The short answer is probably yes. If your ship is 400 gross tonnage or above MARPOL expects you to have equipment that keeps oily bilge discharge under 15 parts per million and on larger ships that equipment has to include an alarm. Do you need a 15ppm bilge on your particular vessel? That depends on size and trade and where you are sailing and the detail is exactly where crews and owners tend to trip up.
This guide covers who needs one and when it has to be working and what inspectors actually look at. For a deeper technical breakdown there is a full guide to 15ppm bilge alarm MARPOL compliance that goes further than we can here.
What the Alarm Actually Does
Oily water collects in the machinery space bilge. Leaks and drips and washdown and condensation all end up there. Before any of it goes overboard it has to pass through an oily water separator and the oil content of what comes out cannot exceed 15 parts per million.
The alarm is the safety net on that process. An oil content meter samples the treated water continuously and if the reading climbs above the limit the alarm sounds. On ships that need it the system also stops the discharge automatically so oily water does not carry on going into the sea while someone is still walking down to the engine room.
Why the Limit Exists in the First Place
Oil in the sea does not stay where you put it. Even small concentrations spread and coat surfaces and harm marine life and a single ship’s bilge discharge is tiny. Multiply that across thousands of ships over decades and it adds up to a real problem.
That is why the limit has been tightened over time. Earlier rules allowed far higher concentrations and the industry gradually moved to 15ppm as the internationally agreed threshold. It is not an arbitrary number. It is the result of years of trying to keep routine operational discharge from becoming a steady source of pollution.
What MARPOL Annex I Says
The rules sit in Annex I of MARPOL and mainly in Regulations 14 and 15. Regulation 14 deals with the equipment a ship has to carry. Regulation 15 deals with the conditions under which machinery space bilge water can actually be discharged.
Put simply one tells you what to fit and the other tells you when you may use it. Plenty of confusion comes from mixing the two up. A ship can have perfectly good equipment and still break the rules by discharging in the wrong place or under the wrong conditions.
Which Ships Need One
Ships of 400 gross tonnage and above but under 10000 must carry oil filtering equipment that keeps effluent at or below 15ppm. For ships of 10000 gross tonnage and above the requirement goes a step further. The equipment must include an alarm that warns when 15ppm cannot be maintained plus an arrangement that automatically stops the discharge.
In practice the line between those two categories matters less than you would think. Modern separators approved to the current standard come with the alarm and the automatic stop built in so most ships end up with one regardless of tonnage. Fitting the full system is usually simpler than arguing about whether you can get away without it.
Smaller Vessels
Below 400 gross tonnage the obligations are lighter but they do not disappear. These ships are still expected to retain oily mixtures on board for discharge to a reception facility or to deal with them in line with the discharge conditions that apply to them.
Here is where owners get caught. Flag states and coastal states can set their own requirements for smaller vessels and those requirements are not always the same. Before assuming a small vessel is exempt, check what your flag administration actually expects. Assuming is how you end up with a detention.
When Discharge Is Actually Allowed
Having the equipment does not mean you can discharge whenever you like. Under Regulation 15 the ship has to be en route. The oil filtering equipment has to be running. The oil content of the effluent without dilution cannot exceed 15ppm. And it cannot be mixed with oil cargo residues.
Special areas carry stricter rules and discharge is prohibited altogether in Antarctic waters. If your trading pattern takes you through any of them the crew needs to know exactly what applies where. A bilge alarm that works perfectly does not help much if the ship is discharging somewhere it should not be.
Type Approval Matters More Than People Think
The equipment cannot just be any separator with a sensor bolted on. It has to be type approved to the standard set out by IMO currently Resolution MEPC.107(49) and the approval has to come from the flag administration or a recognised organisation acting on its behalf.
That approval is documented. The ship carries an International Oil Pollution Prevention certificate and its supplement lists the equipment fitted. If what is actually in the engine room does not match what the paperwork says, that is a finding waiting to happen. Retrofitting or swapping a unit without updating the certificates is a classic way to create one.
Calibration and Record-Keeping
An oil content meter that is out of calibration is worse than no meter at all because it gives you confidence you have not earned. Meters need calibrating within the interval the manufacturer and the approval specify and the calibration certificate should be on board and in date.
Then there is the Oil Record Book. Part I covers machinery space operations and it should reflect bilge discharges and tank transfers and any failure or malfunction of the oil filtering equipment. The monitor also records its own data which the crew may have to produce on inspection. Gaps in either record raise questions fast.
What Port State Control Looks For
Inspectors usually start with the paperwork. That means the IOPP certificate and the supplement and the Oil Record Book and the calibration certificate. Then they go and look at the equipment itself. Does it match the certificate? Is it working? Can the crew actually demonstrate the alarm and the automatic stop?
That last point catches people out. An inspector might ask the crew to show how the system responds or ask when it was last tested. A crew that cannot answer or has not ever really checked is a bigger red flag than a minor paperwork error. Knowing how your 15ppm bilge behaves is part of the job and not just the engineer’s job.
Who Is Responsible on Board
Day to day the engineers run the system and the chief engineer answers for it. But the master carries overall responsibility and the company’s safety management procedures should spell out who tests the alarm and who signs off the records and what happens when something fails.
That last part is often vague. If the alarm trips at two in the morning does the crew know whether to stop or switch to holding tanks or log it and carry on? Clear procedures and spare parts on board and a crew that has actually practised the response make a bigger difference than any certificate on the wall.
Why Ships Get Detained Over Bilge Alarm Basics
The usual suspects are sensors that are fouled and nobody has cleaned them and calibration certificates that quietly expired and alarms that were disabled during a fault and never switched back on and Oil Record Book entries that do not match what the equipment’s own data shows.
None of these are exotic. They are routine maintenance and discipline failures and they are also what leads to detentions and fines. Fixing them mostly takes a checklist and someone who actually owns it and not a bigger budget.
Get those right and the inspection is mostly a formality. For more on equipment built for these requirements visit Rivertrace.
Frequently Asked Questions
1. Which ships must be fitted with an alarm under MARPOL?
Ships of 10000 gross tonnage and above must have an alarm and automatic discharge stop. Ships from 400 to under 10000 gross tonnage must have oil filtering equipment and most modern units include the alarm anyway.
2. What oil content is allowed in bilge discharge?
No more than 15 parts per million measured in the effluent without dilution and only when the other discharge conditions are also met.
3. Can I discharge in a special area?
Special areas have stricter rules and Antarctic waters prohibit discharge entirely. Check the specific requirements for each area on your route.
4. How often does the oil content meter need calibrating?
Within the interval specified by the manufacturer and the type approval. The calibration certificate should be on board and in date.
5. What do inspectors check first?
Usually the IOPP certificate and supplement and the Oil Record Book and the calibration certificate followed by a check that the equipment matches the paperwork and works as it should.