When to use: Commercial kitchens, restaurants, and food-service facilities must intercept fats, oils & grease (FOG) before discharge to the sanitary sewer. Hydromechanical (HGI) units are sized by flow (GPM) from fixture drainage load per PDI G-101 / UPC 1014. Gravity (GGI) in-ground tanks are sized by liquid volume (gallons) from meals served per the EPA / IPC formula. Choose the method that matches your jurisdiction and installation.
Fats, oils, and grease (FOG) discharged from commercial kitchens are one of the leading causes of sanitary sewer overflows in municipal collection systems. When FOG cools in the sewer, it solidifies and accumulates on pipe walls, eventually causing blockages, sewer backups, and overflows that create public health hazards and regulatory liability. Grease interceptors are mandatory for virtually all commercial food service operations, and proper sizing is critical to their effectiveness. Two distinct sizing methodologies apply depending on the type of interceptor: PDI G-101 for hydromechanical units and the EPA/IPC gravity interceptor formula for large in-ground tanks.
The Clean Water Act and local pretreatment ordinances prohibit the discharge of grease and other substances in quantities that could obstruct sewers or interfere with wastewater treatment. Most municipalities have adopted local FOG control programs that require commercial food service establishments to install and maintain grease interceptors, maintain pumping logs, and demonstrate compliance at inspection.
Failure to properly install and maintain a grease interceptor can result in significant fines, mandatory sewer cleaning at the establishment's expense, and even business closure orders. Many municipalities now require grease hauling manifests and have reduced the maximum allowable grease accumulation in the interceptor — typically not exceeding 25% of the working depth in the grease zone — before a required pump-out.
Hydromechanical grease interceptors (HGI) are compact devices installed inside the building, typically under or near the sink, and are rated by their maximum flow rate in gallons per minute (GPM). They use baffles and a flow control fitting to slow the wastewater, allowing grease to float and solids to settle. Standard PDI ratings range from 4 GPM to 100 GPM. HGIs are most appropriate for small to medium food service operations with limited kitchen footprints.
Gravity grease interceptors (GGI, also called grease traps) are large buried precast concrete or fiberglass tanks installed outside the building, typically in a parking lot or service area. They range from 500 gallons to 5,000+ gallons and use retention time and baffling to separate grease. GGIs are required for larger operations or where local codes mandate them. The EPA/IPC gravity sizing formula — V = M × WF × RT × SF — is based on meals served, waste flow per meal, retention time, and a storage factor based on operating hours.
The Plumbing & Drainage Institute (PDI) G-101 standard is the primary reference for hydromechanical grease interceptor sizing in the United States. The method sizes the interceptor by the peak fixture drainage flow, not the average flow, to ensure it can handle the worst-case drainage event (e.g., all sinks draining simultaneously after a meal service).
The calculation proceeds as follows: (1) identify all fixtures draining to the interceptor and their liquid capacity (gallons); (2) assume all fixtures drain in a 1-minute period; (3) apply the 0.75 drainage-load factor, which accounts for the fact that fixtures typically do not fill or drain to 100% capacity; (4) the resulting GPM is the minimum flow rating required; (5) round up to the next standard PDI rating (4, 7, 10, 15, 20, 25, 35, 50, 75, or 100 GPM). The UPC Section 1014 further specifies that the grease retention capacity of a hydromechanical interceptor is 2 pounds of grease per GPM of flow rating.
A grease interceptor only works when it is properly maintained. As grease accumulates in the interceptor, the separation zone diminishes and grease passes through into the sewer. The "25% rule" — pump the interceptor when accumulated grease exceeds 25% of the working depth — is now codified in many municipal FOG ordinances. For a typical 1,000-gallon gravity interceptor serving a busy restaurant, this may mean pumping every 4–8 weeks.
Hydromechanical interceptors accumulate grease much faster given their smaller volume and must typically be cleaned weekly to bi-weekly in high-volume operations. Failure to maintain the interceptor at the specified frequency is one of the most common causes of sewer backups at restaurants. Many municipalities now require electronic monitoring of grease levels and automated alerts to both the establishment and the utility.
The required size depends on the type of interceptor and the sizing method. For a hydromechanical unit (PDI G-101 / UPC 1014), the size is determined by the total volume of connected fixtures divided by 1 minute, multiplied by the 0.75 drainage-load factor, then rounded up to the next standard PDI GPM rating. A typical coffee shop with a 3-compartment sink (75 gal), mop sink (15 gal), and pre-rinse station (20 gal) would calculate: (75 + 15 + 20) × 0.75 = 82.5 GPM, requiring a 100 GPM rated unit. For a gravity interceptor, the EPA/IPC formula based on meals served per peak hour governs. Always verify the required method with the local AHJ, as many jurisdictions have specific requirements.
The required pumping frequency depends on the size of the interceptor, the volume of grease-laden wastewater generated, and the local authority's requirements. Most municipal FOG ordinances require pumping when accumulated grease and solids exceed 25% of the total working volume. For a large-volume restaurant with a 1,000-gallon gravity interceptor, this may mean pumping every 4–8 weeks. Many municipalities now require a minimum quarterly pump-out regardless of accumulation level. Hydromechanical units must be cleaned weekly to monthly depending on the operation. All pump-outs must be performed by a licensed grease hauler and documented in a manifesting log maintained on-site.
In common usage, "grease trap" often refers to a small, indoor hydromechanical device, while "grease interceptor" refers to a large outdoor gravity unit. In code language, both are grease interceptors, but the IPC distinguishes between "grease interceptors" (larger gravity units for institutional or restaurant applications) and "grease traps" (small hydromechanical devices). Some jurisdictions use the terms interchangeably. PDI-rated hydromechanical interceptors are listed and tested per ASME A112.14.3 or PDI G-101 and are rated in GPM. Gravity interceptors are sized in gallons of capacity and are constructed to IAPMO standards or local specifications.
Hydromechanical grease interceptors (HGI) are designed for interior installation and are typically located under sinks or in a utility area close to the cooking equipment. Their compact size, sealed construction, and UL listing for indoor use make them suitable for installation in finished kitchen spaces. Gravity grease interceptors (GGI) — the large precast concrete tanks — must generally be installed outside the building and below grade due to their size, access requirements, and the risk of odor and gas (methane and hydrogen sulfide can accumulate). Some jurisdictions allow indoor GGIs in dedicated vented rooms, but outdoor installation is standard practice.
PDI G-101 bases the required flow rate on the fixture drainage load: the sum of all connected fixture volumes (in gallons) divided by a 1-minute drainage period, multiplied by a 0.75 drainage-load factor. The drainage-load factor accounts for the fact that fixtures typically do not fill to maximum capacity or empty to zero during each cycle. For example, if the connected fixtures total 110 gallons, the required flow rate is 110 × 0.75 / 1 = 82.5 GPM, and the next standard PDI rating is 100 GPM. This approach sizes the interceptor for the worst-case simultaneous drainage event, ensuring the flow-control fitting and interceptor capacity are not overwhelmed during peak operation.
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