Any vessel operating at sea produces bilge water and under international law, that water cannot be discharged overboard without being monitored. The instrument that makes this monitoring possible is the 15ppm bilge alarm.
For ship operators, technical superintendents and purchasing teams, understanding what a 15ppm bilge alarm is, what it does and when it is legally required is a fundamental part of MARPOL compliance. This article explains everything you need to know.
| Key thing to remember A 15ppm bilge alarm is a type-approved oil content monitor fitted to the outlet of a vessel’s oily water separator. It continuously measures the oil content of the processed bilge water and automatically stops discharge if oil concentration exceeds 15 parts per million the limit set by MARPOL Annex I. |
What Is a 15ppm Bilge Alarm?
A 15ppm bilge alarm — also known as an oil content monitor (OCM) or bilge monitor — is a type-approved instrument designed to continuously measure the concentration of oil in bilge water being discharged overboard from a vessel’s oily water separator (OWS).
The name refers to the discharge limit it enforces: under MARPOL Annex I, processed bilge water may only be discharged overboard if the oil content does not exceed 15 parts per million. The bilge alarm monitors this continuously and, if the oil content rises above 15ppm, it automatically activates a shut-off valve to stop the discharge.
The technical requirements for 15ppm bilge alarms are set out in IMO resolution MEPC.107(49) which defines the performance standards, test methods and type approval requirements that all bilge alarms must meet before they can be used on MARPOL-regulated vessels.
What Is Bilge Water and Why Does It Need to Be Monitored?
Bilge water is the water that accumulates in the lowest part of a ship’s hull the bilge from equipment leaks, condensation, drainage from machinery spaces and water used in cleaning. It typically contains oil, fuel residues, lubricants, detergents and general machinery contamination.
Because bilge water is contaminated with oil, it cannot be discharged directly overboard. It must first be processed through an oily water separator (OWS), which uses gravity, coalescence and sometimes filtration to reduce the oil content to below 15ppm.
The 15ppm bilge alarm sits at the outlet of the OWS and verifies that the separated water meets the MARPOL discharge standard before it reaches the sea. Without a functioning, calibrated and type-approved bilge alarm, the vessel cannot legally discharge processed bilge water overboard. For more on the regulatory framework that governs this, see the MARPOL regulation review.
How Does a 15ppm Bilge Alarm Work?
A 15ppm bilge alarm works by continuously drawing a sample of the water stream from the OWS outlet and measuring its oil content using one of several measurement technologies most commonly optical or light-scattering methods that detect the presence and concentration of oil droplets in the water.
The measurement process typically works as follows:
- A small sample of processed bilge water is drawn from the OWS outlet pipe into the monitoring cell
- The oil content of the sample is measured continuously using the monitor’s sensor technology
- The oil concentration reading is displayed on the instrument panel in parts per million
- If the reading remains below 15ppm, the discharge valve stays open and discharge continues
- If the reading rises above 15ppm, the alarm activates and the automatic shut-off valve closes, stopping discharge
- The discharge data time, date, oil concentration and any alarm events is recorded for the Oil Record Book
The alarm must respond quickly enough to prevent any significant quantity of oil-contaminated water from being discharged above the limit. MEPC.107(49) sets specific response time requirements that type-approved bilge alarms must meet.
| How 15ppm is measured 15 parts per million means 15 milligrams of oil per litre of water or approximately 15 millilitres of oil per 1,000 litres of water. This is a very low concentration, which is why the bilge alarm must be sensitive, accurately calibrated and regularly maintained to provide reliable readings at this level. |
When Is a 15ppm Bilge Alarm Required?
Under MARPOL Annex I and the associated MEPC.107(49) resolution, a type-approved 15ppm bilge alarm is required on all ships of 400 gross tonnage and above. The full set of conditions for bilge water discharge and the equipment requirements that accompany them are summarised in the table below.
| Requirement | Detail |
| Vessel type | All ships of 400 gross tonnage and above; all ships certified to carry 200 cubic metres or more of oil in bulk |
| Discharge location | More than 12 nautical miles from the nearest land |
| Vessel condition | Proceeding en route not at anchor or stationary |
| Oil content limit | No more than 15 parts per million (ppm) of oil in the discharge |
| Monitoring equipment | Type-approved 15ppm bilge alarm meeting MEPC.107(49), fitted to the oily water separator outlet |
| Automatic shut-off | The bilge alarm must automatically stop discharge if oil content exceeds 15ppm |
| Oil Record Book | All bilge water discharge operations must be recorded in the Oil Record Book (Part I) |
| Prohibited areas | Discharge is prohibited in special areas under MARPOL Annex I regardless of oil content |
Ships operating in MARPOL special areas including the Mediterranean Sea, Baltic Sea, Black Sea, Red Sea, Gulfs area, Gulf of Aden, Antarctic area and North West European Waters face additional restrictions. In some special areas, the discharge of any bilge water is prohibited regardless of oil content.
What Are the Type Approval Requirements for a 15ppm Bilge Alarm?
A 15ppm bilge alarm must be type-approved by a flag state administration or recognised organisation before it can be fitted to a MARPOL-regulated vessel. Type approval confirms that the equipment meets the performance standards set out in MEPC.107(49), including:
- Measurement accuracy — the alarm must accurately measure oil concentration across the required range, including at concentrations below 15ppm
- Response time — the alarm must respond quickly enough to prevent discharge of oil-contaminated water above the limit
- Automatic shut-off — the alarm must be connected to and capable of activating the OWS discharge shut-off valve
- Recording capability — the alarm must be capable of providing a record of discharge operations for the Oil Record Book
- Robustness — the alarm must function reliably in the marine environment, including variations in temperature, pressure and water characteristics
Type approval certificates are issued by flag state administrations or classification societies acting on their behalf. When purchasing a 15ppm bilge alarm, purchasers should confirm that the equipment holds a valid type approval certificate for the vessel’s flag state.
What Are the Calibration Requirements for a 15ppm Bilge Alarm?
A 15ppm bilge alarm must be calibrated regularly to maintain measurement accuracy. The calibration requirements are set out in MEPC.107(49) and the associated calibration guidelines, which specify:
- Calibration certificates must be checked at IOPP annual, intermediate and renewal surveys
- The accuracy of the 15ppm bilge alarm must be verified by the manufacturer or a person authorised by the manufacturer
- Calibration must take place at intervals not exceeding five years after commissioning, or within the timeframe specified in the manufacturer’s instructions whichever is shorter
- As an alternative to calibration, the bilge alarm may be replaced with a newly calibrated unit
- The calibration certificate, showing the date of the last calibration check, must be retained onboard for inspection
Failure to maintain a valid calibration record is one of the most common findings during port state control inspections and IOPP surveys. An expired or missing calibration certificate can result in deficiency notices, detention or enforcement action even where the equipment itself is functioning correctly.
| Calibration is a compliance obligation, not a maintenance option A 15ppm bilge alarm that has not been calibrated within the required interval cannot be used to demonstrate MARPOL compliance. Vessels found with out-of-calibration or uncertified bilge alarms during port state control inspection or IOPP survey are at risk of detention, deficiency notices and enforcement action. |
Which Rivertrace Product Is the Right 15ppm Bilge Alarm for My Vessel?
Rivertrace offers two type-approved oil content monitors suitable for 15ppm bilge alarm applications, depending on the vessel type and the complexity of the bilge water stream:
SMART BILGE
SMART BILGE is Rivertrace’s dedicated 15ppm bilge alarm, designed for oily water separator discharge monitoring on standard marine vessels. It is type-approved to MEPC.107(49), provides continuous oil concentration measurement, automatic discharge shut-off and Oil Record Book recording capability. It is suitable for machinery space bilge water applications across a wide range of vessel types — from general cargo ships and tankers to offshore support vessels and naval vessels.
SMART PFM 107
SMART PFM 107 is an MEPC.107(49)-approved oil-in-water monitor that uses optical microscopy to differentiate oil particles, gas bubbles and solids in the 1–500 micron range. It is suitable for applications where the bilge water stream contains gas bubbles or solids that could cause false readings in a standard bilge monitor — including produced water applications, drill rig slop tanks and oily water separator discharge on vessels with complex machinery space drainage systems.
| Not sure which product is right for your application? The correct bilge alarm depends on the vessel type, the complexity of the bilge water stream and the flag state type approval requirements. Rivertrace can advise on the correct product selection for your specific application. |
What Should Ship Operators Check to Maintain 15ppm Bilge Alarm Compliance?
For ship operators and technical superintendents, maintaining 15ppm bilge alarm compliance requires ongoing attention to both the equipment and the documentary record. Key checks include:
- Confirm the bilge alarm is type-approved for the vessel’s flag state and that the type approval certificate is held onboard
- Confirm the calibration certificate is current and will not expire before the next IOPP survey or port state control inspection
- Verify that the automatic shut-off valve is connected to and activated by the bilge alarm
- Confirm that the Oil Record Book entries for bilge water discharge are complete, accurate and consistent with actual operations
- Check that the alarm response time meets the MEPC.107(49) requirement
- Confirm that crew responsible for OWS and bilge alarm operation are trained in alarm response, manual override procedures and routine maintenance
- Arrange calibration or equipment replacement before the five-year calibration interval expires
Frequently Asked Questions
What is the 15ppm limit in MARPOL bilge water discharge?
15 parts per million (ppm) is the maximum oil concentration permitted in bilge water discharged overboard under MARPOL Annex I. It applies to processed bilge water from vessel machinery spaces after treatment through a type-approved oily water separator. The bilge alarm monitors this concentration continuously and stops discharge automatically if the limit is exceeded.
Which ships are required to have a 15ppm bilge alarm?
Under MARPOL Annex I, all ships of 400 gross tonnage and above are required to have a type-approved 15ppm bilge alarm fitted to the oily water separator outlet. The alarm must meet the technical requirements of IMO resolution MEPC.107(49).
How often does a 15ppm bilge alarm need to be calibrated?
Under MEPC.107(49) calibration guidelines, the accuracy of a 15ppm bilge alarm must be verified at intervals not exceeding five years after commissioning, or within the interval specified in the manufacturer’s instructions whichever is shorter. Calibration certificates must be checked at IOPP annual, intermediate and renewal surveys and retained onboard for inspection.
What happens if a vessel is found with an uncalibrated or non-functional 15ppm bilge alarm?
A vessel found with an uncalibrated, out-of-certificate or non-functional 15ppm bilge alarm during port state control inspection or an IOPP survey is at risk of deficiency notices, detention and enforcement action. An uncertified bilge alarm means the vessel cannot demonstrate MARPOL compliance for bilge water discharge.
Can a 15ppm bilge alarm be used for FPSO produced water discharge?
A standard 15ppm bilge alarm is designed for oily water separator discharge from a vessel machinery space not for produced water discharge from an FPSO topsides process system. Produced water has a more complex and variable composition that can cause unreliable readings in a standard bilge monitor. A specialist produced water analyser such as SMART PFM 107 or OCD Xtra is required for FPSO produced water applications.