Grease Trap: Definition & Professional Guide
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A grease trap is a plumbing device that intercepts fats, oils, and grease before they enter the drain-waste-vent system, using gravity separation to capture FOG at the fixture point and prevent it from solidifying downstream in sewer lines. Grease traps are required by code in virtually all commercial food establishments and increasingly recommended for residential kitchens where heavy cooking generates significant FOG discharge.
Separation Process: How FOG Interception Works
Grease trap operation relies on a simple physical principle: fats, oils, and grease are lighter than water and naturally float to the surface when flow velocity slows. Wastewater enters the trap through an inlet baffle, which reduces flow speed and directs water downward. As the water moves through the retention chamber, FOG rises to the top and accumulates in a floating layer while heavier food solids settle to the bottom. Clarified water exits through an outlet baffle positioned below the grease layer.
Sizing a grease trap correctly requires calculating the gallons-per-minute flow rate from all connected fixtures, not simply matching the inlet pipe diameter. A trap undersized for the actual flow rate allows water to pass through too quickly for adequate separation, sending grease directly into the sewer. PDI-G101 provides the standard testing and rating protocol that establishes grease removal efficiency at rated flow.
Hot water liquefies grease temporarily, but FOG re-solidifies downstream where the water cools, typically in the building’s main sewer line where repair access is far more difficult and expensive. A properly maintained grease trap captures FOG at the source before this downstream solidification occurs.
Under-Sink, In-Ground & Automatic Grease Traps
Under-sink grease traps are compact units installed in the cabinet below a kitchen sink, handling flow rates of 4 to 50 GPM. Stainless steel construction is standard for residential and light commercial use, with accessible lids for periodic cleaning.
In-ground grease interceptors are large-capacity buried tanks, typically 500 to 2,000 gallons, installed outside the building on the main kitchen drain line. Commercial kitchens with multiple fixtures and high-volume cooking require these larger interceptors to handle aggregate FOG loads.
Automatic grease removal devices use a timer-driven skimming mechanism to continuously remove accumulated grease into a separate collection container. Units from manufacturers like Grease Guardian eliminate manual cleaning cycles and produce a concentrated waste stream that some haulers accept for biodiesel recycling.
Point-of-use traps are small, single-fixture devices installed on individual drains such as floor drains beneath fryers or wok stations, providing targeted FOG capture before wastewater reaches the main interceptor.
Installation, Cleaning, and Drain Services
Grease trap installation and maintenance is part of Bonded Plumbworks’ drain and sewer services. An under-sink residential grease trap installation typically costs $200 to $500, while an in-ground commercial interceptor ranges from $1,500 to $5,000 depending on capacity and excavation requirements. Professional grease trap cleaning costs $150 to $400 per service visit.
FOG accumulation is the leading cause of residential sewer line blockages. When kitchen plumbing drains slow or back up repeatedly despite drain cleaning, Bonded Plumbworks’ technicians evaluate whether a grease trap would address the root cause rather than treating symptoms with repeated clearing.
IPC, UPC & ASME Compliance Standards
IPC Section 1003 and UPC Section 1014 require grease interceptors on all drains receiving FOG-laden waste in commercial food service establishments. ASME A112.14.3 specifies design, material, and performance requirements for grease interceptors. PDI-G101 establishes the testing protocol and minimum grease removal efficiency ratings. Local FOG ordinances in most municipalities mandate specific pumping frequencies, often quarterly, with certified hauler documentation. The 25 percent rule, adopted by most local codes, requires pumping when combined FOG and settled solids exceed 25 percent of total trap capacity.
Stainless Steel, Thermoplastic & Semi-Automatic Interceptors
Zurn manufactures a complete line of grease interceptors from residential under-sink units to large in-ground models, with stainless steel and epoxy-coated options specified by engineers nationwide. Canplas produces the Endura line of thermoplastic grease interceptors widely used in commercial food service for their corrosion resistance and lightweight installation. Watts offers grease interceptors in both gravity and semi-automatic configurations, with sampling ports that simplify the FOG level monitoring required by municipal compliance programs.