April 8, 2025
This blog explores why effluent concentration limits (FOG in milligrams per liter) are not appropriate for grease interceptors.
Proper grease interceptor maintenance is a critical component to any FOG (fats, oils and grease) program. The challenge for jurisdictions is figuring out when an interceptor is full and how often it should be maintained.
One of the solutions, apart from Grease Production Sizing, that many jurisdictions have adopted is to set a numeric concentration limit for the effluent of an interceptor. 100, 200, 250 or 300 mg/l are limits commonly found throughout the U.S. The inspector will take a grab sample downstream of the interceptor and send it off to a lab to measure the amount of FOG.
Below is an illustration of the SV10 inspection port.
What problems occur with this approach?
The effluent sample is not an accurate indicator of grease interceptor performance
First, the assumption that a grease interceptor is designed to meet a specific concentration limit is flawed. To demonstrate this, we need to review how a grease interceptor works. Although effluent concentration limits shouldn’t be used for compliance, we will discuss ways for its use later in the blog. Fats, oils and grease have a “specific gravity” that allows them to float on top of water. A hydrometer - a floating device with a calibrated scale - is used to measure the density of liquids (specific gravity). If water has a specific gravity of 1.0 and lard ranges between .90 and .94, it would be lighter than water and therefore will float when suspended. Interceptors are designed to separate FOG based on the differences in specific gravity which is aptly called “gravity-differential separation.”
Stoke's Law describes the force of friction (drag force) experienced by spherical objects moving through a viscous fluid. It applies to small particles or droplets in laminar flow. The law is useful in determining the velocity of particles in a fluid, which is important in fields like sedimentation, fluid dynamics and environmental engineering.
The mathematical expression of Stoke’s Law is:
This equation is used to estimate how fast particles rise or settle in fluids, which applies directly to how interceptors function to remove FOG from the waste stream. Researchers, when developing the testing standard for grease interceptors in 1944, arrived at a grease droplet, or spherical particle as noted above, of 150 microns. This size is about the width of human hair. These droplets at 150 degrees Fahrenheit will rise at a rate of .05 feet per second. Essentially, grease is rising out of the waste stream, it is not being “filtered” out by any media or membrane within the interceptor. This applies to all grease interceptors, both gravity and hydromechanical.
The studies where this information was pulled from can be found in the Iowa Institute of Research and the Symposium on Grease Removal, Design and Operation of Grease Interceptors.
The size of the grease particles that enter an interceptor is a result of the level of emulsification between the grease and water. Emulsification is the mixing of immiscible liquids (liquids that do not normally mix), which can be caused mechanically or chemically. A good example of this would be oil and vinegar salad dressing.
Mechanical emulsifications occur from a shearing force, such as hand washing or using a dishwasher to violently scrub the dishes or shaking that oil and vinegar bottle. Chemical emulsification occurs when soap or detergents are used along with a shearing force to make the grease and soap bond. Just pouring dish soap into a glass of water does not create this force - try it.
A grease interceptor has no control over the level of emulsification of the liquid entering the tank. They do not cause the grease to coalesce inside the unit, although in a laminar flow environment, coalescence will occur naturally. There is no shearing force in Schier’s tanks.
Since the grease interceptor is designed to collect grease particles 150 microns or greater, it would be possible for the grab sample - taken on the outlet or sample well of the system - to have a concentration of particles less than 150 microns and actually exceed the local limit. The unit, if being maintained regularly, is still functioning correctly. We see this in coffee shops because of the homogenization process that milk goes through to keep the milk solids, milk fat, water, and emulsifiers in suspension. Those particles are not 150 microns, but more like 2-4 microns, making it more difficult to separate in any system - large or small.
Test method 1664A was not designed for grease interceptors
Second, the method that the industry uses to test concentration limits is the EPA 1664A test method, first introduced in 1999. It is used to measure the concentration of oil and grease in water and wastewater samples. Specifically, it quantifies substances that are extractable with n-hexane (such as hydrocarbons, vegetable oils, animal fats, waxes, and related compounds) by gravimetric analysis.
Key applications of method 1664A include:
Wastewater treatment monitoring: Ensuring that oil and grease levels in industrial and municipal discharges meet regulatory limits.
Environmental compliance: Used in the National Pollutant Discharge Elimination System (NPDES) permits to regulate pollutants in discharges.
Environmental impact assessments: Monitoring the contamination of water bodies by oil and grease from various sources.
The EPA test method 1664A was not specifically designed for testing grease traps, but bodies of water such as lakes, rivers and streams. The application of it to grease interceptors did not take into account the effluent as a whole. A study done by WERF in 2008 and an abstract presented by Yi Wang and Dr. Joel Ducoste entitled “Challenges in the measurement of Fat, Oil and Grease in Food Service Establishment Waste Streams” found that the testing method along with protein in the samples causes wide variations in results. Sometimes as much as 40% within the same sample. This makes the test method impossible to confirm whether or not the interceptor is achieving the required effluent limit. Using this method for compliance is suspect, however, it could be used in enforcement when the food service establishment is going way beyond the recommended maintenance period. The EPA 1664A result will not be in the 100’s, it will be in the 1,000s or 10,000s which would be a clear violation of the ordinance or FOG program and therefore enforceable when sampled per EPA guidelines.
Summary
Grease interceptors are designed to separate grease particles based upon a minimum size of 150 microns. Anything smaller could potentially bypass the interceptor. Using effluent concentration limits to determine grease interceptor compliance could yield results that may be outside of set jurisdictional limits because smaller emulsified grease particles that are significantly smaller than 150 microns are escaping the interceptor.
Instead of using numeric concentration limits for compliance, jurisdictions should concentrate on only allowing those grease interceptors that are tested for performance with a certified maximum grease capacity. How often the interceptor should be cleaned is a function of Grease Production Sizing that calculates how much grease the individual food service establishment is producing to arrive at a maintenance schedule. Core sampling interceptors based upon manufacturer’s recommendations should be used to ensure compliance with the FOG Program.