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  • Ken Loucks

How Much FOG Does a Utility Have to Deal With?

Updated: Apr 25, 2019

A utility recently contacted me seeking help in doing a cost/benefit analysis of their fats, oils and grease (FOG) management program. To me, that analysis must start with asking the question; how much FOG does a utility have to deal with?


To answer that question, the jurisdiction wanted to evaluate the difference between a program with 100 percent compliance from all food service establishments (FSE), versus partial compliance versus total non-compliance. Here is what I sent them to consider:


Compliance has two basic components; does the FSE have a grease interceptor, and is the grease interceptor being maintained frequently enough?


To answer that, we would have to begin with evaluating how much grease a given FSE is producing. Let me give an example that should be helpful.


Assume a fast food fried chicken FSE (Chik-Fil-A, KFC, etc.). Let's say they serve the equivalent of about 500 customers or meals per day. If the average amount of FOG produced per meal is 0.035 pounds per meal or customer, then the FSE is producing 17.5 pounds of FOG per day. If they are open everyday then they would produce 122.5 pounds of grease each week, 525 pounds per month and 6,300 pounds per year. Let's consider the ramifications:


Let's say the FSE has a 35 GPM/70 pounds capacity hydromechanical grease interceptor (HGI). 70 pounds total capacity divided by 17.5 pounds of grease produced each day means the HGI should be pumped out every four days assuming the device is 100 percent efficient to its operational capacity. If we assume that the interceptor meets its certified efficiency of 90 percent that would mean that a properly sized HGI being maintained correctly would bypass seven pounds of grease in four days and would therefore have retained 63 pounds.


Now lets say the FSE is having the HGI pumped out every 30 days. The device will have retained 70 pounds and the remaining 455 pounds would have bypassed to the collection system. In a year the FSE will have captured 840 pounds of FOG and will have bypassed 5,460 pounds to the collection system.


Let's assume that the jurisdiction has 1,000 FSEs and that 100% of them produce similar amounts of FOG in a day/month/year and that all have the same grease interceptor.


Properly maintained scenario:

63 lbs captured per FSE every 4 days, annualized (360/4=90) x 1000 = 5,670,000 lbs captured (90%)

7 lbs bypass per FSE every 4 days, annualized (360/4=90) x 1000= 630,000 lbs bypassed to system (10%)


Maintained monthly scenario:

70 lbs captured per FSE every 30 days, annualized (x 12) x 1000 = 840,000 lbs captured (13%)

455 lbs bypass per FSE every 30 days, annualized (x 12) x 1000 = 5,460,000 lbs bypassed to system (87%)


No Grease Interceptors Installed:

525 lbs bypass per FSE every 30 days, annualized (x 12) x 1000 = 6,300,000 lbs bypassed to system (100%)


We can easily draw a conclusion that improperly maintained HGIs, depending on how infrequently they are being pumped out, are hardly any better than no grease interceptors installed at all (87% bypass versus 100%).


What if the FSE has a 1000 gallon gravity grease interceptor (GGI) installed? Since we don't know what the efficiency of a GGI is, because they are not tested for performance, we must use the 25% rule and the only study available that has evaluated GGI efficiency, the WERF report, Assessment of Grease Interceptor Performance (Click here to download free copy), which would put the best efficiency at around 80 percent.


1000 gallons x 25% = 250 gallons (assumes 100% grease and 0% solids, which is not accurate)

250 gallons x 50% = 125 gallons of FOG storage capacity (assumes 50/50 mix of FOG and solids)

125 gallons x 7.263 lbs per gallon = 908 lbs FOG storage capacity (using Lard as common element to certified HGIs)


If the GGI is maintained once per month it will retain no more than 80% of the 525 pounds discharged to it, which is 420 pounds. It will bypass 105 lbs per month.

If the GGI is maintained once every 90 days it will retain 908 pounds of the 1575 pounds discharged by the FSE and will bypass 667 pounds.


Properly maintained scenario:

420 lbs captured per FSE every 30 days, annualized (x 12) x 1000 = 5,040,000 lbs captured (80%)

105 lbs bypass per FSE every 30 days, annualized (x 12) x 1000 = 1,260,000 lbs bypassed (20%)


Maintained every 90 days scenario:

908 lbs captured per FSE every 90 days, annualized (x 4) x 1000 = 3,632,000 lbs captured (58%)

667 lbs bypass per FSE every 90 days, annualized (x 4) x 1000 = 2,668,000 lbs bypassed (42%)


If the goal is to minimize FOG discharges to the collection system and to maximize FOG resource recovery, you can begin to see how a jurisdictions policy on the sizing, selection, installation and maintenance requirements for grease interceptors are crucial!


It should be obvious based on this analysis that the best scenario for a jurisdiction is having 100 percent of their FSEs with performance tested and certified HGIs on a proper maintenance schedule, which would capture 90 percent of the FOG being discharged. Any other scenario yields worse results.


High-capacity HGIs provide the best efficiency's and grease storage capacity's allowing for longer and more affordable pump out frequencies, making them the best solution for FOG pretreatment.


For help in calculating the potential grease production for FSEs, click the following link for a free copy of the Interceptor Whisperer's Hydromechanical Grease Interceptor Sizing and Selection Guide.


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