[Sketch notes: The closed-loop scrubber consists of: (1) the scrubber tower (a vertical vessel) in the exhaust gas line; (2) a wash water circulation pump; (3) a wash water treatment unit (with an alkaline additive dosing system, e.g. caustic soda NaOH); (4) a wash water tank; (5) a sludge tank; (6) a wash water cooler; (7) monitoring instruments (pH, PAH, turbidity, temperature).]

In a closed-loop scrubber, the exhaust gas from the engine enters the scrubber tower where it is sprayed with wash water (fresh water with an alkaline additive). The SOx in the gas is absorbed by the wash water, which becomes acidic. The wash water is collected at the bottom of the tower and recirculated by the pump through a treatment unit where an alkaline additive (caustic soda) is dosed to neutralise the acid and maintain the pH. The treated wash water is cooled and returned to the scrubber. The sludge (from the neutralisation and the soot) is removed and sent to a sludge tank for disposal ashore. The wash water is recirculated, so very little is discharged; any excess is treated and discharged (or stored) as required. The system is monitored (pH, PAH, turbidity, temperature) to ensure the discharge meets the limits.

The fluid used is fresh water with an alkaline additive (caustic soda, NaOH, or sometimes sodium carbonate). Reasons:

1. Fresh water is used because it can be recirculated in a closed loop (unlike sea water which is used in an open-loop system and discharged).
2. The alkaline additive (caustic soda) neutralises the sulphuric acid formed when the SOx is absorbed, maintaining the pH of the wash water and preventing it from becoming too acidic.
3. The closed-loop system can be used in port/ECA waters where the discharge of acidic wash water is prohibited, because the wash water is treated and recirculated.
4. The alkaline additive ensures effective SOx removal and prevents corrosion of the system.

The effectiveness of the scrubbing fluid is maintained by:

1. Continuously dosing the alkaline additive (caustic soda) to neutralise the acid and maintain the pH of the wash water at the required level (monitored by a pH sensor).
2. Recirculating and treating the wash water (removing the solids/soot by a separator/filter) and cooling it before re-use.
3. Monitoring the wash water quality (pH, PAH, turbidity) and adjusting the additive dosing and the treatment.

The sludge (the solid residue from the neutralisation and the soot) is removed by:

1. A sludge separator/settling tank that collects the sludge from the wash water.
2. The sludge is transferred to a sludge tank on board.
3. The sludge is disposed of ashore to a reception facility (not discharged overboard), in accordance with MARPOL.

The system is designed so that the wash water is recirculated and the sludge is collected and disposed of properly.