Ships and submarines collect bilge water in the lowest parts of the hull, where leaks, condensation, seawater, fuel, lubricating oil, and cleaning fluids can mix. Before this water can be pumped overboard, it must be cleaned so oil does not harm marine life or coat the ocean surface. An oily water separator uses physical properties like density, droplet size, and filtration to remove oil from water.
This system is important because even small amounts of oil can spread into a thin film over a large area.
Key Facts
- Oil is usually less dense than water, so it tends to float when the mixture is allowed to settle.
- Density = mass / volume, or ρ = m / V.
- Many marine discharge rules limit oil content to 15 parts per million, written as 15 ppm.
- 1 ppm in water is approximately 1 mg of oil per 1 L of water.
- Flow rate can be calculated with Q = V / t, where Q is flow rate, V is volume, and t is time.
- An oily water separator often uses gravity separation, coalescing filters, oil sensors, and an automatic valve that stops discharge if oil content is too high.
Vocabulary
- Bilge water
- Bilge water is the mixture of water, oil, fuel, and other fluids that collects in the lowest part of a ship or submarine.
- Oily water separator
- An oily water separator is a device that removes oil from bilge water before the water is discharged.
- Coalescence
- Coalescence is the joining of small oil droplets into larger droplets that separate from water more easily.
- Parts per million
- Parts per million, or ppm, is a concentration unit that describes how many parts of a substance are present in one million parts of a mixture.
- Oil content monitor
- An oil content monitor is a sensor that measures how much oil remains in treated water.
Common Mistakes to Avoid
- Assuming the separator destroys the oil. It only separates and collects oil, which must still be stored and disposed of properly.
- Thinking all oily water can be discharged after one filter. The discharge depends on the oil concentration reading and may be stopped if it is above the legal limit.
- Ignoring flow rate through the separator. If water moves too fast, oil droplets may not have enough time to rise, merge, or be trapped by filters.
- Confusing clear water with safe water. Water can look clean while still containing dissolved or tiny suspended oil droplets above the allowed limit.
Practice Questions
- 1 A bilge tank contains 600 L of water with 9,000 mg of oil mixed in. What is the oil concentration in ppm, using 1 ppm = 1 mg/L?
- 2 An oily water separator treats 1,200 L of bilge water in 40 minutes. What is the flow rate in liters per minute?
- 3 Explain why an oily water separator often uses both a settling chamber and a coalescing filter instead of only one separation step.