Equipment that separates oil from bilge water to reduce oil content to 15 ppm before overboard discharge under MARPOL.
Quick facts
Regulation
MARPOL Annex I
In practice
Bypass or disabling of the 15 ppm alarm is among the most serious environmental offences under MARPOL and has resulted in multi-million dollar penalties and imprisonment of officers in various flag and port states. Engineers must ensure the alarm is operational, calibrated, and free of any manual override capability before departure. Maintenance records for the OWS and alarm system are inspected by port state control officers and class surveyors. The Oil Record Book Part I must record every operation of the OWS, including the quantities processed, time, position, and the overboard or retention decision.
Regulatory detail & full definition
An oily water separator is required by MARPOL Annex I on any vessel of 400 gross tonnes or more, and is the primary means of treating bilge water to the 15 mg/kg oil content limit before overboard discharge. The unit typically operates in two stages: an initial gravity separation tank where density difference causes oil droplets to coalesce and rise, followed by a coalescer or polishing stage using oleophilic media. The discharge outlet is connected to a 15 ppm bilge alarm approved under IMO Resolution MEPC.107(49), which automatically closes the overboard valve and routes the flow back to the bilge holding tank if the set limit is exceeded.
Bypass or disabling of the 15 ppm alarm is among the most serious environmental offences under MARPOL and has resulted in multi-million dollar penalties and imprisonment of officers in various flag and port states. Engineers must ensure the alarm is operational, calibrated, and free of any manual override capability before departure. Maintenance records for the OWS and alarm system are inspected by port state control officers and class surveyors. The Oil Record Book Part I must record every operation of the OWS, including the quantities processed, time, position, and the overboard or retention decision.