An oil/water separator relies on a simple principle: oil and water don't mix, and they don't weigh the same. Wastewater flows into the separator and slows down enough for gravity to do the sorting, so heavier solids and sediment settle to the bottom, while oil and grease, which are lighter than water, rise to the top and form a layer that gets held back.
The relatively clean water in the middle layer is what's allowed to continue on to the sewer system, while the oil layer and the settled solids stay behind in the tank until they're pumped out during a service visit.
It's a straightforward system, but it only works as long as there's room left in the tank for that separation to happen. Once the oil layer or the sediment at the bottom gets too thick, the separator can't keep up, which is why routine cleaning matters as much as the separator itself.
