Various processes onboard ships, such as machinery wash-down, maintenance, and leakage generate oily wastewater. This contaminated water flow collects in the bilge of the ship. Marine diesel, lubricating oils, grease, as well as garbage may be present in bilge water. The bilge water is discharged overboard, with oil and grease concentrations in the discharged water limited by national and international regulations.
In the United States territorial waters, the U.S. Coast Guard (USCG) limits the discharge of oil and grease in bilge water to 15 ppm, measured by the U.S. Environmental Protection Agency (EPA) oil and grease measurement method. The USCG further limits bilge water and oily discharges that cause a visible sheen on the water. In Canadian inland waters, the Canadian Coast Guard limits the discharge of oil and grease in bilge water to 5 ppm. Bilge water discharges in international waters are limited to 15 ppm by the International Maritime Organization (IMO). Ship operators who violate the discharge limits are subject to large fines by the jurisdictional authority. Recently, cruise ship lines have incurred record fines for discharge violations.