There are three basic designs for exhaust gas scrubbers: open loop, hybrid (caustic assisted), and closed loop.
In an open-loop system, top, the flue gas is sprayed with seawater that is used only once. The sulfur oxide (SOx) in the gas reacts with the water to form sulfuric acid. The resulting effluent is acidic, but the natural alkalinity of seawater serves to neutralize it. Provided that enough water is used, IMO requirements can be met to discharge directly into the sea. Washwater from the scrubber also can be treated and monitored for compliance.
When a ship operates where salinity is low, like fresh water or brackish water, a caustic agent may be added to the system. This is typically called caustic-assisted scrubbing or hybrid scrubbing, center. The hybrid mode is much like the open-loop system, with the exception that alkalinity is added to the water to assist scrubbing performance and ensure compliance.
In a closed-loop system, bottom, the scrubber liquid is reused. The liquid absorbs heat from the flue gas and is cooled indirectly with seawater in a heat exchanger. During closed-loop operation, no liquid is discharged directly to the sea. Instead, it is kept in tanks for disposal ashore.