Trapping and Venting for Grease Interceptors

From time to time I will get an email that contains a schematic of a grease interceptor installation with the question, "does this look right?"

Usually the email-sender wants to know if the trapping and venting is shown correctly.

The answer requires a knowledge of both the applicable plumbing code and the installation requirements of the specific grease interceptor.

Model plumbing codes require each fixture discharging into a grease interceptor be individually trapped and vented, and require the installation of a vent downstream of the grease interceptor.

Standards also come into play when it comes to determining the type and location of vents upstream of a grease interceptor.

PDI G101 mandates the installation of an vented external flow control. The vent on the flow control is an air intake.  As the waste stream flows through the orifice in the flow control device negative pressure is created, drawing in air from the air intake which is intended to mix with the waste water as it enters the grease interceptor aiding in separation efficiency.

ASME A112.14.3 allows four types of ratings as follows:
  Type A - units with external flow control, with air intake (vent): directly connected
  Type B - units with external flow control, without air intake (vent): directly connected
  Type C - units without an external flow control: directly connected
  Type D - units without an external flow control: indirectly connected

Manufacturer's are required to identify which Type their interceptor is rated to when certifying to the ASME standard. 

PDI G101 and ASME A112.14.3 Type A certified interceptors are required to have a vented external flow control installed upstream of the interceptor.

Okay, so lets take a look at some drawings and see what works and what doesn't.

What's wrong with this installation?

It depends on which model plumbing code applies to the installation.

The International Plumbing Code (IPC) 1002.1 (exception 3) allows a grease interceptor to serve as a fixture trap - where it is intended by the manufacturer to serve as a trap - for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the outlet of the fixture to the inlet of the interceptor is not more than 30 inches and the developed length of the waste pipe from the most upstream fixture outlet to the inlet of the interceptor does not exceed 60 inches.

This drawing for a PDI G101/ASME Type A grease interceptor appears to be compliant with the IPC.  Assuming the interceptor is intended to serve as a fixture trap, there is no requirement to install an additional trap and vent between the fixture and the interceptor. 

The Uniform Plumbing Code (UPC) does not permit a grease interceptor to serve as a fixture trap and also prohibits the installation of a vent between the air intake on the flow control and the grease interceptor. The above diagram would not be compliant with the UPC. There needs to be a trap and vent between the fixture and the vented flow control fitting.

What's wrong with this installation?

Actually, nothing!

The drawing shows a PDI G101/ASME Type A grease interceptor connected to a the three compartment sink that is trapped and vented, an external flow control with air intake and a vent on the downstream side of the grease interceptor.

This installation would be compliant with both the IPC and the UPC.

What's wrong with this installation?

The drawing shows a PDI G101/ASME Type A grease interceptor with an external flow control with air intake, however it also shows a trap on the fixture but no vent for the trap.

This is not compliant with the IPC since the code does not allow double trapped fixtures. The code does allow the interceptor to serve as a trap - assuming that this interceptor is intended to serve as a fixture trap - thus adding a trap to the fixture upstream of the interceptor creates a double trapped fixture installation. Either adding a vent to the trap or removing the trap on the fixture altogether would solve the problem.

The only solution for compliance with the UPC is to add a vent for the trap on the fixture upstream of the vented flow control fitting.

What's wrong with this installation?

This drawing shows a semi-automatic draw-off type grease interceptor but without a vented external flow control.

Since all semi-automatic draw-off grease interceptors are certified to either PDI G101 or ASME A112.14.3 Type A (that I am aware of) an external vented flow control must be shown for compliance with the IPC.

For compliance with the UPC this drawing would have to show both a vented external flow control and a trap and vent for the fixture.

What's wrong with this installation?

Again, there is nothing wrong with this installation, though it may not be immediately obvious as to why.

The unit shown is certified to ASME A112.14.3 Type C (without external flow control) with a built-in or integral flow control and does not require an air intake.

That being the case the interceptor does not require a vented external flow control - jurisdictions unfamiliar with this type of interceptor often question drawings like this and understandably so.

This installation is compliant with both the IPC and the UPC since it shows a trapped and vented fixture discharging through an approved interceptor without external vented flow control with a vent installed downstream.

Air Admittance Valves (AAV)

The UPC does not include provisions for AAVs except as a part of an "Engineered Vent System" under section 912.0, much to the chagrin of engineers around the country. Many states that adopt or adapt the UPC have added an allowance for AAVs so you will have to check with your state to see if they are approved.

The IPC approves the use of and installation requirements for AAVs under section 918.0.

What's important to remember about AAVs is that they only allow for the relief of negative pressure in the drainage system. Therefore, when using an AAV to vent a grease interceptor, it's important that the drainage system has provision for the relief of positive pressure to ensure proper flow.

Boilerplate drawings can be trouble

To be honest, much of the confusion over whether a drawing shows a code compliant installation of a grease interceptor or not, can be the result of boilerplate drawings provided by manufacturers in submittals or installation instructions. Manufacturer's want to provide guidance for a broad range of installations without providing detailed drawings for every single installation variable that can exist. 

When it comes to traps and vents for grease interceptors and the fixtures discharging to them, it's incumbent upon an engineer or contractor to identify and comply with local code requirements regardless what a manufacturer's drawing shows. 

Hopefully after reading this post you will find it easier to identify a drawing that is right for your installation or one that needs to be corrected. 

Steel grease interceptor's costly but last tearout and replacement

Steel grease interceptor's corrode and fail over time.  That's a fact that's generally not in dispute.

How long it can take can vary of course,  but is influenced by restaurant menu, frequency of pumpouts and thoroughness of cleaning.

A corroding steel interceptor feeds FOG deposit formation in collection systems with lots of iron oxide, and when it fails it leaches it's contents and contaminates the surrounding soil.

The average life expectancy for a steel interceptor is roughly five years.

The cost to replace a failed steel interceptor runs in the thousands of dollars, which is why we put together a video; we wanted to show just what it takes to tearout and replace a corroded steel interceptor.

The best long term strategy to prevent the problems associated with failing steel interceptors, of course, is to mandate or approve only interceptors made with non-corrosive materials such as heavy duty thermoplastics or fiberglass. 

Eliminating steel interceptors will benefit all stakeholders including restaurant owners, wastewater authorities and the rest of us.

Check out the video here: https://vimeo.com/130551325

Regulatory compliance manager; fancy title or job description?

Recent activities I have been involved in have made me think that people really don't understand what it is that I do for a living.

There's no secret that I work for a grease interceptor manufacturer, But, what do I do for Schier?

Am I in sales?  No.

Wait, let me correct that.  I believe everyone is a sales person in one way or another.  For example, my wife would say that she is absolutely not cut out to be in sales.  But when she really wants something, she seems to have no problem presenting me with all of the features and benefits that would justify her request - which usually boil down to me having a happy and contented wife.

Hey, who doesn't want a happy and contented wife?

I generally find that preferable to just about anything else in the entire universe as it tends to provide a much "safer" environment at home.

My kids are some of the best sales people I know, with their mom and dad anyway.  They can be very creative in their justifications for why we should do this thing, or buy that thing, or go to such and such a place, all the while hammering home the features of this or that and the benefits that either they specifically would enjoy or that we all would collectively enjoy together.

Sales is really just the art of persuasion.  Some are naturally more gifted at it than others', and they typically end up selling for a living, but everyone has sold someone something at some point in their life.

Okay, so in the sense of doing "sales" for a living, no, I don't do that anymore. In other words, Schier does not pay me to sell anything.  I can't even tell you what our sales look like anymore because I am very much out of that loop.

So what do I do?

You might say that I am an advocate.

Merriam-Webster's dictionary defines advocate as, "one that defends or maintains a cause or proposal", or "one that supports or promotes the interests of another."

Who or what do I advocate for?

I am an advocate for fats, oils and grease (FOG) abatement in wastewater collection systems to reduce or eliminate sanitary sewer overflows (SSO) and their resultant risks to human health and safety.

That is my job description.  It has taken a while for it to come into clear focus, even for me.

What it means is that, while I am paid to do what I do by Schier Products, I am not paid to promote or sell for Schier Products.

I am in fact paid by Schier Products to advocate for what I think is right for FOG abatement in wastewater collection systems.

The advocacy I do is both internal and external to Schier.

When working with Schier I may advocate for some specific product improvements to make field inspections by pretreatment personnel easier or I might advocate for a new product to solve problems I am seeing in the field.

When working with pretreatment programs I may advocate for improving the requirements in an ordinance, or to remove some restrictive language that will hurt enforcement efforts, or to stop mandating products that are untested for performance.

When working with plumbing codes I may advocate for improved less-confusing-language regarding which fixtures should be routed to an interceptor, or I might take a stand against proposed changes that might lead to conflicts with pretreatment requirements or would loosen restrictions that would, in my opinion, be harmful to wastewater collection systems.

When working with standards such as ASME A112.14.3, I might advocate for more accountability in the standard and less prescriptive design requirements that shackle manufacturers and stifle innovation - something very much needed in grease interceptor design and operation, in my opinion.

I think it's important to understand that I see what I do, not as a job, but as a calling.

I believe in the "cause" of FOG abatement.

When you passionately believe in something you are willing to fight for it. You are willing to take actions that you believe in your heart will make a difference for your cause.

I guess that means that you might ruffle a few feathers. You might not "go along" to get along - if getting along means compromising what you believe in.

That being said, I love engaging with the pretreatment community and others who are equally interested in the issues surrounding FOG deposit formation and the exploration of solutions that might mitigate its deleterious effects.

My job title sure can be confusing, but I take my role in the cause seriously and I welcome the challenges, debates, and struggles that are necessary to make a real difference for us all.

Grease Production Sizing

Although sizing grease interceptors, whether gravity or hydromechanical, has always been based solely on flow rate, this strategy falls short in that it does not consider how much grease a specific food service facility could produce.

There are many times when volume or flow rate sizing would lead to the same size interceptor for a subway sandwich shop as for a Mexican grill, however it is well understood by inspectors that these two types of restaurants produce significantly different amounts of fats, oils and grease (FOG).

Schier recommends using a grease production calculation to determine how much grease a particular restaurant is likely to produce in order to ensure that the specified grease interceptor has sufficient grease storage capacity to allow for a realistic and affordable pump out frequency.


 

We developed the above categories for restaurants based on feedback from jurisdictions and pumper contractors around the country, combined with reports such as the Brown Grease Study (Kennedy Jenks 2011) which gives detailed information about restaurant types and menu's as well as grease and solids production data.

The formula for calculating grease production requires three bits of information:
1. Grease production per meal - you simply have to decide by menu type which category (low, medium or high) that the restaurant falls under and then whether the restaurant uses flatware or disposable (plastic or paper) forks, knives, spoons, plates, cups, etc.

2. Meals or customers per day - if this is a franchise they typically have this information available. Independent start-ups may not know this information up front, in which case you may have to make an educated guess or phone a friend or consult a medium. Most of the time you can get close enough that the calculation makes sense.  Just remember that its better to err on the high side.

3. Days per pump-out cycle - this is simply the maintenance cycle you plan to use for pumping out the interceptor.  Most people will not maintain an interceptor that is sized to be cleaned out more often than once per month and most jurisdictions won't let an interceptor be maintained less often than once every 90 days.  Somewhere in between is the sweet spot for your project.

The formula for calculating grease production is very straight forward. You simply take the amount of grease expected per meal (a,b,c,d,e, or f), times the number of meals expected per day, times the number of days between pump outs to arrive at the grease capacity required for the interceptor.

Lets take a couple of real-world examples:

Example 1
McDonalds (medium grease producer, no flatware - category "c")
400 meals per day X 0.025 lbs per meal = 10 lbs FOG per day, or 300 lbs every 30 days, or 600 lbs every 60 days, or 900 lbs every 90 days.

You can also take a grease interceptors' certified capacity and divide it by the amount of grease production per day, to determine the pump out frequency as follows:

10 lbs per day would require:

Schier GB-75, 75 gpm, 616 lbs = 61 days between pump-outs
Schier GB-250, 100 gpm, 1076 lbs = 108 days between pump-outs
Trapzilla TZ-400, 75 gpm, 400 lbs = 40 days between pump-outs
Trapzilla TZ-600, 75 gpm, 600 lbs = 60 days between pump-outs
Mifab BigMax 750, 75 gpm, 150 lbs* = 15 days between pump-outs
Mifab BigMax 1150, 100 gpm, 200 lbs* = 20 days between pump-outs

*based on grease interceptors' actual third party certification (not based on the manufacturer's claims of performance, which cannot be proven).

Example 2
Buffalo Wild Wings (high grease producer with flatware, category "f")
642 meals per day X 0.455 lbs per meal = 29.2 lbs per day, or 876 lbs every 30 days, or 1,752 lbs every 60 days, or 2,628 lbs every 90 days.

29.2 lbs per day would require:

Schier GB-75, 75 gpm, 616 lbs = 21 days between pump-outs
Schier GB-250, 100 gpm, 1076 lbs = 36 days between pump-outs
Trapzilla TZ-400, 75 gpm, 400 lbs = 14 days between pump-outs
Trapzilla TZ-600, 75 gpm, 600 lbs = 20 days between pump-outs
Mifab BigMax 750, 75 gpm, 150 lbs* = 5 days between pump-outs
Mifab BigMax 1150, 100 gpm, 200 lbs* = 7 days between pump-outs

Of course you can increase capacity by increasing the number of interceptors in order to lengthen the pump-out cycle, i.e:

Schier GB-250 (2), 100 gpm, 2152 lbs = 74 days between pump-outs

Using the grease production sizing method is not limited to any specific manufacturer, you can use it for any interceptor.

The only thing to watch out for is the funny-business some manufacturers' play in making unsubstantiated claims of capacity that they want to be used in determining a pump-out frequency.

Sorry Charlie, that's just not going to work.

When in doubt ask them for their certified test reports to see what their actual/real/genuine/true/factual capacity is, then base the pump-out cycle on that.

High Capacity Hybrid HGI Replacement of 1000 Gallon GGI

Replacing a large volume gravity grease interceptor is expensive and time consuming.

Owners like Chipotle Mexican Grill are opting for better performing high-capacity hybrid hydromechanical (HGI) grease interceptors such as Schier's Great Basin series to replace failing concrete gravity interceptors at locations such as this in Belton MO.

This 1000 gallon concrete interceptor is just 11 years old:



https://www.youtube.com/watch?v=Y6Cz3KL838o&feature=youtu.be

Grease interceptors are not called traps anymore

Every now and again the question of whether a grease interceptor is still considered a trap comes up. 

Grease interceptor...grease trap...you say tomayto, I say tomahto - does it really matter?

The distinction is important enough that you should be on the lookout for a fearsome predator known as a lawyer.  These anal-retentive-jot-and-tittle-semantics-fanatics have already picked up the scent of blood in the water on this subject.

Disclaimer: the sharks in suits pictured here are not intended to represent any particular lawyer or lawyers; any similarity to actual individuals is purely coincidental.

Anyways...

It's interesting to see how the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC) have evolved on this issue over time.

UPC
Through the year 1997, the UPC mandated that grease traps have a 2" water seal (the minimum required for a fixture trap), allowed a maximum connection of four fixtures and allowed the grease trap to be used as a fixture trap for a single fixture provided that the distance between the fixture outlet and the grease trap did not exceed 4 feet and the vertical tailpipe or drain did not exceed 2-1/2 feet. 

In 1994 the UPC added or clarified a restriction by stating that, "no fixture shall be double trapped," which created a conundrum. Since the code prohibited double trapping fixtures, and since a grease trap was allowed to serve up to 4 fixtures, if the grease trap is considered a "trap" owing to its 2" water seal, and it can serve as a fixture trap for a single fixture, and it can receive the discharge of up to 4 fixtures, and each of those fixtures must be individually trapped, and each of those fixtures is then routed to a grease trap, then all 4 fixtures are double trapped, which is a violation of the code.

The 2000 UPC eliminated some confusion by removing the language that allowed a grease trap to serve as a fixture trap as had been previously approved and eliminated the requirement for a minimum 2" water seal. Yet this really only added confusion since a grease trap would no longer be required to have a water or trap seal but it was still called a trap.

The 2006 UPC eliminated the term grease trap and introduced a new term, hydromechanical grease interceptor (HGI), to define passive grease interceptors. By eliminating the term grease trap and all references to it, and by not allowing a HGI to serve as a fixture trap for even a single fixture, the UPC eliminated the confusion that had existed over whether a grease trap was a trap.

It's not.

IPC
Prior to 2006 the IPC defined grease interceptors and grease traps, which were distinguished from each other only in that a grease trap had a rated flow of 50 gpm or less while a grease interceptor had a rated flow exceeding 50 gpm. A grease trap intended to serve as a fixture trap in accordance with the manufacturer's installation instructions was permitted to serve as the trap for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the fixture outlet to the inlet of the interceptor did not exceed 30 inches and the developed length of the waste pipe from the furthest compartment outlet to the inlet of the interceptor did not exceed 60 inches (1002.1 Exception 3).

In 2006 the IPC removed the definition for grease trap and changed the definition of grease interceptor, removing any reference to rated flow, making it the term for a passive grease interceptor.  The code language under 1002.1 Exception 3 from previous codes was not amended, retaining the term grease trap, a term which was no longer defined in the code.

The 2009 IPC removed all references to the term grease trap. The code language under 1002.1 Exception 3 was amended, changing the term grease trap to grease interceptor.

The 2012 IPC introduced a new definition of grease interceptor adding two new terms; hydromechanical and gravity.  The term hydromechanical grease interceptor (HGI) was introduced as the new term for a passive grease interceptor while the term gravity introduced a liquid volume-retention time based type of interceptor.  Beyond simply defining the term, the code made no further reference to gravity grease interceptors.  Section 1002.1 Exception 3 remained unchanged.

The 2015 IPC added another subcategory for grease interceptors called fats, oils and grease (FOG) disposal systems and added section 1003.3.6 governing gravity grease interceptors and the new FOG disposal systems. Section 1002.1 Exception 3 remains unchanged. 

The problem I see with the IPC is in the misapplication of a HGI because the section doesn't require the interceptor to have a 2" water seal.  Instead it relies on the manufacturer to stipulate that the interceptor is intended to be used as a fixture trap in compliance with the language of the section. 

That's dangerous to me. 

By dangerous of course I mean like wearing tuna-laced swim trunks and swimming in shark infested waters.

The IPC could eliminate the confusion by simply requiring a 2" water seal for any interceptor approved for installation under section 1002.1 Exception 3.

Time for someone to submit a code change proposal for the next code cycle...anyone?

Bernoulli's Principle Revisited

One of the most popular posts I have written deals with the question of whether Bernoulli's Principle is behind the operation of hydromechanical grease interceptors, as some have argued.

The specific point I was making in that post (Is Bernoulli's Principle Behind Grease Interceptor Performance?) is that there is no evidence that deliberately adding baffles to the interior of an interceptor (to create regions of higher and lower pressure) enhances grease separation.

Gravity differential separation works best in a laminar low-velocity flow environment.

Attempting to enhance the natural separation of liquids with differing specific gravities by incorporating regions of higher and lower pressure by means of internal baffles, doesn't even make sense.

It would be like hooking your car up to a team of horses to add some "horsepower."

I guess technically the horses add horsepower, but do you really think the car is going to go faster!

So why am I writing about Bernoulli's principle again?

Because, there is a proper application of Bernoulli's Principle in grease interceptor design.

Lets begin with a couple of basic concepts in fluid hydraulics.

Liquids are generally non-compressible, which means you cannot change the volume of a liquid by adding pressure. 

The 'conservation of mass' principle requires every bit of mass (volume of liquid) to be accounted for during a process.

Think of a water hose with a spray nozzle attached.  As you close the nozzle the stream tightens up and goes farther.

The tighter the stream the higher the velocity (the speed at which the liquid is moving).

Since neither the mass nor the volume can change under steady-flow conditions, the velocity must change to allow the mass and volume of liquid to move from the larger area in the hose through the smaller area in the nozzle.

When I was a boy, my brothers and I discovered the awesome power of the tight stream from a hose nozzle which could clean dirt and mud from almost anything.

We often tried to clean each others faces off with this same technique - which invariably led to an opportunity to sit quietly for a while to contemplate our "actions".

I took contemplate to mean ponder all the ways to blame my brothers for the trouble we were in, while plotting appropriate avenues of revenge. 

Anyway, what does this have to do with grease interceptors?

The same is true in reverse!

Think of the nozzle as being the waste piping connecting to a grease interceptor.  The volume and mass of liquid is constant but the velocity changes as the liquid moves from the pipe through the interceptor.

Good interceptor designs will take the mass and volume of water and distribute it throughout the cross-sectional area of the interceptor to reduce the velocity - the speed at which the volume is moving through the interceptor - allowing for gravity differential separation (The real difference between Gravity and Hydromechanical grease interceptors).

Now it may be easier to understand why adding baffles inside the interceptor would not make sense.

The baffles add an obstacle inside of the interceptor that the liquid must flow around.  Velocity increases as the liquid moves around the edges of the baffles and while there is an associated pressure drop, the increase in speed and turbulence more than compensate for the pressure drop making it more difficult for gravity differential separation to occur.

Proof?

If you promise not to get mad, I'll demonstrate the point by comparing several PDI certified 20 gpm units to Schier's GB-20:

Yes, the Schier GB-20 holds more water than the other units.  That's why we are looking at lbs per gallon to compare each interceptor's efficiency in capturing and storing grease.  It simply illustrates that the baffles in these PDI certified units do not enhance the interceptors performance.

The Schier unit has no internal baffles - it's wide open. It does a better job of distributing the volume of liquid throughout the units cross-sectional area which reduces velocity and enhances separation.

So now you can rest easy - feel free to even snore a bit - knowing that Bernoulli's principle does figure into good grease interceptor design - just not the way you might have thought.

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.

The Standards that govern Grease Interceptors (all types) in North America

UPDATED (June 2014)

When it comes to grease interceptors there are a variety of recognized standards that have been developed over the years that govern performance requirements, construction requirements or both. This post will explore all of the available standards for North America and the types of interceptors they govern.

PDI-G101
This standard governs passive hydromechanical grease interceptors.

PDI stands for the Plumbing and Drainage Institute which is an association of manufacturers of plumbing and drainage products. In the early 1940's PDI was called the Plumbing and Drainage Manufacturers Association and members of the group joined with representatives of the Quartermaster General, Surgeon General, Army Corp of Engineers and researchers from the Iowa Institute of Hydraulic Research to develop a testing program to establish flow rates and grease holding capacities for uniform rating of grease interceptors.

Using the guidelines established in Iowa, the Research Committee continued the testing program at The United States Testing Company, Inc., which culminated with the first issue of this Standard in 1949 and certification of applicable grease interceptors.

Test apparatus used by PDI and ASME

Interceptors are tested and rated for efficiency and grease storage capacity. Approved interceptors will have a minimum average grease removal efficiency of 90% and will retain not less than 2-1/4 lbs of grease for each one gpm of certified flow rate at either the interceptors Rated Grease Capacity per Table 1 below (with a 12.5% safety factor) or at the interceptors Maximum Grease Capacity by determining the break down point.

Certification to a Rated Grease Capacity allows a manufacturer to test their unit through 13 cycles and whether "full" of grease or not, so long as the interceptor meets the requirements for efficiency and the minimum grease capacity per Table 1, it will be certified at the Rated Grease Capacity per Table 1.

The standard also allows a manufacturer to have an interceptor tested to break down to determine its Maximum Grease Capacity. It must meet the same efficiency requirements and minimum grease capacity as an interceptor with a Rated Grease Capacity and so long as it does, it will be certified.

Interceptors that are certified to a Rated Grease Capacity and those certified to a Maximum Grease Capacity are required to label their interceptors the same according to Table 1 above. This essentially discourages manufacturers from continuing to test an interceptor beyond 13 cycles to determine the interceptors break down point because the interceptor will still be certified and listed to the same performance as a Rated interceptor. This makes it difficult to determine if an interceptor certified to PDI-G101 actually exceeds the minimum requirements.

This standard also mandates the use of an external flow control with integral air vent (which provides air entrainment to aid grease in separating from the entering waste stream) and internal baffles or barriers. Currently the standard does not allow for any alternative design elements, which discourages innovation.
Lastly, the standard provides language that would imply that a manufacturer could have an interceptor tested and rated to flow rates in excess of 100 gpm. Unfortunately PDI does not have the ability to actually perform the testing at this time. Currently the maximum flow rate of any interceptor certified to this standard is 100 gpm.

This is the most widely used, recognized and approved standard in North America.

According to PDI's website (www.pdionline.com) there are 24 manufacturer's with grease interceptors certified to this standard.

PDI G-102
This standard sets requirements for how grease interceptor sensing and alarm devices perform to signal that the interceptor is approaching its rated capacity.

ASME A112.14.3
This standard governs passive hydromechanical grease interceptors.

According to the foreword, “In 1994, the Plumbing and Drainage Institute agreed to work with the American Society of Mechanical Engineers for the development of this Standard. This Standard includes criteria for testing and rating of grease interceptors; general requirements for these appurtenances; and an appendix of valuable sizing, installation, and maintenance data.”

Regarding the scope of the Standard the foreword states, “…this Standard is limited to units of 100 gallons per minute (gpm) or less in rated capacity.”

One of the significant differences between PDI G-101 and this Standard is that while PDI G-101 interceptors are restricted to design requirements that include a vented external flow control and internal baffles, this standard classifies interceptors with these design elements as Type “A” Rated.

Type A – Units with external flow control, with air intake (vent), directly connected
Type B – Units with external flow control, without air intake (vent), directly connected
Type C – units without an external flow control, directly connected
Type D – units without an external flow control, indirectly connected

I am not aware of any manufacturer who has certified an interceptor to Type B at this time.

Schier Products Great Basin series and Thermaco's Trapzilla series are the only grease interceptors currently certified to Type C. Canplas' new Endura XL series, which they are advertizing as available this fall, also utilize built-in flow control conforming to Type C.

I am not aware of any manufacturer who has certified an interceptor to Type D at this time.

While the testing under ASME A112.14.3 is identical to that under PDI G-101, there are a couple of differences in performance and rating requirements:

PDI G-101 requires an interceptor to hold 2-1/4 lbs of grease for each one gpm, while the ASME standard only requires 2 lbs for each one gpm.

PDI G-101 allows for either Rated Grease Capacity or Maximum Grease Capacity, while ASME only allows for Maximum Grease Capacity.

ASME A112.14.3 is listed and approved by the Uniform Plumbing Code (UPC), the International Plumbing Code (IPC) as well as the National Standard Plumbing Code (NSPC) and most independent state plumbing codes.

ASME A112.14.4
This standard was first published in 2001 and is the only recognized standard in North America governing automatic grease removal devices (GRD).

The devices required to meet this standard must automatically remove fats, oils and grease from the separation chamber to a point outside of the GRD in a container capable of holding 150% of the interceptors rated capacity. The removal process is supposed to ensure that the removed FOG is 95% free of water, by volume.

The standard mandates that GRDs first be tested and rated according to ASME A112.14.3 to establish that the interceptor's separation chamber can hold 2 lbs of grease for each one gpm of flow rate. Then the standard employs an additional test to ensure that the GRD's collected grease can meet the required percent of water content, not to exceed 5%.

ASME A112.14.6
This standard was developed to address a growing category of interceptors that not only separate and retain FOG, but internally dispose of retained FOG by means of mass and volume reduction through thermal, chemical, electrical and biological processes.

The standard allows for the use of hydromechanical grease interceptors meeting either PDI G-101/ASME A112.14.3 or gravity grease interceptors meeting IAPMO/ANSI Z1001.

Acceptable performance, under the test parameters of this standard, is an effluent concentration limit of 100 mg/L (PPM) as determined using USEPA Method 1664.

CSA B481
This is a consensus standard published by the Canadian Standards Association in 2007 governing hydromechanical grease interceptors. It is a more comprehensive standard that governs material requirements, construction requirements, test methods and performance requirements, marking requirements and sizing guidelines.

CSA both publishes Standards and offers testing and certification services to those standards for a fee.  They also allow other certified/approved agencies to test and certify to their standards.  Testing agencies have their own identifying mark as follows:

This mark identifies an interceptor that has been tested and rated by CSA Group to CSA B481.

This mark identifies an interceptor that has been tested and rated by IAPMO to
CSA B481.

 

This standard offers two different types of ratings; B481.1 and B481.2.

CSA B481.1 Testing and rating of grease interceptors using lard
This certification specifically requires testing to ASME A112.14.3 for the interceptors flow rate and rated capacity and requires the interceptor to be marked with its removal efficiency, flow rating, and maximum grease containment capacity.

CSA B481.2 Testing and rating of grease interceptors using oil
This certification 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 fill the interceptor with sunflower oil to the point at which 5 test cycles of 12,000 mg/L will fill the interceptor to its rated capacity.

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

For demonstration purposes let’s say an interceptor’s first cycle has an average effluent concentration of 100 mg/L, 2nd cycle its 120 mg/L, 3rd cycle its 130 mg/L, 4th cycle its 175 mg/L, and 5th cycle its 225 mg/L. The interceptors overall average efficiency would be 100 + 120 + 130 + 175 + 225 = 750 divided by 5 for an average of 150 mg/L. The manufacturer would then be required to post the interceptors Effluent Grease Concentration as 150 mg/L on its label.

There is no predetermined concentration limit that triggers a pass or fail. The test simply requires the manufacturer to list the grease concentration limit as an average of the recorded test results.

There are no certified grease interceptors that meet this standard today.

ASTM C 1613
This is a standard specification for Precast Concrete Grease Interceptor Tanks. It governs the type of cement, method of manufacturing as well as structural and physical design and construction requirements including: the shape and capacity of the tank, number of compartments, location of baffles, minimum and maximum liquid height, volume of free airspace, size and location of manhole covers, and so on.

The only testing requirement contained in the standard is for water-tightness. There is no performance test requirement.

This is not an approved standard in either the Uniform Plumbing Code nor the International Plumbing code.

IAPMO/ANSI Z1001
This is the primary standard governing gravity grease interceptors (GGI), recognized by national model plumbing codes and most independent state plumbing codes. It governs design requirements such as a minimum of two compartments, minimum liquid volume of 300 gallons, minimum free airspace, size and location of manholes covers, and so on. It also governs construction material requirements including concrete, fiberglass, polyethylene and coated steel.

The major drawback to the standard is that it does not require a performance test for certified units, but rather only mandates leakage testing.

Future work on this standard should focus on developing a test protocol to justify detention time based sizing methodologies.

Now you know all of the standards that govern grease interceptors today, the types of interceptors or technologies that are covered and the basic requirements contained in each standard.