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.

Grease Interceptors; 2009 UPC vs 2006 UPC

In the 2006 Uniform Plumbing Code the term 'Grease Trap' was replaced by a new term 'Hydromechanical Grease Interceptor' which is defined as:  "A plumbing appurtenance or appliance that is installed in a sanitary drainage system to intercept nonpetroleum fats, oil, and greases (FOG) from a wastewater discharge and is identified by flow rate, and separation and retention efficiency.  The design incorporates air entrainment, hydromechanical separation, interior baffling, and / or barriers in combination or separately..."  Hydromechanical Grease Interceptors are required to control the flow rate entering the device with either an external flow control or a built-in flow control (non-external), either directly connected or indirectly connected to the sewer system.  These devices are allowed a maximum of 720 drainage fixture units (DFU) according to Table 10-2 of the code.

The 2009 Uniform Plumbing Code revised the sizing requirements for Hydromechanical grease interceptors and eliminated the application of Drainage Fixture Units in sizing considerations.  In chapter 10 Table 10-2 was revised to size these devices by either using Gravity Flow Rates or Fixture Capacity.

2009 Uniform Plumbing Code, Table 10-2

There is some confusion about whether to use the One-Minute Drainage Period or the Two-Minute Drainage Period.  Lets clear that up.

Plumbing Engineers (specifiers) rely on the American Society of Plumbing Engineers (ASPE) for instruction and guidance on industry best practices for sizing plumbing piping systems.  In the Plumbing Engineering Design Handbook 2, chapter 1 Sanitary Drainage Systems, ASPE instructs engineers to size their gravity flow waste piping systems at half-full capacity to minimize the generation of pneumatic pressure fluctuations, to prevent self-siphonage of traps, and reduce noise, while providing enough flow to rapidly carry away the soiled water without clogging the pipes or leaving solids in the pipes.

Bearing in mind the instruction to engineers to design their gravity flow sanitary drainage systems at half-full capacity, we can easily see that the Two-Minute Drainage Period more closely reflects the actual flow rates the piping system was designed to deliver.  Thus when sizing an interceptor using the pipe size exiting the building you should use the Two-Minute Drainage Period.

When using Fixture Capacity the same rule applies.  Notice the total load in gallons in the example given is 49.9 gpm.  The One-Minute Drainage time assumes that the tail piece at the bottom of each compartment will allow the full contents of the sink to empty in one minute.  In reality this is not the case.  Most of the multi-compartment sinks either use a 1-1/2" or 2" tail piece.  The maximum flow rate of a 2" tail piece is 24 gpm.  Therefore you should use the Two-Minute Drainage Period which more closely reflects the actual flow rate coming from the fixture.

Whether it takes the sinks a minute or two to drain into the sanitary system makes little difference in the normal operation of a commercial kitchen.  The more important issue to consider when sizing grease interceptors is how much grease they should be expected to separate and store.  The plumbing code makes no allowance for determining this "FOG loading" and as such has done a poor job of correctly sizing grease interceptors.  For information on grease production sizing please see Schier Product's website at www.schierproducts.com.