The bilge of a vessel is the lowest compartment on a vessel.This is where bilge water collects. It may result from condensate, rough seas, rain, leaks in the hull or stuffing box, or other interior spillage. The collected water may contain various contaminants including oil and lubricants and must be pumped out periodically when the bilge holding tank gets filled.
Discharge of bilge liquids may be restricted and for commercial vessels is regulated ininternational waters under jurisdiction of the International Maritime Organization (IMO) through the regulation Marpol Annex I. IMO Resolution MEPC.107 (49) requires for treated bilge water overboard discharge that it does contains no more than
15 parts per million (ppm) oil content .
Other regulations require even stricter discharge standards like e.g. the voluntary DNV-GL Clean(Design) notation that stipulates a maximum 5 ppm of oil content remaining in the water after treatment, prior to pumping overboard.