Why do we still use pork lard for testing and rating hydromechanical grease interceptors? There are so many different oils used in cooking today with varying densities and viscosities; isn’t it time to change from pork lard to something else? These questions come up from time to time and deserve an answer.
The issue at question is whether the results of tests that use lard would vary distinctly from the results of tests that use some other test media. There would likely be some difference, to be sure, but the question is whether the difference is statistically significant or not.
All grease interceptors work off of the same principle in fluid dynamics called gravity-differential separation. Fats, oils and grease (FOG) have densities less than water. Specific gravity is the ratio of the density of a liquid (substance) to the density of water. At room temperature water has a density of about 1.0.
The lard used in testing grease interceptors is required to have a specific gravity of 0.875 +/- 0.005. What you may not realize is that at room temperature lard has a density in the range of 0.91 - 0.92. At 160 degrees the specific gravity of lard will be closer to 0.87 – 0.88, which is why this range is specified in the standards that govern the testing and rating of grease interceptors.
Now consider that the common cooking oils used today have specific gravity's that fall in a range from 0.91 to 0.93 at room temperature (60 deg. F):
According to a 1992 study titled, “Densities of Vegetable Oils and Fatty Acids,” by Hossein Noureddini, B.C. Teoh, and L. Davis Clemmons, all members of the Department of Chemical Engineering at the University of Nevada, the density of several common vegetable oils did in fact decrease as temperature increased. Here is a sample of the data contained in Table 3 of the report:
I think we can safely assume that in a worst-case-scenario the specific gravity's of today’s cooking oils will be similar to that of lard at 160 deg. F. That being the case, what benefit would there be to change from lard to another oil for testing and rating grease interceptors?
Consider also that the rise rate for any given liquid with a density of less than 1.0 will depend upon these factors:
Density of the globule
Size of the globule
Viscosity of the media/globule
Temperature of the media/globule
Velocity of the globule (horizontal)
Flow environment (turbulent or laminar)
Of these factors, only density and viscosity would be distinguishing characteristics between lard and other cooking oils. Viscosity is how easily a liquid pours. Water has very low viscosity while room temperature Honey is very viscous. The more viscous a liquid is the more resistance works against it, slowing its rise rate. It may stand to reason that a fat which is a solid at room temperature may be more viscous than an oil, which is not. At 130 deg. F lard will have a viscosity of 34.3 centistokes, while the previous list of oils will have the following values:
Notice that the viscosity of each of these oils is less than that of lard, which means that we should be able to safely conclude that in a worst-case-scenario these oils will have better rise rates than lard given all other parameters are equal.
The current tests under PDI and ASME adequately evaluate the factors that affect gravity-differential separation resulting in reliable reporting of an interceptor’s efficiency using lard. Lard has been used successfully as a test medium for grease interceptors for over 70 years, by hundreds of manufacturers and thousands of devices, which have millions of installations across the country.
All one has to do to verify whether today's modern HGI is operating efficiently in the real world to to open one up and inspect its contents. If there is FOG collecting inside then you know the device works at least up to its rated capacity.
Changing from Lard to say olive oil, would have a beneficial effect on reported efficiencies because even though their densities are very close, the viscosity of olive oil is much lower than lard. Olive oil would actually be more efficiently removed in a grease interceptor than lard.
I wonder if the question about using lard as a test media is the result of decades of installations of grease interceptors, which while working efficiently up to their rated capacities, are rarely cleaned out as often as they should be (i.e. daily or every couple of days). HGIs are commonly found serving only multi-compartment sinks and are being serviced on a monthly or bi-monthly or even quarterly basis. While it is true that the devices operate efficiently up to their rated capacity, they simply are not being maintained often enough to ensure their ongoing efficient operation. There is simply no test protocol, test media, or test apparatus that can ensure that an FSE will keep their interceptor properly maintained and in efficient operating condition.
Now that you have a better understanding of the role of density and viscosity of the media used for testing and rating grease interceptors, hopefully you will have more confidence in the test method employed by ASME A112.14.3 and PDI G101. Lard is an appropriate media and any alternatives would only improve test results but wouldn’t give anyone assurance that the interceptor would perform better in the field than on the test apparatus.