A grease trap or interceptor sits between kitchen fixtures and the septic tank to remove fats, oils and grease (FOG) before they reach the tank and drainfield, and it is sized under the plumbing code your jurisdiction has adopted rather than a fixed rule of thumb that applies everywhere.
A septic tank already separates solids by settling and floating, but grease that reaches a drainfield coats the soil surface and stops it from absorbing effluent, which is a slow, expensive failure that is much harder to reverse than a tank pump-out. Intercepting FOG at the source, before it ever mixes with the rest of the building's wastewater, keeps that failure mode away from the drainfield entirely.
Interceptors range from small under-sink or in-line units for light kitchen use to large outdoor gravity interceptors for full-service restaurant kitchens, and sizing follows a flow-and-retention-time methodology set by the local plumbing code, typically the Uniform Plumbing Code or International Plumbing Code as adopted and possibly amended locally.
A grease interceptor is normally a plumbing-code requirement enforced through the building permit process, separate from the septic tank and drainfield's own permit under the onsite wastewater program, and it needs its own maintenance schedule, regular pump-out of accumulated grease, independent of when the septic tank itself is due for service.
Find your state's septic regulator and permit sequence.
A new use, an addition, a remodel, a change of tenant, gets compared against what a commercial property is already permitted for. Where a recorded septic file exists for the property, it typically shows the tank size, the treatment components, the drainfield or dispersal area, and the design flow the system was approved for, which is the starting point for figuring out whether a change fits inside the existing approval or needs an upgrade. We publish where county septic records are kept and how many we hold, we list local septic companies by county, and we generate a free, code-referenced septic design for a parcel to use as a starting point.
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The terms are often used interchangeably; broadly, a grease trap tends to refer to a smaller, often indoor unit and an interceptor to a larger outdoor unit, but the function is the same, removing FOG before it reaches the rest of the wastewater system.
Under the flow-and-retention-time methodology in the plumbing code your jurisdiction has adopted, typically the Uniform Plumbing Code or International Plumbing Code. There is no single national number; check with your local plumbing authority.
No, it is a separate pretreatment step ahead of the septic tank, with its own permit and maintenance schedule, not a substitute for tank and drainfield design.
Page last reviewed 2026-08-31.