The Baffling Requirement for Baffles
I am always baffled when I see a requirement for baffles in an interceptor. Why are they required and do they actually improve performance? If you are a pretreatment professional, may I ask you to consider the following question:
What is the role of manufacturers of grease interceptors?
I come back to this line of thinking from time to time when I come across requirements that mandate an interceptor have internal baffles. I simply do not understand why baffles are required.
Remember that all grease interceptors, regardless of type, operate based on gravity-differential separation. The principle is simple, reliable and is based on laws that are immutable. Fats, oils and grease (FOG) are lighter than water and therefore float. Turbulence inside of an interceptor, any interceptor, is bad. Turbulence negatively effects velocity which decreases the amount of time FOG has to separate since the FOG has to break free from the turbulent flow path before it can ascend to the surface.Creating a laminar (turbulence free) flow path in an interceptor is of paramount importance in an effective design. Baffles inside an interceptor create turbulence.
Allow me to illustrate my point like this...
Here are some typical HGIs - same basic designs that have been around since the 1940's:
Notice the vertical baffles that are intended to enhance the performance of these designs.
90% average efficiency
Maximum grease storage capacities are 2 times the flow rate
Grease storage capacities range from 25% to 30% of total liquid volume
It has only been in the last decade or so that manufacturer's began introducing high-efficiency, high-capacity HGIs and you may notice something these devices have in common:
All of these designs are open with no vertical baffles.
Efficiencies range from 93% to 99%
Maximum grease storage capacities range from 10 to 18 times the flow rate
Grease storage capacities range from 50% to 90% of total liquid volume
Okay, so what about gravity grease interceptors (GGI)?
Many seem convinced that somehow a 30 minute retention time and two-chambers ensure that these devices work correctly as commercial grease interceptors. What most people may not be aware of is that GGIs are really residential septic tanks adapted for a different purpose. A comparison between IAPMO/ANSI Z1001 (the standard that governs GGIs) and IAPMO/ANSI Z1000 (the standard that governs septic tanks) reveals little if any real difference between the two tanks.
But, it turns out that even in a septic tank, a vertical baffle forming two chambers is not a good idea!
According to the same book that the UPC derives the 30 minute retention time from titled, Small and Decentralized Wastewater Management Systems (McGraw-Hill, 1998), "Two compartments have been used to limit the discharge of solids in the effluent from the septic tank. Based on measurements made in both single and double compartments, the benefit of a two-compartment tank appears to depend more on the design of the tank than the use of two compartments."
Under the heading Tank Configuration the textbook notes, "Although a divider is used, the rationale for its use is historical more than scientific. Both Seabloom et al. (1982) and Winneberger (1984) have found, on the basis of field measurements, that the performance of a single-compartment tank is equal to or exceeds the performance of a two-compartment tank of the same liquid volume."
When it comes to GGIs, the WERF report titled, Assessment of Grease Interceptor Performance (2008) confirmed with bench testing, that the inclusion of a vertical baffle had a negative effect on the performance of the tank. The tank design that performed the best had an open design with no vertical baffle.
Now you know why I'm baffled when I see requirements mandating the inclusion of internal baffles. Instead of telling us what you want an interceptor to look like, consider telling us what you want it to do and leave the design, research and development to manufacturers.