Your grease trap fills at a rate. It’s steady, it’s measurable, and it’s probably the only number in your kitchen that nobody has ever written down.
How often should a grease trap be cleaned? For most commercial kitchens, every 30 to 90 days. The trigger behind that range is the 25 percent rule: service the trap once the floating grease and the settled solids together reach a quarter of the tank’s liquid depth. Your service date is whichever arrives first, your sewer authority’s calendar minimum or that 25 percent mark.
The rule is a ceiling, not a schedule. Two measurements taken four weeks apart turn it into a date that belongs to your kitchen instead of the quarterly guess everyone starts with.
What the 25 Percent Rule Actually Measures
The rule reads as one sentence: the combined depth of the floating grease layer and the settled solids layer can’t exceed 25 percent of the total liquid depth of the interceptor.
Written as arithmetic, it’s this:
(FOG depth + solids depth) ÷ total liquid depth
Take a tank with a 48 inch liquid column. Six inches of grease floating on top, six inches of sludge settled on the bottom. That’s (6 + 6) ÷ 48, which equals 0.25. That tank is full by the rule, even though 36 inches of it still look like water.
Liquid depth, not tank volume
Operators most often go wrong by treating 25 percent as a share of the tank’s gallon rating. It isn’t. It’s a share of the wetted height, which Austin’s city code defines precisely as the distance from the bottom of the trap up to the invert of the outlet pipe.
That distinction changes the answer. A tank that’s 25 percent full by depth is nowhere near 25 percent full by volume, because grease and sludge don’t stack evenly across the footprint. If you’re calculating gallons, you’ll pump late.
Why looking under the lid gets it wrong
Grease floats and solids sink, and the water in between is what makes the trap work. When you lift the cover, you see the top layer and nothing else. The sludge on the bottom is invisible from above, and in a trap that takes a lot of food debris it’s often the thicker of the two.
A core sampler, sometimes called a sludge judge or a dipstick, is lowered slowly through the whole column and pulled up with the layers intact. The FOG collects at the top of the tube, the solids settle at the bottom, and you read both against the total. That’s one draw, both numbers.
Two cautions before anyone reaches for one. OSHA identifies covers, openings, and larger interceptors as fall and confined space hazards, so an exterior in ground unit is not something a line cook opens on a whim. And most service contracts already include the reading. Ask your hauler to record the two depths on every visit and hand you the sheet.
How Often Should a Grease Trap Be Cleaned in Your Kitchen
A threshold tells you when to stop. It doesn’t tell you when to book. Getting from one to the other takes two readings and a division.
- Take a baseline reading right after a pump out, or on any day you can get a clean measurement. Record the grease depth, the solids depth, and the total liquid depth.
- Take a second reading four weeks later. Same tank, same method, ideally the same person.
- Subtract, then divide by the weeks. That’s your fill rate in inches per week.
- Work out your allowance. Twenty five percent of the liquid depth is the number you can’t pass.
- Divide the allowance by the fill rate, then book the service one to two weeks earlier than the answer.
A worked example from a fryer heavy kitchen with a 48 inch column:
| Step | Reading |
|---|---|
| Baseline, week 0 | 3 inches combined |
| Second reading, week 4 | 9 inches combined |
| Accumulation | 6 inches in 4 weeks |
| Fill rate | 1.5 inches per week |
| Allowance at 25 percent of 48 inches | 12 inches |
| Weeks to the ceiling | 8 |
| Service interval to book | Every 7 weeks |
That kitchen on a default quarterly schedule would spend the last five weeks of every cycle above the threshold. The trap would still drain, the sinks would still empty, and an inspector pulling a core sample on any of those days would write it up.
Run the same math on a low volume cafe and it often comes back at 15 or 20 weeks, which is when the local calendar minimum becomes the binding rule instead. Either way, you now know which of the two clocks you’re actually on. That’s the same logic that decides how cooking volume changes your cleaning frequency everywhere else in the kitchen: measure the thing that drives the soil, then schedule against it.
The Calendar Floor Your Sewer Authority Sets
Two clocks run on every grease trap. The 25 percent rule is the ceiling. Your local wastewater authority sets a floor, a maximum number of days between pump outs regardless of how clean the tank looks. Whichever one you reach first is your service date.
The floors vary more than most operators expect:
| Authority | Published requirement |
|---|---|
| Philadelphia Water Department | Quarterly for most kitchens, monthly for high volume |
| Capital Region Water (Harrisburg) | 90 days minimum for commercial grease traps |
| Dallas Water Utilities | At least every 90 days, or at the 25 percent condition |
| Salt Lake City Public Utilities | 30 days for hydromechanical devices, 90 days for gravity interceptors or 25 percent in the first chamber |
| Austin Water | At least every 90 days, or sooner at 50 percent of wetted height (City Code 15-10-197) |
Austin is the one to notice. Its trigger is 50 percent, not 25. There is no national cleaning interval and no national percentage, and a rule you read on a contractor’s website in another state is not your rule.
In Pennsylvania, the state sets the framework through Act 537 and the Department of Environmental Protection, and then the actual numbers come from your municipal sewer authority. That’s the borough or township authority that bills you, not the health inspector and not the state. One phone call gets you the interval, the measurement point they enforce, and the form they expect. Ask for it in writing and keep it with your cleaning records and logs.
The ceiling itself is a strangely precise number for a rule that no federal agency ever wrote, and who chose it decides whether your own tank deserves it.
Where the 25 Percent Came From, and When It Is Too Strict for Your Tank
Honolulu wrote it, in the late 1990s.
James Baginski, then the Regulatory Control Branch Head at the city’s Department of Environmental Services, worked it out from the published performance data on tested and rated hydromechanical grease interceptors. He calculated each unit’s rated grease storage capacity as a percentage of its total volume, and the results clustered between 25 and 35 percent, regardless of manufacturer or flow rate. The city took the bottom of the range to stay conservative, and the number went into its new FOG program.
Two things then carried it nationwide. The Plumbing and Drainage Institute’s 1998 guide to grease interceptors, reaffirmed in 2010, noted that PDI-G101 certified units may need maintenance once as little as 25 percent of rated capacity is reached. And the 2003 Orange County FOG Control Study recommended it to dozens of California jurisdictions after surveying what other programs were already doing. It spread by consensus, not by experiment.
That history matters for one practical reason. Older interceptors built to the minimum ratings genuinely earn the 25 percent trigger. Newer high capacity units from manufacturers like Schier, Canplas and Thermaco are tested and rated to hold grease at well above 50 percent of total capacity, and some models are rated as high as 90 percent. Applied to one of those, a flat 25 percent rule makes you pay for pump outs the equipment doesn’t need.
If your interceptor is a recent high capacity model, pull the manufacturer’s certified test report and take it to your sewer authority. Some programs will evaluate a rated unit individually and approve a longer interval. Most operators never ask, because most operators don’t know the number is negotiable.
What Fills a Grease Trap Faster Than Any Calendar Predicts
Cover counts are a poor predictor of fill rate. Four kitchen habits are much better ones.
- Fryer and griddle volume. Rendered fat from high heat cooking is the bulk of what reaches the trap. Two restaurants with identical seat counts and different menus can fill at double the rate of each other.
- Garbage disposals. A disposal grinds food solids and sends them straight to the tank, where they settle into the sludge layer that nobody sees. Disposals are the fastest way to shorten an interval, and several FOG programs restrict them outright.
- Dish machine and pot sink temperature. Hot water carries grease past the trap in suspension, then it cools and drops out downstream in your lateral or in the city main. Warm enough to clean, not hot enough to emulsify.
- Scraping discipline. Plates scraped into the bin instead of the sink is the single cheapest intervention available. It’s also the one that decays fastest without a written closing procedure.
One habit to skip entirely: enzyme and bacterial additives sold as a way to stretch the interval. They liquefy grease rather than remove it, and liquefied grease is still grease.
It leaves your trap, cools in the pipe, and hardens somewhere more expensive to reach. The tank reads cleaner and the problem moved. A number of municipal FOG ordinances prohibit additives outright as a substitute for maintenance, and some name enzymes, emulsifiers, surfactants and bacteria by category.
Who Pumps It, and What They Have to Leave Behind
Grease trap pumping is a licensed trade of its own. In Pennsylvania, only haulers registered with the DEP may transport interceptor waste, and it has to go to a DEP approved disposal site. Your janitorial contractor cleans the kitchen around the trap and keeps the floor, the drains, and the surrounding area under control between visits. The pump out itself belongs to the hauler.
The document that proves it happened is the manifest, and you should get a signed copy on the day. It names the hauler and their registration, the generator (you), the volume removed, the disposal site, and the receiving facility’s acknowledgment. Pennsylvania programs generally expect three years of these records available on request.
That paperwork is your defense, not the hauler’s. If a sewer crew traces a blockage back to your lateral, the manifest file is what separates a conversation from a cleanup bill, and it’s the same reason compliance responsibility ultimately sits with the operator no matter who holds the hose.
What a Year of This Should Cost
Once you have an interval, the annual number is straightforward.
- Indoor traps, roughly 30 to 200 gallons: $125 to $180 per scheduled service. Emergency calls run $190 to $240.
- Exterior interceptors around 1,000 gallons: $225 to $450 per scheduled service on contract.
- Contract versus on demand: a standing schedule runs 15 to 30 percent below calling when there’s a problem.
- Regional adjustment: Northeast and West Coast pricing sits 20 to 40 percent above Midwest and Southeast averages, so Pennsylvania operators should budget toward the upper half of every range above.
The fryer heavy kitchen from the earlier example, on a seven week interval, buys about seven and a half services a year. On a 1,000 gallon exterior unit at the middle of the range, that’s roughly $2,500 annually. Moving to quarterly would save about $1,300 of it and put the tank over the threshold for five weeks out of every thirteen, which is a trade plenty of owners make without seeing both halves of it.
Set the Schedule
Three situations, three answers.
Case one: you’ve never measured. Your authority publishes an interval. Obey the published interval starting now, and take your two readings anyway. You need to know whether the floor is protecting you or whether you’re already past the ceiling between visits.
Case two: you fill slower than the floor. Your readings say you’d hit 25 percent later than the local minimum. Keep the local minimum. It’s the binding rule. File the readings, because they’re the evidence behind any request for a longer interval later.
Case three: you fill faster than the floor. Your readings say you’d hit 25 percent before the calendar does. Move the service date up to your own number and stop treating the calendar minimum as permission. An interceptor over the threshold at the moment of inspection is a violation in most programs, and the pump out you skipped costs far less than the one that follows a backup.
The wider cleaning calendar for everything else in the building, from hood dates to floors, works the same way and is covered in the guide to how often a restaurant should be professionally cleaned.
Every kitchen starts on the same quarterly guess. Yours has a fill rate, it takes two readings and one division to find, and once it’s written down, the date stops being a guess.
Frequently Asked Questions
How often does a grease trap need to be emptied?
Most commercial grease traps are emptied every 30 to 90 days. High volume kitchens that fry daily often need monthly service, and low volume cafes can sometimes run quarterly. The correct interval for any specific kitchen is set by whichever comes first: the local sewer authority’s published maximum interval, or the point at which grease and solids reach 25 percent of the tank’s liquid depth.
How do you know when a grease trap is full?
By measurement, not by symptoms. A core sampler lowered through the tank captures the grease layer and the solids layer in one draw, and the trap is full once those two depths together reach a quarter of the total liquid depth. Slow drains, sulfur odors, gurgling fixtures, and grease appearing around covers all mean the trap is well past full, not approaching it.
Is the 25 percent rule measured by volume or by depth?
By depth. The rule compares the combined depth of the floating grease and the settled solids against the total liquid depth of the tank, measured from the outlet pipe to the bottom. Calculating it as a percentage of the tank’s gallon rating gives a different and later answer, which is how kitchens end up over the threshold while believing they’re compliant.
Does every city use a 25 percent rule?
No. It’s the most widely adopted trigger in the United States, but it isn’t universal and it isn’t federal. Austin Water uses 50 percent of wetted height instead.
Some programs enforce a maximum number of inches of accumulation rather than a percentage, and some rely only on a calendar interval. Use the rule for the jurisdiction and the device you actually have.
Can I clean my own grease trap?
Small under sink units can sometimes be cleaned in house, depending on local rules, with the waste going to solid waste rather than any drain. Exterior in ground interceptors are a different matter: they require a licensed hauler, a vacuum truck, and a disposal manifest, and the openings carry fall and confined space hazards that OSHA specifically flags. Never put trap waste down a drain or a storm sewer.
Who is allowed to pump a grease trap in Pennsylvania?
Only haulers registered with the Pennsylvania Department of Environmental Protection may transport and dispose of interceptor waste, and it has to go to a DEP approved facility. Ask any prospective hauler for proof of DEP registration and a sample manifest before booking. Your local sewer authority can usually provide a list of approved haulers for your area.
What records do I have to keep?
A service log and a manifest for every pump out, generally retained for three years. The manifest should show the hauler’s name and registration, your business as the generator, the volume removed, the disposal site, and the receiving facility’s acknowledgment. Add the measured grease and solids depths to your log and you’ll also have the evidence for a schedule change request.
Do enzymes or bacterial additives reduce how often I need a pump out?
No. Additives break grease into smaller particles that travel past the trap and resolidify downstream in your lateral or the municipal main. The tank reads cleaner while the problem relocates to a more expensive place, and a number of FOG programs prohibit their use for that reason. Nothing substitutes for mechanical removal.
Why does my grease trap still smell right after it was cleaned?
Usually because the pump out removed the liquid but not the residue. Baffles stay coated, sludge stays packed in corners, and both keep producing odor. A full service includes scraping and washing the interior walls and baffles, not just vacuuming the tank. Persistent odor after a proper cleaning points to an undersized trap or an interval that’s too long for the kitchen’s actual volume.