Here is a rundown of the different types of grease interceptors and the functions they perform.

Issue: 11/04

Editor's Note: "Back to Basics" is a column that will run periodically in PME reviewing the basic principles of plumbing engineering.

Who do you think was the first person that had their drain clogged from greasy waste? It was probably Nathaniel Whiting or someone like him who must have had to deal with greasy waste and clogged pipes, because Mr. Whiting patented the first grease trap in California in 1884. He may have been the plumber that repeatedly cleaned the drains from that restaurant with the greasy meat loaf. We know Mr. Whiting's design for his grease trap was basically the same then as today's passive- or gravity-type grease interceptors. The passive design works on the principle of the flow of waste flowing through the box or interceptor slow enough to allow enough time for the fats, oils and grease (FOG) to float up to the surface. The inlet and outlet of his invention was below the water surface, so the floating fats, oils and grease were trapped or intercepted in the unit. The fact that he called his invention a "trap" instead of an "interceptor" has led to much debate and confusion in the plumbing industry. Common plumbing industry language refers to a trap as "a fitting or device that provides a liquid seal to prevent the passage of sewer gas without affecting the flow of sewage through the trap." Mr. Whiting's choice of words probably seemed simple and innocent enough that everyone understood what he meant. In hindsight, maybe he should have used a more appropriate term such as grease interceptor.

Automatic grease recovery devices collect grease in a baffled chamber as wastewater flows through the unit. (Photo courtesy of Zurn Plumbing Products Group.)
So why should we care whether it is called a trap or an interceptor? Well it seems the model codes cannot even agree on the proper terminology. The codes have further confused things by referring to traps and interceptors based on the number of fixtures connected or the flow ratings in gallons per minute, and not based on a description or definition of the device. The following definitions are referenced in the two model codes for a grease interceptor and a grease trap.

  • In the Uniform Plumbing Code (UPC):

    Grease Interceptor-An interceptor of at least 750-gallon capacity to serve one or more fixtures and which shall be remotely located.

    Grease Trap-A device designed to retain grease from (1) to a maximum of (4) fixtures.

  • In the International Plumbing Code (IPC):

    Grease Interceptor-A passive interceptor whose rated flow exceeds 50 gpm.

    Grease Trap-A passive interceptor whose rated flow is 50 gpm or less.

This automatic grease interceptor is configured with three skimming discs, one paddle wheel for surface movement, six immersion heaters and two inlets with diffusion boxes. (Photo courtesy of Lowe Engineering.)
The definitions in both codes are not really definitions of an interceptor, they are requirements that determine what size of interceptor is to be used or where it is located. These outdated definitions will be modified in the next edition of the plumbing codes, thanks in part to the input from plumbing pros.

The term "grease trap" will disappear from the code, as the accepted term is "grease interceptor." The new definition for the codes will define the various types of grease interceptors. The definition became confusing when the code tried to make a distinction between exterior concrete grease interceptors and interior grease interceptors. Exterior grease interceptors have historically been concrete tanks with baffles and access covers, located outside the building and designed to intercept waste. There is no consensus standard for concrete grease interceptor performance. The National Pre-cast Concrete Association is sponsoring the development of a consensus standard for concrete grease interceptors through the American Society of Testing and Materials (ASTM). There are currently some product standards for concrete grease interceptors, but they apparently do not address sizing and efficiency. They only address material and structural issues.

The following is a description of the different types of grease interceptors.

Type 1 passive interceptors collect grease as it rises to the top of a baffled chamber when wastewater passes through the unit. (Photo courtesy of Josam.)

Type 1-Passive Interceptor

Passive grease interceptors collect grease as it rises to the top of a baffled chamber when wastewater passes through the unit. The grease that is intercepted must be manually removed from the grease interceptor. This is done by maintenance personnel or by grease rendering companies. A typical 20 gpm (76 lpm) passive-type grease interceptor used by fast food restaurants will be serviced once or twice a week. If a grease interceptor is not properly maintained, it will not intercept the grease, allowing it to pass through into the building drain and building sewer. The separation efficiency of these types of grease interceptors is 95-98% when properly maintained.

A Type 2 pre-cast concrete interceptor detail from a specific job. (Intended only for reference.)

Type 2-Pre-Cast Concrete Interceptor

A pre-cast concrete grease interceptor is a larger interceptor that is designed to collect grease in the same manner as a passive grease interceptor, only on a much larger scale. The most popular sizes are 750 to 2,000 gal. These interceptors can store a larger quantity of grease than a passive interceptor. Pre-cast concrete grease interceptors are more expensive to install and maintain. These interceptors still require maintenance, typically by a grease rendering company.

The cleaning of Type 3 automatic grease interceptors is accomplished by heating the trapped grease during off peak hours and transforming the grease into a liquid that can be removed by skimming or an automatic draw-off. (Photo courtesy of Josam.)

Type 3-Automatic Interceptor

Automatic interceptors, also called grease removal devices (GRD), are designed as passive interceptors, collecting grease in a baffled chamber as wastewater flows through the unit. The collected grease is automatically removed from the interceptor at predetermined times. These units remove up to 98% of effluent grease. Automatic units can be much smaller and the storage capacity can be less, since the grease is automatically removed at regular intervals. The self-cleaning of GRDs is accomplished by heating the trapped grease during off peak hours; this transforms the grease into a liquid (110

This Type 4 biological grease remediation unit system is designed to utilize bacteria to eat suspended fats, oils and grease in the interceptor. (Photo courtesy of Jay R. Smith Mfg. Co.)

Type 4-Biological Grease Remediation Unit

A grease remediation treatment system is designed to utilize bacteria to eat suspended fats, oils and grease in the interceptor. This task is accomplished through the use of live bacteria that have an appetite for fats, grease, sugars, and other complex carbohydrates and proteins. The non-pathogenic bacteria combined with odor-control chemicals is injected in doses of about one ounce per day during off peak hours to liquefy and digest the organic waste, grease and food byproducts. The waste discharge from the biological grease remediation unit is virtually grease free.

An example of a Type 3 point-source automatic grease and oil removal unit servicing a kitchen sink. (Photo courtesy of Thermaco.)

Origins of Grease Through Dish Washing

The pre-wash scrap sink with a food waste grinder is the first stop for dirty dishes in many commercial kitchens. A typical dish-washing station in a commercial kitchen has a pre-wash or scrap sink with an overhead spray and a round sink opening into the mouth of a commercial food waste grinder. This is often where the owner, designer or a food service consultant has determined that the most effective design is to facilitate grinding up the food waste and discharging to the drainage system. Normally, there is a scrap food waste can and a trash can near the pre-wash sink, and the big stuff gets removed from the dishes and thrown into these cans. The local sewer ordinance should be checked to determine if grinding up the food waste is allowed. Some locations have ordinances prohibiting grinding of the food waste because it can cause a problem with the sewer system. There are many areas of the country that prohibit this practice because it puts a burden on the municipal waste treatment facility to separate and treat the additional waste. Where grinding of all of the scrap food is not permitted, there should be a place for a scrap food bin and a trash receptacle near the pre-wash sink. The arrangement of the trash receptacle and scrap food bin needs to be in a convenient location next to or partially under the soiled dish drain board to allow for a smooth sideways flow of dishes.

An example of a Type 3 automatic grease interceptor designed to intercept and remove large quantities of fats, oils, and grease discharged from food service facilities and large commercial/institutional kitchens. These units are relatively small, allowing installation in a kitchen under a sink or other limited space. (Photo courtesy of Lowe Engineering.)

The Pre-Wash Sink Is a Big Contributor to Grease Waste

The pre-wash sink probably receives the most grease-laden waste of all the sinks in the kitchen, yet some local codes have prohibited food waste grinders from being piped through a grease trap or grease interceptor. If the food waste disposal cannot be piped to the interceptor, the most grease-laden waste in the kitchen bypasses the grease interceptor. This can result in problems with blockages in the building sewer and public sewers. When permitted, a pre-wash sink with a food waste grinder should discharge through a grease interceptor. A solid interceptor is required ahead of the grease interceptor to collect large food particles. This also improves the efficiency of the grease interceptor.

We seem to have come a long way from those original clogged drains in the late 1800s, but with all of our new technology and product improvements, it looks like buying stock in Roto-Rooter is still a good idea.