"油水分离罐" can be translated as "oil-water separator tank" or more concisely as "oil-water separator" (when the context makes it clear that it refers to a tank or container).
"Oil-water separator tank" explicitly mentions the tank aspect, which might be useful if there's a need to emphasize that it's a container.
"Oil-water separator" is a more general term that is widely used in various industries to refer to devices or systems designed to separate oil and water, and in many contexts, it's understood to imply a tank or vessel where this separation occurs.