Waste oily water is a by-product created by ocean going vessels which runs into millions of tonnes annually. A mixture of water, oily fluids, lubricants and greases accumulate in the lowest part of each vessel known as the bilges. The fluids come from a variety of sources including engines, piping and other mechanical and operational equipment required on board. Bilges are periodically pumped out to holding tanks which can be transferred to a facility on shore, or the contents can be processed with a separator on board and discharging clean water back to the ocean.
Discharge to the ocean is highly regulated, and the oil content of such water must be accurately monitored and remain within set limits. Bilge water monitors provide this data.

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An uncalibrated or faulty oily water monitor can result in operational and unexpected downtime, along with substantial fines or impounding if discharge water contamination exceeds legal limits. 

With a wealth of Oily Water Separator (OWS) monitoring system experience, AtZ are able to fully manage your calibration schedule as well as manage complete system lifecycle through upgrades and extension measures. 

AtZ offer calibration services or annual calibration and maintenance contracts as a flexible, cost-saving option to ensure your systems continues to operate effectively and meets legislation at all times.

Which vessels have it?

Under MARPOL regulations, all ships over 400 gross tonnes are required to have equipment fitted which measures and limits the discharge of oil into the ocean to a maximum concentration of 15ppm. Some regions including Canada enforce stricter limits, where a limit of 5ppm is set on the Great Lakes. Typically, the hydrocarbon content in a vessel bilge will be in the range of 100-500ppm before any form of processing.

What are the rules and consequences of non-compliance?

There are two versions of rules, applicable based on the construction date of the vessel and equipment. These are:

  • MEPC 60(33) – up to 2005
    • Requires unit to be operational and working within +/- 5ppm accuracy
    • Calibration can be performed by user
    • No regular re-certification requirements
    • Can be repaired, but if replaced – must be with newer equipment standard below
  • MEPC 107(49) – from 2005 onwards
    • Requires unit to be operational and working within +/- 5ppm accuracy
    • Functional test can be performed by user (note this is not a measure of accuracy)
    • Calibration only to be performed by OEM or their appointed agent, certificate issued
    • Recertification required in at least year of vessel IOPP survey (5 yearly)
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Between 2004 and 2014 in the US alone, more than US$200 million was paid by vessel owners in penalties. In addition, more than 17 years of jail time was given to offenders – most of whom were seafarers.

In cases on non-compliance identified by Port State inspections, Coastguard and Harbour authorities – an immediate halt is usually called to vessel operations, and in some cases vessels are impounded until regulations are met and fines paid.

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What is the equipment and how does it work?

Before discharge, oily water on board is first processed through a separator. Either of a static, or centrifugal type – it works to separate the oil from water so that it can be safely discharged. Remaining oil is kept on board for later disposal.

In case the output from the separator is not of a sufficiently low oil content to discharge, the output is diverted and recycled through the system to remove further oil to allow discharge.

Many different techniques exist for measurement of water oil content, but the most common is using light scattering. A representative sample of the discharge water passes through a glass sample tube through which a beam of light passes. Oil droplets in the sample cause refraction of the light and to an extent related to the oil concentration. Based on these changes, an instrument read-out is given.

How is the equipment maintained?

As a delicate optical measuring instrument, accurate readings can only be taken if the equipment is in a good state of repair and clean. The sample tube must be kept free of oily deposits – usually using a small cleaning brush and mildly abrasive paste. Where iron oxide deposits accumulate on the glass from upstream corroded pipework, this can be removed by soaking the instrument in a mild citric acid solution, allowing the deposits to dissolve in fresh water.

For units to MEPC 60(33) a calibration is performed by flooding the measuring cell with a known concentration of simulated oily water sample, which the cell ‘learns’.

For units to MEPC 107 (49) a calibration is performed using a simulated oily water sample of known concentration and a reference cell. The signal gain of the vessel measuring cell is then adjusted to read the same value as the reference cell. For this reason, ONLY the OEM or their appointed Agent with the required equipment can carry out a calibration.

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How we can help?

AtZ are official representatives of Deckma Hamburg and can supply new units, spares, service and of course calibration services to our customers. We have a significant quantity of spare parts, as well as service exchange units which can be used to assure you continuous cover of monitoring capability as required by regulations.


My system is showing a high oil concentration even with a freshwater sample, what is wrong?

Most likely cause is that the sample tube is contaminated with oil and iron deposits. Follow the manufacturer recommendations for cleaning the tube and flushing thoroughly. As the tube gets cleaner, you should see the PPM readings decrease gradually to zero. Worst iron contamination is best removed with soaking the sample tube of the measuring cell overnight in a dilute citric acid solution.
Cleaning should be part of a weekly routine. If the unit is allowed to get too dirty, it may have to be returned for a rebuild and new glass element.

Can I calibrate the unit myself?

For units fitted before 2005 and that are to standard MEPC 60(33), yes
For units fitted after 2005 and to MEPC 107(49) – No, they need to be attended or returned to the manufacturer or Agent for calibration and certification.
Be aware that companies offering a calibration check certificate cannot meet the requirements of MEPC 107(49) and without the manufacturer approved certificate, you will fail inspection.

What is the difference between a functional check and calibration for MEPC 107(49) instruments?

A functional check uses a simulated oily water fluid which causes the measuring cell to activate a high PPM alarm. It will demonstrate correct operation of the measuring cell, alarms and 3-way valve on board. It is ideal for demonstrating correct operation to an inspector. IT DOES NOT constitute a calibration as the instrument is not being adjusted with a reference sample. Any functional check or calibration check certificate issued by an attending company DOES NOT satisfy IMO requirements for calibration.
A calibration requires a calibrated sample fluid and reference cell. OEM and Dealer only software allows adjustment of the vessel measuring cell to match the reference cell value. A calibration certificate is issued by the OEM / Agent to confirm this has been completed.

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