Mandated Design Requirements For Grease Interceptors; Why It's a Bad Idea!

March 25, 2014

Many jurisdictions mandate prescriptive design elements for grease interceptors like, “must have a minimum of two compartments” or “must have a minimum liquid holding capacity of 500/750/1000 gallons”, etc.

Many of the design elements that we find required by jurisdictions are based on the extrapolation of engineering principles such as Stokes law, but most have never been proven to enhance interceptor performance.  Any design for a grease interceptor should of course be based on fundamental engineering principles, but as you will see that is only the beginning of the process of design development.

Back to the beginning

Prior to the 1940’s grease interceptor manufacturer’s each rated their own interceptors and produced them in a variety of sizes and types according to engineer’s specifications or to satisfy plumbing codes.  There was no uniform testing or rating procedure for grease interceptors.

In 1944 the researchers from the Iowa Institute of Hydraulic Research (IIHR) attended the Sixteenth Annual Meeting of the New York State Sewage Works Association where they presented a symposium of four papers, one of which was titled Symposium on Grease Removal - Design and Operation of Grease Interceptors, by F. M. Dawson and A. A. Kalinske.

In this paper, the authors explain how the researchers from IIHR developed the testing and rating method for grease interceptors that the Plumbing and Drainage Manufacturer’s Association (now the Plumbing and Drainage Institute) would later formalize into the standard PDI-G101. 

Fundamentals of Operation

Fats, oils and grease (FOG) float in water owing to a difference in their specific gravity.  Water has a specific gravity of 1.0 while the specific gravity of olive oil is 0.703, lard is 0.875, and vegetable oil is 0.92 which is why they all float.  Interceptors are designed to separate FOG based on the differences in specific gravity which is called “gravity-differential separation”

Stokes law predicts the rise rate of a grease globule in static water based on its size, temperature, viscosity and specific gravity.  For example, in static water with a temperature of 150o F, a grease globule with a size of 150 microns and a specific gravity of 0.90 will have a rise rate of 0.05 feet per second. 

All other factors being the same, a grease globule of 50 microns will take 9 times longer to rise than a grease globule of 150 microns. For this reason researchers concluded that interceptors should be designed to separate grease globules of about 150 microns because, “the rate of rise of globules much less than this size is so small that gravitational separation is impracticable, and globules much larger than this will be easily separated.”

So we see from the beginning that researchers started with the fundamental engineering principles of Stokes law and the differing specific gravities of fats, oils, grease and water.  From this they calculated some basic ratios for an interceptor’s length, width and height and the internal velocity that should allow for the proper separation of FOG according to Stokes law and the differences in specific gravity.

Engineering is never as simple as that.  For example, Stokes law can predict the rise rate of a globule of grease in a static (non-moving) body of water, given that we know the size of the globule, its specific gravity, its temperature, and its viscosity.  If the water is moving do we know if the flow laminar or turbulent?  If it is laminar (turbulent free) the rise rate may still be predictable, but how do you calculate for turbulent flow?  What about velocity?  Velocity is a function of the entering flow rate and the cross sectional area the flow passes through as it moves across the interceptor.  How do we know if a design does a good job in spreading out the entering flow sufficiently to mitigate the velocity and allow for gravity differential separation?

Because no amount of work with a calculator can answer these questions, designs must be tested, their performance evaluated and adjustments made to improve the design until it achieves the desired performance. 

This is why jurisdictions should not mandate specific prescriptive design elements for grease interceptors. How do we know that the mandated design elements work in an interceptor?   

Let’s consider a couple specific design elements typically mandated

Must have a minimum of two compartments – What is the basis for this requirement?  There are many claims of enhanced performance but where are the scientific studies that prove the benefit?  On the contrary studies such as the Assessment of Grease Interceptor Performance by the Water Environment Research Foundation (WERF) published in 2008 raise serious concerns over the inclusion of a baffle wall (creating two compartments) in traditional gravity interceptor designs citing, among other things, short circuiting (unexpected bypass).

Must have a minimum liquid holding capacity of 500/750/1000 gallons – What does the amount of water an interceptor can hold have to do with how efficient the interceptor is?  The answer is nothing directly.  How much grease will a 500/750/1000 gallon gravity interceptor efficiently hold?  The answer is that no one knows. Traditional gravity interceptors that comply with these minimum liquid capacities have an Achilles heal; they are not tested and rated for performance.  The answers to any questions regarding the performance of these interceptors can only be guessed because there is no scientific data that proves that a minimum amount of liquid holding capacity will provide assurance of performance.

A better way

Instead of telling us what you want an interceptor to look like, why not tell us what you want it to actually do?  For instance:

  • How efficient should an interceptor be?  Should it be required to be tested to prove that it meets the requirement?
  • How much grease should an interceptor be required to separate and store?  Should it be required to be tested to prove that it meets the required storage capacity?

If jurisdictions would get out of the design business and instead focus on approving performance standards for interceptors this would create an environment that would encourage innovation in designs with a focus on better efficiency and storage capacities. 

That’s how jurisdictions will get what they really want out of an interceptor!



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