Grease Interceptor Sizing for a Restaurant Tenant
A restaurant tenant is being built out in a mixed-use building. The kitchen has a three-compartment sink (each compartment 500 mm x 500 mm x 400 mm deep), one pre-rinse spray sink, two hand sinks, and a floor drain in front of the cooking line. The tenant proposes a single 20-gallon-per-minute (gpm) hydromechanical grease interceptor with a 40 lb grease capacity, installed indoors under the three-compartment sink, with a 50 mm (2 in.) vent and a cleanout on the outlet. The building's existing 100 mm (4 in.) sanitary line runs 12 m to the main. The owner wants the smallest unit that will pass inspection. The local authority requires that the interceptor be sized by the fixture-capacity method and that no grease-laden waste bypass the unit.
1. Determine whether the proposed interceptor is adequately sized, and show the calculation approach.
Read the worked answer
Use the fixture-capacity (dump) method the authority requires.
Step 1 - Compute the three-compartment sink capacity. Each compartment: 0.5 m x 0.5 m x 0.4 m = 0.10 m³ = 100 L. Three compartments = 300 L. Convert to gallons: 300 L ÷ 3.785 L/gal ≈ 79.3 gal.
The standard fixture-capacity method assigns the sink a flow rating based on its full capacity and a drain time (commonly 1 minute for the calculation, or the code's stated factor). Using a 1-minute drain, the sink alone demands roughly 79 gpm.
Step 2 - Add other fixtures. The pre-rinse spray sink and floor drain contribute additional load; even at modest allowances (e.g., 15–25 gpm combined) the total exceeds 90 gpm.
Step 3 - Compare. The proposed unit is rated 20 gpm with 40 lb grease capacity. That is far below the calculated demand - roughly one-quarter of the sink load alone.
Conclusion: The proposed interceptor is grossly undersized and will not pass. A unit sized to the calculated flow (on the order of 75–100 gpm depending on the authority's exact factors) with a proportionate grease capacity is needed, or an adequately sized outdoor gravity interceptor.
Also flag: the 50 mm vent and outlet cleanout are good practice but do not cure undersizing; and the unit must be installed so no grease-laden waste bypasses it.
2. The contractor proposes to skip the floor drain connection to save money, arguing the floor drain carries only mop water. Decide and justify.
Read the worked answer
The proposal should be rejected. The floor drain in front of the cooking line receives wash-down water that carries grease, oil, and food solids from the cooking area. The authority's stated condition is that no grease-laden waste bypass the interceptor; a floor drain in a commercial kitchen is precisely the kind of fixture that carries grease-laden waste, even if the tenant calls it 'mop water.'
Reasoning:
- Source control: Grease enters the building drain from multiple points, not just the sink. Floor drains, pre-rinse sinks, and dish machines all contribute. Interceptor sizing and selection assume the connected fixtures; omitting one defeats the design.
- Code intent: Interceptors are required for fixtures that discharge grease-laden waste, and the fixture list is determined by what the fixture actually receives, not by the tenant's label.
- Downstream risk: The 12 m run to the main is where grease will congeal if it bypasses the interceptor, causing blockages and backups in a mixed-use building - a liability for the owner.
- Inspection: The inspector will look for all grease-producing fixtures connected. An unconnected floor drain in the cooking area is a predictable correction notice.
Acceptable alternative: connect the floor drain to the interceptor, or provide a separate properly sized interceptor for that area. Cost savings do not override the no-bypass requirement.