Federal regulations and most authorities having jurisdiction require that “free oil” be removed from deck drainage prior to disposal. It is extremely difficult to predict an oil drop size distribution for rainwater or washdown water that is collected in an open drain system, and regulations do not define what size droplet is meant by “free oil.”
Long-standing refinery practice is to size the drain water treating equipment to remove all oil droplets 150 microns in diameter or larger. If no other data are available, it is recommended that this be used in sizing sumps and disposal piles.
