The grease trap is the most ignored piece of equipment in most restaurants — until the day a kitchen sink backs up during service, or a sewer-district inspector hands over a fine. A grease trap only works if it's cleaned on schedule, and the rules that govern it are stricter than most operators realize. This guide covers what a grease trap is, why it has to be cleaned, how often, what goes wrong when it isn't, and what proper professional service actually looks like.
What a grease trap is
A grease trap (also called a grease interceptor) is a holding vessel plumbed between your kitchen drains and the public sewer. Its job is to slow wastewater down long enough for fats, oils, and grease — collectively known as FOG — to separate out before the water continues to the sewer.
The physics are simple. Grease floats and food solids sink, so inside the trap the warm, greasy water cools and stratifies into three layers: a layer of congealed FOG on top, relatively clear water in the middle, and a layer of settled food solids on the bottom. Only the clean middle water is allowed to flow out. Smaller under-sink units hold 20 to 50 gallons; larger in-ground interceptors can hold 750 to 1,500 gallons or more. Either way, the trap fills with grease and solids over time and must be emptied — it does not consume or break down what it captures.
Why FOG rules require cleaning
Fats, oils, and grease are the single biggest cause of sanitary sewer overflows. Poured-out grease cools inside municipal pipes, hardens, and builds up like cholesterol in an artery until the line blocks and raw sewage backs up into streets, homes, and businesses. To prevent this, nearly every city and sewer district runs a FOG control program, and that program is the legal reason your grease trap has to be cleaned.
Under a typical FOG ordinance, food-service establishments are required to install an adequately sized grease interceptor, keep it maintained, and — critically — keep records proving it. Most districts require that every cleaning be documented on a manifest showing the date, the volume pumped, and the licensed hauler who did the work and disposed of the waste. Those manifests are what an inspector asks for. Without them, you can be in violation even if the trap happens to be clean on inspection day.
How often: the one-quarter rule
The most widely used standard for cleaning frequency is the 25% rule (sometimes called the one-quarter rule). It states that a grease trap must be cleaned whenever the combined layer of floating FOG and settled solids reaches 25% of the trap's total liquid depth. Beyond that point, the trap loses its capacity to separate grease effectively, and FOG starts slipping through into the sewer.
In practice, how quickly a trap reaches that threshold depends on its size and your cooking volume, so frequency varies:
- Small under-sink traps in a busy kitchen often hit the 25% mark in two to four weeks, so monthly or even more frequent service is common.
- Mid-size and in-ground interceptors are typically serviced every one to three months.
- Large interceptors at lower-volume operations may go a quarter between cleanings.
Many districts also set a maximum interval regardless of fill — frequently 90 days — meaning the trap must be pumped at least quarterly even if it hasn't reached 25%. The right schedule for your kitchen is whichever comes first. The honest answer is that frequency should be set by measuring your trap, not by guessing, and then adjusted as your volume changes.
What happens if you don't
Skipping grease trap service rarely stays a small problem. It usually shows up in three escalating ways:
- Backups and shutdowns. A full trap stops separating, grease moves into your drain lines, and sooner or later a sink or floor drain backs up — almost always during peak service. A grease blockage can force you to stop cooking and close until it's cleared.
- Fines and violations. Sewer districts issue notices of violation, escalating fines, and re-inspection fees for overdue traps, missing manifests, or FOG discharge into the sewer. Repeat offenders can face significantly steeper penalties.
- Permit and reputation risk. Chronic FOG violations can jeopardize your discharge permit and, in severe cases, your ability to operate. Sewer overflows traced back to a restaurant can also bring cleanup liability and bad press.
Beyond the regulatory side, a neglected trap creates foul odors and accelerates wear on your plumbing — the costs of which always exceed the price of routine service.
DIY versus professional service
It is technically possible to skim a small under-sink trap yourself, and some operators do. But there are real reasons most restaurants don't:
- Disposal is regulated. Grease trap waste cannot go down a drain or into the dumpster. It must be hauled by a licensed transporter to an approved facility. A bucket of scooped grease becomes an illegal disposal problem the moment you try to get rid of it.
- Documentation is the point. A do-it-yourself skim produces no manifest, so even a clean trap leaves you unable to prove compliance to an inspector.
- Partial cleaning isn't real cleaning. Skimming the top FOG layer leaves the settled solids and the hardened sidewall buildup behind, so the trap fills again quickly and never gets a true reset.
Professional pumping empties the entire trap — all three layers — hauls the waste under a tracked manifest, and gives you the records that keep you compliant. For anything larger than a small sink unit, professional service isn't a luxury; it's the only practical way to stay within the rules.
What proper service includes
A complete, compliant grease trap cleaning should cover all of the following:
- Full pump-out of the entire trap contents — floating FOG, the middle water, and the bottom solids — not just a surface skim.
- Scraping and scooping of the hardened grease and solids built up on the walls, baffles, and floor of the trap.
- Inspection of the baffles, inlet and outlet tees, and lid gaskets for damage that would let FOG pass through or cause leaks.
- Measurement and notes on the FOG and solids depth, so your cleaning interval can be tuned to your actual fill rate.
- Licensed disposal of the waste at an approved facility, with the load tracked from your trap to the destination.
- A manifest documenting the date, volume pumped, hauler, and disposal site — the record your sewer district expects to see.
How Certified Restaurant handles it
Grease traps are part of our managed back-of-house program, so you get the cleaning and the paperwork from one accountable partner. We measure your trap, set a service schedule built around your actual fill rate and your district's maximum interval, and dispatch a vetted, licensed, insured local crew to do the full pump-out described above. Every visit produces a compliant manifest, kept on file and ready the moment an inspector asks — and it all lands on the same single invoice as the rest of your services.
You can learn more on our grease trap cleaning service page, or fold it into your certification from the start. Most restaurants begin with reliable used cooking oil collection and recycling, then add grease traps and the rest of the back of house as they grow.
Ready to stop chasing haulers and worrying about manifests? Get certified and let one partner keep your trap clean and your records inspection-ready.