Oily Water Separator or Bilge Oil Separator
A major source of oil pollution in the past from the operation of the ships was discharge into the sea of tank washings from tankers. This was reduced by the discharge of tank washing to a slope tank for settling, and discharge overboard of the water while retaining the sludge for pumping ashore to the refinery, with the next cargo. Crude Oil Washing (COW) eliminates the use of water and enables cargo residues to be pumped ashore during discharge because cleaning is carried out simultaneously with the discharge.
Ballast carried in oil cargo and bunker tanks which is therefore contaminated with oil constitutes another pollution source, unless pumped out via an oily water separator. New regulations require tankers of certain sizes to have segregated or clean ballast tanks. Still oily water mixtures in bilges of engine room and other places is a pollution source.
Hence MARPOL stipulates that the engine room bilges must only be pumped out through suitable oil water processing equipment (bilge oil separator) or retained for discharge ashore.
A complete oily water separator and filter unit for 15 ppm (parts per million) purity is shown in the figure above. The complete unit is first filled with clean seawater, the oily water mixture is then pumped through the separator inlet pipe into coarse separating compartment. Here some oil, as a result of its lower density, will separate and rise into the oil separation space. The remaining oil water mixture now flows down into the fine separating compartment and moves slowly between the catch plates. More oil will separate out on to the under side of these plates and travels outwards until it is free to rise into the oil collecting space. The almost oil free water passes into the central pipes and leaves the separator unit. The purity at this point will be 100 ppm or less. An automatically controlled valve releases the separated oil into a storage tank. Air is releases from the unit by a vent valve. (Vent valve should be kept open while initial filling of oily water separator (ows) with seawater. Close the vent once water starts coming through it.) Steam or electric heating coils are provided in the upper and sometimes lower parts of the separator, depending upon the type of oil to be separated (heating reduces viscous drag of fluid, and also increases the density difference between oil and water which makes separation of oil and water easier).
Where the greater purity is required, the almost oil free water passes through a filter unit. The water passes through two filter stages as shown and the oil removed passes to the oil collection spaces. The first stage filter remove physical impurities present and promotes some fine separation. The second stage filter uses coalescer inserts to achieve the final de-oiling. Coalescence is the breakdown of surface tension between oil droplets in an oil/water mixture which causes them to join and increase in size.
The oil from the collecting spaces is drained away manually, as required, usually about once in a week. The filter inserts will require changing, the period of useful life depends on the operating conditions.
References
“Basic Marine Engineering” by J.K.DHAR