Oil Water Separators (OWS) are typically concrete structures installed below grade. the purpose is to separate oil and grease before water is discharged to sanitary sewer. They are commonly used in the automotive industry for car washes. Of course YouTube is the place I learned all about them, there are good videos with how they work. If you have sludge accumulating in it you might need a pre filter. Check with local regulations regarding permits, also they will need to get cleaned. The person installing them will have all the specifications and should be able to answer your questions for why it is not working properly.

I have quite a bit of experience with them. Couple questions: is it aboveground or underground, what volume is it rated for, what kind of problems are you having with it?

They’re pretty straightforward - your oil/water liquid goes into it, the oil rises to the top, the water flows under a baffle and discharged. The oily layer on top will accumulate so there has to be some sort of process in place that removes that layer periodically