Oil-Water Separator

What this type does

Bilge water collected in the machinery space is a mix of water, fuel and lube oil residue, cleaning chemicals and sludge. An oil-water separator (OWS) strips the free and emulsified oil out of that mixture in stages, using gravity separation followed by coalescing media, before the water can be discharged overboard. It is the mechanical treatment plant; the oil content meter and automatic stopping device fitted downstream is a separate, class-approved instrument that measures what the separator actually achieved and shuts the overboard valve if the result is not good enough. Confusing the two is common on board - a clean OWS with a faulty monitor is still not compliant.

Main components

Gravity separation stage

The first chamber slows the flow so free oil, which is lighter than water, rises and collects at the top while heavier solids settle at the bottom, removing the bulk of the oil load before finer treatment.

Coalescing stage

Plate packs or coalescing media force the remaining fine oil droplets to merge into larger globules that can rise out of the water, which is what allows the unit to reach the low ppm levels gravity alone cannot achieve.

Sludge and oil collection

Separated oil is drained to a sludge or waste oil tank rather than discharged, and this tank needs its own management under the ship's oil record book.

Automatic stopping device and 15 ppm bilge alarm

Fitted downstream of the separator, this unit continuously samples the outlet, alarms if oil content approaches the limit, and automatically closes the overboard discharge valve and diverts flow back to the bilge tank if the limit is exceeded.

Selection and sizing

Capacity is sized to the machinery space bilge accumulation rate, commonly in the range of a few cubic metres per hour on smaller vessels up to considerably more on large ships, with a margin over the expected worst-case leakage rate rather than the average. Materials in contact with oily bilge water are usually corrosion-resistant steel or coated carbon steel, since ordinary mild steel corrodes quickly in this service. Emulsion-breaking capability matters more than raw throughput on vessels that use a lot of detergent-based bilge cleaners, because strong emulsifiers can defeat a separator that would otherwise easily meet the limit on untreated bilge water.

Regulations and class

MARPOL Annex I sets the 15 ppm discharge limit and requires the automatic stopping device to be fitted and functioning whenever the separator is used to discharge overboard. Newer separator types must be type-approved to the IMO performance and test specification in force at the time of installation, and the oil record book Part I must log every discharge, transfer and internal transfer of oily bilge water, which is the document port state control checks first. Class surveys verify the separator and monitor are operable and that seals used to prevent bypassing the system, where fitted by flag or port state, are intact.

Typical faults

What to look for in a supplier

• Current IMO type approval for the specific separator and monitor combination, not the separator alone.
• Documented performance with the detergents and cleaning chemicals actually used on board, since some units struggle with heavy emulsifiers.
• Spare parts availability for coalescing elements and monitor sample cells, which are the routine wear items.
• Commissioning support that includes calibration of the oil content monitor against a certified reference, not just installation.

A spotless engine room log and a bypass pipe found during a port state control inspection have ended careers - never let the separator's slow throughput become a reason to look for a shortcut.