Oily water separator
|This article does not cite any references or sources. (November 2006)|
An oil water separator (OWS) is a piece of shipboard or in general waste water treatment equipment that separates oil from oily waste water such as bilge water before the waste water is discharged.
Bilge water is an almost unavoidable product in ship operations. Bilge water that is generated in proximity to shipboard equipment (such as in the engine room) often contains oil and its direct discharge would result in undesirable transfer of waste oil to the marine environment. By international agreement under the MARPOL convention, most commercial vessels need to be fitted with an oily water separator to remove oil contaminants before bilge water is pumped overboard.
Oily water separator equipment has been a shipboard requirement since the 1970s but recently it has become evident that oily water separators have not been as effective as had been assumed, and alleged improper operation of this equipment by crewmembers (sometimes called the magic pipe) has resulted in criminal prosecutions in the United States and to a lesser extent in Europe.
There are different technologies that can be used to separate oil from waste water. Most economic are those based on the gravity effect, where oil droplets are separated from the water by floating to the surface due to the difference in viscosity between oil and water. This phenomenon is expressed by Stoke's Law. Other technologies employ membrane filtration, centrifugation, adsorption on active carbon granules, heating, ultrasonic energy, electro-coagulation and chemical methods such as Wet Air Oxidation Zimpro Process.
Oil in waste water can be present as free oil, emulsified oil, a chemically stabilized dispersion or molecularly dissolved oil. Oily waste water can also contain suspended solids (silt).
Oil water separators can be manufactured for above ground or under ground use or in the case of ships for onboard use. The shell can be made from concrete, steel, stainless steel or GRP. They can contain a single treatment step or a combination thereof.
Some manufacturers of oil water separator equipment are: Mercer International (USA), San Dizier Environment (France), Conder Environmental Solutions (UK), and Sulzer Chemtec (Switzerland).
Oil water separators can be by-pass separators or full retention separators. By pass separators as the name implies by-pass the treatment step above a certain flow. Full retention oil water separators treat 100% of the flow. Class 1 oil water separators guarantee a maximum of 5 ppm oil in the effluent whereas Class 2 oil water separators guarantee a maximum of 100 ppm oil in the effluent.
Common legislative requirements for OWS design are alarm devices and an automatic closure device which is activated when the full oil storage capacity of the oil water separator has been reached.