Scrubber

A scrubber, or exhaust gas cleaning system, washes SO2 out of engine exhaust so a ship can burn higher-sulphur residual fuel and still meet the MARPOL Annex VI sulphur limits as an approved equivalent under Regulation 4. Open-loop units spray seawater and discharge the washwater; closed-loop units recirculate an alkaline wash and retain residues; hybrids switch between modes. The economics turn on the spread between high-sulphur fuel and VLSFO. Several ports and coastal states have banned open-loop washwater discharge over acidification and metals concerns, narrowing where the open-loop type can operate.