Sized, Set, Plumbed
and Inspection-Ready

Two different pieces of equipment get called the same thing. The small stainless or plastic box under a sink is properly a hydromechanical interceptor — a grease trap — and it is rated in gallons per minute of flow. The large in-ground concrete tank outside, typically 1,000 to 3,000 gallons, is a gravity grease interceptor, rated in gallons of capacity. South Carolina is blunt about which one a commercial kitchen on septic needs: an interior unit may not be used in place of a properly sized exterior grease trap. Grease matters because it is the single most common cause of sewer blockages — the EPA has put it at 47% of reported blockages — which is why nearly every sewer authority runs a formal program with sizing rules, inspections, and penalties.

Any new food service facility — the plumbing code lists restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias, and clubs. Any kitchen fit-out or menu change that adds fryers or a dishwasher, because that changes the sizing basis. Any existing facility that has had a grease-caused malfunction, at which point South Carolina requires immediate compliance as though it were a brand new facility. And on septic, any food service facility served by an onsite system, full stop. Which rulebook applies depends on where your waste goes and which state you are in: on sewer it is your local pretreatment authority, and on septic it is that state’s onsite wastewater regulation — South Carolina’s is considerably more prescriptive than most people expect.

Tank size sets the floor, and tank size comes out of whichever sizing method your authority accepts — so settle that before anyone prices a tank. From there the spread is driven by whether the lids have to be traffic-rated, excavation depth and whether you find rock, the run from the kitchen to the tank, whether a sample manhole is required, and whether the parking lot has to be cut and put back. An under-sink hydromechanical trap is a fixture-level purchase. An exterior in-ground interceptor is a site job with a crane on it, and those are not the same order of magnitude. Plan review with the sewer authority is typically two to six weeks; the install itself is usually one to three days of site work once the tank is on site. The critical path is almost always the precast lead time, so order early — a tank sitting in a yard three counties away has held up more certificates of occupancy than any inspector ever has.

Every authority around here runs a real program with real paperwork, and no two of them are the same. ReWa, the regional authority for the Greenville area we work out of, requires a submittal review form with flow calculations and enrollment in a FOG register where you upload your maintenance manifests — free, but mandatory. OJRSA, covering Seneca, Walhalla, and Westminster out in Oconee, publishes standard details for grease traps, interceptors, and oil-water separators along with a sizing workbook and a sewer use regulation. Cross into North Carolina or Georgia and you are under a different pretreatment ordinance that may size the tank a different way entirely — which is why nobody should price a tank before the governing authority is settled. Restaurants on septic get hit twice: they need both a grease trap and a commercial septic system, and South Carolina prohibits mound systems for facilities requiring grease traps, which closes off the usual workaround on poor soils. One more South Carolina wrinkle worth knowing: since July 2024 the retail food permit comes from the Department of Agriculture, while the septic and grease side sits with SCDES.