What Are the Wash Water Discharge Regulations?

Marine exhaust gas cleaning systems, also known as scrubbers, significantly decrease air pollution but generate harmful wash water containing polycyclic aromatic hydrocarbons (PAH), particulate matter (PM), and other detrimental substances. Therefore, the IMO has implemented various discharge limits for wash water, which ships with a scrubber must abide by. Let us dive into the scrubber wash water discharge limits and how they can be maintained.

The IMO Exhaust Gas Cleaning System Guidelines require ships to abide by several scrubber wash water discharge limits. Parameters such as pH, PAH concentration, turbidity, and temperature should be monitored and recorded continuously, set, and logged with time and the vessel’s position.

pH - With Temperature Compensation

The pH limit for discharge scrubber wash water is a minimum of 6.5 and must not exceed a temperature of 60°C. There are two methods for the shipping industry to comply with this.


Option 1:

The pH limit of the scrubber wash water should not be lower than 6.5, with the exclusion of maneuvering and transit. A maximum difference of 2 pH units is accepted between the vessel’s scrubber wash water inlet and overboard discharge.

Option 2:

When operating a scrubber, the pH of the discharge wash water must be measured externally from the vessel. This must be within a limit corresponding to a pH result of 6.5 or above 4 meters from the vessel. 4 meters are recognized as the point where discharged water and seawater surrounding the vessel will start to mix.

Polycyclic Aromatic Hydrocarbons (PAH)

PAHs are organic compounds with at least two or more integrated aromatic rings. PAHs are naturally found in petroleum. Furthermore, PAHs are a byproduct of combustion processes in a vessel’s engine. Even though most engines today are designed to optimize fuel combustion, exhaust gas streams will always encompass a small amount of inadequately combusted substance. A small amount of this substance is PAHs.

The discharge limit for PAH is based on the concentration of phenanthrene equivalents in the wash water.


Turbidity is used to measure water transparency loss due to suspended solids’ presence. The greater the number of suspended solids in the wash water, the more blurred it will be and the higher the turbidity will be.

In combination with PAH, turbidity is an effective method to determine PM removal performed by the wash water treatment unit. Yet, the turbidity of wash water may be affected by the turbidity of the water entering the scrubber. Therefore, measuring the average turbidity for the last 15 minutes is accepted. The turbidity must not exceed 25 formazin nephelometric units (FNU) or 25 nephelometric turbidity units (NTU).

Monitoring for Compliance

The water monitoring unit installed in scrubbers ensures guaranteed compliance and consistent quality of the treated water, as it monitors the pH, temperature, PAH, and turbidity.

If a vessel does not abide by the scrubber wash water discharge limits, it can either access ports or receive fines. Therefore, it is essential to have adequate water monitoring and an excellent water filtration solution.

See our Marine Scrubber Water Treatment System
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