The Grease Interceptor 25% Rule, Explained: How to Measure It and Set a Pumping Schedule

**The short answer:** under the 25% rule, a grease interceptor must be pumped out before the floating grease plus the settled solids reach 25% of the liquid depth (or capacity) of the unit. You find the percentage by measuring, not by counting days: `(FOG depth + solids depth) ÷ total liquid depth × 100`. If the result is 25 or more, the interceptor was pumped too late and the pumping interval has to get shorter. Most cities then add a calendar backstop on top, so the real rule is "25% or every N days, whichever comes first."

That is the version everyone repeats. The version that decides whether a restaurant gets a notice of violation is messier, because cities do not agree on the denominator, the chamber, the sampling method, or the backstop. This post goes through what the actual documents say.

The formula, as cities publish it

Halifax Water's maintenance diagram shows it as `% Full = (A + B) ÷ C × 100`, where A is the FOG layer, B is the settled solids and C is the total depth of the contents. Its example: 20 cm of FOG and 15 cm of solids in 95 cm of liquid is 36.8% after one month, so the interceptor should have been serviced sooner. Two weeks after a cleaning, 10 cm and 10 cm in 95 cm is 21.05%, so the interceptor was fine and a two-week schedule suits that restaurant (Halifax Water, Grease Interceptor Maintenance diagram).

New Orleans' Sewerage and Water Board uses feet. Its handout asks whether 0.5 ft of FOG plus 1.0 ft of solids in 4.0 ft of interceptor contents meets the rule: 1.5 ÷ 4.0 is 37.5%, so the 25% rule is exceeded and pump-out frequency must go up (Sewerage and Water Board of New Orleans, Grease Traps 101).

The arithmetic is the same everywhere. What changes is below.

What changes from city to city

Sources for the rows above: the Halifax and New Orleans documents linked earlier, plus Pleasanton's Sewer Pipe Blockage Control Program manual, Lancaster's FOG Control Program Manual (Jan 2019), Apex's SOP 002: FOG Inspection Plan, Calgary's grease interceptor guide and Austin's grease trap maintenance page.

Three things in that table trip people up.

**1. The denominator is not always the same.** Tacoma says the chamber, New Orleans says liquid depth in one document and design capacity in another, Pleasanton says usable volume measured at the water level, and Austin measures to the outlet invert. For a tank with straight sides, depth and volume percentages are nearly the same. For a unit with a sloped bottom, a baffle wall or an oversized dead zone above the outlet, they are not. The only safe move is to use the denominator the inspector for that sewer authority uses.

**2. Apex splits the chambers.** A two-compartment interceptor can pass in the outlet chamber and fail at the inlet. The Apex SOP tells the inspector to lower the sludge judge slowly, shake it gently once it passes through the grease mat, and compare against 35% at the inlet and 25% at the outlet. Hydromechanical units get a different test: a screwdriver moved through the wastewater, failing if there is no visible separation of grease and water. A hauler who only reads the first compartment is not checking the thing the inspector checks.

**3. "Pump within 5 days" is its own deadline.** Lancaster's manual says that once an inspector finds more than 25% of the liquid depth full of FOG and solids, the interceptor must be pumped out within 5 days of discovery. The clock starts at the inspection, not at your next scheduled visit.

The calendar side: a formula for days between pumps

The 25% rule tells you when you were late. It does not tell you the interval to schedule. Independence, Missouri publishes a worksheet that does, and it is one of the few places the math is written out (City of Independence, Grease Interceptor Clean-Out Frequency Worksheet, revised 27 July 2022).

Worked example

A barbecue restaurant serves 600 meals a day without flatware (very high, 0.058 lb per meal), so it produces 600 × 0.058 = 34.8 lb of grease a day.

The same 1,500 gal interceptor at a sandwich shop doing 200 meals a day (0.005 lb) fills at 1 lb a day, which gives 1,970 days on paper, so the 90-day maximum applies instead. The formula is a starting estimate. The sludge judge reading at the next visit confirms or overrides it.

How to take the reading so it survives an inspection

What the rule does not tell you

The EPA's National Pretreatment Program fact sheet puts it plainly: required maintenance frequency depends greatly on how much FOG a facility generates and on the best management practices it uses, and a kitchen that adopts them can often cut its pumping frequency (US EPA, Controlling Fats, Oils, and Grease Discharges from Food Service Establishments). The 25% number is a cleanliness threshold, not a schedule. Menu changes, a new fryer, a switch to or from flatware, a catering contract or a seasonal rush all move the interval, which is why a once-a-year "we pump every 60 days" assumption drifts.

How to set this up

For a pumper, the spreadsheet version of this works until about the third utility. After that, the thresholds, deadlines and denominators live in different heads.

The grease trap pumping software guide lays out the full build. For this specific rule, the pieces are:

FAQ

**Is the 25% rule a law?** Not by itself. It is a threshold written into individual city, utility or district rules, and sometimes only into guidance. Pleasanton's manual says it adopted the rule as a standard from its wastewater treatment provider, the Dublin San Ramon Services District. Always read the local ordinance or the restaurant's discharge permit.

**Does the 25% apply to traps under the sink?** Usually not in the same form. Small hydromechanical traps are generally on a fixed short cycle. Pleasanton's manual calls for weekly inspection and cleaning at least every 2 weeks, and Apex tests hydromechanical units by whether grease and water visibly separate rather than by a depth percentage.

**What if the interceptor passes at 24% on the day but fails a week later?** The rule is a snapshot. That is why cities add calendar caps (30 days in Halifax, 90 days in Lancaster, quarterly in New Orleans) and why a reading of 24% is a signal to shorten the interval, not a reason to extend it.

**Can I extend the interval if every reading is low?** Where the utility allows it, yes, up to the calendar maximum. The Independence worksheet caps the cycle between 30 and 90 days. Keep the readings, because they are the evidence for the longer interval.

**Who is liable if the line backs up?** The sewer authority's documents are clear that the establishment owner pays. New Orleans says the Sewerage and Water Board holds business owners liable for all costs of removing FOG from the sewer system, including damage to adjoining property, streets and sewer repair. Lancaster says costs for maintenance, repair and pumping fall on the establishment owner.

**Why does Austin use 50%?** Austin's code (section 15-10-197) is written around grease traps pumped at least every 90 days, or sooner when grease and solids make up 50% or more of the wetted height measured from the trap bottom to the outlet invert. It is a different rule, not a typo of the 25% rule.