Exhaust gas cleaning system that uses seawater as the alkali source to remove SOx, then discharges treated wash water overboard.
Regulatory detail & full definition
Compliant under MARPOL Annex VI Reg. 14 as an equivalent to low-sulphur fuel. Effluent must meet IMO MEPC.259(68) limits (pH > 6.5 at the discharge point, 4 m from discharge; PAH and turbidity limits). Banned in Singapore, China, German waters, several Baltic and US states; charters often spell out where open-loop is permitted.