Depending on the operating area of a ship, it may be preferable to have an open or a closed loop system. Many ships operating close to shore, or spending a lot of time in control zones will opt for a closed loop system. Ships that spend most of their time on the open sea opt for an open mode system.

Open Loop
Single pass raw seawater, scrubber effluent returned to the sea.

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Closed Loop
No overboard discharge permitted. Washwater is recirculated with NaOH added to neutralize sulphur. Solution is processed and contaminants are stored for disposal ashore.

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