Grease Interceptors 101

The basics of design and common mistakes.

First, I would like to start by saying Happy Holidays and New Year to all. I don’t know about you, but, for me, 2016 was a very difficult year, and I am not sorry to see it go. I am hopeful that 2017 will be much better. Then again, that’s how I prefaced my December 2015 article, so luck has certainly not been on my side. Plus, since our new president was just elected, nobody knows what we have in store for 2017 — let’s hope it is good!

Anyway, on to happier subjects. I and many others have written about grease interceptors, probably because their design requirements are quite vague and interpretation varies all over the map.

Most of you reading this know that there are different styles of grease interceptors. There are the large gravity grease interceptors, concrete for many years, but made of plastic in more recent years. These large central units are often buried outside or hidden in a parking garage. You would think after being around so long that the plumbing associated with these units would be long established and well defined, but it is not. Here are a few examples.

Does the unit need a running trap as shown in the 25-year-old detail by the county of Los Angeles? Most jurisdictions would say no because the interceptor is trapped internally. Does the interceptor require a vent? Most inspectors would say yes because the unit itself is trapped and a trap needs a vent, but if you ask the inspectors in Los Angeles City, they would say no, due to the fact that each fixture upstream of the trap is individually vented. If you do provide a vent, how do you size it? Is it half the size of the connected waste piping? Or do you size it based on the number of fixture units connected upstream? It can be done either way with dramatically different results. I would argue the former since, as noted above, each connected fixture is already trapped and the vent just serves to allow for the displacement of air — it is not protecting a 1'' trap seal.

Sizing of gravity grease interceptors was a mystery for many years, since thePlumbing Drainage Institute (PDI) sized the units based on meals served per hour coupled with drain down time. However, since PDI created this procedure, the health departments of the country have mandated indirect waste connections on prep sinks and the like, so drain down time cannot be controlled. Some years ago, I was successful in getting a sizing table written in to Chapter 10 of the UPC relating fixture units to grease interceptor size, but the IPC still does not have such a table so it remains a mystery in that code.

The location of gravity grease interceptors is critical. If there is no place to bury them outside they are often located in parking garages. This is fine if the waste hauler has a means of access to the unit with their vacuum hose, but often they are mistakenly buried in the parking garage where no such access exists, and a grease truck is much too tall to drive into a parking garage with 8'-4'' ADA clearance.

Los Angeles County grease interceptor detail

Some of these large interceptors (basically the plastic ones) come with vent connections on the tank itself. If such is the case, they should be piped to the vent system and not left plugged, otherwise it may void the interceptor warranty. Further, some of plastic units can be ordered with a dip tube service connection that can be piped to the exterior of the build for service. This is a very handy feature indeed. Be aware that there is a maximum lift height of the vacuum pump on the service truck, so don’t locate the unit more than one story below grade. Older service trucks have weaker vacuums, and there’s no telling who will show up.

Where large central grease interceptors are entirely impractical, such as my old stomping ground of New York City, localized interceptors become the most practical solution. These take two basic forms, hydromechanical grease interceptors (HGIs) or automatic grease removal devices (GRDs). 

HGIs are generally made from lined steel and are much smaller than gravity grease interceptors. There are no moving parts, but there is a series of baffles that catch the grease in the waste stream creating a hydromechanical process. 

Hydromechanical grease interceptor

GRDs are similar to HGIs except they have a motorized wheel that skims the floating grease off the top of the waste stream and deposits it in a removable container that makes maintenance quite simple.

Both HGIs and GRDs are sized based on the flow rate of the waste stream in gpm. Determining the gpm is tricky since the piping is sized based on fixture units. Hunter’s Curve doesn’t work because it is intended for water flow rates, not drainage.

If you are designing in the UPC, there is a table in the Code for sizing the interceptors based on pipe size and associated peak flow rate, and then assigns interceptor sizes based on one-minute and two-minute drain down times. If you use this chart, I would use the one-minute column, or your interceptor might be undersized. Drain down can’t be controlled to two minutes due to the presence of air gaps on many of the fixtures. Since the interceptor comes with a flow control in the inlet, if your drainage flow rate exceeds the rating of the interceptor the waste will back up and end up on the floor. Note that HGIs are generally located recessed into the floor or on the floor below if that is possible. If the device serves only one fixture like a pot sink,it can often sit above the floor. 

Automatic grease removal device

GRDs are trickier to size than HGIs. If you use this piece of equipment, I would get the vendor involved in the sizing. Like HGIs, they can be recessed in the floor, located on the floor below, or sit on the floor next to a single fixture. 

Both HGIs and GRDs are vented on the outlet. As with the gravity grease interceptors, the sizing of the vent is ambiguous. I would size it as half the size of the inlet with a 2'' minimum, but your AHJ might want you to use the connected fixture units so it is worth checking with your AHJ.

Timothy Allinson, P.E., LEED AP, is vice president of Engineering at Murray Co., Mechanical Contractors, in Long Beach, California. He holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a LEED accredited professional. Allinson is a past-president of ASPE, both the New York and Orange County chapters. He can be reached at

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