What Is the Best Method of Separation for Oil and Water: A Selection Guide Answering the core question: What is the best method of separation for oil and water? There is no single best method; the right one is set by the droplet size of the oil in the water, and everything else follows from that. Free oil, droplets above 150 micrometres, separates by gravity alone in an API 421 separator, reaching 50-150 mg/L. Dispersed oil of 20-150 micrometres needs a plate pack interceptor