Gravity Interceptor Sizing
A gravity interceptor is a tank. Wastewater sits in it long enough for grease to float and solids to sink. A larger kitchen, with more sinks and a higher flow, needs more volume so the grease has time to separate.
The right volume is a plumber's calculation and a local code question. Grease.org describes the idea so a restaurant knows why a five-gallon under-sink box and a multi-thousand-gallon outdoor tank are both called traps, and why they are pumped on different schedules.
The range kitchens actually have is wide. A small interceptor under the sink may hold about five gallons. A large in-ground trap may hold several thousand gallons and use more than one baffle. Both are traps. They are pumped on different schedules because they fill at different rates. The volume itself is a plumber's calculation.
Sizing is time and volume
A gravity interceptor is a large tank, usually outside, that slows kitchen wastewater long enough for fat to rise and solids to sink. Sizing is the question of how big that tank must be for the kitchen you actually run. A sizing sheet starts from the fixtures: sinks, dishwashers, floor drains, and the flow each one can send. It also asks whether the flow arrives as a steady rinse or as a dump from a full sink. A tank that is large on paper and short on retention time will pass grease. A tank that is enormous and never pumped will pass grease too, once it is full.
The twenty-five percent rule is an operating rule, not a sizing rule. When the floating cap and the settled sludge together reach about a quarter of the liquid depth, the remaining water path is too short and the tank is due for a pump-out. A correctly sized tank still needs that measurement. Sizing without an interval is how a new restaurant fails its first inspection. GREASE.ORG pumps and cleans the tank. A plumber or an engineer stamps the drawing that chose the tank. Those are different jobs, and the kitchen needs both.
Retrofits are where sizing goes wrong. A menu that adds a fryer program, a new dishwasher, or a second prep sink can outgrow a tank that was legal on opening day. The symptom is a short interval and grease at the outlet even after a careful pump-out. The fix is a resized tank or a reduced flow, not a stronger degreaser. Degreaser carries grease through a gravity tank that was already marginal.
Tell the pumper the tank volume and where the manway is. Tell the designer the real fixture count, not the count from a brochure. Gravity sizing is unglamorous arithmetic. It is also the reason a large outdoor tank either works for years or becomes a monthly emergency.