An oil-in-water monitor is an instrument that continuously measures the concentration of oil in a water stream. It is used wherever oil must be detected in water before that water is discharged, processed or reused from the bilge water systems of marine vessels to the produced water treatment packages of offshore FPSOs, and from industrial cooling water circuits to exhaust gas scrubber wash water systems.
This article explains what an oil-in-water monitor is, how the different measurement technologies work, when each type is used and what the key terms mean written for engineers, purchasing managers and ship operations teams who need a clear, plain-English starting point.
| Key thing to remember An oil-in-water monitor does not just measure it also acts. A correctly specified and integrated monitor automatically stops discharge if oil concentration exceeds the permitted limit, records the discharge data for the compliance record, and activates an alarm to alert operators. The measurement is only one part of the system. |
What Is an Oil-in-Water Monitor?
An oil-in-water monitor also referred to as an oil content monitor (OCM), an oil-in-water analyser or a bilge alarm is an instrument that measures the concentration of oil suspended or dissolved in a water stream, typically expressed in parts per million (ppm).
The monitor draws a sample of the water being monitored, passes it through a measurement cell, and analyses it for the presence of oil using one of several optical measurement technologies. The result is a continuous, real-time reading of oil concentration in the water stream.
In regulatory applications such as MARPOL bilge water discharge monitoring or FPSO produced water discharge monitoring the monitor is connected to a discharge shut-off valve. If oil concentration rises above the permitted limit, the monitor automatically closes the valve to stop discharge and logs the event in the compliance record.
What Are the Main Types of Oil-in-Water Monitor?
Oil-in-water monitors use several different measurement technologies, each with different strengths, limitations and applications. The right technology depends on the water stream being monitored, the oil types expected to be present and the regulatory standard that must be met.
| Technology | How it works | Strengths and limitations | Typical applications |
| Light scattering | A beam of light passes through the water sample; oil droplets scatter the light and the scattered signal is correlated to oil concentration | Widely used; cost effective; susceptible to interference from gas bubbles and solid particles | General marine bilge water, cooling water monitoring |
| Optical microscopy | A camera captures images of individual particles in the water; software classifies each particle as oil, gas or solid based on size, shape and optical properties | Highly accurate; differentiates oil from gas and solids; more complex than light scattering | FPSO produced water, OWS discharge, drill rig applications |
| Ultraviolet fluorescence | UV light causes hydrocarbon compounds to fluoresce; the fluorescence signal is correlated to oil concentration | Sensitive to dissolved and dispersed aromatic hydrocarbons; less sensitive to non-aromatic oils | Exhaust scrubber wash water monitoring, process water with aromatic hydrocarbons |
| Infrared absorption | Infrared light is absorbed differently by oil and water; the difference is used to calculate oil concentration | Accurate across a range of oil types; suitable where water composition is stable | Laboratory analysis and some in-line process water applications |
In practice, most marine and offshore oil-in-water monitoring applications use light scattering or optical microscopy technology. The choice between them depends primarily on whether the water stream contains gas bubbles or solids that could cause false readings in a simple light scattering monitor.
How Does an Oil-in-Water Monitor Work Step by Step?
The measurement process in a typical optical oil-in-water monitor works as follows:
- A sample of the water stream is drawn from the process pipe at the monitoring point and directed into the monitor through a sample conditioning system
- The sample conditioning system reduces pressure, stabilises temperature and removes large solids before the sample reaches the measurement cell ensuring the sample is within the monitor’s specified operating range
- Inside the measurement cell, the sample passes through a beam of light. Oil droplets in the water scatter or absorb the light differently from clean water, producing a measurable signal
- The monitor’s electronics convert this signal into an oil concentration reading in ppm, which is displayed on the instrument panel and transmitted to the control system via an analogue output
- If oil concentration is within the permitted limit, the discharge valve remains open and monitoring continues
- If oil concentration rises above the alarm set point, the monitor activates an alarm, closes the discharge shut-off valve automatically and logs the event with a timestamp
- All readings, alarm events and discharge operations are recorded continuously in the data log, which forms part of the compliance record
| What is parts per million? Parts per million (ppm) is the unit used to express oil concentration in water. One ppm means one milligram of oil per litre of water or one millilitre of oil per 1,000 litres of water. The MARPOL bilge water discharge limit of 15ppm is therefore 15 milligrams of oil per litre of processed bilge water. This is a very low concentration, which is why accurate calibration and regular maintenance of the monitor are essential. |
What Is the Difference Between a Bilge Alarm and a Produced Water Analyser?
The terms bilge alarm, oil content monitor, oil-in-water monitor and produced water analyser are all used to describe instruments that measure oil in water but they are not interchangeable. Each type is designed for a different water stream, a different regulatory standard and different operating conditions.
15ppm bilge alarm
A 15ppm bilge alarm is a type-approved oil content monitor designed specifically for monitoring oily water separator discharge from a vessel’s machinery space under MARPOL Annex I and MEPC.107(49). It measures oil concentration against the 15ppm discharge limit and automatically stops discharge if the limit is exceeded. It is the most common type of oil-in-water monitor on marine vessels.
MEPC.107(49) approved oil-in-water monitor
A SMART PFM 107 type monitor is also MEPC.107(49) approved but uses more advanced measurement technology optical microscopy that differentiates between oil droplets, gas bubbles and solid particles. This makes it suitable for water streams that are more complex than standard bilge water, including produced water from FPSO topsides systems and oily water separator discharge on vessels with more variable bilge water composition.
Produced water analyser
A produced water analyser such as the OCD Xtra is designed for the more complex and variable composition of FPSO produced water, which may contain multiple crude oil types, gas bubbles, suspended solids and production chemicals. It measures oil concentration across a broader range (0 to 200ppm) and can be calibrated on up to six different oil types, making it suitable for applications where crude composition varies. For a full explanation of how produced water monitoring works on an FPSO, see the Produced Water Discharge Monitoring for FPSOs explainer page.
What Are the Key Components of an Oil-in-Water Monitoring System?
An oil-in-water monitor is not a standalone instrument it is part of a wider monitoring system. The complete system typically includes:
- Sample tap and isolation valve draws a representative sample from the process pipe at the correct monitoring point
- Sample conditioning system reduces pressure, stabilises temperature, controls flow rate and removes large solids before the sample reaches the measurement cell
- Oil-in-water analyser the instrument itself, containing the measurement cell, light source, detector and electronics
- Alarm and signal outputs discrete alarm output to the control room; analogue output (typically 4 to 20mA) transmitting the oil concentration reading to the DCS or ICSS
- Discharge control valve connected to the monitor’s alarm output; closes automatically when oil concentration exceeds the alarm set point
- Data logging system records all readings, alarm events and discharge operations with timestamps for the compliance record
All components must be correctly specified, installed and integrated for the monitoring system to function as a compliance tool. An analyser that is installed but not connected to the discharge shut-off valve cannot prevent a compliance breach automatically.
What Is Calibration and Why Does It Matter?
An oil-in-water monitor must be calibrated against the specific oil type present in the water stream being monitored. Different oils have different optical properties they scatter or absorb light differently so a monitor calibrated against one oil type may give inaccurate readings when the oil type changes. For a full explanation of calibration requirements and methods, see Calibration of Oil Content Monitors.
Calibration is not a one-time event. All oil-in-water monitors require periodic recalibration to maintain accuracy, and several events can trigger a requirement for early recalibration:
- A change in crude oil type or composition in the water stream
- Consistent discrepancy between monitor readings and laboratory grab sample results
- Maintenance or replacement of the measurement cell or sensor
- Expiry of the calibration interval specified by the manufacturer or the applicable regulation
| An uncalibrated monitor cannot provide a defensible compliance record A monitor that has not been calibrated against the correct oil type may give readings that are systematically higher or lower than the actual oil concentration. This means the compliance record may show the discharge was within the permitted limit when it was not or it may trigger unnecessary alarms. Calibration is a regulatory obligation, not a maintenance option. |
In Which Applications Are Oil-in-Water Monitors Used?
Oil-in-water monitors are used wherever oil concentration in water must be measured for compliance or process protection purposes. The most common applications across marine, offshore and industrial environments include:
- Bilge water discharge monitoring on marine vessels under MARPOL Annex I and MEPC.107(49)
- Produced water discharge monitoring on FPSOs and offshore platforms under field permit conditions
- Oily water separator outlet monitoring on marine vessels and offshore installations
- Cooling water monitoring for heat exchanger leak detection in marine and industrial systems
- Boiler feed water and condensate monitoring for oil ingress detection
- Deck drain discharge monitoring on offshore platforms and FPSOs
- Slop tank wash water discharge monitoring on tankers and FPSOs
- Hydrocarbon storage tank drainage monitoring
- Exhaust gas scrubber wash water monitoring under MEPC.259(68)
- Industrial process water monitoring for hydrocarbon leak detection
How Do You Select the Right Oil-in-Water Monitor?
The right oil-in-water monitor depends on several factors that must be confirmed before any equipment is selected:
- What water stream is being monitored bilge water, produced water, cooling water, wash water or process water?
- What regulatory standard or permit applies to the discharge from this stream?
- What oil types are present or likely to be present in the water stream?
- What oil concentration must be measured and what alarm threshold is required?
- Are gas bubbles or suspended solids present in the water stream that could cause false readings in a simple light scattering monitor?
- What are the temperature, pressure and flow conditions at the monitoring point?
- Does the monitor need to interface with a discharge control valve or control system?
- Is hazardous area certification required for the installation location?
Rivertrace provides application engineering support to help operators, EPC teams and purchasing managers select the right oil-in-water monitor for their specific application from bilge water compliance monitoring on marine vessels to produced water discharge monitoring on FPSO projects.
Frequently Asked Questions
What is an oil-in-water monitor?
An oil-in-water monitor is an instrument that continuously measures the concentration of oil in a water stream in real time, expressed in parts per million (ppm). It is used to monitor water quality before discharge or reuse, to detect hydrocarbon leaks in process and cooling water systems, and to demonstrate compliance with MARPOL and permit discharge limits. It is also referred to as an oil content monitor, OCM or oil-in-water analyser.
How does an oil-in-water monitor measure oil concentration?
Most oil-in-water monitors use optical measurement technology. A light beam is passed through a sample of the water stream in a measurement cell, and the way the light is scattered or absorbed by oil droplets in the water is used to calculate oil concentration. Different technologies light scattering, optical microscopy and ultraviolet fluorescence are suited to different water streams and applications.
What is the difference between a 15ppm bilge alarm and a produced water analyser?
A 15ppm bilge alarm is a type-approved monitor designed for bilge water discharge from a vessel machinery space under MARPOL Annex I and MEPC.107(49). It measures against the 15ppm discharge limit. A produced water analyser is designed for the more complex composition of FPSO produced water, which may contain multiple crude types, gas and solids, and is governed by field permit conditions rather than a single global MARPOL standard.
Why does an oil-in-water monitor need to be calibrated?
Different oil types have different optical properties, so a monitor calibrated on one oil type may give inaccurate readings when monitoring water containing a different oil. Calibration establishes the correct relationship between the monitor’s measurement signal and the actual oil concentration for the specific oil type present in the water stream. Regular recalibration is required to maintain accuracy over time.
What happens when an oil-in-water monitor reads above the permitted limit?
When oil concentration exceeds the alarm set point, the monitor activates an alarm to alert operators and where correctly integrated with process controls automatically closes the discharge shut-off valve to stop the discharge. The alarm event is logged with a timestamp in the compliance record. Discharge can only resume when oil concentration returns to within the permitted limit and the cause of the high reading has been investigated.