The technical guidance on this page reflects these published codes and standards, which your local authority (AHJ) adopts, amends, and enforces. It is educational, not engineering or legal advice — confirm the current requirements for your jurisdiction.
Have a grease trap pumped before the combined grease and solids reach 25% of its liquid depth (the 25% rule). Many jurisdictions require gravity interceptors pumped at least quarterly and small hydromechanical traps monthly, but your local sewer authority sets the binding schedule.
Fats, oils, and grease, collectively called FOG, pour down the drain as warm liquid and then cool and congeal inside pipes and sewers. Over time they build up, restrict flow, and cause sanitary sewer overflows. A grease trap or grease interceptor is a plumbing device that slows wastewater long enough for grease to float to the top and solids to settle to the bottom, so only relatively clean water continues to the sewer.
That is why nearly every municipality with a sewer system runs a FOG control program and requires food service establishments to install, maintain, and document a grease interceptor. The EPA regulates these discharges under the Clean Water Act, but the specific rules, sizing, and pump-out frequency are set locally by your sewer authority or authority having jurisdiction (AHJ).
Traps come in two broad types. Small hydromechanical grease interceptors (HGIs), often installed indoors under or near a sink, are compact and fill quickly. Larger gravity grease interceptors (GGIs), typically buried outside, hold far more volume and are pumped less often. Both work on the same principle of separation, and both are governed by the same maintenance standard covered next.
The core maintenance standard across the industry is the 25% rule, sometimes called the one-quarter rule. It states that when the combined thickness of the floating grease layer and the settled solids reaches 25% of the trap's total liquid depth, the device must be pumped out and cleaned.
The logic is capacity. A grease interceptor is only designed to retain grease up to a point; once accumulation passes roughly a quarter of the working volume, the trap can no longer separate incoming wastewater effectively and grease begins to carry over into the sewer line. The 25% threshold builds in a conservative safety margin and appears in industry standards and municipal codes nationwide. It is measured with a dip stick or sludge-judge probe lowered into the trap, not guessed at from the top, since the settled solids layer at the bottom is invisible from above.
The 25% rule sets the trigger; your fixture type, size, and menu determine how fast you hit it. A high-volume fryer-heavy kitchen with a small under-sink trap can reach 25% in a week, while a large exterior interceptor may take months. Local codes translate this into a maximum interval regardless of fill:
| Interceptor type | Typical minimum pump-out | |
|---|---|---|
| Small hydromechanical (indoor) trap | Often every 30 days | |
| Large gravity interceptor (exterior) | Often every 90 days | |
| Any device at the 25% level | Immediately, whichever comes first |
Treat these as common examples, not universal law: your permit or ordinance states the binding frequency for your address. When in doubt, pump on the more frequent schedule and adjust based on how full the trap actually is at each service.
Grease trap pumping is done by a licensed liquid-waste or grease hauler. They fully pump the device, scrape the walls and baffles, inspect for damage, and haul the waste to an approved disposal or rendering facility. A proper cleaning removes the entire contents, not just the floating grease, so solids do not accumulate at the bottom over time.
Because grease requirements are AHJ-driven, the single most useful step is to contact your municipal sewer authority, wastewater utility, or building department and ask for the local FOG ordinance. It will tell you the required interceptor size, the mandated pump-out frequency, the recordkeeping period, and the penalties for a blocked or overflowing trap, which can be substantial.
Build the schedule into your kitchen's routine the way you would any other preventive maintenance, alongside hood cleaning and equipment service. A well-maintained grease trap protects your plumbing, keeps you compliant, and avoids the far larger cost of a backup during service. For the broader kitchen build, see our guides on flooring and equipment selection.