Coalescer/Separator A coalescer/separator system consists of two elements, creating a two stage filtration process. When contaminated fuel enters the vessel, particulate (rust, scale, dirt, and other contaminants) is removed and water is coalesced by the first stage filter (coalescing is when the water droplets collect on the media and then fall into the sump to be removed). Any free water is separated by the second stage element providing clean, dry diesel to your engine and equipment.