Does an Aboveground Interceptor Have to Be Upstream of a Belowground Interceptor?

Stephen Lucich, PE, CPD, an Engineer with Colvin Engineering Associates in Salt Lake City, is working on a retrofit project to renovate an existing space into a commercial kitchen. Due to space limitations, they are considering using aboveground grease interceptors, and when investigating the code requirements, Stephen found the following in the 2018 edition of the International Plumbing Code (IPC) in Section 1003.3.1: 

“Where lack of space or other constraints prevent the installation or replacement of a grease interceptor, one or more grease interceptors shall be permitted to be installed on or above the floor and upstream of an existing grease interceptor” (bold added).

Stephen is wondering if the bolded text means that the aboveground interceptor shall be upstream of a belowground interceptor, thus creating a double interceptor installation, or if it is just an option. He consulted our experts on ASPE Connect, and here’s what they had to say.

Recommended, not Required

The general consensus was that an aboveground interceptor can, not shall, be located upstream of an existing interceptor.

The 2015 IPC Commentary reads as follows: “The last line of  [Section 1003.3.1] is intended to cover situations where gravity-type grease interceptors cannot be installed or the existing gravity interceptor is determined to be to small, where replacement with a larger unit is not possible for a variety of reasons (i.e., not enough space, too costly, etc.). Where these situations are encountered, the code allows grease interceptors (typically the hydromechanical type) to be installed upstream of the existing gravity interceptor (or where a gravity interceptor would be located) in order to satisfy the jurisdictional requirements for a gravity interceptor.”

Double Interceptors Can Increase Efficiency

Regarding double interceptors, this installation can provide much better efficiency in capturing grease. Dade County, Florida, requires grease traps to be provided in series to increase the efficiency to 99 percent. Some jurisdictions in the Northeast also specifically call out for a hydromechanical interceptor inside the building, upstream of the outside gravity interceptor, to help protect the lines leading to the outside gravity grease interceptor from clogging.

One manufacturer has always recommended that where more than one interceptor is installed, they should be installed in series. In the case of gravity grease interceptors, it does not matter if one is upstream or downstream of the existing interceptor, but it is preferred to put the larger one first. As far as installing the newer interceptor upstream, some could argue that the newer interceptor would be more efficient than the older, which could mean it could capture and retain more wastewater grease and solids and decrease the load on the older downstream unit.

As always, you should consult with the local authority having jurisdiction (AHJ) for approval.

For More Information

ASPE offers two on-demand webinars on grease interceptors: “Grease Interceptors” and “Sizing Grease Interceptors.” Our experts also suggest contacting a manufacturer such as Schier, Mifab, or Zurn Green Turtle.

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