A gravity grease interceptor has a liquid depth of 40 inches. Take as given the maintenance rule in force on this project: the unit is to be pumped when the combined accumulation of floating grease and settled solids reaches 25 percent of the liquid depth. A technician measures 6 inches of grease floating on top and 3 inches of solids on the bottom. What is the correct call?
- AIt is due now, because the floating grease layer measured by itself has already passed the limit the maintenance rule sets for the unit.
The rule given combines the two layers rather than testing each separately, and 6 inches of grease alone is well under the 10-inch trigger in any case. This reads one number and stops.
- BIt is due now, because 9 inches of combined accumulation is more than 25 percent of the liquid depth.
This compares 9 inches against a trigger that was never computed. Twenty-five percent of a 40-inch liquid depth is 10 inches, so 9 inches has not reached it.
- It is not due yet, because the combined 9 inches is just under the 10-inch trigger, though it should be scheduled soon
- DIt is not due, because only the settled solids on the bottom are counted toward the trigger.
Counting only the settled solids ignores the grease layer, which is the entire reason the interceptor exists. It would let a unit fill with grease and still read as compliant.
Why C is correct
Twenty-five percent of 40 inches is 10 inches, and the rule counts grease and solids together, giving 6 plus 3 for 9 inches. The measured accumulation is below the trigger, so the unit is not yet due, but the margin is small enough that the next pump-out belongs on the calendar.
What this question is testing
Whether the candidate can convert a percentage rule into an actual dimension before comparing it, and whether they read carefully enough to combine both accumulation layers as the stated rule requires. It also checks whether they can deliver a defensible answer on a close call, including the judgment that a unit near its trigger needs its service interval shortened.
On the job
Service technicians make this call from the top of an open interceptor with a sludge judge and a flashlight, and the owner is standing there hoping to skip a pump-out. Doing the percentage arithmetic out loud settles it. Two other things matter in the field: measure the grease layer before you disturb it, and remember that once the combined depth is close, the interval you are using is too long for the kitchen load.
Memory technique
Grease on top plus sludge on the bottom, both count toward the quarter.
Exam tip
Turn a percentage into inches or gallons before you compare anything to it; most errors happen when the comparison is made in mixed units.
Where to look it up
Chapter 10, the interceptors and separators section on grease interceptor maintenance; the specific percentage trigger is normally set by the local sewer authority. UPC and local FOG ordinances differ, so confirm the rule in force.