What Happens to FOG When It Enters the LA Sewer System

FOG stands for Fats, Oils, and Grease. It is the collective term for all the lipid-based byproducts of commercial cooking — rendered animal fat from carnitas and BBQ, frying oil from fryers and deep-fry stations, butter and dairy residue from baking operations, and the emulsified fat that comes off dishes during washing.

When it leaves your kitchen warm and liquid, it looks harmless. The problem begins about 50 to 100 feet downstream, once the temperature drops and the FOG starts to cool. At that point it transitions from liquid to semi-solid — and it sticks to whatever pipe wall it touches first.

Once FOG coats a pipe wall, the buildup compounds over time. Each new wave of warm grease adds another thin layer on top of the existing deposit. Calcium compounds in the water bind with the fat molecules to form a harder, soap-like material that resists normal water flow. Left unchecked, this is how a 12-inch sewer lateral narrows to 4 inches over a few years — without a single sign visible at surface level.

In severe cases, FOG accumulations from multiple upstream sources combine in main sewer trunks and form what engineers call fatbergs — masses of congealed grease, wet wipes, and debris that can grow to the size of a car or larger. The UK has had fatbergs weighing over 130 tons. Los Angeles, with one of the highest restaurant densities of any city in the United States, is not immune.

When a sewer pipe becomes fully blocked, sewage has nowhere to go but up. A Sanitary Sewer Overflow (SSO) can push untreated wastewater into streets, storm drains, and waterways. In Los Angeles, storm drains connect directly to the LA River and eventually to Santa Monica Bay — which is why a sewer overflow upstream can result in a beach closure at Santa Monica, Venice, or Malibu within 24 to 48 hours.

FOG from commercial kitchens is identified as the leading cause of SSOs in LA County.When an overflow occurs, LA Sanitation uses sampling and flow tracing to identify contributing FSEs in the upstream zone. If your facility is found to have been discharging FOG without a functioning interceptor — or with one that was not properly maintained — you can be held financially liable for cleanup and mitigation costs that can exceed $50,000 per event.

The City of Los Angeles Bureau of Sanitation operates a formal FOG Control Program that applies to all Food Service Establishments connecting to city sewer lines. The program has three core requirements:

• 1
  Install a properly sized interceptor: Sizing must comply with local plumbing code and be approved by the Bureau of Sanitation before final occupancy permit is issued.
• 2
  Maintain it under the 25% rule: Service the interceptor before the combined FOG and sludge layer reaches 25% of capacity. Service records must be retained for three years.
• 3
  Use a licensed waste hauler: The FOG waste must be transported by a California-licensed waste hauler to an approved disposal or rendering facility. The manifest is your proof.

Non-compliant FSEs face civil penalties starting at $500 per day until the violation is corrected. Repeat violations or those that contribute to an SSO event carry significantly higher penalties and can result in permit suspension.

When FOG is properly collected by a licensed hauler, it does not go to a landfill. In California, most recovered FOG goes to one of two places:

Rendering facilities process animal fat into usable byproducts — including tallow for industrial applications and biodiesel feedstock. Wastewater treatment plants in some cases accept FOG for anaerobic digestion, converting it into biogas that generates electricity on-site.

Properly managed FOG is not waste — it is a recovered resource. That is why California invests in enforcement: keeping it out of the sewer system means it can be put to productive use instead of causing expensive infrastructure damage and environmental harm.