Purpose and Regulatory Drivers
Fats, oils, and greases (FOG) discharged from commercial cooking operations congeals in sewer lines at ambient temperatures, causing blockages, sewer overflows, and sanitary sewer system (SSS) capacity loss. Most municipal wastewater authorities enforce FOG pretreatment programs under Clean Water Act authority, requiring food service establishments (FSEs) to install and maintain grease interceptors sized to reduce FOG in the effluent to below 100–200 mg/L (jurisdiction-specific limit). Failure to comply results in fines, forced closure, and retroactive installation requirements.
Types of Grease Interceptors
Hydromechanical Grease Interceptors (HGI)
HGIs (also called "grease traps" in common usage) are small, flow-through devices installed under sinks or nearby in the floor. They operate by slowing the flow velocity so that FOG floats to the surface and is retained, while water passes through an internal baffle and out the effluent port. ASME A112.14.3 and PDI G101 govern HGI performance, material, and testing. HGIs are rated by flow rate (GPM) — common sizes are 10, 20, 35, 50, and 100 GPM. They are typically used for smaller food service operations or individual fixture groups (two or three compartment sinks, mop sinks).
Gravity Grease Interceptors (GGI)
GGIs are large underground concrete or fiberglass tanks (500 to 5,000+ gallons) that operate on the same gravity separation principle but with substantially longer retention times. ASME A112.14.4 and EN 1825 govern GGI performance. GGIs are required for large food service operations — full-service restaurants, commercial kitchens, hotel kitchens, hospitals, and institutional cafeterias. They are installed outdoors below grade, accessible by manway, and require pumping by a licensed waste hauler at regular intervals.
Sizing Methods
PDI Flow-Rate Method (for HGIs)
PDI Standard G101 provides the standard sizing method for HGIs. The procedure:
- Identify all fixtures draining to the interceptor (three-compartment sinks, prep sinks, dishwasher pre-rinse, etc.). Do not route floor drains, restroom fixtures, or mop sinks through grease interceptors — these dilute the FOG concentration and reduce efficiency.
- Calculate the total fixture volume: for a three-compartment sink with individual compartments of 24" × 24" × 14" depth, each compartment volume = (24 × 24 × 14) / 231 = 34.7 gallons. Total = 104 gallons.
- Apply the PDI formula: GPM rating = (Total volume × drain factor × fill factor) / 1 minute. Drain factor accounts for drainage rate (typically 1.0 for kitchen fixtures at normal head); fill factor accounts for typical fill level (75% = 0.75). Example: GPM = 104 × 1.0 × 0.75 = 78 GPM → select 100 GPM HGI.
Retention Time Method (for GGIs)
GGIs are sized to provide sufficient hydraulic retention time (HRT) for FOG separation. Minimum HRT per most FOG programs is 30 minutes; 45–60 minutes is preferred for high-grease operations. Required tank volume = Peak Flow (GPM) × Retention Time (minutes). For a restaurant generating 120 GPM peak flow with 30-minute HRT: Tank volume = 120 × 30 = 3,600 gallons. Select the next standard size: 4,000-gallon GGI.
Peak flow for GGI sizing is typically the sum of all connected fixture GPM ratings operating simultaneously, reduced by a diversity factor. Many FOG programs specify the diversity factor (commonly 0.25 for restaurants) based on operational data.
Fixture Unit Method
Some jurisdictions use a fixture-unit approach similar to sanitary DFU sizing, assigning FOG load units to each kitchen fixture and sizing the interceptor from a table. This method is less technically rigorous than the retention time method but may be required by specific AHJs. Always check the local FOG program requirements before applying any sizing method — local rules supersede PDI and ASME standards.
Installation Requirements
HGIs must be installed as close as practicable to the connected fixtures to prevent FOG from cooling and congealing in the drain line upstream of the interceptor. The inlet water temperature must not exceed 140°F — above this temperature, animal fats emulsify and pass through the interceptor rather than floating out. Dishwasher discharge (150–180°F) must never be routed through an HGI; grease-laden pre-rinse and pot washing discharge are the appropriate connections.
GGIs should be sized with a minimum 2-inch inlet-to-outlet drop to prevent hydraulic short-circuiting. Two-compartment designs (primary separation chamber plus secondary polishing chamber) provide better performance than single-compartment units. Venting is required to prevent odors and pressure buildup — GGI vents must connect to the building sanitary vent system or be independently vented to exterior.
Maintenance and Pumping Intervals
HGIs require cleaning every 1–4 weeks depending on FOG load; many FOG programs require cleaning logs. The "25% rule" — pump when the combined FOG and solids layer occupies 25% of the interceptor volume — is a widely adopted guideline. GGIs require pumping by licensed waste haulers every 1–3 months for high-volume operations, up to every 6 months for low-volume operations. Manifests for FOG disposal must be retained for regulatory inspections.
Common Installation Mistakes
The most frequent design and installation errors: (1) routing floor drains through grease interceptors (dilutes FOG, causes interceptor to be oversized, passes grease-free wastewater through unnecessarily); (2) locating HGI too far from fixtures (FOG congeals in connecting drain, causing blockages upstream of interceptor); (3) omitting the vent on GGIs; (4) undersizing — always err toward the next larger standard size, as FOG loads in commercial kitchens routinely exceed design estimates; (5) using HGIs where GGIs are required — most FOG programs require GGIs for new full-service restaurants regardless of size.