The International Maritime Organization sets a limit of 15 parts per million of oily waste that can be present in clean water discharged within 12 nautical miles from shore. That is roughly equivalent to a grain of table salt in 2 liters of water. That figure goes up to 100 ppm beyond 12 nautical miles from land.

The removal and disposal of bilge water is highly regulated, and fines are heavy for non-compliance. The arm of the law is also long: In 2016, the U.S. government fined the operator of the Caribbean Princess U.S. $40 million for pumping oily waste overboard while in U.K. waters.

Depending on the size of vessel, there are several methods for removing bilge water. It can be pumped onshore for safe disposal, but the waste belongs to the ship owner, from the cradle to the grave, and so whatever that third-party treatment company does with it is still the ships responsibility. Small vessels can evaporate the water with electric or propane-fired heaters and dispose of the oily waste separately, again retaining responsibility.

Larger ships are fitted with bilge filters and oily water separators, which use the gravity differential between oil and water to separate the chemical sludge from the water, which is then discharged at sea while the waste is carried back to port. This is often challenging, especially in rough seas and high-humidity areas. Ships bilge water tanks are small and easily fill up in a short time. For that reason, there is always pressure on crew to handle bilge water, Fatih Onder, an Istanbul-based sales and purchase shipbroker for Optima Shipping Services, told LubesnGreases.

Apart from actual water, the waste also contains oils that have leaked from a number of places, such as fuel oil purifiers, fuel lines and from spills. Lubricating oils find their way into the bilge well from engines, oil pumps and compressors. They also run off from winches, hatch covers and crane wires.

Maritime law dictates that any lubricant used in an area where there is contact with the sea must be biodegradable, minimally toxic and have a low propensity to accumulate in organisms They are known as environmentally acceptable lubricants, or EALs, often made with synthetic esters. Closed systems, such as engines, do not require EALs and can be mineral-oil based.

Although only part of the bilge waters contaminants, lubricant in the bilge water can cause oxidation damage, cautioned Onder and added that oxidation is not the only problem. The idea of having clean bilges is not just to avoid any problems during […] inspections. If an inspector finds out there is oil in the bilge, then he or she would be tracing that leak, which could cause more problems, even detentions of a ship.

Other incentives for reducing leakage of lubricant are its resale value. While the cost of leaks is fairly small, the volumes used onboard larger ships are substantial. Lubricant is included in the sale of a secondhand ship and depending on the quantity can be worth up to $10,000, Onder said. It pays to make sure that any lubricant in a ships bilge wells is a drop in the ocean and not in a bucket.