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!

Why Effluent Concentration Limits for Grease Interceptors is not Appropriate

Proper grease interceptor maintenance is a critical component to any FOG abatement program.  The challenge for jurisdictions is figuring out when an interceptor is full and how often it should be maintained.

One of the solutions that many jurisdictions have adopted is to set a numeric concentration limit for the effluent of an interceptor such as 100 ppm (parts per million) or 100 mg/L (milligrams per liter).  An inspector will take a grab sample from the effluent of an interceptor and measure the amount of FOG in the sample using EPA method 1664A to determine if the interceptor is in compliance or whether an enforcement action is required to get the interceptor back into compliance.

There are two problems with this approach, lets address them both.

The first problem with using effluent concentration limits for determining grease interceptor compliance issues is that grease interceptors are not designed to meet a specific effluent concentration limit and never have been. When you understand how grease interceptors are designed to work you'll understand why it would be inappropriate to use an effluent concentration limit to determine interceptor compliance issues.

Up until the early 1940’s there was chaos in the interceptor world because each manufacturer rated their own interceptors and produced them in a variety of sizes and types according to engineer’s specifications or to satisfy plumbing codes.  There was no uniform testing or rating procedure for grease interceptors.

In 1946 the Iowa Institute of Hydraulic Research (IIHR) published a bulletin on its activities and had this to say regarding grease interceptors, “The institute has for many years been engaged in research and testing relating to grease interceptors…and has been instrumental in developing testing standards and methods of rating for such interceptors.  During the war large numbers of grease interceptors were required in the kitchens of army cantonments and similar establishments.  With the cooperation of the construction branch of the Army Engineers and the interceptor manufacturers, a standard test method was developed.” (1)

Prior to the publication of the aforementioned bulletin, in 1944 the researchers from the IIHR attended the Sixteenth Annual Meeting of the New York State Sewage Works Association where they presented a symposium of four papers, one of which was titled Symposium on Grease Removal - Design and Operation of Grease Interceptors, by F. M. Dawson and A. A. Kalinske.(2)

In this paper, the authors explain how the researchers from IIHR developed the testing and rating method that the Plumbing and Drainage Manufacturer’s Association (now Plumbing and Drainage Institute) would later formalize into the standard PDI-G101. 

The performance requirements that came from this testing and were later formalized in PDI-G101 (and later in ASME A112.14.3 and CSA B481) mandated that an interceptor must have an average efficiency of 90% and be capable of storing at least 2 lbs of grease for each 1 gpm of flow rate.  

Fundamentals of operation
Fats, oils and grease (FOG) float in water owing to a difference in their specific gravity.  Water has a specific gravity of 1.0 while the specific gravity of olive oil is 0.703, lard is 0.875, and vegetable oil is 0.92 which is why they all float.  Interceptors are designed to separate FOG based on the differences in specific gravity which is called “gravity-differential separation”(2). 

Stokes law predicts the rise rate of a grease globule in static water based on its size, temperature, viscosity and specific gravity. For example, in static water with a temperature of 150 deg F, a grease globule with a size of 150 microns and a specific gravity of 0.90 will have a rise rate of 0.05 feet per second.

All other factors being the same, a grease globule of 50 microns will take 9 times longer to rise than a grease globule of 150 microns. For this reason researchers concluded that interceptors should be designed to separate grease globules of about 150 microns 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 will be easily separated.”(2)

The size of the grease globules entering an interceptor is a result of the level of emulsification between the grease and water.  

Emulsification is the mixing of immiscible liquids (liquids that don't naturally mix), which can be caused mechanically or chemically.  

Mechanical emulsification occurs by washing dishes by hand in a sink and then pulling the drain plug or in a dishwasher by means of spray jets and drainage action.  

Chemical emulsification occurs when soap or detergents are used.  Soaps are surfactants and detergents contain surfactants which act to chemically bond with oil droplets making it easier for them to be separated from each other and more difficult for them to coalesce together again.  

A grease interceptor has no control over the level of emulsification of the entering grease and waste water and are not designed to cause grease to coalesce inside the unit, although in a laminar flow environment some coalescence will naturally occur.

A grease interceptor is designed to separate grease from waste water that is a minimum of 150 microns in size with an average efficiency of 90% and have a storage capacity of at least 2 lbs of grease for each 1 gpm of its certified flow rate.

150 microns = .15 mm
50 microns = .05 mm

The smallest object the human eye can detect is around .04 mm.  What this means is that if you observe the effluent from an interceptor you could potentially see very small grease globules escaping even though the interceptor is being properly maintained is in functioning correctly. 

If a grab sample taken from the effluent from an interceptor contains a significant amount of very small grease globules (anything less than 150 microns) the sample could produce results that indicate an effluent concentration of greater than 100 mg/L even though the interceptor is operating properly.  

The second problem with using effluent concentration limits for determining grease interceptor compliance issues is that EPA test method 1664A is flawed yielding a high degree of variability in test results.

In 2008 the Water Environment Research Foundation (WERF) released a comprehensive study of grease interceptor's in real world installations titled Assessment of Grease Interceptor Performance. Regarding the use of EPA test method 1664A WERF stated, “as an indirect outcome of this study, the FOG concentration measurement results showed that the EPA Method 1664 displayed significant variability when measuring known concentrations of total oil and grease. Variability in the measured concentration was approximately 40%, making it impossible to confirm or refute whether the grease interceptor is properly achieving the required effluent limit.”(3)  

Summary

Grease interceptors are designed to separate grease based on a minimum size of 150 microns.  Anything smaller is expected to bypass the grease interceptor because to design an interceptor to separate out smaller grease globules would require the unit to be dramatically larger and impracticable to install and maintain.  

Using effluent concentration limits to determine grease interceptor compliance could and likely will yield results that may be outside of set jurisdictional limits but may have been the result of highly emulsified grease and waste water containing a significant amount of grease that was less than 150 microns in size escaping the interceptor.  

Finally, EPA test method 1664A is flawed, potentially yielding test results with as much as a 40% error rate, according to WERF.

Instead of using numeric concentration limits for compliance enforcement on grease interceptors, jurisdictions should instead concentrate on allowing only grease interceptors that are certified with known efficiencies and storage capacities and require them to be maintained regularly and properly.  

How often they should be cleaned is a function of how much grease a restaurant produces and how much grease storage capacity the interceptor has.  Here is where I will plug Schiers grease production sizing method (check out the 2014 catalog at www.schierproducts.com), because it helps a jurisdiction and an owner be able to figure out how much grease is being produced so a proper maintenance schedule can be established.

References

(1) The University of Iowa Studies in Engineering, page 60, The Iowa Institute of Hydraulic Research, 1946, http://ir.uiowa.edu/uisie/30

(2) Symposium on Grease Removal, Design and Operation of Grease Interceptors, Water Environment Federation, Sewage Works Journal, Vol. 16, No. 3, F.M. Dawson and A. A. Kalinske, Iowa Institute of Hydraulic Research, 1944, http://www.jstor.org/stable/25029790

(3) Assessment of Grease Interceptor Performance, page 116, Water Environment Research Foundation, 2008, http://www.ndwrcdp.org/documents/03-cts-16t/03cts16taweb.pdf