Selecting HGIs by Flow-Rate Alone Isn't Enough

February 23, 2016

The American Society of Plumbing Engineers is currently working on edits to their four published Data Books. In case you may not be aware, these Data Books form what is essentially the “bible” of the plumbing engineering community.

I am privileged to be working on Chapter 8 of Data Book four along with my friend Max Weiss (Plumbing and Drainage Institute).

One of the issues that we have been fleshing out is the section titled “Guidelines to Sizing”. In all of the copies of this Data Book that I have, this section has always centered on providing instruction on how to determine the total flow rate that would be discharged by the connected fixtures into the grease interceptor, and rightly so. We want to select an interceptor that is capable of receiving all of the discharge of the fixtures routed to it.

Okay, so now that we’ve done that, let me ask a question; how much grease capacity does the selected interceptor have and how often will it have to be maintained by the owner?

This is the rub, in my opinion. Sizing and selecting of grease interceptors, regardless which type, have never considered anything more than the flow rate of the discharging fixtures.

Grease interceptors (hydromechanical at least) come with two critical pieces of information; maximum flow rate and grease storage capacity. Why then does the sizing and selection of grease interceptors only consider one factor; flow rate? It’s as if the industry is blind to the fact that every certified hydromechanical grease interceptor also has a certified grease storage capacity.

But when a pretreatment inspector walks into a restaurant to inspect the grease interceptor they don’t normally inspect the piping connecting to the interceptor, nor the flow control orifice to ensure that it is the correct size. The first thing they inspect is whether the interceptor is “full” or not. They ask simple questions like, “when was the last time this device was serviced?” And they get answers like, “I don’t remember” or “what is that thing?”

What happens when the FSE finds out that the interceptor that was selected based solely on its maximum flow rate has to be maintained two, three or even four times a week to ensure it is in compliance and operating correctly? Sadly this situation is very real and has been repeated tens of thousands of times over the past several decades.

But how might things improve if an engineer had the opportunity up front to consider another factor in selecting the right interceptor? What if the engineer had access to information which would allow him to evaluate how much grease a particular FSE might produce over a given period of time and then select an interceptor that had enough grease storage capacity to allow for a longer maintenance cycle?

Let’s say that the engineer is working on a strip mall tenant improvement that will be adding a small Mexican style Taqueria. His flow rate calculation determines that he needs a grease interceptor with minimum 50 gpm. He turns to his handy dandy ASPE Data Book 4 and turns to Chapter 8 and finds a supplemental sizing method that shows him how to determine how much grease this Taqueria might produce. For this example we'll use 500 pounds of fats, oils and grease (FOG) per month.

Now, he starts reviewing all of the available grease interceptors that have a minimum of 50 gpm flow rate and he finds that the majority have a grease storage capacity of only 100 pounds. What does he do?

He can select one of these units, of course, and then let the owner know that the unit selected will likely have to be maintained on a weekly basis. “But,” the owner says “it costs me $100 to have the interceptor maintained, that will cost me $5,000 per year, just to have my interceptor cleaned out!”

What if that the same engineer dug a little deeper and found another interceptor with the same flow rate, but which held 249 pounds of grease? This interceptor, he tells the owner, “could be maintained twice a month saving you half the cost.”

The owner is still not satisfied so the engineer goes back to the drawing board and finds multiple units on the market at 75 gpm flow rates (higher but still approved) but which offer much larger capacities; 559 lbs, 616 lbs and even 635 lbs. Each of these units would provide a monthly clean out schedule at a relatively affordable price. The owner agrees and everyone is happy, right?!

Here’s the catch. Just because the engineer selected an interceptor with enough grease storage capacity to allow this particular facility to have a monthly maintenance cycle doesn’t mean that this particular facility will have a monthly maintenance cycle!

There are many factors that determine an effective cleaning frequency including menu type, cleaning practices in the kitchen, which fixtures are routed to the interceptor and so on. The point is, regardless what interceptor was selected or on what criteria the selection was based, cleaning frequency will always be determined from field inspections by the pretreatment department.

A grease interceptor that has been selected because it has the flow rate required to handle the effluent from the fixtures routed to it is a good first step. A grease interceptor that has been selected because it also has enough grease storage capacity to allow for a longer maintenance frequency is a very good second step.

I would think that any engineer would be happy with this kind of two-step sizing and selection process for grease interceptors. If you would like to see this supplemental sizing methodology included in ASPE Data Book Four, Chapter 8 Grease Interceptors, let me know and I'll make sure your voice is heard.

For more information see: Grease Production Sizing


  • Cormac Fitzpatrick

    May 12, 2016

    There is another key point I would add. Having visited many sites with both HGIs and GGIs installed here in Ireland I think one of the main influencing factors is the appearance of the surface layer. Grease has an amazing ability to give the false impression that a trap is saturated, or even to cake up thick like its been there for months. But a little “stirring” shows that the actual layer is < 1 inch thick. Therefore unless sludge judges or even a simple stick are routinely used by ALL inspecting authorities, they could be making demands for more frequent pump outs unnecessarily.

  • Interceptor Whisperer

    Mar 16, 2016

    @Rob, thanks for you comment. Solids most definitely become a problem with grease interceptors, especially when and FSE treats their device like a garbage can – and some do that and worse. I like to point out to stakeholders that they really are called “grease” interceptors for a reason. While HGIs are tested and rated for their efficiency and grease holding capacity most offer “some” amount of solids capacity but they are not truly designed to fill up with solids. If the FSE is a high solids producer I suggest they install a solids interceptor between their HGI and the upstream fixtures to try to mitigate the volume. I do agree that most jurisdictions don’t want a lot of extra food waste in the form of solids added to their collection systems – despite the claims from some that this extra solids loading is somehow a good thing.

  • Interceptor Whisperer

    Mar 16, 2016

    @Jason; thanks for your questions. Hyromechanical grease interceptors are not tested and rated to meet any effluent concentration. Effluent concentration limits for grease interceptors cannot be reliably tested (EPA 1664A has a known variability in test results of up to 40%) and I am usually campaigning against this evaluation method. Regarding the 25% rule, if your jurisdiction tightly enforces that rule on constituent FSEs then they will not enjoy the benefits of high capacity HGIs like Schiers’ Great Basin. Many jurisdictions are figuring out ways to let FSEs go longer between pump outs when they use a high-capacity HGI and can demonstrate that a longer pump-out schedule is appropriate. I have some older posts dealing specifically with the issue of the 25% rule and effluent concentration limits that you may enjoy.

  • Rob

    Mar 11, 2016

    This is all good information, however, I feel like all the attention is going to grease and how much an interceptor can hold. What about solids? Solids cause sewer issues as well. If you look at the distance from the bottom of the effluent tee to the floor of the device, you don’t have much. Where I live code says that’s 8 inches. So that means once those solids levels achieve 8 inches or more it’s time for a pump out, regardless of how much grease the device is holding.

  • Jason Mouring

    Mar 11, 2016

    The Great Basin series catalog lists retention capacities expressed in pounds, percent of volume, and inches. Up to the percentages listed and/or total grease height and total solids height in the specs, are these GB units meeting their 100 ppm limit for the certified listing for performance? Some AHJ’s limit capacity to a “25% rule” when inspecting for compliance. If any more than 25% total volume of the interceptor is achieved (grease+solids), the unit is deemed to be too full and not in compliance, and must be pumped. Please clarify. Thanks!

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