Grease interceptors (commonly called grease traps) are required on the drain lines of virtually every commercial kitchen to prevent fats, oils, and grease (FOG) from entering the municipal sewer system. FOG congeals in sewer lines, causing blockages, sanitary sewer overflows (SSOs), and expensive municipal cleanup operations. The EPA estimates that FOG is the single largest cause of sewer overflows in the United States, and local sewer authorities aggressively enforce pretreatment requirements on foodservice establishments.
Sizing a grease interceptor correctly requires understanding the difference between point-of-use traps and in-ground interceptors, the flow rate calculation methods prescribed by the Plumbing and Drainage Institute (PDI) and the Uniform Plumbing Code (UPC), and the local sewer authority's specific requirements. An undersized interceptor passes grease into the sewer and triggers code enforcement. An oversized interceptor wastes money on installation and more frequent pumping of a larger tank. This guide covers the engineering fundamentals and code requirements that govern grease interceptor sizing for commercial foodservice operations.
Point-of-Use Traps vs. In-Ground Interceptors
Point-of-use grease traps (also called hydromechanical grease interceptors, or HGIs) are small devices installed directly under a sink or in the drain line close to the fixture. They are rated by flow rate in gallons per minute (GPM), typically ranging from 7 to 100 GPM. HGIs are listed to PDI G101 and work by slowing the flow of wastewater through an internal baffle system that allows grease to float and solids to settle. They require frequent cleaning, often daily or weekly, depending on the volume of FOG generated. Common brands include Watts, Zurn, and Josam.
In-ground gravity grease interceptors (GGIs) are large concrete, fiberglass, or steel tanks buried outside the building, typically ranging from 500 to 5,000 gallons. They provide extended retention time (typically 30 minutes minimum) for gravity separation of FOG from wastewater. GGIs are the standard requirement for full-service restaurants and high-volume foodservice operations. They require periodic pumping by a licensed waste hauler, typically monthly or quarterly depending on size, usage, and local regulations. GGIs must meet the requirements of the local plumbing code and sewer authority, which often reference IAPMO or ASME standards.
The choice between HGI and GGI depends on the jurisdiction, the type and volume of cooking, and the physical constraints of the site. Many jurisdictions mandate in-ground interceptors for any establishment with a commercial cooking operation, while some allow point-of-use traps for limited food preparation (coffee shops, delis without deep fryers). Always check with the local sewer authority before selecting the interceptor type, as their requirements often exceed the minimum plumbing code provisions.
Sizing Calculation Methods
The most widely used sizing method for in-ground gravity grease interceptors is the fixture flow rate method from the Uniform Plumbing Code (UPC) Appendix H or local sewer ordinance. This method sums the drainage fixture unit (DFU) values of all fixtures connected to the interceptor, converts to a peak flow rate in GPM, and then applies a retention time to determine the minimum tank volume. The formula is: Tank Volume (gallons) = Flow Rate (GPM) × Retention Time (minutes). Most jurisdictions require a 30-minute minimum retention time, and some require 60 minutes for high-FOG operations.
An alternative approach used by some jurisdictions is the meals-served method, which bases the interceptor size on the maximum number of meals served per peak hour. This method uses factors like 5-8 gallons per meal per hour depending on the type of establishment (fast food vs. full service). The PDI method for hydromechanical interceptors (PDI G101) rates devices by their tested flow rate and grease retention capacity, so sizing an HGI involves matching the device's rated GPM to the calculated peak flow from the connected fixtures.
When calculating the connected fixture load, include all fixtures that could discharge FOG: three-compartment pot sinks, prep sinks, pre-rinse sprayers, floor drains in the cooking area, and automatic dishwashers. Do not connect restroom fixtures or handwash sinks to the grease interceptor, as they add unnecessary volume without contributing FOG. Some jurisdictions require food waste disposers (garbage disposals) to discharge through the interceptor, which increases the solids load and requires more frequent pumping.
Grease Interceptor / Trap Sizing Calculator
Calculate grease trap GPM rating and capacity per IPC standards. Enter sink compartments, drain time, and dishwasher connections.
Maintenance and Compliance Requirements
Grease interceptor maintenance is not optional. Most sewer authorities require a maintenance schedule as a condition of the operating permit, and failure to comply can result in fines, surcharges, or forced closure. In-ground interceptors must be pumped when the combined thickness of the floating grease layer and settled solids reaches 25% of the total liquid depth (the "25% rule"). For a typical restaurant, this means pumping every 30-90 days depending on volume and menu type. Establishments with heavy frying operations (fish and chips, fried chicken) accumulate FOG faster than those with lighter cooking.
Point-of-use traps require much more frequent attention. Most HGIs should be cleaned daily or at minimum weekly, depending on the flow volume. Cleaning involves removing the accumulated grease from the trap's internal baffles and disposing of it properly. Staff often neglect this task during busy service periods, leading to trap overload and FOG pass-through. Many jurisdictions now require automatic grease removal devices (AGRDs) that skim grease continuously into a collection container, reducing the reliance on manual cleaning.
Documentation is critical. Keep a maintenance log showing the date, service provider, volume pumped, and condition of the interceptor at each service visit. Many jurisdictions require this log to be available for inspection and submitted quarterly or annually. Some municipalities have adopted monitoring programs that use sensors to measure the grease layer thickness in real time and alert the operator when pumping is needed. Investing in proper maintenance avoids the far greater cost of sewer line blockages, SSO cleanup, and regulatory penalties.
Grease Interceptor / Trap Sizing Calculator
Calculate grease trap GPM rating and capacity per IPC standards. Enter sink compartments, drain time, and dishwasher connections.
Installation and Piping Best Practices
Proper installation is essential for interceptor performance. In-ground interceptors should be located as close to the kitchen as possible to minimize the length of the inlet pipe, which can accumulate grease before it reaches the tank. The inlet and outlet pipes must be properly sized to handle the peak flow without surcharging, and the inlet should include a flow control device or fitting to prevent excessive velocity from disturbing the separation process inside the tank. Most manufacturers specify a maximum inlet velocity of 1 foot per second during peak flow.
The interceptor must be accessible for pumping and inspection. Provide risers from the tank access covers to grade level, with traffic-rated covers if the interceptor is in a drive or parking area. Buried interceptors with no risers are nearly impossible to maintain and will eventually fail. The tank must be structurally rated for the burial depth and any traffic loads above it. In areas with high groundwater, the tank must be anchored to prevent flotation when pumped empty.
Sampling ports are required by many sewer authorities on the interceptor outlet pipe to allow effluent quality testing. The sample port is typically a tee with a removable cap, located between the interceptor outlet and the connection to the sanitary sewer. Some authorities also require a sampling port on the inlet side to compare influent and effluent quality. Install cleanouts at changes of direction in the piping to allow maintenance access for jetting and clearing blockages.