Grease Trap

A grease trap is a plumbing device that intercepts and captures fats, oils, and grease (FOG) before they enter the wastewater disposal system. Also called grease interceptors, grease recovery devices, or grease converters, these required installations prevent FOG from clogging municipal sewer lines and protect wastewater treatment facilities from contamination.

How Grease Traps Work

Grease traps operate using basic gravity and buoyancy principles. Wastewater flows into the trap’s interior compartments, which slow the flow rate to allow separation. Solid food scraps sink to the bottom, grease and oil float to the top due to their lighter density, and the cleaned middle layer of water exits to the sewer system. This separation process requires a minimum retention time of 30 minutes to work effectively.

Most grease traps are constructed from stainless steel, plastic, concrete, or cast iron. The design dates back to the Victorian era, with Nathaniel Whiting receiving the first patent in the late 1800s. Modern standards from the Plumbing and Drainage Institute (PDI) and the American Society of Mechanical Engineers (ASME) ensure traps meet current plumbing codes and performance requirements.

Types and Sizing

Two main installation types serve different kitchen operations. Under-sink or point-of-use traps are smaller units typically installed beneath three-compartment sinks in lower-volume kitchens. In-ground gravity interceptors range from 500 to 2,000+ gallons and handle high-volume commercial operations.

Grease traps are sized by flow rate measured in GPM (gallons per minute) and storage capacity in gallons or pounds. Small operations might need 4-7 GPM units, while large restaurants require 50+ GPM capacity. Sizing depends on the number of fixtures draining into the trap, kitchen volume, and types of food prepared. A licensed plumber and local codes determine the correct size for your operation.

Hydromechanical grease interceptors (HGIs) and Automatic Grease Removal Units (AGRUs) represent modern alternatives that use motorized mechanisms to automatically remove collected grease, reducing manual cleaning frequency.

Maintenance Requirements

Professional cleaning every 1-3 months is standard industry practice, with many municipalities requiring quarterly minimum service. The industry’s “one-fourths rule” mandates cleaning when the trap reaches 25% full of FOG materials to prevent overflow and maintain proper function.

Never add bleach, emulsifiers, or enzymes to grease traps—these chemicals harm the natural bacteria needed for breakdown and can actually push grease further into sewer lines. Only bacteria additives specifically designed for grease traps are typically permitted.

The “brown grease” removed from traps is considered hazardous waste in many jurisdictions and must be disposed of by licensed professionals. Most restaurants generate 300-400 pounds of this waste annually, which typically goes to landfills, though some facilities offer specialized recycling services.

Regulatory Compliance

Most local governments and health departments require commercial kitchens to install and maintain grease traps. These regulations protect municipal sewer infrastructure from costly blockages and prevent environmental damage to waterways. Failure to maintain your grease trap results in health code violations, fines, emergency service costs significantly higher than routine maintenance, and potential restaurant shutdown.

Proper waste management practices help reduce your grease trap load. Using can liners and garbage bins to dispose of solid food waste before it reaches sinks extends time between cleanings and prevents system overload.