On April 18-19 the Western States Alliance hosted their first "official" FOG Forum in beautiful Bend Oregon. I was asked to give a presentation on how to properly size and select grease interceptors. During the presentation I made the comment that the 30 minute retention time has no basis in scientific data or research that would justify its use for sizing GGIs.
I was in the middle of explaining that the origin of the 30-minute retention time was uncertain but that the justification for its use was founded upon a textbook titled, Small and Decentralized Wastewater Management Systems. published in 1998 by McGraw-Hill. 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)."
During this portion of the presentation an audience member said, "actually the 30 minute retention time was first introduced in the Uniform Plumbing Code in 1982 in Appendix H." Normally I can quickly ascertain what is wrong with a statement like that but honestly, at the time by brain cramped on me. So naturally I spent the next several days meditating on what I should have said.
Since I should have had an answer to that statement but didn't, I decided to go back and review the Appendix H sizing method and to confirm what I already knew; that it was not based on a 30 minute retention time.
Here is the exact section from the 1982 UPC Appendix H:
Notice that there is a calculation for retention time and that it does not use 30-minutes. Instead notice that there are two options; for commercial kitchen wastes with dishwasher you use 2.4 hours (144 minutes) or for single service kitchens use 1.5 hours (90 minutes). Of course the time increases with storage factors added into the calculation.
Okay, so the question remains; where did the 30-minute retention time come from?
Prior to the 1982 edition of the UPC and the then new Appendix H, the EPA published a Design Manual titled, Onsite Treatment of Wastewater Disposal Systems (1980). The manual provided the following formula for sizing GGIs:
The sizing formula provided in the EPA manual is similar to the Appendix H method but obviously not identical. Both though, lead to very, very large GGIs with retention times well in excess of 30-minutes.
IAPMO formed a task group (TG) on grease interceptors to make recommendations for updating the requirements for these devices in the 2006 edition of the UPC. Among other things, the TG recommended replacing the Appendix H sizing method because, "No sound technical basis could be found to justify retaining the meals per peak hour sizing criteria found in Table H-1 (in Appendix H)." Instead the TG recommended a Drainage Fixture Unit (DFU) sizing methodology for GGIs that was based on a 30-minute retention time as supported by the very textbook from McGraw-Hill I noted at the beginning of this post. In other words the basis for using a 30-minute retention time was and still is anecdotal.
I have not found any evidence to support a technical or scientific basis that justifies using a 30-minute retention time in sizing GGIs. Anecdotal evidence cannot be the basis of any sizing methodology. The fact that these devices are not tested and rated for performance is the very reason that sizing is still an issue that I, for one, consider unresolved. That's why I have a bone to pick with GGI sizing.