Technical Resources

What is a Precast Concrete Grease Interceptor?

Precast grease interceptors are large basins with strategically designed chambers which separate fats, oils, and grease (FOG) and solid sediment from free flowing water.

Grease interceptors differ from grease traps, which are much smaller and are typically located under kitchen sinks. Grease interceptors are designed to handle much larger volumes of water and are typically buried underground near the affected building to prevent foul smells from affecting the indoor environment.

There are two distinct grease interceptor designs: Baffle and Stand Pipe. Baffle designs are simpler and work well for naturally slow-flowing water. Stand Pipe systems utilize a crossover pipe to slow the flow of water and adequately increase retention times to meet municipal standards.

Precast grease interceptors can handle anywhere from 500 to 15,000 gallons of moving water. FOGs rise to the top and are blocked from flowing through the interceptor by barriers which descend from the top of the basin, whereas solid sediment falls to the floor and is blocked by barriers rising from the bottom.

The best solution results from a size and design that accommodates relatively slow rate of flow. It is important to refer to state and municipal guidelines for sizing and collection specifics.

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Design Considerations

  • Shape and design to collect and remove waste in accordance with state and municipal standards
  • Concrete strength and design that withstands the pressure of vehicular traffic if located beneath a roadway
  • Concrete that can handle applicable thermal effects
  • Easy access for waste removal and maintenance
Precast Concrete Advantages – For Designers
  • Unmatched durability and strength
  • High quality concrete mix designs
  • Documented QA/QC programs
  • Easily customizable to meet project needs

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Precast Grease Interceptor Sizing

Proper grease interceptor sizing should be determined based on the anticipated volume of wastewater that must flow through the system.

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Below is some helpful information regarding sizing methodology.

U.S. EPA Sizing Method

(D) x (GL) x (ST) x (LF)x(HR/2) Grease Interceptor Liquid Capacity =


D = Number of seats in dining area
GL = Gallons of wastewater per meal, normally 5 gallons
ST = Storage capacity factor – minimum of 1.7, onsite disposal, 2.5
HR = Number of hours open
LF = Loading factor
a) 1.25 interstate freeways
b) 1.0 other freeways
c) 1.0 recreation areas
d) 0.8 main highways
e) 0.5 other highways

Reprinted from the EPA Design Manual, Onsite Wastewater Treatment and Disposal Systems.

State, City and Municipality Requirements

Each state Department of Transportation, city and even municipality may have different requirements and regulations that govern grease interceptors. It is important to work with a producer that is familiar with these requirements and can supply a product that meets all local standards.

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It is important to work with a producer that is familiar with these requirements and can supply a product that meets all local standards.

ASTM C1613 Standard Specification for Precast Concrete Grease Interceptor Tanks

This specification covers design requirements, manufacturing practices, and performance requirements for monolithic or sectional precast concrete grease interceptor tanks.

Glossary of Terms

Baffle Wall
A barrier that, when implemented properly into the design of a grease interceptor, blocks grease, fats, oils, and solid sediment from flowing through to the sewer system.
An acronym for fats, oils, and grease; common materials that block sewage pipes and systems.
Baffle System
A very basic design which dates back to the 1800s, and is still in use today due to its effectiveness. If the flow of water is slow enough, this design allows the FOG to naturally separate and rise to the top as sediment sinks to the bottom of the first and middle chambers. Another baffle then stops the FOG from traveling any further in the system as the water flows through the second chamber and exits the precast grease interceptor.
Stand Pipe System
A system that utilizes a crossover pipe to convey water from the first chamber into the second chamber. This design slows the flow, affording extra time for the FOG to separate from the water as it flows through the precast grease interceptor. The pipes are often removable, and are reachable from the access covers to facilitate regular cleaning.
Retention Time
The time it takes for fats, oils, and grease to separate from water molecules.
Storage Capacity
The amount of fats, oils, grease, and sediment that can be retained in a grease interceptor between cleanings while still affording acceptable flow of water.
Casting concrete in a reusable mold or “form” which is then cured in a controlled environment, transported to the construction site and maneuvered into place.
Soil Bearing Capacity
The capacity of soil to support the loads that are applied to the ground above.