How often should a restaurant grease trap be pumped?

Clean the trap before FOG and solids reach 25% of its total capacity. This is the widely used "25% rule," and the City of Portland's Bureau of Environmental Services states it plainly: a grease interceptor should be cleaned "when 25 percent of the wetted height is occupied by grease and settled solids."

The reason is physics, not bureaucracy. A trap works by holding water long enough for grease to float and food bits to sink, so only the cleaner middle layer flows out. When the combined grease and solids fill a quarter of the tank, the water's path shortens, it moves through faster, and separation fails. At that point the unit is passing grease even though it is only one-quarter "full."

Waiting until a trap is half or fully packed is the most common mistake restaurant owners make. By then grease has already been escaping into the line for weeks. The 25% mark is an early-warning threshold, not a deadline you can push.

To use the rule you have to measure. A "dip test" with a sludge-judge stick or a dipstick tells you the depth of the top grease cap and the bottom solids. Compare that to the tank's working depth. If grease plus solids is approaching one-quarter, schedule a cleaning before the next service period.

Most kitchens land on one of two common minimums depending on the type of unit they have.

• Hydromechanical grease interceptors (HGIs), the smaller under-sink or in-floor units, are typically pumped about every 30 days. They hold far less, so they hit 25% fast.
• Gravity grease interceptors (GGIs), the large in-ground concrete or fiberglass tanks outside the building, are typically pumped about every 90 days. Their size buys more time between cleanings.

Portland's program sets exactly these as floors: hydromechanical interceptors at minimum every 30 days, gravity interceptors at minimum every 90 days, with more frequent service required if the trap fills faster.

Treat 30 and 90 days as ceilings on the interval, not targets. A busy fryer-heavy kitchen can fill a small trap in two weeks. The 25% measurement always overrides the calendar: if you hit a quarter full in 18 days, your real interval is 18 days, regardless of what the minimum allows. The calendar number is the longest you should ever go, not a promise that the trap can wait that long.

Here is the part that trips up owners: there is no single national pumping number. The actual required interval is set by your local FOG (fats, oils, and grease) ordinance, administered by the city or county sewer authority that your kitchen drains into. Phoenix and Maricopa County point to the jurisdiction's own rules rather than publishing one fixed frequency, so the 25% rule and the 30 / 90-day standards are the practical benchmarks to plan around until you confirm the local number.

Local ordinances exist because every sewer system has different capacity and different problems. Some cities require quarterly pumping as a hard minimum no matter how slowly your trap fills. Others tie the schedule to your trap size and food type. Many also dictate how the trap must be cleaned. A full pump-out, removing all liquid, grease, and solids, is the accepted method; skimming only the top grease and leaving the water and sludge does not reset the trap.

The plumbing code sets the baseline that local rules build on. The International Plumbing Code (IPC), Chapter 10 requires grease interceptors to be "maintained in efficient operating condition by periodic removal of the accumulated grease," and bars discharging the removed grease back into any drainage system. Your city ordinance fills in the specific frequency and reporting on top of that requirement.

To find your rule, contact your municipal wastewater or pretreatment program and ask for the FOG ordinance and any permit tied to your business. In Maricopa County, food establishments also fall under county environmental health requirements for indirect waste and grease control, which inspectors check.

Pumping the trap is only half the job. You also have to prove you did it, and most FOG programs require you to keep your service records for about three years. Portland's program directs interceptor owners to retain cleaning and maintenance records for three years and make them available on request.

Each service record should capture the date of the cleaning, the company that performed it, the volume of waste removed, and where that waste was hauled and disposed. Licensed grease haulers issue a manifest or service ticket for exactly this purpose. Keep them in one binder or digital folder so you can hand them over when an inspector asks.

These records protect you in two ways. They are the evidence a health or sewer inspector wants to see, and they are your own data: a year of tickets shows how fast your specific trap fills, which lets you tighten or relax your schedule with confidence instead of guessing.

Skipping or stretching pump-outs is expensive in ways that dwarf the cost of a cleaning. Grease is the single largest cause of sewer blockages. According to the EPA's Report to Congress on Impacts and Control of CSOs and SSOs, grease accounted for 47% of reported sewer blockages, far more than tree roots or debris. The same report found that blockages were behind 48% of sanitary sewer overflows with a known cause, the largest single trigger.

That national pattern hits your kitchen first. When a neglected trap passes grease, the grease cools and hardens on the inside of your building's drain lines and the sewer lateral. Flow narrows, then stops. The result is a backup, often a floor drain or mop sink overflowing during your busiest service. The EPA notes that sewer overflows like these release raw sewage that threatens public health, which is precisely why authorities enforce FOG rules.

Backups bring odors, too. A trap sitting past its service date turns septic; the breakdown of trapped grease and food produces hydrogen sulfide, the rotten-egg smell that drifts up through floor drains and into the dining room.

Then there is the inspection. An overdue trap, a missing service log, or a grease-passing unit is a standard citation for health and pretreatment inspectors. It can mean fines, a compliance order, repeat re-inspections, and in serious cases a threat to your permit to operate. A clean trap with three years of tickets behind it is one of the easiest line items to pass.

To set your own schedule, start with the minimum for your unit type, then let your own measurements correct it.

1. 1Identify your trap. Is it a small hydromechanical unit (start near a 30-day interval) or a large gravity interceptor (start near 90 days)? Note its capacity.
2. 2Confirm the local rule. Call your city or county wastewater program for the FOG ordinance and any required frequency or reporting. The local number wins if it is stricter.
3. 3Measure between pumpings. Dip-test the trap on a set day each week or month and log the grease and solids depth. The moment it nears 25%, that is your true interval.
4. 4Set a recurring date with a licensed hauler. Lock in standing service so it never gets forgotten in a busy week, and adjust the frequency once your dip-test data shows the real fill rate.
5. 5File every manifest for at least three years.

Two kitchens with identical traps can need very different schedules; a high-volume fryer operation fills a trap far faster than a salad-focused cafe. That is why the measured 25% point, not a generic calendar, should drive the final number. For more on what FOG is and why it hardens in your pipes, see our what is FOG (fats, oils, and grease) page, and for choosing the right unit see what size grease interceptor a restaurant needs. If a trap is already overdue and backing up, our commercial kitchen drain overflowing page covers the immediate steps.