Grease Trap Sizing

Two numbers, and they are not the same number

Every trap carries two ratings and people mix them up constantly. Flow rate, in gallons per minute, is how fast water may enter before separation stops working. Capacity, in pounds or gallons, is how much grease and solids the box can hold before it needs pumping. A unit rated at 25 gallons per minute and 50 pounds of grease is telling you two different things about two different failures. Exceed the flow rate and the trap fails immediately, every service period, because water is moving through faster than it can cool and stratify. Exceed the capacity and the trap fails gradually, over weeks, as the working volume fills with grease from the top and solids from the floor. The first failure is a plumbing problem and gets fixed with a flow restrictor or a bigger unit. The second is a scheduling problem and gets fixed with a shorter interval. If somebody hands you a trap recommendation with one number on it, ask which one it is, because you need both to know whether the thing will work in your kitchen.

Where the fixture count goes wrong

The common way sizing gets botched is counting the fixtures that are obvious and missing the ones that are not. The three-compartment sink is always counted. The mop sink often is not, and a mop sink carries floor grease and detergent straight to the trap. Floor drains under a fry station get left off constantly because they are not fixtures in the usual sense, and they are frequently the dirtiest inlet in the building. Wok ranges with a rinse trough, combi ovens that drain condensate and fat together, tilt skillets, rotisserie catch pans, the pre-rinse spray at the dish station: each of these is a real input. Then there are the fixtures that must not be on the trap at all. Toilets and urinals never tie into a grease trap, and in most jurisdictions neither does a garbage disposer, because a disposer turns solid food waste into a slurry that fills the bottom of the box faster than anything else in the kitchen. Getting the list right before anybody sizes anything is most of the work, and it takes a walk through the space with a tape measure rather than a look at a drawing.

Undersized traps and oversized traps both fail, differently

An undersized trap is the failure everyone expects. Water blows through, grease goes to the lateral, the interval collapses to weeks, and eventually the line backs up onto the kitchen floor during service. What people do not expect is that oversizing has its own failure mode. A trap far larger than the kitchen’s load holds wastewater for a very long time between services, and stagnant grease and food solids sitting in a warm box go septic. That produces hydrogen sulfide, which smells like rotten eggs, comes back up through the floor drains into the dining room, and eats the concrete and any steel in the structure from the inside. We have opened oversized in-ground units with badly spalled walls and a corroded frame at an age where the box should have looked new. Bigger is not automatically safer. The right size is the one matched to what the kitchen actually drains, serviced at an interval that keeps the contents moving.

Indoor unit or in-ground interceptor

Sizing usually decides this for you. Small hydromechanical units, the ones that sit under a sink or recessed in the floor, generally top out in the range where a modest kitchen lives. Past that, you are into an in-ground interceptor, which is a buried concrete or poly structure sized in the hundreds or thousands of gallons and serviced from the surface through manhole covers. The tradeoffs are real in both directions. An indoor unit is cheap to set, easy to reach, and has to be serviced more often, in the building, with the smell in the building. An in-ground interceptor holds far more, gets serviced from a truck in the parking lot without anybody walking through the dining room, and requires you to have somewhere outside to put it with truck access. In dense Miami storefronts and in older Broward strip centers, that last constraint is often the one that settles the whole discussion. We would rather have that conversation while the kitchen is still on paper than after the slab is poured.

What South Florida adds to the calculation

The water table here is shallow, in a lot of Miami-Dade only a few feet down, and that changes the sizing conversation the moment the trap goes in the ground. A large empty concrete interceptor sitting in saturated soil wants to float, so it needs enough mass, the right bedding and sometimes an anti-buoyancy detail to stay where it was set. Poly tanks are lighter and more prone to it than concrete. Groundwater also finds any joint or penetration that was not sealed properly, and an in-ground box slowly filling with groundwater between services is a box with less working volume than the drawing says it has. The other local factor is heat. Grease stays softer in a warm box, which helps at the pump, but warm standing wastewater turns septic faster, which argues against long intervals on a generously sized unit. None of this changes the fixture math, but it changes which trap type and which placement make sense once the math is done.

Worth knowing

If your trap needs pumping every few weeks, sizing is usually the reason, not the cleaning. Get the fixture count and the flow rate checked before you pay for another short interval.