The health inspector is coming Thursday. Your line cook just mentioned a slow drain in the prep sink. And somewhere in the parking lot, the in-ground interceptor is sitting quietly — either fine or two weeks past due — and you're not entirely sure which.
For most restaurant operators, grease trap service is the maintenance item that gets scheduled, then postponed, then ignored until something forces the issue. By then, you're dealing with a backup in the middle of a dinner rush, an odor that drifts into the dining room, or a compliance notice that lands on a Friday afternoon.
The "how often" question isn't complicated. But the real answer depends on a few variables that most generic service schedules never account for — and getting it wrong in either direction costs you.
How a Grease Trap Fills Up
When wastewater leaves your sinks, dishwasher, and floor drains, it flows into the grease trap before hitting the sewer line. Inside the trap, the water slows. That's by design. As it sits, physics separate the contents: fats, oils, and grease (FOG) are lighter than water, so they float up and form a cap across the surface. Food solids are heavier, so they sink to the bottom. The relatively clean water in between eventually flows out to the sewer.
The trap does its job as long as that middle layer stays the largest zone. The moment the grease cap on top and the solids bed on the bottom take up too much of the tank, separation stops working. Wastewater moves through too fast, FOG pushes downstream into your pipes, and the problems start building where you can't see them.
The 25% Rule — The Number Your Schedule Should Be Built Around
The benchmark the industry uses is the 25% rule: your grease trap needs pumping when the combined depth of the grease cap and settled solids reaches 25% of the trap's total liquid depth.
In practical terms, that means a trap holding 24 inches of liquid hits its service threshold once grease and solids together occupy 6 inches of that space. A technician can measure this directly during a routine visit. You don't have to wait for a backup to know where you stand.
Most local environmental and health agencies tie their compliance schedules to this same number. Some cities set a blanket 60- or 90-day interval, but the intent is identical: get pumped before you hit 25%, not after you've already caused a backup or triggered a line inspection.
How Volume, Trap Size, and Cuisine Type Change Your Interval
The 25% threshold arrives at very different speeds depending on how your kitchen runs. Four variables drive your real-world service window: weekly covers, trap size, cuisine type, and equipment load.
| Weekly Covers | Typical Interval (Standard 750–1,000 gal Trap) |
|---|---|
| Under 500 | Every 60–90 days |
| 500–1,000 | Every 45–60 days |
| 1,000–2,000 | Every 30–45 days |
| 2,000–3,000 | Every 21–30 days |
| 3,000+ | Every 14–21 days |
These ranges are for a standard in-ground interceptor. Smaller traps shift the intervals shorter. Larger interceptors (1,250–1,500 gallons) may allow one band longer — but only if the 25% measurement confirms it.
Cuisine type moves the number significantly. A burger restaurant doing 1,200 covers per week lands in the 30–45 day range because of fry oil volume. A ramen or sushi spot with that same cover count and minimal frying often stretches to 45–60 days. The food that goes through your kitchen drives the grease load, not just the headcount. High-FOG menus — anything with a lot of frying, BBQ, or oil-heavy saucing — can shorten your interval by a full band compared to lighter fare.
Heavy equipment use also matters. A high-temperature dishwasher that runs constantly, or a prep sink with a garbage disposal, pushes more FOG load through the trap than a lower-volume setup with the same menu.
Under-Sink Traps vs. In-Ground Interceptors
Not every commercial kitchen has an in-ground interceptor. Smaller operations — fast-casual counters, cafés, and food trucks — often use under-sink grease traps that hold 20 to 50 gallons. These fill up fast.
For under-sink units in an active kitchen, weekly to biweekly service is the norm. Some jurisdictions require it by code. There's almost no buffer between a functioning under-sink trap and one that's pushing grease downstream, and the small capacity means the 25% mark arrives in days rather than months.
Many operations that start with an under-sink unit end up moving to an in-ground interceptor as volume grows. The difference in service cost alone makes the case — weekly pump-outs on a small trap add up quickly against quarterly service on a properly sized interceptor.
Warning Signs You're Already Behind Schedule
A properly maintained grease trap doesn't announce itself. When it starts making itself known, the 25% mark has already passed.
| What You're Noticing | What's Happening in the Pipe |
|---|---|
| Slow drainage in prep or dishwash sinks | Grease has moved past the trap and is coating downstream lines |
| Sulfur or rotten-egg odor near drains | Trapped waste is decomposing inside the full tank |
| Gurgling sounds from kitchen drains | Partial blockage downstream is creating back-pressure |
| Visible grease in floor drain wastewater | FOG is bypassing the outlet baffle, the trap is past capacity |
| Backup in sinks during peak service | Grease buildup has restricted flow through the main line |
Any of these signals means the service interval has already slipped. At that point, you need a pump-out before the next inspection — not before the next scheduled service date.
What Happens When You Push Past the Limit
The first thing to go is your downstream pipe. Grease that makes it past the trap cools against the interior pipe wall and adheres. Hot dishwasher water loosens it briefly during service, but it re-solidifies a little further downstream. That cycle repeats. Each pass deposits another thin layer, narrowing the pipe bore until even relatively clear water has trouble moving through.
The second problem is the grease itself. As the cap ages inside an overdue trap, it begins to saponify — converting from liquid FOG into a waxy, soap-like solid. A trap that should take 45 minutes to clean can run twice as long and cost significantly more when the grease has had time to solidify. Some badly neglected traps need hydro jetting of the downstream line after the pump-out just to restore flow. That's a second truck and a second bill.
Compliance is the third layer. Most municipalities require pump-out records on demand. An inspector who finds a trap that's six months overdue — when your kitchen's volume clearly called for six-week service — can issue a violation notice whether your system is currently backing up or not.
How to Slow the Fill Rate Without Skipping Service
You can't eliminate FOG production in a working kitchen. But you can reduce how much of it reaches the trap.
Scraping and dry-wiping pans and plates before rinsing is the single biggest lever. A pan scraped into the trash before the first rinse sends a fraction of the grease load that a pan dumped straight into the wash sink does. Sink strainers catch food solids before they settle in the trap. Proper fryer oil disposal — skimming into a collection container, not the floor drain — makes a measurable difference over a month.
Train the staff who do the dishes. FOG management at the source is entirely a training issue. And it doesn't take long — a five-minute walkthrough of what goes in the trash versus what goes in the sink is the kind of habit that adds two or three weeks to your service interval without cutting a corner.
Frequently Asked Questions
The Restaurants That Avoid Problems Are the Ones That Kept a Schedule
It's not the ones with the biggest interceptors or the lightest menus. It's the ones that figured out their actual interval, wrote it down, and never skipped a service. A missed pump-out doesn't just delay the work — it makes the next visit harder, risks damage to the downstream line, and sets up the conditions for an emergency call at the worst possible time.
Know your volume. Know your trap size. Track the 25% mark. The math handles the rest.