The IMO Exhaust Gas Cleaning System Guidelines require ships to abide by several scrubber wash water discharge limits. Parameters as pH, PAH concentration, turbidity, and temperature should be monitored and recorded continuously set and logged with time and the vessel’s position.
1. pH (with temperature compensation)
The pH limit for discharged scrubber wash water is a minimum of 6.5, and it must not exceed a temperature of 60 ˚C. Ships have two methods to comply with this.
Method 1) The pH limit of the scrubber wash water should not be lower than 6.5 with the exclusion of maneuvering and transit. Here, a maximum difference of 2 pH units is accepted between the vessel’s scrubber wash water inlet and overboard discharge.
Method 2) When operating a scrubber, the pH of the discharged wash water must be measured externally from the vessel. The discharged wash water is measured, and 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.
2. Polycyclic Aromatic Hydrocarbons (PAH)
PAHs are made up of organic compounds with at least two or more integrated aromatic rings. PAHs are naturally found in petroleum. Likewise, PAHs are a byproduct produced by combustion processes in a vessel’s engine. Although 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. To control the total amount of unsafe and harmful PAH discharged into sea, an upper limit of 50µg/l above that at scrubber wash water inlet is related to a 45t/MWh flow rate, equal to that of an open-loop scrubber.
Turbidity is used to measure transparency loss in water as a result of the presence of suspended solids. 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, it is accepted to measure the average turbidity for the last 15 minutes. The turbidity must not exceed 25 formazin nephelometric units (FNU) or 25 nephelometric turbidity units (NTU).
Monitoring for Compliance
All scrubbers are installed with a water monitoring unit (WMU), which constantly monitors the pH, temperature, PAH, and turbidity in relation to the IMO scrubber wash water discharge limits. This ensures guaranteed compliance and consistent quality of the treated water.
If a vessel does not abide by the scrubber wash water discharge limits, it can either be permitted access to ports or receive fines. Therefore, it is essential to have adequate water monitoring as well as an excellent water filtration solution.
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