Environmental Technology

The Oil/Water separator

Malcolm Latarche
Malcolm Latarche

09 November 2018

The Oil/Water separator

The first demand as regards the operational waste oil from machinery is that the ship must be fitted with adequate holding tank capacity for any waste that cannot be dealt with by way of discharge or incineration. Most ships generate large amounts of oily waste (waste contaminated with oil) and waste oils (such as spent lubes or sludge from fuel and lube treatment systems).

As well as the oil in the bilge water there will be grease, detergents and cleaning fluids along with contaminants that may have been removed from fuel and lube treatment systems, some of these may present more of a hazard to the marine environment than oil does. Prior to the introduction of regulations, all of this waste 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 and resultant waste must be retained on board for disposal ashore.

The 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 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.

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 and crew members may find themselves having to operate and service unfamiliar those that use membranes, flocculation or absorption filters means valuable time must be spent searching out manuals and attempting to make sense of them.

A choice of system technology

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 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.

Many crew members are familiar with this type of equipment which is also used for preparing fuels and lubes before use by removing sludge and homogenising the fuel or lube. 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.

Other technologies are also used for cleaning bilge water including absorption and adsorption, flocculation, biological and membrane separation. Absorption and adsorption are very similar physicochemical processes and for the purpose of this guide 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.

Avoiding problems with separators begins long before the device is switched on and involves a proper plan for managing waste streams and doing as much as possible to prevent emulsions forming especially if they are chemical emulsions resulting from the use of cleaning chemicals and detergents.

So called primary emulsions in which larger drops of oil are dispersed in water generally separate through gravity within 24 hours. Secondary emulsions caused by turbulent conditions where oil droplets are very fine become stable and will not separate easily. Solid material should also be prevented from contaminating the bilge as much as possible. Not only does it promote emulsification it also creates false alarm situations and shuts down the separator requiring crew intervention to restart the separation process. Filters and removal of solids before treatment will allow the separator to operate more effectively and for longer.

Oil-in-water monitors may be fooled by suspended solids such as rust and scale which are quite innocuous but they may not detect the presence of some chemicals which could be toxic to marine life when discharged into the sea. The monitor is a crucial component of separators and is often not an in-house product of the separator maker.