Short definition
A grease interceptor — sometimes called a grease trap when it’s a small under-sink unit — is a tank that separates fats, oils, and greases (FOG) from kitchen wastewater before the water enters the public sewer or septic system. King County requires one for any business with a Food Business Permit, including small operations like delis and coffee shops. Residential single-family homes do not need one.
What it is
A grease interceptor works on density. Cooled wastewater flows through the tank slowly enough that fats float to the top, settled solids drop to the bottom, and a layer of cleaned water in the middle exits through a baffle and continues to the sewer. The retention time is typically 30 minutes at design flow.
Two scales exist:
Gravity grease interceptors (GGI) are large outdoor in-ground tanks, 750 gallons and up, used for restaurants and any commercial kitchen handling significant FOG. They’re sized by flow rate and the drainage fixture units (DFU) they serve, with a minimum two-compartment design under UPC Chapter 10.
Hydromechanical grease interceptors (HGI) are smaller under-sink units, 25 to 100 gallons, used in very small operations or food prep with low FOG load. Internal baffles plus an air-injection or flow-control device speed the separation in a small footprint.
The fixture’s only job is keeping FOG out of the public sewer (or septic system). A clogged sewer main from accumulated restaurant FOG is one of the most common — and expensive — utility-side failures, which is why utilities and health departments enforce grease-interceptor rules aggressively.
Why it matters to a homeowner
For most homeowners in Washington, this is vocabulary you’ll never need. Residential single-family homes don’t require a grease interceptor. A standard kitchen P-trap and the disposer (if you have one) are the only fittings between your kitchen sink and the side sewer.
Two situations where a homeowner does need to think about it:
A licensed home-based food business. Cottage food, catering, baking businesses operating out of a residential kitchen may trigger an interceptor requirement under King County food-permit rules. The threshold and the exact requirements depend on the license tier and the specific operation. Verify with King County Public Health or your county’s equivalent before assuming the home plumbing is sufficient.
Buying a converted commercial space. If you’re buying a property that was once a restaurant, deli, or coffee shop, the old grease interceptor on the property may need decommissioning or maintenance reset. The tank doesn’t disappear when the business closes; it stays in the ground until someone formally removes it or repurposes the connection.
Otherwise, this term shows up in the permit and food-license context for friends and neighbors running food businesses — knowing what it is helps you read their plumbing quotes.
Washington note
Washington adopts the Uniform Plumbing Code (UPC) Chapter 10 for traps and interceptors, with state amendments. The county and city rules layer on top:
- King County requires a grease interceptor for any business with a Food Business Permit. The list explicitly includes small operations: delis, coffee and ice-cream shops, pizza parlors, caterers. Source: King County Public Health “Plumbing Requirements for Food Establishments.”
- Seattle Public Utilities maintains a Fats, Oils, and Grease (FOG) program for all commercial kitchens, with cleaning-frequency expectations and inspection authority.
- Residential single-family in Seattle and King County: no grease-interceptor requirement under standard residential plumbing permits.
For other counties — Snohomish, Pierce, Spokane — the rules generally mirror King County, but verify with the local health department before relying on prescriptive text. A licensed cottage food or catering business may push you across the threshold even on residential premises.
Common failure modes
- Overdue cleanout — the interceptor backs up and FOG escapes to the sewer; the operator gets a notice. Cleanout schedules are jurisdiction-specific, typically every 30 to 90 days for active kitchens.
- Bypass or illegal connection — kitchen drain bypasses the interceptor. Inspector finds out; fines plus retrofit order.
- Wrong sizing — undersized interceptor overflows during peak flow.
- Damaged baffle — separation fails; FOG to sewer.
Common variants and what a grease interceptor is not
- Grease interceptor (large, outdoor) vs. grease trap (small, under-sink). Terminology overlap. Code uses “grease interceptor” for any FOG separation device; trade uses “trap” for the small under-sink units.
- Gravity (GGI) vs. hydromechanical (HGI). GGI relies on residence time in a large tank; HGI uses internal baffles and flow control to separate in a small footprint.
- Grease interceptor vs. P-trap. A P-trap is a fixture trap for sewer-gas blocking. A grease interceptor traps food fats. Different functions.
- Grease interceptor vs. oil-water separator. Oil-water separators handle vehicle and industrial oils; grease interceptors handle food fats. Different code requirements.