Why the Oily Water Separator Is Fitted

1. The duty of the machine

Warning: Operating rule

The maker's manual, the approved operating procedure, the SMS, the class rules, the IOPP Certificate and its supplement, and the Chief Engineer's orders always override anything in these notes. No oil is to be discharged to sea outside the regulation. The overboard valve is locked and the key is held by the Chief Engineer.

An oil/water separator is fitted so that a ship can pump out its bilges, its oil tanks and any other oil-contaminated space without putting oil into the sea. That is the whole purpose, and it is a legal purpose rather than an operational convenience. Without the separator the only lawful way to get rid of machinery space bilge water would be to retain it until the ship reached a shore reception facility, which no ship can do indefinitely.

The regulation behind it is MARPOL 73/78, the international convention for the prevention of pollution from ships. Annex I of that convention deals with oil, and the oily water separator is the piece of equipment Annex I assumes you have.

2. Current MARPOL requirements for an OWS

For machinery-space bilges, 15 parts per million is the current MARPOL Annex I oil-content criterion.

Equipment approval: Annex I Regulation 14 sets the oily-filtering-equipment requirements for ships to which it applies. The IMO's revised specifications for 15 ppm Bilge Separators are in resolution MEPC.107(49), which superseded MEPC.60(33). A compliant installation may combine gravity separation, coalescing/filter stages, a 15 ppm Bilge Alarm and the required control/stopping arrangements. Approval applies to the equipment and arrangement as certified; check the ship's IOPP Certificate and supplement, flag instructions, and maker's manual.

Discharge operation: Annex I Regulation 15 limits machinery-space bilge discharge from ships of 400 gross tonnage and above to an effluent oil content not exceeding 15 ppm without dilution, when the ship is en route and the other conditions in the regulation are met. Additional equipment and automatic stopping requirements apply in Special Areas. The 15 ppm value is therefore both the approved equipment performance basis and the operational discharge criterion; an alarm or separator reading does not by itself authorize discharge.

Never use a general teaching note to decide whether a real discharge is permitted. Verify the vessel's certification, applicable Annex I provisions, operating procedure, and the current instructions of its Administration.

3. Why the law exists

It is easy to treat the regulation as paperwork. It is not, and the reason is straightforward.

Free oil and oily emulsions discharged into a waterway interfere with natural processes such as photosynthesis and re-aeration. They induce the destruction of the algae and plankton that fish life depends on. A discharge close inshore damages bird life and pollutes beaches, and the damage is visible to the public in a way that a discharge in mid-ocean is not.

The applicable discharge criterion for machinery-space bilges is the 15 ppm MARPOL rule described above, not a 100 mg/L allowance. A suspected or unauthorized discharge can trigger investigation and penalties under the laws and procedures applicable to the ship and coastal State; the outcome is jurisdiction- and fact-specific. Treat the separator, alarm, stopping device, records, and the ship's approved procedure as pollution-prevention controls, not as permission to discharge whenever the equipment is running.

4. The three parts of an installation

A complete oily water separator installation is not one vessel. It is three things working together:

1. The separator. A coarse separator, usually built with baffle plates, whose job is to accumulate the small oil particles and let them rise. This is the gravity stage.
2. The filter. The unit holding the coalescence filters, which take out the droplets the gravity stage could not lift.
3. The control unit. The control device and the monitoring device together — a test chamber, a mixing or sampling pump, a light source, and a controller that receives inputs for discharge rate, ship's speed and oil content.

The control unit is the part that makes the installation legal rather than merely effective. A separator that works but cannot demonstrate that it works — cannot show the oil content, cannot alarm, cannot stop the discharge — does not satisfy the regulation.

5. Where the rest of the plant sits

Three things surround the separator and are dealt with in later chapters:

• The bilge system that feeds it, and the discipline that keeps the overboard connection closed.
• The oil content monitor that watches the effluent and trips the discharge when the limit is exceeded.
• The sludge and incinerator arrangements that deal with the oil the separator takes out.

The separator is therefore the middle of a chain that begins at the bilge suction and ends at a shore reception facility or an incinerator. Any link in that chain failing turns the whole installation into a retention system by default.