The Grease Trap: How It Actually Works.
Most restaurant operators know they have a grease trap. Almost none know what's happening inside it. Here's the actual science — what FOG is, how it separates from wastewater, what the baffles do, and why pouring grease down a drain is a decision that costs thousands.
THE BASIC PROBLEM
FOG Doesn't Stay Liquid. That's the Entire Issue.
In a commercial kitchen, fats, oils, and grease leave the sink at 130–160 °F — fully liquid, mixed with dishwater, invisible. They flow through the drain line the same as any other wastewater. Nothing looks wrong.
The problem starts the moment that water cools. Fats and greases are hydrophobic — they don't dissolve in water. As temperature drops, they separate, congeal, and coat the interior walls of whatever pipe they're traveling through. By the time wastewater reaches underground sewer temperatures (60–70 °F in Southern California), FOG has solidified into a wax-like layer that narrows pipe diameter with every additional discharge.
This is why commercial kitchens can't discharge FOG directly into the sewer. It accumulates, hardens, and eventually blocks the line. The grease trap exists to catch it before it gets there.
THREE SUBSTANCES, THREE BEHAVIORS
Not All FOG Is the Same.
Fats — solid or semi-solid at room temperature. Butter, lard, shortening, and animal fat. These congeal fastest and form the hardest deposits inside drain lines. A 20-quart stockpot of beef tallow dumped down a sink can coat 30 feet of pipe in a single pour.
Oils — liquid at room temperature but still hydrophobic. Vegetable oil, canola, olive oil, and fryer oil. They float on wastewater and accumulate as a slick layer on the trap's surface. Fryer oil is the highest-volume FOG contributor in most commercial kitchens.
Grease — the byproduct of cooking with fat and oil. Rendered pan drippings, sauces, emulsified dressings, and the residue that coats dishes during pre-rinse. Grease carries food solids with it, forming the sludge layer at the bottom of the trap.
INSIDE THE TRAP
How a Grease Trap Actually Separates Waste.
A grease trap is a holding tank with internal baffles that slows wastewater flow long enough for physics to do the work. There are no filters. No chemicals. No moving parts. It's pure density separation — FOG floats because it's lighter than water, and food solids sink because they're heavier.
Here's what the inside looks like at any given moment:
Fats, oils, and grease float to the surface and form a solid or semi-solid cap. This is the layer that gets measured to determine when cleaning is required. As the kitchen continues discharging, this layer grows downward.
The middle layer — relatively clear water that has shed its FOG upward and its solids downward. This is what exits the trap through the outlet pipe and flows into the sewer. When the trap is functioning correctly, this water meets discharge limits.
Food particles, sediment, and heavy organic material settle to the bottom. Over time, this layer builds upward — reducing the effective volume of the effluent zone and compressing the separation space.
THE KEY COMPONENTS
Every Part of the Trap Serves the Separation Process.
A grease trap has no moving parts and no consumable media. Every component exists to control flow — slowing wastewater down, directing it through the right path, and keeping FOG from reaching the outlet.
Inlet Baffle
Flow Control
Directs incoming wastewater downward into the middle of the tank, below the FOG layer. Without the inlet baffle, fast-moving dishwater would push floating grease straight toward the outlet — defeating the trap entirely. The baffle forces water to enter low, giving FOG time to separate and rise.
Outlet Baffle
Containment
Blocks floating FOG from exiting the trap. The outlet pipe draws water from below the surface — well beneath the grease cap. If the outlet baffle is damaged or missing, grease passes straight through and enters the sewer line. This is the single most critical component during inspection.
Flow Diffuser
Velocity Reduction
Present in larger interceptors, the diffuser spreads incoming flow across the full width of the tank. High-velocity water from a busy pre-rinse station creates turbulence that remixes separated FOG. The diffuser breaks that energy, keeping the separation layers intact.
THE 25% RULE
Why 25% Is the Line — and What Happens When You Cross It.
The 25% rule is the standard used by sewer authorities across Southern California — including OCSD, LA County Sanitation, and LASAN. It means: the combined depth of the floating grease cap and the settled sludge layer must not exceed 25% of the trap's total liquid depth.
This isn't a suggestion. It's the enforceable threshold in most FOG pretreatment programs. Your sewer authority can measure it during an inspection, and if the combined FOG and solids exceed 25%, you're in violation — regardless of when you last had service.
Why 25%? Because that's the point where the effluent zone — the clean-water layer in the middle — no longer has enough volume to separate incoming FOG effectively. Above 25%, grease begins passing through the outlet with the discharge water. The trap stops working as designed, and FOG enters the sewer line.
Cleaning frequency depends on kitchen volume, menu type, and trap size. High-volume restaurants with fried food programs may hit 25% in 30 days. A low-volume café might take 90 days. The only way to know is to measure — or to set a program based on your kitchen's historical output.
Do
- Pump the trap before FOG + solids exceed 25% of liquid depth
- Keep every hauler manifest on file — inspectors check the paper trail
- Scrape plates and use strainer baskets before rinsing to reduce load
- Schedule service based on your kitchen's actual output, not just the 90-day maximum
Don't
- Wait until drains back up to schedule a pump-out
- Pour chemical degreasers into the trap — they liquefy grease and push it downstream
- Run hot water to "flush" FOG through the trap — it re-solidifies in the lateral
- Rely on kitchen staff maintenance as a substitute for licensed hauler service
TRAP CONDITION COMPARISON
Maintained vs. Neglected — The Difference Is Visible.
Before — Neglected Trap
- FOG layer exceeding 25% of trap capacity
- Outlet baffle partially or fully blocked
- Compressed effluent zone — no effective separation
- Odor, slow drains, and health code violation risk
- Missing or expired service manifests
After — Properly Maintained
- FOG layer well below 25% threshold
- All baffles clear and functioning correctly
- Clean separation: FOG cap, clear effluent, settled solids
- Zero violations on last health or sewer district inspection
- Current manifests and service log on file
GREASE TRAP TYPES
Three Types of Traps. Same Physics. Different Scale.
The type of grease trap installed depends on kitchen volume, available space, and local code requirements. All three use density separation. The difference is capacity, location, and how they're serviced.
Under-Sink
Passive Grease Trap
Small-capacity units (typically 20–50 gallons) installed directly below the sink or dishwasher. Common in low-volume operations — cafés, small delis, food courts. The kitchen staff is usually responsible for daily or weekly maintenance. Professional cleaning is required on a schedule determined by the local sewer authority.
Capacity: 20–100 gallons. Location: Under sink or in the floor, inside the kitchen. Service: Manual cleaning weekly; professional pump-out monthly to quarterly.
In-Ground
Gravity Grease Interceptor
Large buried tanks (500–2,000+ gallons) installed outside the building, typically in the parking lot or service area. These handle the combined discharge of an entire kitchen and are the standard for full-service restaurants, hotels, and institutional kitchens. Serviced by a licensed hauler with a vacuum truck — the same service that requires a state manifest.
Capacity: 500–2,000+ gallons. Location: Buried outside, accessed via manhole cover. Service: Professional pump-out on 30–90 day cycles depending on volume.
Automatic
Grease Removal Device (GRD)
Mechanical units that skim or reheat collected grease and deposit it into a separate container for disposal. These are the only trap type with moving parts. They reduce the volume of FOG that reaches the main interceptor but do not eliminate the need for periodic professional service. Some jurisdictions accept GRDs as a supplement — not a replacement — for a gravity interceptor.
Capacity: Varies. Location: Under or adjacent to the sink. Service: Operator maintenance daily; professional service per manufacturer schedule and local code.
THE REAL MATH
Pouring Grease Down the Drain Is a $10,000 Decision.
A properly maintained grease trap, serviced on schedule, costs $300–$600 per visit. A grease line backup that closes your kitchen for a day costs $5,000–$15,000 in lost revenue alone — before the emergency service call, the sewer authority fine, the health department re-inspection, and the remediation. The trap isn't an expense. It's the cheapest insurance in the building.