Merchant ships are required to be provided with oily-water separating equipment that meets the requirements of the International Performance Testing Standard A. 393 (x) prescribed by the International Maritime Organization (IMO), where the discharged oil contents of the effluent measured at the outlet of the equipment are controlled to 15 ppm or 100 ppm according to the classification of ships by gross tonnage. However, investigations are being undertaken to find international solutions centring on the Marine Environment Protection Committee (MEPC) of IMO to improve the not-necessarily-satisfactory-service-performance of oily-water separating equipment, as is evident in cases reported overseas. In this study, the actual service performance of the existing oily-water separating equipment in Japan was investigated and a study on new oily-water separating techniques was conducted with expertise acquired as outlined below.
In view of the fact that the engine room bilge in an actual ship contains suspended solids (SS), detergents and other materials in addition to fuel oil and lubricating oil, it is difficult to clear the discharged oil control requirement of 15 ppm at all times, inasmuch as the existing oily-water separating system relies only on a gravitational separation system comprising multiple parallel plates and a filter-type coalescer.
The newly developed oily-water separating mechanism relying on oil flocculation and froth flotation has displayed its usefulness for oily water containing oil particles and suspended solids with a particle size smaller than 10 µm, and thus is promising for application to oily-water separating equipment to be designed for clearing the 15 ppm discharge requirements.