Grease traps or interceptors are passive devices required by municipalities to stop grease, fat, oil, wax or debris from entering the city's sanitary sewer system.

Such materials cause blockages in the system, which cause backups and overflows. Traps and interceptors are designed to separate greasy materials from wastewater so that they can be removed before they enter the sewer system.

All restaurants, caterers, school cafeterias and other commercial cooking facilities must avoid discharging grease into the municipal sewer system.

Grease interceptors must receive wastewater from all contributory sources, such as pot sinks, dishwashers, floor drains and mat washing area drains before draining to the sanitary sewer system.

Typical building codes require all such new or rebuilt facilities to install a grease interceptor to pre-treat grease entering a sewer. All units should be fitted with a standard final-stage sample box. Interceptors must typically be sized for at least a 30 minute peak wastewater flow detention time from all contributory sources.

On occasion, a grease trap will be required in place of an interceptor. Traps are typically required to have a rated flow capacity sufficient for the same 30-minute flow time.

All new, and rebuilt, as well as existing kitchen facilities, must dedicate a specific area for the washing of floor mats and related equipment, bermed to ensure wastewater is not discharged to the storm drain system but to a grease interceptor discharging to the sanitary sewer.

Outdoor wash-down area drains must be protected from rain water intrusion. All garbage disposal units must be eliminated.

Usually, grease interceptors must be installed by a state-certified plumber. For grease interceptors and traps to function properly they must also be regularly serviced and maintained by a qualified contractor.

Please note that the following information is provided as a guide only, and is not to be used as a substitute for a professional evaluation of any business. Grease traps must be located at least 10 feet from hot water faucets. All hot water must cool to 120° F before entering the grease trap. The size of the grease trap must comply with the Uniform Plumbing Code. Any sink or disposal that can carry grease must empty into the trap.

The following illustration depicts a typical restaurant grease interceptor installation.

Grease Interceptor Sizing Calculations
Sizing should always be left to a trained professional. This section is intended as a guide to familiarize the reader with some basic sizing rules. Requirements are determined by the volume of waste water that the interceptor can handle compared to demand. Unit selection depends on the type desired and its capacity rating in gpm. The Plumbing and Drainage Institute (PDI) defines standard sizes that are roughly equivalent to gpm of flow.

The National Plumbing Code lists drainage fixture-unit values of various plumbing fixtures based on drain outlet or trap size. These values are converted to gpm discharge rates assuming 7.5 gpm discharge to represent one fixture-unit. This information is illustrated below.

The discharge rate will help to determine the model needed.

1. Calculate volume fixture in cubic inches
   
2. Convert this volume to gallons
   
3. Multiply by .75
   
4. Calculate flow rate and drainage period (1 or 2 minutes)
   

Dishwashers:
A separate grease interceptor is recommended for each dishwasher.
The size is determined by the dishwasher's capacity.