Accuracy should trump easy-peasy

In January the ASME A112.14.3 sub-committee will once again meet to try to update the standard, which has remained unchanged since its publication in 2000.

One of the issues that the committee has struggled to come to consensus on is the incremental test cycles and the skim tank procedure.

At the risk of boring the reader to death - something I normally try to avoid - let me explain the procedure and the inherent problem that I believe needs to be addressed.

This is what the ASME A112.14.3 test apparatus looks like, conceptually anyway.

Once the flow rate is calibrated from the test sinks to the desired flow rate for the interceptor being tested, the testing procedure involves opening the drain valve on the test sinks and "let 'er rip".

The flow from the test sinks will pass through the grease interceptor and into the skim tank, after which any accumulated lard found in the skim tank is skimmed off (hence the clever and sophisticated name of the tank), dewatered and weighed. The weight of the lard taken from the skim tank is deducted from the weight of the lard introduced into the test sinks and the difference representing the efficiency of the interceptor during that specific test cycle.

Now, the skim tank is supposed to have a waste outlet connected to the bottom of the tank at one end and be trapped to retain approximately 18 inches of water in the tank. The tank is also provided with a four (4) inch bottom drain to permit draining and cleaning.

The purpose of the waste outlet being trapped to retain 18 inches of water in the tank is so that the tank volume does not drop below 18 inches during a test cycle.  In practice what this means is that the flow from the grease interceptor can actually pass through the skim tank during a test cycle since the waste outlet is typically not closed off during testing.

Is this a problem?

Perhaps not, but since the point of the testing procedure is to determine with the greatest accuracy possible the efficiency of a grease interceptor, this doesn't seem right to me.

In case you didn't quite follow all that, allow me to clarify the issue.

The skim tank is just that a tank intended to capture the effluent from the grease interceptor during a test run in order to measure the amount of lard that escaped the interceptor which will determine the efficiency of the interceptor.

If the tank is open during a test run it is possible for some of the lard that enters the skim tank to escape the skim tank.  Any lard which escapes the skim tank is automatically counted as lard that must be in the grease interceptor, which has the potential of reporting a higher than actual efficiency of the interceptor for a given test cycle.

Since the interceptor will be tested over a minimum of 13 cycles, the problem can be compounded resulting in a potentially significant higher than actual average efficiency for a certified grease interceptor.

The solution would seem to be a simple one; keep the skim tank closed during each test cycle.

I have proposed just such a radical idea to the committee before but have received resistance from some who cite the benefit of the "automatic" nature of the testing when the tank is allowed to remain open during each test cycle.

I take the term "automatic" to basically mean, "easy-peasy", which is actually short for "easy-peasy-lemon-squeezy", which means "very easy" and makes me think about vodka and a drink called a Lemon Drop...

I doubt that the proponent of the argument intended committee members to conjure up images of
alcoholic drinks, but I did anyway.

Further arguments have been proposed that at higher flows and a two-drop series (allowed by the standard) that a lack of  human intervention at the required time would cause the skim tank to overflow, invalidating the test.

It's hard to not respond with a bit of sarcasm to that argument, but I'll abstain and simply say, "hogwash".

The committee has already agreed to enlarge the size of the skim tank for testing grease interceptors at flow rates over 50 gpm.  The size of the larger skim tank will allow for two complete drops (test increments) at 100 gpm, which is 400 total gallons of water. There is no risk of overflowing the skim tank unless the "human interaction" fails to drain the skim tank prior to another test cycle.

If testing personnel fail to drain the skim tank down between two successive two-drop tests at 100 gpm, then I would agree that the test results would be invalid, requiring the interceptor to be tested over from the beginning - and the testing personnel fired for incompetence (after they clean up the mess, of course).

Anyways, it seems to me that accuracy in testing should be the most important issue - definitely more important that what's easy-peasy.

I'll update this post if any progress on the issue is made at the next committee meeting.

The Inherent Problems with CSA B481.2

You may be familiar with Canadian Standards Association's (CSA) standard B481 which governs hydromechanical grease interceptors.

What you may not know is that B481 contains two alternative ratings designated as B481.1 and B481.2

All passive hydromechanical grease interceptors currently certified to CSA B481 are all listed to B481.1, which is basically the CSA version of ASME A112.14.3 and PDI G101.

By now you must be dying to know what B481.2 is, right?

By "dying to know" what I mean, of course, is that you have at least a mild curiosity if not a feigned interest; something more than an out right I-could-care-less feeling about the subject.

Hey, I'll take what I can get.

Anyway, assuming that you are in fact curious or at least feigning interest in the topic, lets take a look at this alternate rating and notice some inherent problems with it.

B481.2 uses effluent concentration sampling for testing the interceptor. The test requires the interceptor to be filled with water first, then have crushed granite added up to the interceptors maximum solids capacity (unless the interceptor is not designed to capture and contain solids), then  the interceptor is filled with sunflower oil to the point at which 5 test cycles of 12,000 mg/L will fill the interceptor to its rated capacity.

The interceptor is then tested 5 cycles with the oil mixture in the prescribed concentration and the interceptor’s effluent is sampled multiple times during the cycle. The average concentration of the oil in the effluent grab samples is recorded as the efficiency for the interceptor in mg/L.

Here are the problems with this rating:

Problem 1
5.3.3 Flow calibration - the standard does not specify any particular flow rates for testing, but instead, refers to B481.0 which specifies a range of flow rates with a lower limit of 7 gpm (26 L/m) and an upper limit of not more than 100 gpm (380 L/m). Thus a manufacturer can simply pick any flow rate for any interceptor and having tested to that flow rate, report the results of the effluent testing without regard to the flow rate at which the interceptor achieved the rating.

Here's the problem...

B481.0 requires the interceptor be marked with its flow rate according to either B481.1 or B481.2, but not both. Therefore a manufacturer can have an interceptor certified to B481.1 at 100 gpm (380 L/m) and can list that flow rate on its label but it doesn't have to list the flow rate used to achieve the effluent testing results under B481.2.

It's one or the other.

For example, let's say a manufacturer tests and certifies their interceptor to B481.1 at 100 gpm.  They can then list that flow rate on their label.  Now lets say they want to test to B481.2 and they discover that the interceptor's effluent concentration is 300 mg/L at 100 gpm. Now lets say they want to show 100 mg/L as their results but can only achieve that threshold at 20 gpm.

They can show the 100 mg/L test results, but are not required to label the interceptor with the lower flow rate used to achieve these test results.

How will anyone know that the interceptor didn't meet the effluent concentration at the flow rate listed on the label?


Problem 2
5.3.1 Solids preloading - B481.2 requires the interceptor to have solids (in the form of crushed granite) preloaded up to the interceptors maximum solids capacity unless the interceptor is not designed to capture and contain solids.

Here's the problem...

B481.2 does not provide a test protocol to determine a grease interceptors maximum solids capacity. 

Problem 3
5.3.4 Oil preloading - B481.2 requires the interceptor to have oil (in the form of sunflower oil) preloaded, according to section 6.2.3, based on the interceptors total rated oil storage capacity minus the amount of oil that will be added during testing.

Here's the problem...

B481.2 does not provide a test protocol to determine a grease interceptors total rated oil storage capacity.

Problem 4
The test protocol under B481.2 mandates that the water and the oil used to conduct the test be at room temperature. We would expect to find lower effluent concentration results from cooler temperature oil water mixtures owing to higher viscosity and flocculation which would be more resistant to mechanical sheering forces and turbulence inside the interceptor. This undermines the credibility of the test results as a predictor of real world installation performance.

Aside from these inherent problems there are two other issues to note:

1. No approved testing facility in North America has ever conducted the testing protocol under B481.2.

2. We are aware of only one jurisdiction in the US or Canada that actually requires compliance with B481.2. This apparent lack of jurisdictional support only serves to undermine the viability of this rating which dampens the incentive for manufacturers' to certify interceptors to this rating on their own.

As you can see there are a number of problems with certification to CSA B481.2 and these problems are serious enough that jurisdictions should avoid policy decisions that would include mandating compliance with this rating until these problems are adequately addressed in future amendments to the standard.

What's Wrong with Semi-Automatic Draw-Off?

The contents of a grease interceptor, especially one that has not been cleaned out recently, is nothing short of breathtaking!

By 'breathtaking' of course I mean one whiff makes your eyes water and gives you the kind of nausea common on commercial fishing charters, while you reel backwards in a desperate fight for oxygen...and a gas mask.

I had the same experience one time when I accidentally smelled one of my teenagers dirty socks.

It's the kind of experience you are not likely to forget and if you are a restaurant owner it's one you are not likely to repeat - at least very often.

As the saying goes, "necessity is the mother of invention," and not opening a grease interceptor is considered, at least by some, a necessity.  

Enter the semi-automatic draw-off grease interceptor, which as one manufacturer's literature states, "permits removal of accumulated grease without cover removal."

Yeah baby, the holy grail of indoor grease interceptors, right?!

Not so fast.

Here are two major reasons that semi-automatic draw-off is not the right solution:


1. Typical cleaning instructions:

• Run full stream of hot water (preferably 140 deg F or higher) for at least two minutes
• Turn off hot water for three to five minutes - to allow grease to liquify
• These interceptors come with a shut-off valve connected to the outlet - step three is to close the shut-off valve
• Most units come with a draw-off valve on top connected to a hose or pipe - step four is to open this valve and place a container under the hose or pipe
• Now run hot water through the unit at 1.5 to 2.5 gpm causing the unit to fill, raising the accumulated and now liquified grease into an internal cone and out the draw-off connection
• Continue to run the hot water through the interceptor until clear water appears then shut off the flow
• Close the draw-off valve and open the shut-off valve and the interceptor is now supposedly clean and ready for use again

As an owner you have to do this process each and every time you service the interceptor.  How many full buckets of the interceptor's contents - which, by the way contain the very odor the owner was hoping to avoid in the first place - would it take for the owner to give up ever wanting to service the interceptor again?

Another thing to consider is that some places employ teenagers whom will be tasked with this job.

Teenagers!

I raised three of them myself and I can tell you, based on my experience trying to teach them to make their beds every day, that I have grave concerns about trusting them to properly follow the instructions above...ever.

If you are a teenager reading this post and you feel that my comment does not apply to your bed-making skills then I apologize for lumping you in with my kids, and basically every other teen that has ever been born.


2. How are accumulated solids removed?

It is a well established fact that grease interceptors collect solids, which have to be removed regularly because they decompose inside the interceptor creating all kinds of problems not the least of which is the unforgettable odor I mentioned earlier.

To remove the solids you have to remove the cover, which should cause you to stop and think, "hey, wait a second, why would I want a semi-automatic draw-off if I have to take the cover off the interceptor to clean out the solids anyway?"

Exactly!

So what is the right application for a semi-automatic draw-off?

Let me see, that would have to be a restaurant that does not serve food of any kind and that does not employ teenagers whom would be tasked with the responsibility of maintaining the interceptor and that has an owner that doesn't mind bucket full after bucket full of the interceptors contents stinking up the joint.

So, doing the math, there are approximately 990,000 restaurants in the US according to the National Restaurant Association, and the total number that do not serve any food of any kind would be roughly 0.1% or about 990.  Of those, the number that do not employ teenagers or that have a masochistic owner would be...lets see...divide by...carry the zero...

We're working with very small percentages here, but I think the answer is one.

Whatever the number is, it is too small to justify the existence of the semi-automatic draw-off.

For most restaurants, there is a better way.

A High-efficiency and high-capacity grease interceptor installed outside with all of the kitchen fixtures routed to it, properly sized and properly maintained is the real solution!

ps. teenagers make great restaurant employees, just don't expect them to make their beds every day, or really ever.

Understanding Rated Grease Capacity versus Maximum Grease Capacity

The grease interceptor revolution is about challenging the status quo.

It's about asking tough questions and thinking radically different about the answers.

It's about listening to the people who are affected by the products we make.

The revolution is about making High-capacity grease interceptors because contractors, engineers, restaurant owners and jurisdictions are demanding superior performance.

As more manufacturers see the light and join the revolution it's important that you understand what to look for to make sure that an interceptor that claims to be high-capacity really is high-capacity.

The journey to enlightenment begins with an understanding of the testing and rating requirements of both nationally (US) recognized and plumbing code approved standards and ends with an understanding of some of the tricks of the trade used to disguise an interceptors performance to make it appear to be something that it is not.

Testing and rating requirements

In most jurisdictions across the US, hydromechanical grease interceptor's must be tested and rated to either PDI G-101 and ASME A112.14.3.

Both standards are similar in most respects including the same testing and rating protocol, however there is a key difference between them; PDI allows for either Rated Grease Capacity (RGC) or Maximum Grease Capacity (MGC), while ASME currently only allows for MGC.

What's the difference between RGC and MGC?

An interceptor certified to MGC has actually been tested all the way to its breakdown point while an interceptor tested to RGC has not.

Let me quote from PDI G-101 so you can see the difference between the two ratings:

7.6.2 Determination of Test Breakdown Point (Maximum Grease Capacity)

The test failure, or breakdown point of the interceptor, shall be established at the increment preceding two (2) successive increments in which either the average efficiency is less than ninety (90) percent or the incremental efficiency is less than eighty (80) percent.

7.7 Efficiency Determinations (Rated Grease Capacity)

The grease shall be removed from the skimming tank and the efficiency of the interceptor shall be computed at the (13) thirteenth increment. This provides at least a twelve and one-half (12.5) percent safety factor on the ratio of the rated grease retention capacity to flow rate as indicated in Table 1:



To put the difference between RGC and MGC into perspective lets review the actual test data from NSF for Schier Product's GB-250:




You can see at the 13th increment the GB-250 had an average efficiency of 95.2% and had separated 247.46, which under PDI would allow it to be certified at 200 lbs grease storage capacity according to Table 1.

Under ASME A112.14.3 (which this unit is certified to) there isn't an option for RGC so you have to test to failure (prior to 2007 this wasn't an option under PDI either).

As you can see the GB-250 suffered two successive increments (59 and 60) with incremental efficiencies below 80%. The test breakdown point (the increment that is to be used for certification of the interceptor) is increment 58 at which the GB-250 had an average efficiency of 92.8% and separated 1076 lbs of grease at 100 gpm.

The revolution is gaining momentum

As I said earlier, other manufacturers are apparently starting to see the light.

As new products emerge to compete in the arena of high-capacity grease interceptors though, it's important not only to know how grease interceptors are rated, but also how a manufacturer reports their ratings.

Tricks of the trade

How do you know if the grease capacity shown on a specification or submittal sheet is the interceptor's actual certified capacity or an unsubstantiated claim?

To be honest it can be difficult.

One manufacturer uses the term Grease Design Capacity to confuse or imply that this is the interceptors actual certified capacity. An investigation into the testing and rating of the interceptor revealed that it was certified as RGC, which means the interceptor wasn't tested to failure and therefore their posted 'design' capacity is uncertified and unsubstantiated.

One manufacturer uses the term Greasy-Sludge Capacity. What does that mean? I'm not sure. The amount listed is more than their certified grease storage capacity. The number appears to be a combination of grease and solids capacities combined. But since neither PDI nor ASME test for solids there is no way this number can be substantiated.

Another manufacturer doesn't use pounds at all, instead opting to show their grease capacity in gallons. When you take the gallons and convert it into pounds, you end up with a lot more claimed grease capacity than their apparent certification.

When in doubt ask for the test report.

At the end of the day there is really only one way to know what the actual grease storage capacity of an interceptor is and that is to ask the manufacturer for the test reports.

In case you were wondering, testing agencies are not obligated to share the certification reports for any of their manufacturer's (I was informed of this in a very lawyer-like manner when I asked). That means you have to ask the manufacturer themselves.

Both PDI and ASME require interceptor tests to be documented on Test Form #1, which records each and every test increment along with incremental and accumulated efficiencies. The test report will tell you whether the interceptor was tested only to 13 test increments and is therefore rated at RGC or if the interceptor was tested to failure and you can see for yourself the interceptors MGC.

Schier Products believes wholeheartedly in the interceptor revolution and as such is working on posting the NSF test reports for each certified unit on its website here : http://www.schierproducts.com/Grease%20Interceptors%20Great%20Basin.html. In the mean time, if you would like to see a test report for any of Schier's certified units, just send me an email and I'll get them to you right away.

Competition is a good thing and manufacturers who want to join the revolution are welcome, but now you know what to look for to be sure that the interceptor that claims to be high-capacity really is high-capacity.

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).

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)