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Where Did The 30 Minute Retention Time Come From?

It seems today that it is universally agreed to that gravity grease interceptors (GGI) are supposed to be sized based on a 30-minute retention time. Have you ever wondered where the 30-minute retention time came from? Is there a technical justification for its use? If not then what?

Commercial GGIs are actually residential septic tanks. The reason they work as septic tanks is because they are sized for very long retention times (typically 24 hours) and only have to deal with relatively low flow rates. The genesis of their use as commercial grease interceptors is difficult to pinpoint, however we do know that beginning with the 1982 edition of the Uniform Plumbing Code (UPC) a methodology for sizing commercial kitchen grease interceptors was developed and placed within Appendix H. The method of sizing calculated meals per peak hour, waste flow rate, retention time and a storage factor. Appendix H was included with each subsequent edition of the UPC through 2003.

In 2006 the UPC removed Appendix H and changed the way GGIs were sized to be based on Drainage Fixture Units. A table was created that assigned a liquid volume amount for the interceptor to predetermined DFU loads. These loads were developed to try to ensure that the liquid amount would provide at least a 30 minute retention time. 

The 30 minute retention time was taken from a textbook titled Small and Decentralized Wastewater Management Systems, published in 1998. The authors of that textbook provided the following technical argument for a minimum 30-minute retention time when using a septic tank as a commercial grease interceptor, "Typically, skimming or interceptor tanks are used to trap oils by flotation and grease by cooling and flotation. The contents of the tank serve as a heat exchanger cooling the incoming liquid, which helps to solidify the greases. For flotation to be effective, the interceptor tank must detain the fluid for an adequate period of time (typically greater than 30 minutes)." 

That's it. No research. No studies. No evidence that 30 minutes is the right amount of time. No investigation into the effectiveness of a septic tank as a commercial grease interceptor. Just a statement that 30 minutes or more is adequate to allow for greases to separate in the tank.

Here are some questions to ponder:

How does a 30-minute retention time account for differing flow rates throughout the day? What effect do higher temperature surges have on velocity and flow pattern within the tank? At what point does the accumulated grease and solids effect the separation efficiency? If you travel past the horizon will you fall off the earth? Can you breathe through your nose and mouth at the same time?

You just tried that didn't you?

Since there has never been a technical justification for the use of GGIs, the Water Environment Research Foundation funded a study and published a report titled, "Assessment of Grease Interceptor Performance" in 2008, to investigate the performance of these devices in real world installations. The study found that GGIs were generally not sized correctly, regardless of whether using the older Appendix H or the newer DFU sizing method, and that they frequently suffered from short circuiting. You can read the report yourself by following this link: Assessment of Grease Interceptor Performance

The problem with the Appendix H sizing method is that it often resulted in over-sized interceptors leading to the generation of lethal amounts of H2S gas as well as elevated BOD and TSS in interceptor discharges. The problem with the current DFU sizing method is that it often leads to undersized interceptors that short circuit frequently, especially during periods of high flow rates.

If we don't test these devices to understand exactly how they work, the things that negatively effect efficiency and how much actual capacity they have for storing grease and solids, how can we possibly deduce that a 30-minute retention time is adequate for any particular GGI? The point is that before we go all-in with mandating the use of GGIs, shouldn't we at least do some basic testing to ensure they actually work? 

I for one think you can breathe through your nose and mouth at the same time, though I admit that I've only been trying it for the last few minutes.

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