Head to Head Comparison; Hydromechanical Versus Gravity Interceptor

April 08, 2014

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

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