The Baffling Requirement for Baffles

I am always baffled when I see a requirement for baffles in an interceptor. Why are they required and do they actually improve performance?

If you are a pretreatment professional, may I ask you to consider the following question:

What is the role of manufacturers of grease interceptors?

Let me ask the question differently:

Are there pretreatment programs, basically anywhere in the country, that staff a research and development department that is capable of developing and testing primary pretreatment device technologies in order to justify policy requirements regarding these devices?

Cue Jeapordy theme...

I come back to this line of thinking from time to time when I come across requirements that mandate an interceptor have internal baffles. I simply do not understand why baffles are required.

Remember that all grease interceptors, regardless of type, operate based on gravity-differential separation. The principle is simple, reliable and is based on laws that are immutable. Fats, oils and grease (FOG) are lighter than water and therefore float.

Turbulence inside of an interceptor, any interceptor, is bad. Turbulence negatively effects velocity which decreases the amount of time FOG has to separate since the FOG has to break free from the turbulent flow path before it can ascend to the surface.

Creating a laminar (turbulence free) flow path in an interceptor is of paramount importance in an effective design.

Baffles inside an interceptor create turbulence.

Allow me to illustrate my point like this...

Here are some typical HGIs - same basic designs that have been around since the 1940's:

Notice the vertical baffles that are intended to enhance the performance of these designs.

Facts:

• 90% average efficiency
• Maximum grease storage capacities are 2 times the flow rate
• Grease storage capacities range from 25% to 30% of total liquid volume

It has only been in the last decade or so that manufacturer's began introducing high-efficiency, high-capacity HGIs and you may notice something these devices have in common:

Schier GB-250 (100 gpm, 1076 lbs) - Canplas XL 100 (100 gpm, 1058 lbs) - Thermaco TZ1826 (100 gpm, 1826 lbs)

All of these designs are open with no vertical baffles.

Facts:

• Efficiencies range from 93% to 99%
• Maximum grease storage capacities range from 10 to 18 times the flow rate
• Grease storage capacities range from 50% to 90% of total liquid volume

Okay, so what about gravity grease interceptors (GGI)?

Many seem convinced that somehow a 30 minute retention time and two-chambers ensure that these devices work correctly as commercial grease interceptors.

What most people may not be aware of is that GGIs are really residential septic tanks adapted for a different purpose.

A comparison between IAPMO/ANSI Z1001 (the standard that governs GGIs) and IAPMO/ANSI Z1000 (the standard that governs septic tanks) reveals little if any real difference between the two tanks.

But, it turns out that even in a septic tank, a vertical baffle forming two chambers is not a good idea!

Image scanned from textbook

According to the same book that the UPC derives the 30 minute retention time from titled Small and Decentralized Wastewater Management Systems (McGraw-Hill, 1998),  "Two compartments have been used to limit the discharge of solids in the effluent from the septic tank. Based on measurements made in both single and double compartments, the benefit of a two-compartment tank appears to depend more on the design of the tank than the use of two compartments."

Under the heading Tank Configuration the textbook notes, "Although a divider is used, the rationale for its use is historical more than scientific. Both Seabloom et al. (1982) and Winneberger (1984) have found, on the basis of field measurements, that the performance of a single-compartment tank is equal to or exceeds the performance of a two-compartment tank of the same liquid volume."

When it comes to GGIs, the WERF report titled Assessment of Grease Interceptor Performance (2008) confirmed with bench testing, that the inclusion of a vertical baffle had a negative effect on the performance of the tank. The tank design that performed the best had an open design with no vertical baffle.

Now you know why I'm baffled when I see requirements mandating the inclusion of internal baffles.

Instead of telling us what you want an interceptor to look like, consider telling us what you want it to do and leave the design, research and development to those best able to create something that will do what you want.

Head to Head Comparison; Hydromechanical vs. Gravity Interceptor

Have you ever wondered how a gravity grease interceptor and a hydromechanical grease interceptor would perform given the same test parameters?  In other words, if you put an equal sized gravity grease interceptor and hydromechanical grease interceptor through the same test, would they perform comparably?

Wait no longer, we have just such a comparison for you.   

In their 2008 report, Assessment of Grease Interceptor Performance, the Water Environment Research Foundation (WERF) conducted experimental testing on a 300 gallon gravity grease interceptor at a 20 minute retention time (15 gpm) and a one hour retention time (5 gpm).  

They experimented with different configurations to compare the results against the "standard" interceptor configuration which is based on IAPMO/ANSI Z1001.

Experiments began with an "initially cooled tank (approximately 70 deg. F) with an influent temperature of approximately 110 deg. F at a 1000 mg/L corn oil concentration."  The brand of oil was Mazola Corn Oil with a specific gravity of about 0.92.

Both influent and effluent samples were taken from each of the experiments and recorded on Table 5-6 shown below:

Notice that at 15 gpm the standard configuration (1. from Table 5-6) achieved a removal efficiency of 78% based on an influent concentration that averaged 950 mg/L. Reducing the flow to 5 gpm (3. from Table 5-6) increased the removal efficiency in the standard configuration to 90% based on an influent concentration of 1,028 mg/L.  

For comparison purposes I am going to have to use some data from the NSF test results from a couple of Schier Great Basin grease interceptors - hey, if other manufacturer's would share the data from testing their interceptors I'd happily use that, but so far no such luck!

So lets start by comparing a 15 gpm hydromechanical unit to the performance of the standard configuration above at 15 gpm.

Schiers' GB-15 was tested by NSF International to ASME A112.14.3  meeting the following requirements:

• Test media; lard with a specific gravity of 0.875 heated to 150-160 deg. F 
• Test sinks; filled with 150-160 deg. F water 
• Ratio lard to water; 1 lb of lard for each 10 gallons of water in all test sinks combined
• Interceptor; flow calibrated with a temperature of 150-160 deg. F

The GB-15 has a total liquid capacity of 16 gallons and was tested at 15 gpm.

The ratio of lard to water puts the emulsified influent at a concentration level of 11,660 mg/L for each test cycle. The GB-15 had an average efficiency of 95.5% after 26 repeated test cycles corresponding to an average effluent concentration level of 493 mg/L.  This is vastly improved performance over the standard gravity interceptor.

Lets compare the gravity interceptor to something a little closer in volume.

Schier's GB-250 was also tested by NSF International to ASME A112.14.3 with all of the required parameters noted earlier.  The unit has a total of 250 gallons and was tested at 100 gpm.

The emulsified influent concentration entering the GB-250 was 11,820 mg/L for each test cycle. The unit had an average efficiency of 92.8% after 58 repeated test cycles corresponding to an average effluent concentration level of 854 mg/L.  Again this is a huge improvement in performance over the comparably sized gravity interceptor.  What's amazing is that this level of efficiency was achieved at a flow rate of 100 gpm.

The performance of both the GB-15 and the GB-250 at their certified flow rates exceeded the best performance of the standard gravity interceptor (when it was tested at only 5 gpm).

It could be argued that the gravity interceptor was tested with corn oil with a specific gravity of 0.92 which is lighter than the lard used in the ASME test (0.875), making the comparison unfair. While there is a difference in specific gravity between the two media, the difference is less than 5%.  

A grease globule with a specific gravity of 0.90 and a temperature of 68 deg. F can rise 3 inches in 1 min 3 seconds.  Change the specific gravity to .0875 and it will rise slightly faster, while a specific gravity of 0.92 will rise slightly slower over the same distance.

Compare that with these other differences in the head to head comparisons:

• The gravity interceptor is 1875% larger than the GB-15 and both were tested at 15 gpm
• The GB-250 is 17% smaller than the gravity interceptor and was tested at a 660% higher flow rate.

The more relevant issue to consider is the influent concentration.  The ASME test uses drastically more grease than would be expected in the field, in order to accelerate the test. While it is far more typical to see influent concentrations below 1200 mg/L (WERF (2008), Lesikar et al. (2006)), to use a concentration that low for testing a grease interceptor would extend the testing period by a factor of months.  This makes the results that much more impressive!

In an earlier blog post I explained the problem with the standard IAPMO/ANSI Z1001 design for gravity interceptors and the short circuiting that these devices suffer at higher flow rates.  There is no doubt that uncontrolled influent is the problem, creating high velocity zones and turbulence exacerbated by higher flow rates.

Any hydromechanical grease interceptor tested and rated in accordance with PDI-G101, ASME A112.14.3, or CSA B481 can be relied upon to be more efficient, up to their certified flow rate and grease storage capacity, than comparably much larger standard gravity interceptors.

Hyrdromechanical interceptors are simply more effective grease interceptors at higher flow rates; when you really need an interceptor to perform!

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.