How often to pump a grease trap / interceptor (25% rule)

A plain-language guide to the 25% rule and common U.S. municipal pump cadences for restaurants and food-service establishments. Not legal advice — local ordinance wins.

Related: How to choose a grease-trap / FOG hauler.

Who this is for

If you run a restaurant, cafe, school kitchen, hotel kitchen, or any food-service establishment with a grease trap or grease interceptor, you need a clear answer to “how often do we pump?” The wrong answer shows up as backups, failed inspections, or enforcement from your city’s pretreatment / FOG program.

This page explains the 25% rule and common U.S. municipal pump cadences in plain language. It is not legal advice. Rules are set by your city, county, or utility — always check your local ordinance. EPA’s National Pretreatment Program prohibits solid or viscous pollutants that obstruct sewers (40 CFR 403.5); day-to-day pump schedules are local, not a single federal day-count.

The 25% rule explained plainly

Many U.S. pretreatment programs use a 25% rule: floating FOG (grease) on top plus settled solids on the bottom should not collectively exceed 25% of the device’s liquid depth (sometimes phrased as design hydraulic depth or capacity). When that threshold is met or exceeded, pump — often even if your calendar date has not arrived yet. Annapolis pretreatment materials note that this rule is used by programs across the country (Annapolis Grease Interceptor Maintenance PDF).

Simple measurement formula: (Grease layer depth + Settled solids depth) ÷ Total liquid depth × 100 = % full

Worked example from a real municipal design standard: Pocatello Design Standards §600 provides a clear illustration: a grease interceptor with a 60-inch total liquid depth contains 6 inches of floating grease and 10 inches of settled solids. (6 + 10) ÷ 60 × 100 = 26.7% → exceeds 25% → clean immediately.

(Source: Pocatello Design Standards §600, Section 600 – Sanitary Sewer)

A second example from the Muscatine IA FOG BMP Manual shows a 4-foot deep interceptor with 0.5 ft FOG + 1.0 ft solids = 37.5%, which fails the 25% threshold.

(Source: Muscatine Iowa FOG BMP Manual)

Multiple programs—including Annapolis MD and Rogers Water Utilities AR—formally adopt this (grease+solids)/depth method and instruct operators to pump at 25%.

Typical ordinance cadences (calendar max + whichever comes first)

Municipal schedules almost always use a dual trigger: a maximum calendar interval and a 25% threshold. Whichever occurs first determines the pump date.

City / Source Calendar Max 25% Trigger Additional Notes
Nashville Metro Water Services FOG Policy 2025 Interceptors: complete pump of both chambers at least every 90 days (with DMD/DMA, not less often than every 180 days). Grease traps: minimum every 30 days Pump when surface FOG + settled solids reach 25% of liquid depth Records + manifest on site minimum 3 years. Partial pump / reintroduction prohibited.
Pocatello BMP Light/low-volume interceptors may go less frequently ≤25% good; 25–50% fair/poor → revise schedule; >50% noncompliance → immediate clean Most interceptors cleaned every 60–90 days. Rarely less than every six months—owner must demonstrate with records. Passive undersink traps: weekly typical.
Orange County Sanitation District (OCSD-25) Not less than every 6 months Maintain so 25% not exceeded; quarterly when frequency not established; adjust with data Records ≥2 years.
Muscatine IA FOG BMP Manual Outdoor interceptors often ≥4×/year; many monthly At least every 90 days OR when FOG+solids reach 25% Under-sink traps weekly / no less than biweekly. Records ≥3 years.
Rogers Water Utilities AR FOG Policy (Mar 2024) Minimum pumping twice per year (180 days) Authority sets frequency per user; 25% rule defined Local authority determines user-specific interval.
Annapolis MD — 25% rule used by pretreatment programs across the country; (grease+solids)/depth; pump at 25% States the rule is widely adopted nationally.

Key takeaway: Always pump whichever comes first—the calendar date or the 25% measurement.

How to measure / decide your interval

  1. 1. Pull the interceptor access lid (when safe and per your hauler’s SOP).
  2. 2. Measure three depths using a marked stick or pole:

- Floating grease layer thickness (from top of liquid to top of grease mat). - Settled solids thickness (from bottom of liquid to bottom of solids layer). - Total liquid depth (from top to bottom of liquid zone).

  1. 3. Calculate the percentage using the formula above.
  2. 4. Compare to 25%. If ≤25%, your current interval is working. If >25%, schedule a pump before the next inspection.
  3. 5. Document the measurement in your on-site log (see Recordkeeping section).

Pocatello’s inspection criteria provide a practical grading scale: ≤25% good; 25–50% fair/poor → revise schedule; >50% noncompliance → immediate clean. (Source: Pocatello BMP Fact Sheet)

When to shorten the interval

Several operational factors can push you toward more frequent pumping, even if your calendar interval hasn’t elapsed:

Example: Nashville’s policy requires grease traps to be pumped every two weeks if FOG+solids exceed 25%. Muscatine notes that many operators run monthly interceptor pumps, and outdoor grease interceptors are often serviced four or more times per year. (Sources: Nashville FOG Policy 2025; Muscatine IA FOG BMP Manual)

If your measurements consistently land in the 15–20% range, you may safely extend toward your ordinance’s maximum. If you hover near or above 25%, stick to the shorter end of your city’s range.

Recordkeeping

Every municipality requires documented proof of pumping and inspection. Common requirements across the verified sources:

Source Minimum Record Retention Required Content
Nashville Metro Water Services 3 years on site Manifests, pump dates, measured depths, hauler signature. Partial pump/reintroduction prohibited.
Orange County Sanitation District ≥2 years Pump dates, volumes, 25% measurement records.
Muscatine IA ≥3 years Pump dates, measured FOG+solids depths, interceptor location.
Haysville KS Manifest per city code Includes inches of floatable grease, settleable solids, and total liquid depth per pump event. No decanting/return allowed.
Florida DEP Per hauler manifest Service Manifest (DEP Form 62-705.300(3)); originators keep signed manifests.

Always keep a physical or digital log on-site that includes the date, measured depths, calculated percentage, hauler name, and any corrective actions. This documentation is often the first thing inspectors request.

Red flags

If any of these appear, contact your local pretreatment program immediately and adjust your interval downward.

Sources

Summarizes federal authority: 40 CFR 403.5(b)(3) prohibits solid/viscous pollutants causing obstruction/interference in POTWs; day-to-day pump schedules are local, not federal.

Prohibits pollutants that cause interference; local pretreatment programs implement FOG controls.

Interceptors: complete pump both chambers at least every 90 days; with DMD/DMA may go longer but not less than every 180 days. Grease traps minimum every 30 days; pump at 25% FOG+solids. Records 3 years; partial pump prohibited.

Most interceptors cleaned every 60–90 days; light/low volume may less; rarely less than every six months. Inspector criteria: ≤25% good; 25–50% fair/poor revise schedule; >50% noncompliance immediate clean. Passive undersink traps weekly typical.

25% capacity rule: grease on top + solids on bottom cannot total over 25% of total volume. Example: 60" deep, 6" grease + 10" solids = 26.7% → violation → clean immediately.

25% rule defined as combined FOG+solids ≤25% of design hydraulic depth. Maintain so 25% not exceeded; not less than every 6 months; quarterly when frequency not established; adjust with data. Records ≥2 years.

25% rule compliance; example FOG 0.5 ft + solids 1.0 ft / depth 4.0 ft = 37.5% fails. Interceptors at least every 90 days OR when FOG+solids reach 25%; outdoor often ≥4×/year; many monthly; under-sink traps weekly / no less than biweekly. Records ≥3 years.

States 25% rule used by pretreatment programs across the country; (grease+solids)/depth; pump at 25%.

Licensed haulers, Service Manifest (DEP Form 62-705.300(3)); no single statewide pump-day interval — local rules still apply. Statute: https://www.flsenate.gov/Laws/Statutes/2026/0403.0741

Complete pump minimum every 90 days or more as needed unless director approves longer; no decanting/return; manifest includes inches of floatable grease, settleable solids, and total liquid depth.

Defines 25% rule; minimum pumping twice per year (180 days); authority sets frequency per user.

Included for context on approved haulers and manifest requirements.

Affiliate disclosure

FOG Desk may earn a referral fee when you hire certain listed haulers or related services through links on this site. We only list partners that meet our hauler checklist. Recommendations are not endorsements by any city or utility. Always verify licenses with your local pretreatment program.

Related pages

Choose a FOG hauler — checklist

No prices invented. No reviews fabricated. Local law wins over this summary. Not legal advice.