Grease Traps and Their Role in Protecting Sewer Systems

Grease traps are mechanical interceptor devices installed in commercial and institutional food service plumbing systems to capture fats, oils, and grease (FOG) before those substances enter the municipal sewer network. Their proper installation, sizing, and maintenance are governed by a layered framework of plumbing codes, local pretreatment ordinances, and environmental regulations. Failures in this infrastructure contribute directly to sewer blockages, sanitary sewer overflows (SSOs), and regulatory enforcement actions against food service establishments. The sewer listings reference provides broader context on the service providers and contractors operating in this sector.

Definition and scope

A grease trap — also called a grease interceptor when referring to larger, in-ground units — is a plumbing device designed to intercept and retain FOG and food solids from wastewater before that wastewater discharges into a sanitary sewer. The device operates on the principle that fats, oils, and grease are less dense than water and will separate and float when flow velocity is reduced.

The plumbing industry distinguishes between two primary device categories:

1. Passive grease traps (hydromechanical grease interceptors, HGIs): Small, typically under-sink or under-counter devices with flow rates between 10 and 50 gallons per minute (GPM). These are defined and sized under ASME A112.14.3 or PDI G101 standards, both widely referenced in local plumbing codes.

2. Gravity grease interceptors (GGIs): Large, exterior in-ground tanks — commonly ranging from 500 to 2,000 gallons in capacity — used where higher FOG loads are generated. These are governed under ASME A112.14.4 and must comply with local pretreatment program requirements issued under the U.S. Environmental Protection Agency (EPA) General Pretreatment Regulations (40 CFR Part 403).

The International Plumbing Code (IPC), maintained by the International Code Council (ICC), and the Uniform Plumbing Code (UPC), maintained by the International Association of Plumbing and Mechanical Officials (IAPMO), both include grease interceptor requirements under their fixture and drainage sections. Jurisdictions adopt one or the other — or a local amendment of either — establishing the baseline for permit review.

How it works

The functional sequence of a grease trap or interceptor follows four discrete phases:

1. Inlet and baffling: Wastewater enters through a baffle or inlet tee that slows flow velocity and prevents turbulence from re-emulsifying settled grease.
2. Retention zone: The reduced-velocity chamber allows lighter FOG material to rise and accumulate at the surface, while heavier food solids settle to the bottom as sludge.
3. Effluent outlet: Clarified wastewater — the middle layer — exits through an outlet baffle or dip pipe positioned below the floating grease layer and above the settled solids, discharging into the building drain and ultimately the municipal sewer.
4. Accumulated waste: The trapped FOG cap and settled sludge remain in the device until pumped out by a licensed grease hauler. Without regular pump-out, FOG breaks through the outlet and defeats the device entirely — a condition inspectors refer to as "carry-over."

Hydromechanical units are typically rated for a 24-hour FOG retention efficiency of not less than 90%, per PDI G101 testing protocols. Gravity interceptors depend on hydraulic retention time, which is sized to the incoming flow load during the design event.

Common scenarios

Grease trap requirements apply across a defined set of facility types and operational contexts. The sewer directory purpose and scope page outlines how this category of infrastructure fits into the broader sewer service landscape.

Food service establishments (FSEs): Restaurants, cafeterias, catering kitchens, and institutional food preparation facilities (schools, hospitals, correctional facilities) are the primary regulated class. Most municipal pretreatment ordinances define FSEs by SIC code or NAICS code and impose mandatory interceptor installation as a condition of sewer connection permit approval.

Retrofit installations: When an existing FSE exceeds FOG discharge limits established in a local pretreatment program, the utility authority may issue a compliance order requiring installation of an interceptor where none previously existed. These orders typically cite EPA pretreatment program authority under 40 CFR Part 403.5.

Grease hauler manifest compliance: Pumped FOG waste is classified as a non-hazardous industrial waste in most states. Disposal must follow a manifest trail traceable to a permitted receiving facility. The receiving facility — typically a wastewater treatment plant or rendering operation — must accept the waste under its own discharge permit.

Inspection and enforcement: Municipal sewer utilities and local plumbing inspection departments conduct periodic interceptor inspections. Violations typically fall into three categories: under-sizing relative to flow, inadequate pump-out frequency, and illegal bypass of the interceptor through improper drain routing.

Decision boundaries

Determining the appropriate device type, size, and maintenance schedule involves a structured evaluation process rather than a single universal standard. The how to use this sewer resource page describes how the reference network is organized around these professional distinctions.

HGI vs. GGI selection turns on flow rate, available space, and local code mandate. HGIs are appropriate for low-volume single-fixture applications; GGIs are required where total connected fixture flow rates exceed the HGI rated capacity or where the local pretreatment ordinance mandates outdoor interceptors for all FSEs above a threshold seating count (a common threshold is 25 seats, though local ordinances vary).

Sizing authority: Sizing calculations follow either the IPC (flow rate method) or UPC (drainage fixture unit method), depending on the adopted code. The local building or plumbing department determines which method applies at permit submission.

Maintenance frequency: Pump-out intervals are not standardized nationally. The "25% rule" — requiring pump-out when combined FOG and solids fill 25% of the interceptor's working capacity — is referenced in IAPMO guidance and adopted verbatim in multiple municipal ordinances as the enforceable maintenance standard.

Permit triggers: New construction, tenant improvements involving kitchen plumbing, and change-of-use from non-food-service to food-service occupancy each independently trigger interceptor permit review under IPC Section 1003 and equivalent UPC provisions.

References

• U.S. Environmental Protection Agency — General Pretreatment Regulations, 40 CFR Part 403
• International Code Council (ICC) — International Plumbing Code
• International Association of Plumbing and Mechanical Officials (IAPMO) — Uniform Plumbing Code
• ASME A112.14.3 — Hydromechanical Grease Interceptors (American Society of Mechanical Engineers)
• ASME A112.14.4 — Gravity Grease Interceptors
• Plumbing and Drainage Institute (PDI) — PDI G101 Testing and Rating Standard for Grease Interceptors
• U.S. EPA — Pretreatment Program Overview