A ship’s bilge water is a mixture of discharges and leakage from a wide variety of sources, which drain to the lowest compartment (bilge) of a vessel. Bilge water includes seawater accumulation, normal water leakage from machinery, and wash downs. It can also contain a variety of constituents such as solids, cleaning agents, solvents, fuel, lubrication oil, and hydraulic oil.

Vessels can use a variety of bilge water collection systems such as:

• Waste oil collection tanks partitioned by baffles into oily and clean sides  
• Waste oil collection tanks without partitioning where gravity separation of oil occurs

The water phase of the separated bilge water is usually pumped overboard as necessary, regardless of the distance from shore. The lower portion of the liquid in the oily water tank can be pumped overboard outside of 50 nautical miles (nmi) from shore.

In the wake of the Exxon Valdez disaster, the Oil Pollution Act amended the Clean Water Act to prevent oil dumping by ships. Oil discharge within 12 nmi from shore must leave no “visible sheen” and measure less than 15 parts per million (ppm). Beyond 12 nmi from shore, ships may release oily waste that measures less than 100 ppm. The remaining oily waste must be retained on board and disposed of at a shore-based facility. Ships must also record the disposal of oily residues and bilge water.