Properly sizing a grease interceptor, also known as a fats, oils and grease (FOG) waste interceptor, requires an open mind and a willingness to accept that, for the most part, it has been done incorrectly. Several commonly used methods for sizing an interceptor include the Environmental Protection Agency (EPA) calculation method and the Uniform Plumbing Code (UPC) calculation method, as well as many manufacturers developing their own sizing methods, typically based on one of these two methods.
The problem with these methods is that they miss the No. 1 variable: the grease production/meal for the specific type of restaurant. This becomes clear when you analyze where most food sales occur. According to the Small Business Chronical, fast-food restaurants average 50 percent to 70 percent of their overall sales from the drive-through window, with another 17 percent of sales coming from carryout sales1.
Together, it is an astonishing 67 percent to 87 percent of total fast-food sales. Other restaurant types also have significant sales volume attributed to carry-out sales, as well as catering.
One thing to keep in mind: cooking does not create the need for an interceptor. It is the washing of cookware, serving pans, utensils, flatware and plates that creates the requirement. Sewer overflows associated with blockages caused by FOG waste is on the rise because the need for an interceptor is based on the FOG waste entering the sanitary drainage system.
Any restaurant that serves with dishes and flatware also will have a slightly higher rate of FOG discharge to the drainage system, compared with restaurants that serve with disposable containers and flatware. Every one of these variables is important to consider when sizing an interceptor. Unfortunately, no sizing methodology has ever included every variable discussed.
Types of Food Service Operations
The 2011 Brown Grease Supply Study2 sampled more than 60 restaurants of varying types in real-world operation. The information obtained in the study established grease production per meal values. This study provided very accurate values because it included the human element often ignored in laboratory testing. The study also allows us to classify food service operations (FSOs) into several categories based on their grease production/meal value.
Variables do exist to increase the FOG production values, such as FSOs serving with flatware, or using a grease hood or fryer. Any restaurant that has a grease hood or fryer is automatically moved up one classification.
The classifications breakdown is as follows (for specifics on these classifications, refer to the 2011 Brown Grease Supply Study2):
I did my own research over two years regarding sales volume for the different types of FSOs, which allowed me to establish an average for the meals sold/hour3. From this, I took the different classifications and averaged the numbers for those FSOs. I also identified several FSO types that stood out because of their simplicity and low FOG production values.
Based on the Brown study and my research, I made several assumptions: full-service restaurants would serve with flatware; fast-food/quick-service restaurants do not serve with flatware; these two groups make up the most significant portion of FSOs; and most other FSOs would require additional details to size an interceptor properly.
The benefit of combining this information is that I was able to establish an average number of meals served per hour for the different FSO classifications as follows (the first four are “stand-alone” FSOs, and the classification “Other FSOs” has FSO types requiring more detailed design methodology and each one has a specific FOG production value):
A 30-day period for FOG waste production/service interval was selected based on industry data that suggests this is the longest storage period designers should use because the waste turns septic beyond this time frame. For example, a fast-food/quick-service Asian food restaurant that is open 12 hours per day and has a grease hood/fryer would have a FOG production assumption equal to 835 lb. over 30 days.
Here is what the calculation would look like:
Hours x meals sold/hour x grease production/meal value x service interval = lb. of FOG
12 x 40 x .058 x 30 = 835.2 lb.
Several years ago, the average weight of FOG waste was listed at 6.4 lb./gal. Today, however, that number is closer to 7 lb./gal. due to the industry using more oils for cooking than lard. If you base the storage capacity on 7 lb./gal., you end up with a storage capacity requirement of approximately 119 gal. If you elect to use the 6.4 lb./gal., the resulting storage capacity requirement is 130.5 gal. I prefer to have a little buffer, so I use the 6.4 lb./gal. figure.
Hydromechanical vs. Gravity
The designer should establish a reasonable grease production assumption. This is the first and most important value because without it, you have no foundation for any further logical decisions. Using the example of the fast-food/quick-service Asian food restaurant with an average of 12 service hours/day, it will look similar to this:
FOG waste production: 835 lb./30 days = 131 gal. (rounded up)
Hydromechanical interceptors. For a hydromechanical unit, the only other step is to select an interceptor with an appropriate storage capacity; otherwise, it will result in excessive maintenance costs. Some design professionals have included three-compartment sink discharge calculations in their designs. This is not required for a hydromechanical unit because regardless of the size of the three-compartment sink, the flow through the interceptor is regulated by the flow control.
This assumes a direct connection from the three-compartment sink and a direct connection to the sanitary drainage system. Without a direct connection, you also would have to calculate for an appropriate length of pipe to store the FOG waste as it is metered through the flow control fitting.
Gravity interceptors. This type of interceptor requires more consideration. Though the plumbing codes typically do not provide much direction for sizing a grease interceptor, the International Plumbing Code (IPC) does state that 30 minutes of retention/separation time is needed. Calculating the gallons required for separation to occur is done by multiplying the gal./minute discharge (gpm) rate of the connected fixtures by the retention/separation time since both are expressed in minutes.
Figuring the flow rate and retention/separation is not the end of the equation. The next step is to add in the appropriate storage capacity, otherwise there will not be enough liquid capacity for separation to occur. Although for years many in the industry indicated up to 25 percent of the overall capacity could be used for storage, this line of thinking was flawed.
Consider this scenario: the FSO has a grease production rate of 835 lb. (or 131 gal.)/30 days, has a three-comp sink with bowls measuring 16 in. x 16 in. x 12 in. (30 gpm), and the design professional specified a 1,000-gal. gravity grease interceptor. Here is what the result would be:
Step 1. 30 gpm x 30 = 900 gal. required for retention/separation.
Step 2. 1000 x 25 percent = 250 gal. (a 25 percent value).
Step 3. 250 gal. – 131 gal. = 119 extra storage (if 25-percent rule used).
Step 4. 1000 – 250 = 750 gal.
Step 5. 900 – 750 = negative 150 gal. (there is not sufficient capacity for retention/separation to occur).
Actual total capacity required for separation to occur including storage:
900 gal. (retention/separation in Step 1) + 131 gal. (storage capacity from calculated FOG production) = 1,031 gal.
Sizing a Grease Interceptor for a Strip Center
The only way to properly size a single interceptor for use at a strip center, or a “community interceptor,” would be to have all the variables and apply them to the design.
Assume we have these variables for four total food service operations in a strip center, each with 12 hours of service daily: Fast-food/quick-service Asian food restaurant (with grease hood/fryer); Fast-food/quick-service deli (no grease hood/fryer); Full-service Italian food restaurant (with flatware); and Fast-food/quick-service ice cream parlor (with grease hood/fryer).
FOG waste discharge based on national averages for the FSO types and service hours breaks down like this: Fast-food/quick-service Asian food restaurant: 835 lb. of FOG waste; Fast-food/quick-service deli: 72 lb. of FOG waste; Full-service Italian food restaurant: 655 lb. of FOG waste; and Fast-food/quick-service ice cream parlor: 655 lb. of FOG waste.
The total projected FOG waste for the strip center is 2,217 lb./30 days.
A hydromechanical unit could be used with the given FOG waste assumption and most likely it would require that several would be needed in series to deal with the anticipated load. For this discussion, we will be sizing for a gravity interceptor. The first step is to calculate the total possible gpm flow rate for all FSOs: Fast-food/quick-service Asian food restaurant: 18 in. x 18 in. x 16 in. three-compartment bowls = 51 gpm; Fast-food/quick-service deli: 16 in. x 16 in. x 12 in. three-compartment bowls = 30 gpm; Full-service Italian food restaurant: 20 in. x 20 in. x 18 in. three-compartment bowls = 70 gpm + 2 gpm from dishwasher = 72 gpm; and Fast-food/quick-service ice cream parlor: 16 in. x 16 in. x 12 in. three-compartment bowls = 30 gpm.
Total possible gpm discharge for the strip center is 183 gpm. The required volume for separation/retention: 183 x 30 = 5,490 gal.
In the example strip center, four FSOs have a total FOG waste of 2,217 lb./30 days or 347 gal./30 days. There is a total possible discharge rate of 183 gpm x 30. The resulting number would be the overall capacity required for a gravity interceptor in gallons:
Total capacity required: 183 x 30 = 5,490 gal. + 347 gal. = 5,837 gal.
Most likely a standard size interceptor of 6,000 gal. would be used. The result is an appropriately sized interceptor, with a little extra capacity, for 30 days between service intervals.
How is a community interceptor for a strip center sized if all the variables are not known? The answer to size the interceptor based on a full-flow condition for the common grease line size since a given size pipe cannot flow more than full flow. Those flow rates break down like this:
Grease production will still need to be calculated for this example. For a strip center, assume that every tenant space could be an FSO with the highest grease production values (such as a full-service Asian food restaurant using flatware, for instance). Using that information with an average of 12 hours of operation per day, the result would be 1,080 lb./30 days for each tenant space, based on national averages.
Apply this to a different strip-center scenario: The client is requesting a design with a community interceptor but the only information they can provide for the project is there are no defined tenants for the eight-unit strip center. Since the only real way to design this will be based on full-flow conditions, the design should technically never fail.
Apply the grease production assumptions for worst-case scenario results in the following: 1,080 x 8 = 8,640 lb. of possible FOG waste or 1,350 gal. Most likely a 4-in. or 6-in. line will be chosen for the community grease waste; assume a 4-in. line is chosen in this scenario. The 4-in. pipe has a full-flow rate of 125 gpm; the code requires a 30-minute retention/separation time.
Retention/separation calculation: 125 x 30 = 3,750 gal.
Storage capacity calculation: 1,350 gal.
Total capacity required: 3,750 + 1,350 = 5,100 gal.
The discharge rate from the three-compartment sinks was not available, so the full-flow rate for the pipe has to be used. If the designer chooses a 6-in. community grease waste, it would have resulted in the following:
Retention/separation calculation: 375 x 30 = 11,250 gal.
Storage capacity calculation: 1,350 gal.
Total capacity required: 11,250 + 1,350 = 12,600 gal.
Carefully consider the size for a “community grease waste” line since the charge for maintenance service on a gravity interceptor is based on the overall capacity of the interceptor. The cost of grease interceptor maintenance is typically never considered in business plans.
Design professionals and contractors need to do a much better job of educating their clients on these somewhat hidden expenses. With this knowledge, they can adjust their sale prices accordingly to not only cover their operational costs but also provide a profit.
I believe community grease waste is just a bad idea. The sizing methodology discussed is included in calculators at www.jrscidt.com.