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Gravity grease interceptor vs hydromechanical grease trap: which to spec
Two devices, same job at different scales: pick by flow, space, and what the local FOG ordinance will accept.
Short answer
Pick a gravity grease interceptor for a full-service restaurant with real cooking-line volume and a yard or lot where a tank can be buried and a pump truck can reach it; pick a hydromechanical grease trap for a small, low-flow kitchen with no outdoor room. The single biggest deciding factor is not the fixture math, it is the local FOG ordinance and the AHJ: many jurisdictions now require a gravity interceptor for any new full-service restaurant regardless of fixture count, and that requirement overrides the sizing calculation. Where the code leaves it open, decide on flow and available space.
Gravity grease interceptor vs Hydromechanical grease trap: side by side
| Factor | Gravity grease interceptor | Hydromechanical grease trap |
|---|---|---|
| Rated by | Volume, in gallons | Flow, in GPM |
| Typical size | 750 to 3,000+ gallons | 20 to 100 GPM |
| Location | Outdoor or below grade; needs pump-truck access | Indoor, under or near the sink |
| Upfront cost | Higher; excavation, tank, H-20 rating under traffic | Lower; compact unit fits indoors |
| Install | Slower; set tank, prove watertight before backfill | Faster; mounts near the fixture |
| Flow control | Not used the same way | Required; vented fitting with air intake |
| Service interval | Often monthly to quarterly | Often weekly to monthly |
| Sizing / standard | IPC peak flow x 30 min retention; local FOG worksheet | PDI G101, ASME A112.14.3; ~2 lb grease per GPM |
| Best use | Full-service restaurant, high volume | Small kitchen, single line, tight space |
Which should you pick?
Choose Gravity grease interceptor when
- A full-service restaurant or high-volume cooking line where peak flow times a 30 minute retention rarely fits in a small trap
- You have outdoor or below-grade room and clear access for a vacuum truck
- The local ordinance names a minimum tank size or requires a gravity interceptor for a new restaurant
- You want a longer service interval and retention that holds through the dinner rush
Choose Hydromechanical grease trap when
- A small, low-flow kitchen served by a single line or one compartment sink
- No outdoor space to bury a tank; the unit has to go indoors near the fixture
- Lower upfront budget and a faster install matter
- Calculated peak flow lands within a standard GPM rating with grease capacity to match
Bottom line
It depends on flow, available space, and what the AHJ will accept, and often the local FOG ordinance makes the call for you by naming a minimum tank or requiring gravity on any new full-service restaurant. As a rule, low flow plus no outdoor room points to a hydromechanical trap; a real cooking line plus a place to set a tank points to gravity. Whichever you install, the maintenance rules are the same: pump at the 25 percent rule (grease plus solids reaching a quarter of the liquid depth) or the permitted interval, do a full pump-out rather than a skim, and keep the disposal manifest the FOG program asks for.
FAQ
What is the difference between a grease trap and a gravity grease interceptor?
A grease trap, or hydromechanical interceptor, is the small indoor unit rated by flow in gallons per minute and served by a vented flow control fitting. A gravity grease interceptor is the large in-ground tank rated by volume in gallons, commonly 750 to 3,000+ gallons. Both slow wastewater so grease floats and solids settle; they differ in scale, rating basis, and location.
Which is cheaper, a gravity interceptor or a hydromechanical trap?
The hydromechanical trap is cheaper up front and faster to install because it is a compact unit that fits indoors. A gravity tank costs more because it needs excavation, a large tank, an H-20 traffic rating if it sits under a drive or lot, and a watertight test before backfill. The trap is serviced more often, though, since it holds less.
Can a full-service restaurant use a hydromechanical grease trap?
Sometimes, but often not. High cooking-line volume usually pushes peak flow past what a trap can hold, and many jurisdictions now require a gravity interceptor for any new full-service restaurant regardless of fixture count. When the local ordinance names a minimum tank size for the use, that overrides the fixture math, so confirm the requirement with the AHJ before speccing a trap.