Learn the basics in this quick to read article

Understanding Oil in Water Monitoring

by Gillian Lovering

Where does oily water come from?

Given the drive to decarbonise shipping that is part of the fight against climate change, it might be thought that the issue of oil pollution at sea could eventually disappear. That scenario is however not likely to occur for a very long time – if ever.

Aside from its use as a fuel, oil has so many other uses ranging from pharmaceuticals through plastics and synthetic fibres to road surfacing. Even the new campaigns against plastics will not diminish the need for them in the majority of applications. Therefore, carriage of crude oil by ship will likely be a fact of life for decades to come.

While the most publicised pollution incidents are usually related to oil cargo spills from damaged vessels, operational use of oil as fuel and for other purposes is a pollution threat from every vessel at sea. Although there are promising replacement fuels for ships at sea these will not become mainstream overnight, and it is likely that oil will be the main fuel for years to come. Oil and oil products will still be needed as lubricants in the engines, pumps, hydraulic systems and other machinery that is essential for a ship to operate.

And where oil is used onboard a ship it will eventually become a waste product that will need to be prevented from polluting the oceans.


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Most of the waste oil will end up mixed with bilge water. This is not a sign of a ‘dirty’ ship, far from it in fact as in the most well-maintained engine rooms, the bilge wells will gradually fill with a mixture of water, oils and chemicals that result from everyday activities such as maintenance and cleaning as well as from other sources.

These other sources include the pipework of machinery that can carry hydraulic fluids as well as both seawater and freshwater, leaking pumps and valve glands, solvents used for cleaning machinery when carrying out repairs, accidental spills, overflowing tanks. Then there is the inevitable rust and general dirt and grime that is always present even in the best maintained ships.

The mix of products in the bilge can mean that the list of contaminants can be long and include some heavy metals and undesirable chemicals albeit it normally only in minute amounts. Below the engine room floor plates there are many pipes serving different systems from heat exchangers, boilers, freshwater generators along with fuel lines between tanks and the engines both main and auxiliary. Bilge can also accumulate in other areas of the ship including in cargo holds and bow thrusters.

At intervals, the mixture accumulated in the bilge wells will be pumped to a holding tank for treatment by the oily water separator (OWS). If this was not done the bilge well would overflow into the engine room causing a severe safety hazard.

Bilge from cargo holds often contains nothing more than cargo residue and hold wash water but may contain oil originating from forklifts used in cargo handling and from hatch cover and crane hydraulic systems. In such cases, the bilge will be pumped to the main bilge holding tank for treatment. There should be strainers within the bilge well to separate oil large solids that have entered the space and so prevent bilge pumps from blockages and more importantly to reduce the material being sent to the oily water separators for treatment.

 On ships fitted with scrubbers and from some NOx reduction systems there will be oily waste that comes from the wash water. The exhaust gases from ships will contain soot, unburnt fuel and other oil-based compounds so as well as washing out the SOx from, the wash water will also become tainted with these oil products and will need cleaning before discharge. More oily water is produced on some tankers when tanks are washed.

Oily water, MARPOL and other regulations

Most would consider MARPOL to be the source of most regulation against pollution and to some extent this is true but oil waste from ships from operational sources had been regulated under an earlier Convention OILPOL since 1958, some 20 years before the first edition of MARPOL was drafted.

Pollution of the seas by oil is covered by Annex I of MARPOL. Because of the recent preoccupation at the IMO with ballast water, energy efficiency and emissions to air, any changes to the requirements of Annex I have generally been related to administrative matters rather than introducing any new requirements.

As well as pollution by oil and related substances covered in ANNEX I of MARPOL, the same topics are also included in the US EPA’s VGP introduced in 2008.

The US also has its own oil pollution prevention and response laws in the form of the Oil Pollution Act 1990, better known as OPA 90. All ships arriving into US waters are obliged to obtain insurance against any possible pollution they may cause. This is generally referred to as ‘OPA 90 insurance’ and can usually be arranged in conjunction with a P&I club.

Prior to the introduction of regulations, all oily water would generally have been disposed of at sea. Today, all vessels above 400gt are required to filter the waste so as to reduce the oil content to a maximum of 15ppm (Canadian rules on the Great Lakes have a maximum of 5ppm) before discharging it at sea. Some classification societies also demand a higher standard of 5ppm to comply with their voluntary Clean Design notations. After filtration, any resultant waste must be retained on board for disposal ashore.

The oil water monitoring on ships filtering is done by a bilge- or oily-water separator – a piece of equipment that has gained an unenviable reputation in recent years. As well as the separator, all vessels subject to the regulation must also be fitted with an oil content monitor (OCM) and bilge alarm to detect if the treated bilge water being discharged meets the discharge requirements. Separators used on board ships are not generally unique pieces of equipment design specifically for marine use but will be versions of separators used in many industries ashore.

It is generally accepted that separators have not performed as well at sea as they do in applications ashore. There are many reasons for this, including the fact that the waste products are less easy to deal with, the conditions at sea with constant movement in many planes affecting operation and the fact that installed systems often lack the capacity to meet the demands placed on them.

As a consequence, they require constant monitoring and frequent cleaning and overhaul, which has made them unpopular with many seafarers. This, coupled with the operators’ desire to reduce the cost of disposing of treated waste ashore, has led to several instances where the separator has been by-passed and waste discharged illegally overboard.

These are the so-called ‘magic pipe’ incidents that lead to regularly-reported prosecutions by port state control regimes and heavy fines and imprisonments especially in the US. It should be noted that the US authorities have no jurisdiction in case of illegal discharges outside of US waters and the fines and imprisonments are not for the acts themselves but for presenting the US authorities with falsified records, most notably the Oil Record Book, which is an offence under US law.

The limit of 15ppm oil allowed in discharges from separators was established in 1993 but did not extend to emulsified oils. Ten years later the requirements were amended to include emulsions with guidelines for equipment performance laid down in MEPC 107(49) introduced in January 2005.

Separator technology

The MARPOL regulations may lay down a maximum limit of oil allowed in discharges, but they leave the means of achieving this open. As a consequence, several technologies are used across the diverse range of separators available.

Early separators were mostly of the gravity separation type that employ plate or filter coalescing technology to separate oil and water. The bilge water is usually heated gently to improve separation with the oil gradually settling out above the water content. The oil is then pumped to the holding tank and the water discharged to sea after passing through the oil content monitor (OCM). Without further refinements, gravity separators can have difficulty in meeting the 15ppm standard especially when the bilge water contains emulsified oils which do not separate easily.

Centrifugal separators also work using the different densities of oil and water but with the centrifuge greatly multiplying the gravity effect as the centrifuge accelerates. This type of separator is more efficient and can generally deal with emulsified oils. They are more compact than gravity-type separators but have the disadvantage of requiring power to operate the centrifuge and, because of their moving parts, often have a higher maintenance requirement.

One way for separator performance to be improved is to add a polishing device into the circuit. Several makers’ current systems include a polishing stage but for older vessels, adding a polishing unit between separator and monitor will improve the performance sufficient to prevent alarms sounding constantly.

Absorption and adsorption are very similar physicochemical processes and can be considered together. In both cases, the bilge water is forced through the sorption media in a reactor or contactor vessel and the oil is removed. When the sorption material has reached its full capacity, it is removed and replaced with fresh material. Some sorption materials can be regenerated onboard, but others will need to be delivered to shore. Popular absorption materials include bentonite and zeolite used as substrates or in cartridges. Typically, 100m3 of bilge water will require 10kg of media.

Flocculation and coagulation make use of an emulsion-breaking chemical to treat emulsions after any free oil has been separated. The chemical breaks down the emulsion and the released oil comes together to form flocks which can then be skimmed off leaving the remaining water to go through further filtration stages. This method tends to produce large amounts of sludge and requires an outlay on the chemical reagent.

Biological treatment employs microbacteria in a bioreactor to literally consume the organic chemicals in the oil, converting it to carbon dioxide and water. It is a slow but effective treatment for oil and emulsions as well as also removing some of the other solvents often found in bilge water. Capital outlay can be high but operating costs are low. Care must be taken to avoid overload on the microrganisms and maintaining operating temperature within the safe range to avoid destroying them.

Membrane technology, ultrafine filtration and reverse osmosis are all physical means of preventing oil and other large molecules from remaining with the water that can pass through the filter barrier. They are efficient but require attention to prevent blocking of the filter or membrane.

Oil Content Monitoring

Under MARPOL regulations, ensuring that oil/water separators function correctly and no discharge exceeds the permitted 15ppm limit, an oil content monitor must be used. The monitor must be fed with a continual flow from the same outlet of the separator as the overboard discharge pipes.

In the event that the monitor detects a level above 15ppm, the separator is either shut down automatically or a valve is operated which sends the outlet water back to the bilges. In normal operation, a monitor may be fooled by suspended solids such as rust and scale which are quite innocuous, but they may not detect the presence of some clear liquid chemicals that could be toxic to marine life when discharged into the sea.

It is also possible to fool the monitor by various deliberate means including closing the valve from the separator so that the sight glass of the monitor is continuously monitoring a clean sample. This may not be possible if the monitor includes a flow-measuring component.

The monitor is a crucial component of separators and is often not an in-house product of the separator maker. There are a small number of specialist monitoring device manufacturers which produce systems for measuring many of the other different discharges permitted from ships such as ballast and scrubber washwater. Many of these monitoring systems have versions that are designed to prevent any deliberate cheating, and some can take a data feed from navigation systems to record precise locations as to where and when separation was carried out.

A number of different technologies are used in oil content monitoring (see the guide)

Scattered Light technology is a popular choice for OCMs on ships because of its relatively low cost and being unaffected by water colour. On the downside, excessive solids can affect readings and the systems must also be calibrated to specific oil types which can lead to deviations if the oil varies. These monitors measure the intensity of light as it passes through water to indicate oil concentration. Oil particles present in the sample water will scatter (or ‘refract’) light in correlation to the levels of oil in water: the higher the oil content, the more light is scattered. Solids will also scatter light, but their irregular shapes do not refract light in the same way as oil’s spherical molecules. Using multiple angles of sensors highlights the shape of the molecules and enable solids to be discounted.

Absorbance technology is not a common choice for shipboard systems due to high cost but they are more accurate approaching laboratory standards of accuracy. Absorbance measurement technologies make use of the fact that different particles absorb light of different wavelengths. Hydrocarbons absorb energy at a specific wavelength (3.4μm) and the amount of that wavelength energy absorbed is proportional to the level of hydrocarbons in a sample. By measuring the amount of energy of specific wavelengths absorbed, the concentration of various oils and solids can be determined.

Fluorescence Is used in some OCMs for bilge water and is also a reliable technology for scrubber washwater monitoring. Fluorescence is an accurate technique for detecting oil in water. Developed in the 1960s to detect contaminants in public water supplies, it can identify particles in the range of parts per billion – a far lower concentration than that encountered in most industrial applications. A straight beam of ultraviolet light at a known wavelength is directed into a glass tube filled with the sample. When oil droplets are excited by ultraviolet light, they will emit light at a different wavelength. This is known as fluorescence light. Each oil molecule has a specific fluorescence wavelength light. In this way, the oil’s presence can be accurately detected. As each species of oil reacts to different light wavelengths, using a single wavelength is ideal in applications dealing with a single, known species of oil. However multiple types of oil and contaminants can lead to inaccurate readings, as they can fluoresce in the same way as the oil being measured.

Microscopy is a good system for measuring oil content in bilge water and as it also detects gas bubbles present it is also useful for scrubber washwater monitoring from both SOx and NOx from EGR systems. Microscopy is the determination of oil concentration by measuring droplet size through image processing and recognition software. A high-speed camera takes pictures of the sample at a rate of several images per second. These images are analysed against a predefined library of components that can determine the visible differences between oil, gas and solids.

Oil Discharge Monitoring Equipment

Large oil tankers above 40,000dwt have been prevented from using cargo tanks for ballast purposes since the early 1980s and must be built with segregated ballast tanks. Smaller product and chemical tankers often do not have tank capacity to segregate ballast. Such vessels are permitted to use seawater ballast in cargo tanks but conditions for discharging the ballast, which will inevitably contain some oil contamination, are strict to reduce the polluting effect to a controlled minimum.

Even though the larger tankers should not have oily ballast, they will still have oily bilge water to dispose of and also the contents of the slop tanks where water used to clean the cargo tanks is stored. This might be disposed of ashore at some ports where shore facilities are provided but in the majority of cases discharge overboard is the only option before loading the next cargo.

As with non-tanker vessels, bilge water if treated alone must comply with the rules laid down in MEPC 107(49). However, in both large and small tankers the discharge of other oily water must be managed according to rules and guidelines set out in IMO resolution MEPC.108(49). Under this resolution, all oil tankers of 150gt and above must have approved Oil Discharge Monitoring Equipment (ODME). A more recent resolution MEPC 240(65) for Bio Fuels, became effective on 1 January 2016.

Unlike the arrangements for bilge water where the discharge of treated water is permitted if the oil content is 15ppm or less, the discharge criteria for other wastewater from tankers requires a more sophisticated computing arrangement. This is because the rate of discharge of oil is limited to 30 litres of oil per nautical mile regardless of ship size and the total quantity of discharge must not exceed 1/30000 of the total quantity of the residue formed cargo.

The technology used to measure the oil content in the water is the same as that used in the 15ppm OCMs of other ship types, but the computing functions and flow rates require additional components and technologies. It is these other components that control the separation and discharge operations. Whereas in the bilge treatment, the operation can continue providing the oil content is not above 15ppm, the ODME must cease the overboard discharge if the 30 litres per nautical mile limit is exceeded (as could happen if the ship is moving slowly) or if the overall limit is reached.

There are other restrictions on discharge beyond those mentioned above and these require that the vessel must be underway, it should not be in any special area where discharge is prohibited, and it must be 50 nautical miles away from any land. These restrictions should be monitored and recorded utilizing a GPS input to the data and the record of ODME operation must be retained on board for a minimum of three years.

Scrubbers and their particular problems

In a scrubber, the sulphur oxides in the exhaust are passed through a water stream reacting with it to form sulphuric acid and are removed from the exhaust gas which then passes out of the system. Sulphuric acid is highly corrosive but when diluted with sufficient alkaline seawater it is neutralised and the wash water can be discharged into the open sea after being treated in a separator to remove any sludge. The alkalinity of seawater varies due to a number of reasons. In estuaries and close to land it may be brackish and closer to neutral and in some areas the water may naturally be slightly acidic.

In the shipping sector, wet scrubbers are divided into two types; open loop and closed loop which were developed separately but which are now usually combined into a hybrid system that can employ the most appropriate technology depending upon prevailing circumstances. In an open loop scrubber seawater is used as the scrubbing and neutralising medium and no additional chemicals are required. The exhaust gas from the engine or boiler passes into the scrubber and is treated with seawater. The volume of seawater will depend upon engine size and power output but equates approximately to around 40m3 per MWh.

An open loop system can work perfectly satisfactorily only when the seawater used for scrubbing has sufficient alkalinity. Fresh water and brackish water are not effective, and neither is seawater at high ambient temperature. For this reason, an open loop scrubber is not considered as suitable technology for areas such as the Baltic where salinity levels are not high. MARPOL regulations require the wash water to be monitored before discharge to ensure that the PH value is not too low.

A closed loop scrubber works on similar principals to an open loop system but instead of seawater it uses fresh water treated with a chemical (usually sodium hydroxide but some systems others) as the scrubbing media. This converts the SOx from the exhaust gas stream into harmless sodium sulphate. Unlike the flow through method of open loop scrubbers, the wash water from a closed loop scrubber passes into a process tank where it is cleaned before being recirculated. The fresh water can either be carried in tanks or else produced on board if a freshwater generator is installed on the ship.

The hybrid system is a combination of both wet types that will operate as an open loop system where water conditions and discharge regulations allow and as a closed loop system at other times. Hybrid systems are proving to be the most popular because they can cope with every situation.

A wet scrubber will produce around 0.6 tonnes of sludge for every 100 tonnes of fuel used in the engines. This is not permitted to be discharged to the sea and must be retained. A closed loop scrubber system may produce more sludge. In a ship, without a scrubber, much of what becomes sludge would have been released into the atmosphere and either remain there or end up deposited in the ocean or on land.

Even after treatment for sludge removal on board, some residues will be present in the washwater. Some of this will be unburned fuel but at levels well below the 15ppm or 5ppm detectable by oil content monitors used for bilge treatment monitoring.

Washwater will normally contain some metals which may be undesirable, but their source is debatable. Vanadium – the most commonly found metal in washwater – is likely to have been present in the fuel used and would have been emitted in non-scrubber equipped ships in at least the same quantities.

Many of the other metals were almost certainly present in the seawater taken in by the scrubber. This is especially true of zinc and copper which are found in anodic protection systems and antifoulings on ships’ hulls and which will be deposited in the ocean by all ships whether scrubbers are installed or not.

Another component of washwater is Polycyclic aromatic hydrocarbons (PAHs). These are organic compounds with two or more fused aromatic rings. PAHs occur naturally in oil and are also produced as by-products of fuel combustion.

Some of these compounds are carcinogenic and can accumulate in edible shellfish, which gives them a pathway to humans and other species. It is acknowledged that PAHs can form due to incomplete combustion of fuel oils and although engines and boilers are designed to optimise the combustion of fuel, exhaust gases will always contain a proportion of incompletely combusted material.

Although some PAHs will be present in the washwater, many of the heavier and more toxic types will be removed especially if they are bound to soot and particulate matter. By contrast, any PAHs present in the exhaust of an unscrubbed ship will be released directly into the atmosphere and will be deposited naturally in the ocean. It should also be noted that PAHs found in the oceans could also have come from non-maritime uses even far inland where they will be washed to the sea by rivers and land runoff.

For a scrubber system, the IMO guidelines require that the washwater discharged from an open loop scrubber is within a certain range of the water taken in for scrubbing use.

Because of this, the monitor must measure more than one characteristic. An ideal monitoring system for a scrubber would measure the polycyclic aromatic hydrocarbon (PAH), turbidity, temperature and pH at both the inlet and the outlet points although under current rules only the pH needs to be measured at the inlet.  

The pH value is of particular importance because if there is too great a variance between inlet and outlet readings, the pH of the washwater must be adjusted. This can be done by neutralisation or by dilution. The former involves adding chemicals and the latter the pumping of large volumes of seawater. 

Washwater monitoring

Scrubber washwater discharges to sea must be monitored but rather than monitoring the specific emissions rate of SO2 in g/kW h, the ratio of parts per million-sulphur dioxide to percentage-carbon dioxide (SO2 ppm/CO2 %) is allowed.

The guidelines first appeared as MEPC.170(57) in 2008. In 2009 a revised version of the Guidelines for Exhaust Gas Cleaning Systems, – IMO Resolution MEPC.184(59), was adopted and replaced 170(57) in July 2010. This reflected changes to Annex VI and included SO2/ CO2 ratios relating to various levels of sulphur-in-fuel as by the Emission Control Areas were established with different rates than the global cap. A further version MEPC.259(68), was adopted on 15 May 2015.

Opponents of scrubbers have pushed for scrubbers to be banned or more severe restrictions put on their use. Some countries and individual ports have been swayed by these arguments and imposed bans in territorial waters or defined areas. As of yet, no national flag has imposed a ban on ships under it. The list of restrictions is lengthening and although it may be of nuisance value to the shipowner, the amount of time spent on the high seas is now where the economics of scrubber use is determined. After a voyage of several days or weeks, half a day in territorial waters at destination ports where the scrubber cannot be used is of small consequence.

Bleed-off from EGR systems used for controlling NOx has always been subject to the 15ppm oil content limitation if it was to be discharged to sea and with the introduction of Tier III this was required to be done by an OCM meeting the MEPC.107(49) guidelines.

In view of the fact that when the new Baltic and North Sea NOx ECAs were adopted, the IMO was planning to introduce the Global sulphur cap in 2020 rather than in 2025, it was judged a good time to bring the guidelines up to date. In 2018, MEPC.307(73) adopted Guidelines for the discharge of exhaust gas recirculation (EGR) bleed-off water, valid for ships with an engine international air pollution prevention (EIAPP) certificate issued after June 2019.

The EGR wastewater handling regulation calls for the specific handling of condensate of exhaust gas depending on the fuel oil sulphur content before it is discharged overboard as bleed-off water. Ships that are operating with 2020 sulphur compliant fuels are obliged to be fitted with an OCM type-approved to MEPC.107(49) or (if the bleed-off water is combined with the washwater from a SOx scrubber) an MEPC.259(68)-compliant meter that measures PAHs, turbidity and pH. Ships that are SOx scrubber fitted and using fuel with a sulphur content above 0.5% must be fitted with an OCM that is MEPC.259(68) compliant.