust gas to remove particulates, soot and sulphur compounds before the gas re-enters the engine air intake. The wash water collects these contaminants and must be monitored for oil content before it can be discharged overboard.
What regulations apply to EGR water bleed-off discharge?
EGR water bleed-off is classified as an oily mixture under MARPOL Annex I and is subject to the same 15ppm oil content limit and type-approved monitoring requirement as bilge water from the machinery space. The applicable technical standard for the oil content monitor is MEPC.107(49). Discharge must not take place if oil content exceeds 15ppm, and discharge must be automatically stopped by the monitor if the limit is exceeded.
Which Rivertrace product is recommended for EGR water bleed-off monitoring?
The SMART PFM 107 is recommended where the EGR bleed-off water contains gas bubbles, particulates or soot that could cause false readings in a standard light scattering monitor. The SMART BILGE+ is suitable where the bleed-off characteristics are similar to standard bilge water. Both products are type-approved to MEPC.107(49).
Is EGR water bleed-off monitoring the same as exhaust scrubber wash water monitoring?
No. EGR water bleed-off monitoring is governed by MARPOL Annex I and MEPC.107(49) — the same standard that applies to bilge water discharge — and focuses on oil content. Exhaust scrubber wash water monitoring for SOx scrubbers is governed by MEPC.259(68) and requires monitoring of PAH, turbidity, pH and temperature. Both monitoring requirements may apply simultaneously on vessels fitted with both systems.
When does the Tier III NOx limit apply and what does it require?
The Tier III NOx limit applies to marine diesel engines installed on vessels built from 2016 onwards when operating within a designated NOx Emission Control Area — currently the North American ECA and the US Caribbean Sea ECA. Tier III requires an approximately 80% reduction in NOx emissions compared to Tier II, which cannot be achieved without dedicated NOx reduction technology such as EGR or SCR.
