An oil-water separator is a device that removes oil, grease, and flammable liquids from wastewater before it reaches the sewer. It uses gravity so lighter oil floats to the top while water flows out below. Auto shops, garages, and car washes need one to keep petroleum out of the drain system.
An oil-water separator keeps oil and fuel out of the sewer. Waste water from a shop floor runs through the separator first. The device pulls out oil, grease, and flammable liquids, then lets the cleaner water pass on to the drain. It is required gear at businesses that work with vehicles and petroleum.
How it separates oil from water
The separator works on a simple idea: oil floats on water. Wastewater flows into a large tank or vault and slows down. In that calm space, lighter oil and grease rise to the top while heavier water sinks. Some units add coalescing plates. These give tiny oil droplets a surface to gather on, so they join into larger drops and rise faster. The clean water leaves through an outlet set below the floating oil layer. The trapped oil is pumped out and hauled away.
Where code requires one
The Uniform Plumbing Code spells out the rule in section 1017.0, Oil and Flammable Liquid Interceptors. It requires an interceptor at repair garages, gas stations with grease pits, and factories that produce oily or flammable waste. The code sets a design standard too. Each interceptor must hold at least an 18-inch water seal. It must be vented on the sewer side, and never tied into a flammable vapor vent. These rules protect both the sewer and the building.
Not the same as a grease trap
An oil-water separator is not a grease interceptor. Both sit between a drain and the sewer, but they catch different waste. A grease trap handles fats, oils, and grease from a kitchen, which is food waste. An oil-water separator handles petroleum oil and flammable liquid from vehicles and machines. It is also different from a sand interceptor, which drops out heavy grit instead of oil. Using the wrong one, or none at all, can send hazardous liquid into the public sewer and bring fines. A commercial plumber can size and install the right interceptor.