MEPC.259(68) – Exhaust Gas Cleaning Systems

What Is MEPC.259(68) and What Does It Require for EGCS Compliance?

MEPC.259(68) is the 2015 IMO Guidelines for Exhaust Gas Cleaning Systems, adopted at the 68th session of the Marine Environment Protection Committee. It establishes the framework under which ships can use an exhaust gas cleaning system as an equivalent means of meeting the MARPOL Annex VI sulphur oxide emission limits rather than switching to low-sulphur fuel.

MEPC.259(68) has two compliance dimensions that are sometimes confused: the air emission monitoring dimension, which governs how the EGCS demonstrates that exhaust gas emissions are within the SOx equivalent limit, and the washwater discharge monitoring dimension, which governs what the washwater from the scrubber must measure before it can be discharged overboard. For scrubber-equipped ships, both dimensions must be met simultaneously. For Rivertrace, the washwater discharge monitoring dimension is the commercially relevant area — the SMART ESM measures the washwater parameters required by MEPC.259(68) and its successor MEPC.340(77) continuously at the EGCS discharge point.

 

Key thing to remember MEPC.259(68) applies to EGCS installed before 1 June 2022 that have not been subsequently materially modified. EGCS on ships with keels laid on or after 1 June 2022, and new EGCS installations made on or after that date, fall under MEPC.340(77) — the 2021 guidelines that supersede MEPC.259(68). Do not assume MEPC.259(68) applies fleet-wide: confirm which guideline applies to each vessel individually.

 

What Are the Two Compliance Dimensions of MEPC.259(68)?

MEPC.259(68) covers both the air emission performance of the EGCS and the quality of the washwater it produces. These are two separate compliance questions that must both be answered continuously whenever the EGCS is operating. The table below sets out both dimensions and their relevance to different equipment categories.

 

	Air emission compliance dimension	Washwater discharge dimension
What it governs	How ships using EGCS demonstrate compliance with MARPOL Annex VI SOx emission limits by alternative means — using a scrubber rather than switching to low-sulphur fuel	What the washwater from the scrubber must measure before it can be discharged overboard
Measurement method	Continuous monitoring of the SO2(ppm)/CO2(%) ratio in the exhaust gas at the EGCS outlet — a ratio of 21.7 or below demonstrates equivalent compliance with the 0.5% or 0.1% sulphur fuel limit	Continuous monitoring of PAH, turbidity, pH and temperature in the washwater at the discharge point
Who it is relevant to	Ship operators and flag state Administrations confirming that the EGCS meets the emission standard as an alternative to compliant fuel	Ship operators, instrumentation engineers and port state control authorities verifying that the washwater discharge meets the parameter limits
Rivertrace product relevance	Rivertrace does not manufacture exhaust gas analysers for the SO2/CO2 ratio monitoring function	The SMART ESM from Rivertrace measures and records PAH, turbidity, pH and temperature continuously for washwater compliance monitoring under MEPC.259(68) and MEPC.340(77)

 

The existing library page for MEPC.259(68) on this site focused on the air emission dimension and the SO2/CO2 ratio monitoring method. The sections below give equal treatment to the washwater discharge dimension, which is the compliance area relevant to the SMART ESM and the monitoring obligations that ships’ officers, technical superintendents and port state control inspectors review.

 

What Washwater Parameters Must Be Monitored Under MEPC.259(68)?

Continuous monitoring of the washwater discharge is required whenever the EGCS is operating in or near ports, harbours or estuaries. The five washwater parameters and their limits under MEPC.259(68) are set out in the table below. These parameters and limits are identical under MEPC.340(77) — the difference between the two resolutions lies in the documentation and approval requirements, not in the measurement criteria.

 

Parameter	What it measures	Limit	Monitoring	Notes
PAH	Polycyclic Aromatic Hydrocarbons	Maximum 50 µg/l PAH above inlet water PAH level, expressed as phenanthrene equivalent	Continuous	Reliable measurement requires clean optical path, representative inlet water sampling and calibration of the inlet and outlet measurement points
Turbidity	Suspended particulate matter in the washwater stream	Maximum 25 FNU above inlet water turbidity on a 15-minute rolling average basis	Continuous	Inlet water turbidity must be measured and subtracted; the rolling average basis means a brief spike above limit does not immediately trigger a breach
pH	Acidity of the washwater discharge	Minimum 6.5, or within 2 pH units of ambient seawater pH, whichever is less restrictive	Continuous	The ambient seawater pH basis allows some flexibility in areas with naturally lower seawater pH; inlet water pH must be measured where the ambient basis is used
Temperature	Temperature of the washwater discharge	No absolute limit; must be recorded and reported as part of the discharge record	Continuous	Supporting measurement that forms part of the compliance record; also relevant for sample conditioning system design
Nitrates	Nitrate content of the washwater	Equivalent to no more than 12% NOx removal from the exhaust stream, or 60 mg/l as NO3 expressed as NO2, whichever is greater	Addressed through EGCS design certification rather than continuous onboard monitoring	Generally managed at the EGCS design and certification stage; does not require a separate continuous onboard nitrate analyser

 

Monitoring must be continuous — not periodic sampling or spot checks. The monitoring data must be recorded in a way that supports review during port state control inspection. The monitoring instrument must generate an alarm when any parameter exceeds the permitted limit and must connect to the EGCS control system to allow automatic discharge shutdown if required.

 

What EGCS Types Does MEPC.259(68) Apply To?

MEPC.259(68) applies to all types of wet exhaust gas cleaning system — open-loop, closed-loop and hybrid configurations:

Open-loop EGCS

Open-loop systems draw in seawater, use it to scrub exhaust gas and discharge it overboard after monitoring. Continuous washwater monitoring at the discharge point is required under MEPC.259(68) whenever the system is discharging. The monitoring must confirm that PAH, turbidity and pH are within the permitted limits before discharge continues.

Closed-loop EGCS

Closed-loop systems recirculate freshwater with an alkaline additive in a closed circuit and produce a smaller bleed-off stream. The bleed-off must be monitored before discharge under MEPC.259(68). Where the bleed-off is retained onboard for shore disposal rather than discharged overboard, the EGCS may not require continuous overboard discharge monitoring during closed-loop operation, but bleed-off monitoring is required before any overboard discharge takes place.

Hybrid EGCS

Hybrid systems can switch between open-loop and closed-loop operation. When operating in open-loop mode, full washwater discharge monitoring is required. When switching to closed-loop mode, the mode change must be recorded. The monitoring system must be capable of supporting compliance in both operating modes.

 

How Does MEPC.259(68) Differ From MEPC.340(77)?

MEPC.340(77) is the 2021 guideline that supersedes MEPC.259(68) for newer EGCS installations. The washwater parameter limits are identical under both resolutions — the differences lie in the administrative and documentation requirements. For a full side-by-side comparison, see What Is MEPC.340(77) and How Does It Differ From MEPC.259(68)? The key differences are summarised in the table below.

 

	MEPC.259(68)	MEPC.340(77)
Adopted	2015	2021 — in force from 1 June 2022
Applies to	EGCS installed before 1 June 2022 (unless subsequently materially modified)	EGCS on ships with keels laid on or after 1 June 2022; new EGCS installations from June 2022; material modifications to existing EGCS
Washwater parameters	PAH, turbidity, pH, temperature, nitrates — same as MEPC.340(77)	Same parameters and limits — no change to the measurement requirements
Monitoring system approval	Not explicitly required as a standalone Administration approval step	Administration approval of the discharge water monitoring system explicitly required
Recording frequency	Continuous monitoring required — specific minimum frequency not stated	Continuous recording at not less than 0.0111 Hz (one data point per 90 seconds minimum)
EGCS Record Book	Not named as a mandatory document in MEPC.259(68)	EGCS Record Book explicitly required — must include maintenance, servicing, malfunction and residue management records
Overall compliance burden	Lower administrative requirement — established framework for pre-June 2022 installations	Higher administrative requirement — stronger documentation and evidence obligations

 

Confirm which guideline applies to each vessel before assuming MEPC.259(68) MEPC.259(68) applies to EGCS installed before 1 June 2022 that have not been materially modified since. A vessel built in 2015 with a new scrubber retrofitted in 2024 falls under MEPC.340(77) for that EGCS — the applicable guideline is determined by the EGCS installation date, not the ship’s build date. Operators should confirm the applicable guideline for each vessel with the flag state Administration or classification society.

 

What Records Are Required Under MEPC.259(68)?

MEPC.259(68) requires continuous monitoring data to be recorded and available for port state control inspection. While the named EGCS Record Book is a specific MEPC.340(77) requirement, MEPC.259(68) vessels should maintain equivalent records covering the same categories. For a full explanation of the complete evidence chain required for scrubber washwater compliance, see What Records Does a Ship Need to Prove Scrubber Washwater Compliance?.

The core records required under MEPC.259(68) for the washwater discharge dimension are:

• Continuous monitoring data for PAH, turbidity, pH and temperature throughout all discharge operations
• Alarm event log covering every high-parameter alarm, the reading that triggered it and the action taken
• Discharge event log with vessel position and speed data to confirm discharge did not occur in restricted waters
• Monitoring instrument maintenance and calibration records
• Records of any monitoring instrument malfunctions and the response taken under MEPC.1/Circ.883/Rev.1

 

How Does Rivertrace Support MEPC.259(68) Compliance?

The SMART ESM from Rivertrace measures PAH, turbidity, pH and temperature continuously at the EGCS washwater discharge point, supporting compliance with both MEPC.259(68) and MEPC.340(77). It is designed for open-loop, closed-loop and hybrid scrubber configurations and provides continuous data logging, automatic optical path cleaning and data export for compliance record keeping.

For the full regulatory framework governing EGCS washwater discharge including geographic restrictions, monitoring failure procedures and the OSPAR Decision 2025/01 timeline, see the exhaust scrubber washwater discharge monitoring explainer page.

 

Frequently Asked Questions

What is MEPC.259(68)?

MEPC.259(68) is the 2015 IMO Guidelines for Exhaust Gas Cleaning Systems, adopted at MEPC 68. It establishes the framework for ships to use EGCS as an equivalent means of meeting MARPOL Annex VI SOx emission limits rather than switching to low-sulphur fuel. It covers both the air emission performance of the EGCS and the washwater discharge monitoring requirements that apply to the washwater produced by wet scrubber systems.

What washwater parameters does MEPC.259(68) require to be monitored?

MEPC.259(68) requires continuous monitoring of four washwater parameters: PAH at a maximum of 50 µg/l above inlet water PAH level, turbidity at a maximum of 25 FNU above inlet water turbidity on a 15-minute rolling basis, pH at a minimum of 6.5 or within 2 pH units of ambient seawater pH, and temperature which must be recorded and reported. Nitrates are addressed through EGCS design certification rather than continuous onboard monitoring.

Does MEPC.259(68) still apply or has it been replaced by MEPC.340(77)?

MEPC.259(68) remains the applicable guideline for EGCS installed before 1 June 2022 that have not been materially modified. MEPC.340(77) applies to new EGCS installations from June 2022 and to material modifications of existing EGCS. Both guidelines require monitoring of the same washwater parameters at the same limits. MEPC.340(77) adds Administration approval of the monitoring system, a minimum data recording frequency and mandatory EGCS Record Book requirements.

What is the difference between the air emission and washwater dimensions of MEPC.259(68)?

The air emission dimension governs how the EGCS demonstrates SOx equivalent compliance — measured by the SO2/CO2 ratio in the exhaust gas, which must not exceed 21.7. The washwater discharge dimension governs what the washwater produced by the scrubber must measure before discharge — PAH, turbidity, pH and temperature must be within the specified limits. Both dimensions must be met simultaneously whenever the EGCS is operating.

Does MEPC.259(68) apply to closed-loop scrubber systems?

Yes. MEPC.259(68) applies to open-loop, closed-loop and hybrid EGCS. For closed-loop systems, the bleed-off stream must be monitored for the washwater parameters before any overboard discharge. Where the bleed-off is retained onboard for shore disposal rather than discharged, continuous overboard discharge monitoring may not be required during closed-loop operation, but monitoring is required before any bleed-off is discharged.

Which Rivertrace product supports MEPC.259(68) washwater compliance?

The SMART ESM from Rivertrace continuously measures and records PAH, turbidity, pH and temperature for open-loop, closed-loop and hybrid scrubber systems, meeting the washwater monitoring requirements of both MEPC.259(68) and MEPC.340(77).

 

Related content → Exhaust Scrubber Washwater Discharge Monitoring: EGCS Compliance, Regulations and Geographic Restrictions → What Is MEPC.340(77) and How Does It Differ From MEPC.259(68)? → What Records Does a Ship Need to Prove Scrubber Washwater Compliance? → SMART ESM: Washwater Monitor product page

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For more information on the MARPOL marine environmental regulations or the Marine Environment Protection Committee (MEPC) guidelines visit our Regulations section