Seawater used to remove sulphur oxides (SOx) from exhaust emissions in exhaust gas cleaning systems (EGCS) commonly known as exhaust scrubbers is processed as wash water before being discharged. Under MEPC.259(68), this wash water must be continuously monitored to ensure it meets the environmental standards required for overboard discharge.
Rivertrace’s SMART ESM measures and records all required wash water parameters PAH, turbidity, temperature and pH in real time, providing the compliance monitoring and data logging required by MEPC.259(68).
| Key thing to remember
MEPC.259(68) requires ships using open-loop exhaust scrubbers to monitor wash water continuously at the discharge point. The monitor must measure PAH, pH, turbidity and temperature and maintain a continuous record. A scrubber operating without a functioning, compliant wash water monitor is not compliant with MEPC.259(68) and the ship may be detained during port state control inspection. |
What Is an Exhaust Gas Scrubber and Why Is Wash Water Monitoring Required?
An exhaust gas scrubber or Exhaust Gas Cleaning System (EGCS) is a technology used on ships to remove sulphur oxides (SOx) from engine and boiler exhaust gases. Scrubbers allow ships to use high-sulphur fuel oil (HSFO) while complying with the IMO 2020 global sulphur cap (0.5% sulphur limit), which came into force on 1 January 2020 as an alternative to switching to low-sulphur fuel.
In an open-loop scrubber, seawater is sprayed into the exhaust gas stream. The seawater absorbs the sulphur dioxide and neutralises it, producing a wash water stream that contains sulphur compounds, polycyclic aromatic hydrocarbons (PAH), heavy metals and particulates.
This wash water cannot be discharged overboard without treatment and monitoring it must meet the limits set by MEPC.259(68) before discharge is permitted. The wash water monitoring system is therefore a mandatory part of any open-loop EGCS installation.
What Are the Three Types of Exhaust Scrubber and How Does Each Affect Monitoring?
Exhaust scrubbers operate in three configurations, each with different monitoring implications:
| Type |
How it works |
Monitoring implication |
| Open loop |
Seawater is drawn in, used to scrub exhaust gases and discharged overboard after treatment |
Requires full wash water monitoring per MEPC.259(68) at the discharge point; banned in some ports and coastal waters |
| Closed loop |
Fresh water with alkaline additive circulates in a closed circuit; no continuous overboard discharge |
Monitoring required for bleed-off discharge; less affected by port bans on open-loop discharge |
| Hybrid |
Switchable between open-loop and closed-loop operation depending on location |
Monitoring requirements apply in open-loop mode; operator must be able to switch modes for restricted areas |
The majority of installed scrubbers are open-loop systems, where the MEPC.259(68) wash water monitoring requirement applies in full. Hybrid systems must be capable of meeting the monitoring requirement whenever operating in open-loop mode.
What Does MEPC.259(68) Require for Wash Water Monitoring?
MEPC.259(68) the IMO’s 2015 guidelines for exhaust gas cleaning systems sets out the wash water discharge criteria that must be met and the monitoring parameters that must be continuously measured, recorded and reported. The key parameters and limits are summarised in the table below.
| Parameter |
MEPC.259(68) Limit |
Notes |
| PAH (Polycyclic Aromatic Hydrocarbons) |
Maximum 50µg/l PAH above the inlet water PAH level |
Measured as phenanthrene equivalent |
| pH |
Minimum 6.5 (or pH within 2 of ambient seawater) |
Indicates acidity of the wash water at the point of discharge |
| Turbidity |
Maximum 25 FNU above inlet water turbidity |
Indicates the level of particulates in the wash water |
| Temperature |
To be recorded and reported |
The temperature of the wash water at the point of discharge |
| Nitrites |
Monitoring recommended where freshwater is used |
Relevant for closed-loop and hybrid scrubber systems |
Monitoring must be continuous not periodic grab sampling and the data must be recorded in a way that allows port state control authorities to review the discharge record during inspection. The monitoring system must also be capable of generating an alarm if any parameter exceeds the permitted limit.
| Port bans on open-loop scrubber discharge
A growing number of ports, coastal states and regional authorities have banned or restricted the discharge of open-loop scrubber wash water in their waters including ports in China, Singapore, the United States (in certain areas), Belgium and Germany. Ships operating in these areas must either switch to closed-loop mode or use compliant fuel. The wash water monitoring system must be capable of confirming that discharge has stopped when entering a restricted area. |
What Does the Rivertrace SMART ESM Monitor?
The SMART ESM is Rivertrace’s dedicated wash water monitor for exhaust gas cleaning systems. It measures and records all parameters required by MEPC.259(68) in real time:
- PAH Polycyclic aromatic hydrocarbon concentration, measured as phenanthrene equivalent, against the 50µg/l limit above inlet water PAH level
- Turbidity Particulate content of the wash water, against the 25 FNU above inlet water turbidity limit
- pH Acidity of the wash water at the discharge point, against the minimum pH 6.5 limit (or within 2 of ambient seawater pH)
- Temperature Wash water temperature at the discharge point, recorded for the compliance log
The SMART ESM provides a continuous data log that satisfies the MEPC.259(68) record-keeping requirement and supports port state control inspection. It also generates alarms when any parameter exceeds the permitted limit, enabling the ship’s crew to take corrective action before a compliance breach occurs.
Why Do Ships Use Exhaust Scrubbers Instead of Low-Sulphur Fuel?
The IMO 2020 sulphur cap requires ships to use fuel with a maximum sulphur content of 0.5% globally, or 0.1% in Emission Control Areas (ECAs). Ships have two main compliance routes:
- Switch to low-sulphur fuel oil (LSFO) or marine gas oil (MGO) straightforward compliance but higher fuel cost
- Install an exhaust gas scrubber and continue using high-sulphur fuel oil (HSFO) higher capital investment but lower fuel operating cost
For many ship operators particularly those with high fuel consumption vessels such as large container ships, bulk carriers and tankers the economics of scrubber installation can be favourable over the long term. The wash water monitoring system is a mandatory element of the scrubber installation and must be operational whenever the scrubber is in use.
What Are the Consequences of Non-Compliant Wash Water Discharge?
Discharging open-loop scrubber wash water that does not meet the MEPC.259(68) limits or operating a scrubber without a functioning wash water monitor can result in:
- Detention during port state control inspection if the monitoring system is non-functional or records show non-compliant discharge
- Financial penalties from flag state authorities or port state control for MARPOL violations
- Reputational damage particularly where wash water discharge incidents attract media or environmental group attention
- Prohibition from using the scrubber in future some authorities may restrict scrubber use following a compliance failure
The marine industry faces increasing scrutiny from port state control and environmental authorities over scrubber wash water discharge. A functioning, calibrated and MEPC.259(68)-compliant wash water monitoring system is the primary defence against enforcement action.
How Does Exhaust Scrubber Monitoring Relate to EGR Systems?
Exhaust Gas Recirculation (EGR) is a related but different technology used to reduce nitrogen oxide (NOx) emissions rather than sulphur oxide emissions. EGR systems also produce a wash water stream that requires monitoring before discharge. For vessels fitted with both scrubbers and EGR systems, separate monitoring may be required for each discharge stream. See our EGR systems application page for more information on EGR wash water monitoring requirements.
Frequently Asked Questions
What is MEPC.259(68) and what does it require?
MEPC.259(68) is the IMO’s 2015 guidelines for exhaust gas cleaning systems. It sets the wash water discharge criteria for open-loop exhaust scrubbers, requiring ships to continuously monitor PAH, pH, turbidity and temperature at the wash water discharge point and maintain a full compliance record. The guidelines also set the discharge limits for each parameter.
What parameters must be monitored in exhaust scrubber wash water?
MEPC.259(68) requires continuous monitoring of four parameters: PAH (polycyclic aromatic hydrocarbons, measured as phenanthrene equivalent, maximum 50µg/l above inlet water); pH (minimum 6.5 or within 2 of ambient seawater pH); turbidity (maximum 25 FNU above inlet water turbidity); and temperature (recorded and reported). Nitrite monitoring is recommended where freshwater is used.
Is a wash water monitor required for all types of exhaust scrubber?
The full MEPC.259(68) monitoring requirement applies to open-loop scrubbers, which discharge wash water continuously overboard. Closed-loop systems do not discharge continuously but must monitor bleed-off discharge. Hybrid systems must be able to meet the open-loop monitoring requirement whenever operating in open-loop mode.
Can open-loop scrubbers discharge wash water in all ports?
No. A growing number of ports, coastal states and regional authorities have banned or restricted open-loop scrubber wash water discharge in their waters. These include ports in China, Singapore and parts of the United States and Europe. Ships must be aware of applicable restrictions and capable of switching to closed-loop mode or compliant fuel when entering restricted areas.
What does the Rivertrace SMART ESM measure?
The SMART ESM measures and records PAH, turbidity, temperature and pH in real time all four parameters required by MEPC.259(68). It provides continuous data logging for the compliance record and generates alarms when any parameter exceeds the permitted limit.
