Bilge Water Discharges

Canadian Inland Water regulations authorize the treatment of oily bilge water through approved equipment overboard with the ship making way. The required effluent quality must be better than 5 parts per million (ppm) of oil in the discharge water, while the ship is moving.

The regulations class oil and oily water as a pollutants that mustn't be discharged from a ship. Therefore, since the regulations authorize the treatment of oily bilge water, the effluent that falls within the regulated discharge requirements becomes then a discharge free of oil under the oil pollution prevention regulations.

How oil-free is then the 5ppm bilge water discharge from a ship? Are compliant bilge water discharges a pollution threat for the Great Lakes - Seaway system?

1 liter is a cube with sides of 100mm each, or equal to 1 Million cu.mm; therefore 1 cu.mm of oil in 1 liter of bilge water is 1ppm; 5ppm is one cube with sides of 1.7mm in 1 liter of bilge water. Which means that the upper limit for compliant bilge water discharges is a small drop of oil in 1 liter of bilge water. This quantity of oil is evenly dispersed in the effluent, in very fine droplets, too fine to coalesce into larger oil droplets to form visible pollution.

So not only do the regulations require fairly clean water, but in addition the ship has also to "make way".

Here is a quick calculation on "typical" worst case discharges from Canadian Lakers. One of the most common bilge separator sizes installed is a 16GPM system, which processes bilge water at about 3,600 liters per hour equal to 3.6 liters per second. A ship making way at 2 knots, the ship does 2 nautical miles per hour, equal to about 3.6km/hr or about 1 meter per second. Therefore, in the worst case under the regulations this ship discharges 1 liter bilge water per second containing an oil drop of 1.7mm over a distance of 1 meter traveled. Since the actual oil content must be below the 5ppm limit and the ship's speed is typically above the 2KN mentioned, therefore a smaller "drop of oil" gets stretched further than the 1 meter mentioned.

I am quite comfortable stating that compliant bilge water discharges are not a pollution threat for the inland waters. Bilge water discharges are not point sources like refinery effluents or run-off from roadways and parking lots, or any other oil pollution from land based sources.