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  • Ken Loucks

Selecting HGI's by Flow Rate Alone Isn't Enough

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 was privileged to work on Chapter 8 of Data Book four along with my friend Max Weiss (Plumbing and Drainage Institute), which deals with grease interceptors.

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 more 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.” Let's say 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. This sizing and selection method is called Grease Production Sizing and its now apart of the newest edition of the ASPE Data Book 4 (2017 release).

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