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. 

The 25% Rule; where did it come from?

Have you heard of the 25% rule?

In case you are unaware or have been off-planet for a while let me explain.

The 25% rule is used primarily by pretreatment authorities to determine when a grease interceptor (or oil separator) is full. A fairly universal definition would be:

"The total depth of the floating grease layer plus the settle-able solids layer cannot exceed 25% of the total liquid depth of the interceptor."

Determination is made by taking a core sample with something like a Sludge Judge or Dipstick Pro (I know of a few jurisdictions that use florescent light covers (clear plastic tubes) from Home Depot or Lowe's, with a rubber stopper).  The device is lowered slowly through the fats, oils and grease layer all the way to the bottom through the solids layer of the interceptor and then capped or plugged and slowly removed and set aside to rest.  This allows the captured FOG to collect at the top of the device while the solids settle at the bottom.

A measurement is taken, typically in inches, from the top of the FOG layer to the bottom of the device, which represents the tanks total water column.  Then the FOG and solids layers are each measured, also typically in inches, and added together.  If the combined FOG and solids layers are equal to or greater than 25% of the total water column then the interceptor is considered full.

For example lets take a typical gravity interceptor in the field like the one pictured here on the left. 

The technician is using a Dipstick Pro which appears to be showing a 48" water column and the technician is showing by the spread of his fingers a FOG depth of about 6" (okay, I'm guessing on that, but I have fairly large hands - not like Wilt Chamberlains, but I wear a large golf glove - and when I spread my fingers like his against a ruler its about 4.5" and adding a bit for the extra FOG above and below his fingers its about 6" or so, give or take).  If the solids layer at the bottom of the device is also 6" that would be a combined 12" of FOG and solids.  When you divide 12 by 48 you get 25%.

This particular interceptor is full, hence the stunned demeanor from the restaurant owner/manager as the technician gives him the bad news.

But wait, there appears to be quite a bit of space left in the interceptor for collecting even more FOG and solids, so how do we know that this interceptor is actually full (a question this restaurant owner/manager probably asked the technician)?

Here's the thing, it's not that the rule is a scientifically based determination of efficiency breakdown, or that the EPA has mandated it, it's more like a generally accepted rule-of-thumb that many jurisdictions have adopted.

Where did it come from?

Good question.

I've been looking for the answer for a while now and no one seems to know.

Seriously.

In 2011, while preparing for a presentation at the Pacific Northwest Grease Summit in Bellevue Washington, I wondered if there was a correlation between the capacities of certified hydromechanical grease interceptors and the 25% rule.

I took all of the major manufacturers certified units (JR Smith, Zurn, Mifab, Watts, Josam, and Wade) and did some very basic math.  For example, if a unit was certified at 20 gpm with 40 lbs grease capacity using lard, I converted the amount of lard in the interceptor, when it was full, into gallons and then divided that by the amount of water the unit could hold.

It didn't matter which manufacturer's unit I checked, the results were very similar and all within a narrow range.  The maximum capacity for storing grease before failure in each unit I checked was between 25% and 35%.

This is further supported by the Plumbing and Drainage Institutes 1998 (R-2010) paper Guide to Grease Interceptors - Eliminating the Mystery, in which they stated that PDI-G101 certified interceptors may need maintenance when as little as 25% of their rated capacity has been reached.

Fast forward to my more recent research in which I have been emailing jurisdictions, googling the internet and searching all available forums for any clue as to the origins of the 25% rule.

Honolulu appears to be one of the early if not earliest users of the rule.  A post on the Yahoo Pretreatment Coordinators forum said that the jurisdiction did the same calculations as far back as 1995 as they were developing their FOG program.  It was stated that the jurisdiction chose 25% to be conservative and it became the rule for grease interceptor maintenance enforcement in their new FOG program.

Many of the jurisdictions in Orange County California use the 25% rule based on a recommendation in the Orange County FOG Control Study which was not based on any science, but rather on a survey of FOG control programs around the US, many of which were using the 25% rule or similar standard such as maximum inches of accumulation of FOG or solids.

The 25% rule appears to be ubiquitous in FOG programs and ordinances, not because of any scientific or technical merit, but rather it seems to have its footing in the idea that 'everyone else is doing it' so it must be right.

The problem now is that the rule is well entrenched in these FOG programs and ordinances making it difficult for jurisdictions to be flexible with newer technologies that hold more grease in comparable foot prints to traditional designs, in some cases matching the storage capacities of much larger gravity style interceptors.

Schier Products Great Basin, Thermaco Trapzilla and other products looming on the horizon are capable of storing grease and solids to well over 50% of their liquid volume, but jurisdictions are challenged to figure out a way to allow an owner to actually benefit from these higher capacities, owing to limitations set on themselves through enacted policy.

Hopefully by understanding how we got where we are, jurisdictions can gain insight into how to either avoid the pitfalls in setting universal capacity limits and/or perhaps, correct any problems that may have been created in enacted policies that inadvertently punish owners who would choose to use newer more efficient higher capacity technologies.

Anyone out there that has more information on the history of the 25% rule please message me and I'll update this post. 

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.

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.

The Grease Interceptor Revolution

They say that imitation is the sincerest form of flattery, which sounds good to me, although I'm not sure who 'they' is.

If it's true, then the Schier team should be blushing, based on whats been happening in the world of grease interceptors recently.

Let me explain (or as Ricky Ricardo used to say, "esplain").

Schier Products is leading a grease interceptor revolution!

Prior to 2006, when Schier launched the Great Basin series, there had been little change in the way grease 'traps' were manufactured and certified. 

Prior to the year 2000, the only standard governing these devices was PDI G-101, which was originally published in 1949 and was based on research conducted at the Iowa Institute of Hydraulic Research (IIHR) in the early 1940s.

Even before the PDI standard was launched, the IIHR had been conducting testing on grease interceptors and had developed a rating system.  Any type of grease interceptor, before it could be installed in an army camp kitchen, had to have a rating certificate from IIHR.  

The image to the left is a schematic of the IIHR test apparatus.  

What was significant about the testing and rating that had been developed at the IIHR was the initial focus on army camp kitchen installations.

It was the army which had requested a standard that could be incorporated into its specification for grease interceptors, MIL-T-18361 (cancelled in 1982), because in the absence of a standard, ratings were determined by each manufacturer for their own interceptors.

IIHR Certified interceptors were required to meet the following minimum performance standards under the test parameters:

• Must have a minimum 90% average efficiency
• Must separate and store 2 lbs of grease for each gpm of flow at the minimum average efficiency.

Since the focus of the research was grease interceptor sizing and rating for army camp kitchens, the researchers created Table B (at right) listing the type of fixtures and recommended minimum rate of flow capacity for any interceptor that would be connected to those fixtures.

This created a problem.

It placed emphasis on how much flow an interceptor could receive while meeting the minimum efficiency and storage capacity requirements.

At left is a typical laboratory test data report for a commercial grease interceptor tested by IIHR.

Notice that the interceptor was tested to failure at multiple different flow rates!  

Why is that?

The goal was to test the interceptor to find the maximum flow rate that the interceptor could be certified to, while meeting the minimum requirements for efficiency and grease storage capacity.

How is that any different than today?

As I said earlier, when it comes to grease interceptor technology, until more recently things were operating pretty much exactly the same.

In 2000 ASME published an alternative standard for grease interceptors, ASME A112.14.3, but unlike the PDI version, this new ASME standard was consensus based and much more flexible with innovative technologies while still requiring the same performance testing as PDI G-101.

The advantage of the ASME standard is that, unlike PDI, a manufacturer does not have to use a vented external flow control and air entrainment as mandated by PDI. Instead, ASME allows for non-vented external flow control, integral (or built-in) flow control, or even an indirect connection.  

At last, an industry recognized and approved standard that lets manufacturer's innovate their interceptor designs to create better performing technology.

Prior to the launch of the new Great Basin series, Schier's R&D team took a radically different view of grease interceptor design based on the flexibility offered under ASME A112.14.3:

• Why create an interceptor that merely meets the minimum performance requirements in the standards?  
• Why not create a design that has a higher average efficiency than 90%?  
• Why not create a design that can hold five or 10 lbs of grease for each gpm of flow rate?  
• What happens to lemmings?

This thinking led Schier to designs that had substantially higher efficiencies and storage capacities than traditional PDI G-101 certified interceptors.  

Schier has certainly not been alone in the revolution!  

Thermaco's Trapzilla grease interceptors have been available for several years and also dramatically outperform traditional PDI G-101 certified interceptors.

Whats finally happening now is that other manufacturer's have 'seen the light' and are introducing certified interceptors intended to compete with Schier's higher effiencies and storage capacities.

While competition is good for the industry, not all competition is the same. 

What is 'design capacity'?  Why do some manufacturer's list different grease storage capacities on the same specification sheet?  Why do some manufacturers use their own terminology instead of recognized industry terms for grease and solids capacities? How do you know how a grease interceptor actually performed during testing?

I will address these questions and more in my next post on grease interceptor testing and ratings - Understanding Rated Grease Capacity versus Maximum Grease Capacity. 

When you know and understand the testing and rating systems in approved standards and what to look for in a certification you'll be better informed when specifying or approving a grease interceptor.

Is Bernoulli's Principle Behind Grease Interceptor Performance?

When it comes to how grease interceptor's work, there are many theories espoused that are supposed to be based on principles of fluid dynamics, however, when tested under controlled conditions many of these theories are debunked.

For example, I have heard the argument that Bernoulli's principle is behind hydromechanical grease interceptor performance. On its face the argument sounds plausible. I would, however, like to test the theory with historical research and current experience from thousands of tests conducted at our plant utilizing our own ASME A112.14.3 (PDI G-101) test apparatus.

In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure.

Bernoulli's principle is commonly used as a rudimentary explanation of how airplanes fly and the way 'lift' works on a wing.

It also explains how curve balls, sliders, and sinkers work in baseball.

Some argue that baffles inside a grease interceptor, which increase velocity, serve to create regions of higher pressure underneath regions of lower pressure aiding in separation efficiency as grease in the higher pressure region is forced to rise towards the region of lower pressure based on Bernoulli's principle.

What the research says
In the early 1940s, research began at the Iowa Institute of Hydraulic Research to formalize a grease interceptor testing and rating system, initially for the construction branch of the US Army Engineers. The Plumbing and Drainage Manufacturer's Association took that research and in 1949 launched the first commercial standard for grease interceptors, PDI G-101.

In 1944 a symposium of four papers was presented at the Sixteen Annual Meeting of the New York State Sewage Works Association, one of which was titled, Symposium on Grease Removal, Design and Operation of Grease Interceptors, by Francis Murray Dawson and Anton Adam Kalinske.

Frank Dawson was Director of the Iowa Institute of Hydraulic Research (1936 - 1944), Dean of the College of Engineering (1936 - 1959) and Professor of Engineering (1936 - 1959) at the University of Iowa. He had worked very closely with the military on various earlier and concurrent projects and was considered an expert on fluid dynamics as well as the hydraulics and pneumatics of plumbing drainage systems.

As a result of the research conducted by the IIHR, any kind of interceptor, before it could be installed in an Army camp kitchen, had to have a rating certificate from the Institute (Iowa Institute of Hydraulic Research, Bulletin 30, 1946).

Dawson et al, established gravity differential separation as "the basic principle of grease interception." He defined this as, "the liquid greases and fats separate from the waste water in the interceptor, when the velocity of flow is reduced, owing to the difference in specific gravity."

"Since grease separation is due to gravity differential, a quantitative analysis of what occurs as waste water flows through an interceptor may lead to the establishment of some basic design data. 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. 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," Dawson said.

Dawson then went on to explain the application of Stokes law, providing the formula and the basic calculations which led to the determination that 150 microns was the minimum size grease globule an interceptor could reasonably be sized to capture 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 size will be easily separated."

Bernoulli's principle was developed by Daniel Bernoulli and published in his book Hydrodynamica in 1738. It is a well known principle in fluid dynamics, yet Dawson et al. never made mention of it applying to grease interceptors. In fact, while Dawson et al. argued there was a benefit to some kind of baffle near the inlet with louvers to distribute the flow (throughout the cross-sectional area of the interceptor) and give it a gentle upward motion, he criticized the use of baffles in the interceptor body as, "undesirable since they induce turbulence."

Any positive effect that could be argued for baffles in the application of Bernoulli's principle is necessarily offset by the negative effect of the resultant turbulence created by the baffles.

ASPEs Plumbing Engineering Design Handbook 4, Plumbing Components and Equipment, Chapter 8, Grease Interceptors, states, "The ideal separation basin is one that has no turbulence, short-circuiting or eddies. The flow through the basin is laminar and distributed uniformly throughout the basin's cross-sectional area."

Distributing the incoming flow throughout the cross-sectional area of a grease interceptor is the key to reducing forward velocity. While flow-rate measures volume, velocity measures speed and reducing the speed of the flow is critical to good interceptor design as was indicated by Dawson et al. in their research and confirmed more recently by ASPE.

Practical experience
Schier has literally run thousands of tests on interceptors, because it’s one thing to sit at a desk calculating the effects of all of the various principles of fluid dynamics on a grease interceptor, but where the leather meets the road is when you take your engineered design to the test bench.

In all of our thousands of tests, we have never found a benefit to adding internal baffles to somehow take advantage of Bernoulli's principle. Instead we experienced far worse consequences in the resulting turbulence.

By far, our greatest successes in design effectively control and distribute the entering flow throughout the interceptors cross-sectional area, while simultaneously creating a laminar flow pattern in a wide open vessel (no baffles).

The argument that Bernoulli’s principle is somehow necessary to hydromechanical grease interceptor performance is a theory that cannot be proven and has no support in any research that has ever been done going back as far as the original testing and rating protocols developed in the early 1940s.

Nothing has ever been written by PDI, ASPE or any other research body that supports the idea that Bernoulli’s principle is behind hydromechanical grease interceptor performance.

Bernoulli's principle does explain how curve balls, sliders and sinkers work though, in case you're interested!

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.

 

Grease Interceptors; Hydromechanical, what is that?

The 2006 Uniform Plumbing Code made some interesting changes in chapter 10 relating to grease traps that have some people confused.  Lets clear it up.

For years the term "Grease Trap" was used to define "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."  The device was commonly used as a point of use interceptor servicing a specific fixture.  Until the 2006 code changes these devices were limited to a maximum of 4 drainage fixtures.

Now we have a new term "Hydromechanical Grease Interceptor".  It 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..."  These devices are required under the code 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.

That definition may need a bit of explaining.  Lets simplify it.  A Hydromechanical Grease Interceptor is a device that separates and stores fats, oils, and grease (FOG) and is commonly used inside a restaurant for point of use grease separation.  These devices are "generally" installed inside.  Flow rates for these devices range from 10 gpm to 100 gpm.

Are all Hydromechanical Grease Interceptors alike?  The short answer is no.

Manufacturers have used the same old technology in grease traps for decades and the standard that has governed this type of grease interceptor is PDI-G101.  The basic design  requires the interceptor to have baffles inside (units are typically made out of steel) and an external flow control with an air vent.  The baffles are intended to extend the flow path while the vent (or air in-take) on the flow control provides a source of air to mix with the in-fluent as it enters the interceptor intended to aide in the separation process.  A certified device is required to separate two (2) lbs of grease for each one (1) gpm of the interceptors certified flow rate at a minimum of 90% average efficiency.  For example a 20 gpm unit is required to separated and store 40 lbs of grease and be at least 90% efficient up to the units rated capacity.

The plumbing code has another approved standard for Hyrdromechanical grease interceptors called ASME A112.14.3-2000.  This is an open standard that allows a manufacturer to be innovative, potentially developing designs that may be more efficient and separate more grease than the minimum requirements.

Schier Product's Great Basin Series and Thermaco's Trapzilla series are just two examples of manufacturers that have developed interceptor designs that are a significant departure from older technologies.  Both of these series are certified to ASME A112.14.3 Type C.  In both cases their designs could not be certified to PDI-G101 owing to their lack of an external vented flow control and internal baffling.  They also meet or exceed the minimum performance requirements of PDI certified interceptors.

Hydromechanical Grease Interceptors; the new term for a grease trap.