The real difference between Gravity and Hydromechanical grease interceptors

Are you one of those who thinks that Gravity Grease Interceptors (GGI) are better than Hydromechanical Grease Interceptors (HGI) because they are so much bigger?  Have you bought into the myth that GGIs can handle all of the flow of a commercial kitchen while HGIs should only be allowed in small kitchens with few fixtures?

You are not alone.  

After decades of use, people just accept assumed differences between GGIs and HGIs as actually existing.

I'd like to point out the similarity between these two types of grease interceptors because this will make it easy to identify mythical assumed differences for what they are. 

They both use gravity-differential separation
Have you ever jumped into the air and not landed on the ground again?  Unless you live somewhere other than on planet earth, you are subject to gravity the same as the rest of us.

If there is no gravity then the earth doesn't revolve around the sun, the moon doesn't revolve around the earth and there is no point in reading any further because there is no such thing as human beings either, which means no one needs to be concerned about how grease interceptors work.

My guess is that we have all proven that gravity exists at least once in our lives.

An object with a specific gravity of less than one will float in water while an object with a specific gravity of greater than one will sink.  This is also very easy to prove.  Here is a list of some common specific gravities:

If you are going to experiment with any of these, I recommend you properly dispose of them when you are finished as most of these are prohibited from discharging to sanitary sewer systems, at least here on earth (if you are not here then you may need to check your local planetary requirements on that).

Gravity-differential separation simply means to use the differences in specific gravities of restricted pollutants, such as fats, oils and grease (FOG) as the means of separation from water inside an interceptor.

Grease poured into a static body of water will rise very quickly to the surface based on the size of the bubbles formed, their specific gravity, their viscosity and the temperature of the grease and water. The rise rates of the bubbles is predictable according to Stokes law.

The difference between a static body of water and a grease interceptor is that the water in the interceptor is not static.  The discharge from a connected fixture will have a flow rate that must be dealt with in a grease interceptor.

A Symposium on Grease Removal titled Design and Operation of Grease Interceptors by Frank Dawson and A.A. Kalinske, published in 1944 explains the fundamentals of gravity-differential separation in grease interceptors as follows:

"For simplicity let us assume that pure grease and water enter near the bottom of a rectangular-shaped interceptor L feet long, B feet wide, and with a water depth of D feet.  The interceptor will do a good job of separation if, as the flow goes through the interceptor, the mean velocity of flow is such as to permit the grease globule to rise a vertical distance D in a length of L feet."

Dawson et al. went on to say, "If we neglect, for the moment, the presence of turbulence we see that the controlling item in the sizing of the interceptor for any particular rate of flow is the rate of rise of the grease globules. If the size of the globules is known, the maximum velocity of rise can be calculated readily from known principles of fluid mechanics, by equating the buoyant force on a globule to the force of fluid resistance."

If I keep quoting Dawson et al. we are all going to be asleep soon. So let me paraphrase from here. Velocity is the speed that the fluid is moving across the interceptor.  If the fluid moves too quickly the smaller grease globules are less likely to have time to rise an adequate distance to remain in the interceptor.  This makes velocity a key to good interceptor design.

The American Society of Plumbing Engineers (ASPE) established three inches as the minimum distance D that a grease globule must rise in a length of L in an interceptor to be retained (ASPE Plumbing Engineering Design Handbook 4, Plumbing Components and Equipment, Chapter 8 Grease Interceptors).

Gravity-differential separation applies to any grease interceptor regardless of manufacturer or type. HGIs must be designed based on this principle and so do GGIs.

No exceptions.

So what is the assumed difference?
That HGI's use controlled flow and some other stuff, most of which is bunk (counter-current flows, bernoulli's equation, etc.), to do what GGIs do with retention time.

Assumption debunked.

HGIs control the incoming flow and distribute it throughout the cross-sectional area reducing the fluid velocity to allow for gravity-differential separation at the maximum rated flow of the interceptor.

GGI's do not!

This is basic physics.

GGIs simply take whatever flow is dumped into them and everyone assumes that, no matter what, the GGI can handle it.

Sorry, it ain't possible.

Uncontrolled inlet velocity and the associated turbulence causes short-circuiting at higher flow rates in GGIs. This is not to say that GGIs don't work.  At lower flow rates they work very well because turbulence decay happens rapidly at low flow rates.  At higher flow rates though, turbulence decay happens over an exponentially longer time.

Experiments by the Water Environment Research Foundation, included in their 2008 report Assessment of Grease Interceptor Performance, demonstrated that residence times of at least one hour were more conducive to turbulence decay in the standard IAPMO approved GGI (Z1001) design.

When everyone finally steps back long enough to think clearly about how all grease interceptors must actually work - we will finally be able to start focusing on what is really important in an interceptor (hint - it isn't how much water it can hold).

When everyone starts focusing on how efficient a grease interceptor is and how much grease it can hold instead of how much water it can hold, we wont need to worry about whether its called an HGI or a GGI because all grease interceptors will be just grease interceptors.

HGIs will not hold just 2 lbs of grease for each 1 gpm of flow rate - because to keep up with competition, they will have to hold 5 times as much or 10 times as much or even more!

If GGIs are the FOG abatement solution of the future then the earth doesn't revolve around the sun and the moon doesn't revolve around the earth and we don't exist either, which is weird because my back hurts.

How do Gravity Grease Interceptors Work and How Efficient are they?

Many jurisdictions across the United States either favor exterior gravity interceptors or outright mandate them, some going as far as disallowing hydromechanical grease interceptors altogether.  

I get it.  I understand why.  Jurisdictions are frustrated!

For many decades grease traps (now called hydromechanical grease interceptors - HGI) have been the primary pretreatment device prescribed by plumbing codes to be installed in commercial food service establishments to prevent grease from entering the collection system.

If they work so well, why have jurisdictions been having so many issues with the buildup of FOG in collection systems?  

A simple inspection of the restaurants in a lot of jurisdictions will reveal a significant number of undersized indoor grease interceptors that are connected only to a multi-compartment sink and are not being maintained frequently enough.  

Hey, Americans are some of the best ditch divers (going from one extreme to another) in the world.  If the problem is allowing restaurants to install undersized grease interceptors inside the kitchen, then the "fix" is only allowing swimming pool sized gravity interceptors outside, right?!

How efficient are these giant interceptors?  How much grease will they hold?

I have asked jurisdictions across the country from sea to shining sea these same questions for the past 5 years, and you know what the answer always is?  "I don't know" - that's the answer. No one knows.  

How is it possible that we have jurisdictions mandating and in some cases only allowing the installation of interceptors whose efficiency and grease storage capacities are unknown?

I suppose the most common argument I hear is that basic physics justify their approval.  Okay, what physics are we talking about?  Stokes law.  The universally accepted answer as to why gravity interceptors work is because of Stokes law and retention time.

Really?  Okay, so what is Stokes law and how does it work?

Stokes law explained
The American Society of Plumbing Engineers (ASPE), Plumbing Engineering Design Handbook 4, Plumbing Components and Equipment, covers grease interceptors in chapter 8. Regarding Stokes law it has this to say on page 153: 

"An examination of this equation shows that the vertical velocity of a grease globule in water depends on the density and diameter of the globule, the density and viscosity of the water, and the temperature of the water and FOG material. Specifically, the grease globule's vertical velocity is highly dependent on the globule's diameter, with small globules rising much more slowly than larger ones. Thus, the larger the globule, the faster the rate of separation."


Table 8-1 Droplet Rise Time

Travel Time for 3" Distance at   68 deg F (hr:min:sec)			Travel Time for 3" Distance at   68 deg F (hr:min:sec)
Droplet Diameter (microns)	Oil (rise time) SG 0.85		Droplet Diameter (microns)	Oil (rise time) SG 0.90
300	0:00:12		300	0:00:15
150	0:00:42		150	0:01:03
125	0:01:00		125	0:01:27
90	0:01:54		90	0:02:54
60	0:04:12		60	0:06:36
50	0:06:18		50	0:09:18
40	0:09:36		40	0:14:24
30	0:17:24		30	0:25:48
20	0:38:46		20	0:58:08
15	1:08:54		15	1:43:22
10	2:35:02		10	3:52:33
5	10:02:09		5	15:30:14
1	258:23:53		1	387:35:49

In the table above ASPE calculated the different rise rates of grease globules based on size in microns, and two different specific gravities (SG) - 0.85 and 0.90, showing how long it would take them to rise 3 inches at 68 deg. F.

Quoting ASPE again:

"Due to reliance on gravity differential phenomena, there is a practical limitation to interceptor effectiveness. In terms of grease/oil globule size, an interceptor will be effective over a globule diameter range having a lower limit of 0.015 centimeter (150 microns)."

Stokes law is fairly straight forward with one catch; it calculates the rate of rise of a grease globule in static (not moving) water. Does a grease interceptor, whether HGI or gravity, contain static water? Obviously not.  Either type of interceptor will have fixtures draining waste water into them at some flow rate.  How does this flow of waste water affect the calculation of Stokes law?

We need to understand the difference between laminar and turbulent flow before attempting to ascertain their affect on the calculation of Stokes law.

According to the Encyclopedia Britannica:

Turbulent Flow

• Turbulent flow is fluid flow in which the fluid undergoes irregular fluctuations, or mixing. The speed of the fluid at a point is continuously undergoing changes in magnitude and direction, which results in swirling and eddying as the bulk of the fluid moves in a specific direction.
• Laminar flow is fluid flow in which the fluid travels smoothly or in regular paths. The velocity, pressure, and other flow properties at each point in the fluid remain constant. Laminar flow over a horizontal surface may be thought of as consisting of thin layers, all parallel to each other, that slide over each other. 

It is impossible to accurately predict the rise rate of a globule of grease in a turbulent flow environment because the turbulent flow acts to re-entrain the grease into the flow path negating the buoyant forces that are acting to lift the grease globule to the surface (depending on its size of course).

The advantage of a laminar flow environment is that the flow has very little effect (adding drag) on the buoyant forces acting on a grease globule.

Quoting again from page 155 of the same ASPE handbook:

"The ability of an interceptor to perform its primary function depends on a number of factors. These include the type and state of FOG in the waste flow, the characteristics of the carrier stream [turbulent vs. laminar], and the design and size of the unit."

Quoting from a paper published in 1944 titled Symposium on Grease Removal, Design and Operation of Grease Interceptors, by F.M. Dawson and A.A. Kalinske: 

"For simplicity let us assume that pure grease and water enter near the bottom of a rectangular-shaped interceptor L feet long, B feet wide, and with a water depth of D feet. The interceptor will do a good job of separation if, as the flow goes through the interceptor, the mean velocity of the flow is such as to permit a grease globule to rise a vertical distance D in a length of L feet."

Let's go back to our ASPE handbook:

"The ideal separation basin is one that has no turbulence, short-circuiting, or eddies.  The flow through the interceptor is laminar and distributed uniformly throughout the basin's cross-sectional area."

Distributing the flow uniformly throughout the basin's cross-sectional area reduces velocity which allows more time for a grease globule to separate inside the interceptor before reaching the outlet and escaping.  The velocity of the flow is reduced proportional to the extent to which it is uniformly distributed throughout the basin's cross-sectional area. 

The question is; do gravity interceptors have a laminar flow path and do they uniformly distribute the flow throughout their cross-sectional areas?

IAPMO/ANSI Z1001 Gravity Grease Interceptor

Here is what the standard gravity interceptor design looks like; the design that is mandated by IAPMO/ANSI Z1001 (formerly PS-80) and approved by model plumbing codes and most jurisdictions:

According to the Water Environment Research Foundations' 2008 report Assessment of Grease Interceptor Performance:  

Figure 5-18

"The standard configuration [IAPMO/ANSI Z1001] displayed substantially poorer performance than any of the other simulations (56.7%). Close observation of the velocity contours (Figure 5-18) of the cross-section within the reactor suggests that the cause for this reduced performance may be due to higher local velocities near the inlet, baffle, and outlet pipes, all contributing non-quiescent (turbulent) flow conditions...Further examination of the velocity contours in the direction of flow (Figure 5-19) suggests several regions of high velocity along the bottom of the tank and along the side walls."


WERF is essentially saying that at higher flow rates the lack of any control over the flow in gravity interceptors exacerbates turbulence and horizontal velocity leading to short-circuiting.

Figure 5-19

The simulations and bench tests pictured in Figure 5-18 and 5-19 represent a 20 minute retention time, which would be 50 gpm entering a 1000 gallon gravity interceptor. That is obviously a significant amount of flow and in fairness is higher than the average flow rate from most FSEs.

WERF's 2008 report also documented Grease Interceptor Influent Fluid Flow Analysis conducted on several different food service establishments (FSE) in Table 4-3 page 4-8.  The data collected included the total flow to the grease interceptor, the maximum flow-rate measured, the average flow-rate for the measurement period, and the size of the interceptor, among other things.

The outlier on maximum flow-rate across all restaurants was 45 GPM at a ‘full service steak house’. The average flow-rate at this same restaurant was 9.8 GPM.  The second highest flow-rate recorded was 35 GPM at a ‘full fare – Italian’ restaurant and the average flow-rate for this FSE was 9.4 GPM. The average flow rate for all restaurants was 2.8 gpm.

It is important to understand that the flow rates from these FSEs have low averages because restaurants don't typically fill up all their sinks and dump them repeatedly all day long.  Normal kitchen operations have some flow associated with both cooking and cleaning throughout the day, however, when they pull the drain plugs at the end of a meal period or the end of the day - this is when they will have a significant amount of flow at higher temperatures.

Based on all of the factors that effect grease interceptor performance, we would expect that a gravity grease interceptor would perform well at the average flow rates of most restaurants. However, we should also expect them to have problems with short circuiting at higher flow rates, and this is in fact what WERF reported finding in their analysis of interceptors in real world installations.  

Conclusion
On page 156 of the handbook we've been referring to here, ASPE explains:

"you can improve the grease interceptor by increasing the interceptor volume or reducing flow and subsequently lowering horizontal velocity and increasing retention time within the interceptor."

Therefore, gravity interceptors will actually need to be bigger in order to prevent short circuiting at higher flow rates - a decision that has consequences both positive and negative (i.e. better efficiencies but H2S gas generation).

Hydromechanical interceptors, though smaller in size and volume, have the advantage of being able to control the entering waste stream creating a laminar flow environment and distributing that flow more or less uniformly throughout their cross-sectional area, thereby reducing horizontal velocity and increasing flow through time.  This is why they are a viable alternative to larger gravity interceptors.

The challenge is to ensure that they are sized to have all of the kitchen fixtures routed to them, in order to avoid inadvertent bypass, and then to make sure they are maintained properly -(both subjects of another post).

Mandated Design Requirements for Grease Interceptors; Why its a Bad Idea!

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 150^o 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!

Article published in WA-APWA Magazine Fall 2012 Issue

Adam Gallion, pretreatment inspector for Southwest Suburban Sewer District in Burien Washington, approached me earlier this year to help document an installation of an alternate gravity interceptor design with a "WERF" style internal piping configuration and to write an article for submission to the Washington State Chapter of APWA (American Public Works Association) quarterly magazine.  Our article was published in the Fall 2012 issue!  Please follow the link below to read the article:

http://www.apwa-wa.org/Uploads/NewsMagazine/Washington_Fall2012_RICH.pdf
(see page 30 of the magazine for the article)