Grease traps and grease interceptors are common controls for fats, oils, and grease (FOG) from foodservice wastewater, but requirements vary by adopted plumbing code, local amendments, sewer authority ordinance, product listing, and site conditions. FOG can congeal in sewer lines, reduce capacity, and contribute to sanitary sewer overflow risk, so many local pretreatment programs regulate foodservice discharges closely.
Planning grease-control equipment requires understanding the difference between point-of-use hydromechanical devices and larger gravity interceptors, then checking which sizing method and product requirements the local authority actually accepts. This guide describes common planning concepts, but it is not a substitute for the adopted code, PDI or ASME standard text, manufacturer instructions, sewer authority review, permit submittal, or a licensed plumber or plumbing engineer.
Point-of-Use Traps vs. In-Ground Interceptors
Point-of-use grease traps (also called hydromechanical grease interceptors, or HGIs) are devices installed near one or more fixtures and rated by flow rate in gallons per minute (GPM). Product listing, required markings, flow-control fittings, installation details, and maintenance instructions must be verified against the current listing and manufacturer data.
In-ground gravity grease interceptors (GGIs) are larger tanks commonly installed outside the building. Their required capacity, retention time, traffic loading, access risers, materials, sampling points, and pumping interval are controlled by the adopted code, sewer authority, manufacturer, and site conditions.
The choice between HGI and GGI depends on the jurisdiction, menu, fixture list, dishwasher and disposer rules, available space, maintenance program, and sewer authority review. Check local requirements before selecting the interceptor type.
Sizing Calculation Methods
Local authorities can require different sizing methods for grease-control equipment. Some methods start with fixture volume or fixture flow, some use drainage fixture units or meals served, and some require an exterior gravity interceptor regardless of a simple local flow screen. The accepted method must come from the adopted code, local ordinance, AHJ, sewer authority, and product listing.
Hydromechanical interceptors are selected from listed product data rather than from app arithmetic alone. PDI and ASME sources provide testing, rating, marking, sizing, and installation context, but final selection still needs the specific standard text, current listing, manufacturer instructions, and local acceptance.
When defining connected loads, verify which fixtures are allowed or required to discharge through grease-control equipment: three-compartment sinks, prep sinks, pre-rinse stations, floor drains, dishwashers, and food-waste disposers can be treated differently by different authorities. Restroom and non-FOG fixtures generally require separate review.
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
Maintenance requirements are controlled by the local sewer authority, permit, manufacturer, and service data. Many programs require a maintenance schedule, pumping manifests, inspection records, or corrective action when grease and solids accumulation exceeds a local threshold.
Point-of-use devices and gravity interceptors can have very different cleaning access, service frequency, and record-keeping requirements. Dishwasher discharge, food-waste disposers, solids loading, staff practices, and menu type can change accumulation rates.
Keep maintenance records that match the local program requirements and manufacturer instructions. The only reliable schedule is based on the actual unit, actual FOG and solids accumulation, required inspection interval, and local enforcement program.
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.