What is the 25% rule for grease traps?

What the rule says

A grease trap or interceptor must be cleaned when the combined volume of FOG and settled solids reaches 25% of the unit’s capacity.

It sits alongside Metro Water Services’ calendar minimums — at least every 30 days for grease traps, at least every 90 for interceptors — and the two work together. Whichever comes first is your service point.

The part most kitchens get wrong

It is not 25% grease. It is 25% grease plus solids, combined.

A grease trap accumulates in two directions. Fats, oils and grease float to form a layer at the top. Food particles, sediment and heavier material settle at the bottom. The functional volume — the space actually available to separate grease from water — is what is left between them.

Measure only the floating layer and you will read 15% and feel comfortable while the combined figure is well past 25%. Kitchens producing high volumes of rice and pasta are particularly exposed to this, because those contribute heavily to the settled layer while the grease layer looks unremarkable.

Why 25% and not more

Because a grease trap works by giving wastewater enough time and space to separate before it leaves.

Fill a quarter of the unit with accumulated material and the water moves through the remaining volume faster, with less time to separate. Grease that should have been captured passes through into the sewer. By the time a unit is half full it is barely functioning as a separator at all — it is a box that water passes through, while still looking like an installed grease trap.

The 25% figure is where separation efficiency starts falling off meaningfully. It is a performance threshold, not an arbitrary number.

How to stay under it

Have it measured at every service and recorded. Two or three visits of data tell you your real accumulation rate, and from there the interval is evidence rather than guesswork.

Check under-sink units weekly. If yours is more than half full at the weekly check, the frequency needs to increase — that is the built-in early warning.

Watch for the things that accelerate accumulation. Fried chicken and fish. Large volumes of rice and pasta. Extended hours. Seasonal peaks. Any change in menu that adds fried items should trigger a review of the interval.

Do not let the minimum become the plan. If you hit 25% in eighteen days, your cycle is eighteen days. The 30-day minimum is a backstop for slow-filling units, not permission.

What happens if you exceed it

Two problems, and the operational one usually arrives first.

Operationally, a unit past 25% is passing grease into the sewer line, where it cools and accumulates. That ends as a blocked line, and blocked lines in commercial kitchens tend to happen at the worst possible time — mid-service, with a full dining room.

From a compliance standpoint, Metro Water Services enforces the FOG program, including manifest use and three-year record retention. A service history showing consistent readings above threshold is a documented pattern of non-compliance, in your own records.

The records point

Every service should produce a certified hauler manifest, and those must be retained for three years. Manifests are what answer an inspector’s questions — a clean trap with no documentation is simply an undocumented trap.

Bottom line

Measure FOG and solids together, service at 25% regardless of what the calendar says, check under-sink units weekly, and keep every manifest for three years. Call (615) 234-9048 to get a schedule built around your actual accumulation rate.