What Plumbing Designers Need to Know About Selecting a Grease Interceptor

Three items need to be considered to properly dispose of grease waste: number of people being served, type of establishment, and pumping interval.

by Siddharth Bhat, PE, CPD, GPD, LEED AP 

One of the unique plumbing elements in most commercial kitchen designs is the presence of grease waste. Knowing how to properly dispose of it is critical because letting grease waste into the facility’s wastewater stream without treatment can expose building owners to charges from city officials. A grease interceptor is the method commonly used to alleviate this problem.

This article provides a brief overview of the factors that affect the size of a grease interceptor to give consulting engineers the background to have educated discussions with their clients, making them aware of the impact of having an interceptor system while incorporating their expectations into the design.

Three factors have to be considered when selecting a grease interceptor. These can be categorized as A, B, and C:

A: Number of people who need to be served or number of plates to be used
B: Type of establishment
C: Pumping interval

Number of People to Be Served or Plates to Be Used

The amount of grease generated in a facility is directly proportional to the number of people being served—more people would equate to more food being prepared. Factor A can be estimated in different ways. For an assembly room in which people may be present only once in a while, you may determine the exact number of people based on the number of seats. For restaurants or cafeterias, you can reach out to the owner regarding information about the typical number of meals served. For a correctional facility, you would have more robust data in terms of the number of inmates present in the facility. Some manufacturers also provide a ballpark number based on the square footage of the area served and the type of establishment if other parameters are not available; however, this may lead to an overestimation in certain situations.

Some manufacturers also provide sizing based on the plumbing fixtures present in the kitchen. Each plumbing fixture has a certain drainage fixture unit factor associated with it along with the gallons per minute (gpm) depending on drain time. A one-minute drain would have a higher gpm as compared to a two- or three-minute drain. Knowing the fixtures that will be used provides helpful information to assist in the proper sizing of grease interceptors.

Type of Establishment

Another important item to note is the type of establishment. Grease-producing ability varies depending on the type of food items that are cooked. For example, a cafeteria most likely would produce less grease per serving as compared to a barbecue restaurant due to the ingredients present and method of preparation. ASPE’s Plumbing Engineering Design Handbook, Volume 4, Table 8-3 provides example grease production values for restaurants in four categories: low grease producer, medium grease producer, high grease producer, and very high grease producer.

The values in Table 8-3 are based on whether flatware is provided or not, which is another element to discuss with the owner. Also, the type of flatware is important. Plastic flatware is typically thrown out, but metal flatware needs to be cleaned before being used for the next customers. The waste from this flatware and the cleaning method also contributes to the grease production from the facility, which needs to be taken into account for sizing.

Pumping Interval

A last important factor is the pumping interval. The grease interceptor needs to be cleaned of accumulated grease at fixed intervals, and greater time intervals require larger grease interceptors. The sizing method in ASPE PEDH Volume 4, Chapter 8 categorizes these intervals into weekly, monthly, bimonthly, and quarterly maintenance. These interval times need to be coordinated with owner, and the owner should be informed about the impact on cost and size for each of the maintenance intervals.

Depending on their size, grease interceptors may be installed belowground outside the building or under the sink in the kitchen area. If located belowground, you may need to consider the water table level in the location. Based on the selection, you may need to coordinate deadman loading anchors with the structural engineer to ensure proper installation.

If the interceptor is installed below a sink, one way to clean it is a portable pump. Some manufacturers also provide accessories such as pump ports that allow trucks outside to pump grease out from higher floors if needed. If the interceptors are located below sinks, you also should consider the width or span of the sink to ensure that parts of grease interceptor do not protrude and cause a tripping hazard.

Some manufacturers also provide grease monitoring and alarm contacts. These control systems monitor the grease level and raise an alarm when the level exceeds a predetermined setpoint to alert the owner about the necessity of cleaning out the grease. These control systems can also be tied to a building automation system if needed.

It’s also a good idea to have your selection verified by the code official early in the design by documenting the above factors and providing a summary of the calculations. Some authorities may also request installation drawings and plans for their verification. Providing all of this information beforehand prevents changes later in the design that can cause costly change orders.

About the Author

Siddharth Bhat, PE, CPD, GPD, LEED AP, is a Mechanical and Plumbing Engineer at DLZ Corporation with specialization in correctional facility design.

The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.

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