 The vast majority of these many equipment models, manufacturers, and types start with some sort of gravity separation of bilge water.Simply letting oil and water sit is called decanting, and this does not always meet the 15 ppm criterion, which is why each manufacturer has added additional features to his equipment to ensure that this criterion can be met.The separation that takes place inside the OWS allows oil that floats to the top to be automatically skimmed off to a sludge tank or dirty oil holding tank.