Oil/water separators used by automotive repair or fleet maintenance businesses to pre-treat waste water usually rely on flow-through, gravity fed systems. Inside the separator, oil gets trapped in a layer over the top of the waste water. As dirt, sand and sweepings fall out of the waste water stream, they form a layer of thick sludge along the bottom of the separator. Once these layers reach sufficient amounts, they need to be removed by skimmers, pumps, or other methods.
Generally, separators should be monitored on a routine schedule and collected oil should be removed as appropriate, so that it operates properly. The required oil/water separator capacity should always be available (i.e., oil should not continually accumulate in the separator over a period of time that would compromise required storage capacity if an oil release were to occur within the drainage area).
Want to know more about OWS compliance? Checkout these resources:
Best Environmental Practices for Auto Repair and Fleet Maintenance
EPA Guidance Chapter 5: Oil/Water Separators