A homeowner in Marysville called about a gurgling drain and a smell that wouldn't quit. My brother Mark handles most of our grease and plumbing calls, and his first thought going in was a grease buildup somewhere in the line. He opened the system and found something else entirely: a 1977 septic system that had backed up far enough to reach the distribution box and soak the soil under the porch. Same warning signs, completely different cause. A gurgle or a smell tells you something's wrong. It doesn't tell you what, and grease is one of the more common wrong guesses.
A grease interceptor is a plumbing device that catches fats, oils, grease, and food solids before they travel further into the sewer or septic system. Some units serve a single kitchen fixture. Others serve an entire commercial kitchen. People still call the smaller indoor units "grease traps," and most modern systems fall into one of two categories: hydromechanical or gravity interceptors, each with its own capacity and maintenance needs.
Inside the unit, grease-laden wastewater slows down and separates. Fats and oils rise because they're lighter than water. Food solids sink. Internal baffles hold both in place while the wastewater underneath moves downstream. That separation only works while the chamber still has room. Once grease and solids fill too much of it, the unit stops doing its job and FOG starts moving into pipes it was never supposed to reach.
Hydromechanical interceptors are what most people picture: smaller, often installed indoors, rated for specific flow rates. Gravity interceptors are the larger tank-style systems installed outside a commercial kitchen, relying on volume and time rather than mechanical flow control. Automatic grease removal devices go a step further and pull collected grease out on their own, though they still need regular inspection.
Most single-family kitchens don't need a grease interceptor. That's not permission to pour bacon grease down the drain. Household FOG still builds up in plumbing, sewer lines, and septic systems over time. It just doesn't usually trigger a code requirement the way a commercial kitchen does.
Washington runs on the 2021 Uniform Plumbing Code under Chapter 51-56 WAC, but the actual requirement shifts by city. Seattle requires an interceptor for any commercial kitchen with a King County Food Business Permit. Everett has its own grease-removal requirements for commercial kitchens. Arlington, Redmond, Bellingham, and the other communities we cover each have their own version of the same rule. If you're opening or remodeling a food-service kitchen, confirm the local requirement before you pick a size or type.
Two restaurants with similar kitchens can fill their interceptors at completely different rates, so a flat number never sits right with me. Local rules matter more than intuition here:
|
Location / System |
Cleaning Requirement |
|
Everett, trap-style system |
At least weekly unless the City approves a longer interval; also cleaned once grease and solids hit 25% of liquid depth |
|
Everett, interceptor-style system |
At least every 90 days unless the City approves otherwise; same 25% threshold applies |
|
Seattle |
Follow manufacturer specs; without those, buildup shouldn't exceed 25% of the interceptor |
|
Other cities we serve |
Check local sewer or plumbing code directly instead of assuming Seattle or Everett rules apply |
If a kitchen hits that 25% mark early, Mark cleans it early. Waiting for a date on a calendar when the unit is already full doesn't protect anyone.
A few patterns show up before a full backup does: greasy wastewater returning through a sink or floor drain, a drain running slower than it used to, or a greasy smell that comes back within days of a cleaning. Any one of those is worth a look. Mark checks both the interceptor and the downstream line whenever he sees buildup, especially after a backup already happened once.
That Marysville system is the reminder I keep coming back to. Gurgling and a bad smell pointed toward grease. The real cause was decades old and had nothing to do with a kitchen. We don't guess from symptoms alone, on grease calls or septic calls, because the wrong guess costs a homeowner time and money chasing the wrong fix. Read the full Marysville case for what we found once we actually opened that system up.
If your interceptor's cleaning history is a mystery, or a drain keeps acting up and you're not sure why, that's exactly the kind of call Mark takes. We'll open it, tell you what's actually in there, and go from there.
More on grease interceptor types and local cleaning requirements is on our blog.
Mark Nikolin, Field Specialist, Septic Solutions LLC
TSM: People throw around a "grease trap" like it's one piece of equipment. Is it?
MN: Not even close. Hydromechanical grease interceptors are the smaller units people usually picture, installed indoors, rated for specific flow conditions. Gravity grease interceptors are much bigger, tank-style systems installed outside commercial kitchens that separate grease and solids using volume and time instead of internal flow control. Then there are automatic grease removal devices, which separate FOG and physically move the collected grease into a container on their own. That automation doesn't mean you skip service. It still needs regular attention.
TSM: How does separation actually happen inside these units?
MN: Grease-laden wastewater slows down once it enters the interceptor, and that slowdown is the whole point. Fats and oils are lighter than water, so they rise. Food solids settle toward the bottom. Baffles inside the unit and the way the outlet is arranged hold both layers in place while the wastewater in between keeps moving. That only works as long as the unit has usable capacity. Once grease and solids build up too far, separation gets worse and more FOG ends up downstream than it should.
TSM: Who actually needs one of these installed?
MN: Most residential kitchens don't, though household grease shouldn't go down the drain regardless. Commercial kitchens are a different story, and the requirement changes by city. Washington runs on the 2021 Uniform Plumbing Code, but Seattle, Everett, and other jurisdictions layer their own rules on top. Seattle requires an interceptor for any business with commercial kitchen plumbing fixtures and a King County Food Business Permit. Everett has a separate requirement of its own. I never assume one number applies everywhere.
TSM: Any signs a business shouldn't ignore between service visits?
MN: Greasy wastewater coming back up through a sink or floor drain. A drain running slower than it used to. A greasy smell that keeps returning no matter how much you clean. Any of those means the interceptor needs a look before its scheduled date, not after.
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