Gravity grease trap, Atlantic Metalworks, formerly Prima Stainless Steel Supply www.primasupply.comGrease Interceptors - Grease Traps
Grease trap types, specifications, sources, installation

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Grease interceptors: this article describes the different types of grease interceptors or grease traps used to prevent drain & sewer system clogging by catching & permitting the removal of FOG (fats, oil, grease) from a building's plumbing system.

The article describes traditional gravity and baffle operated grease traps, hydromechanical grease traps, and automatic grease removal units or AGRUs. Sources for grease interceptor installation, specifications, maintenance, & building codes as well as producers & sources of these grease interceptor or grease trap products are included.

Page top photo: a traditional baffle and gravity type grease interceptor produced by Atlantic Metal Works (formerly Prima Supply).

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Grease Trap Requirements & Installation References

Zurn Grease Recovery Appliance #Z1192 - www.zurn.com 2014Fats, oils, and grease that are poured down building drains cause serious clogs in normal drain traps, drain piping, sewer piping and even public sewers & pumping stations. Fats, oils and grease, referred to in plumbing trades as FOG have been cited for as causing more than sixty percent of public sewer backups in New York City, at an annual cost of $4.65 million in 2013. (Gregory, The New York Times, 2014).

Illustrated at left: Zurn's Z1192 Grease Recovery Appliance (see AGRU below).

[Click to enlarge any image]

Types of Grease Interceptors or "grease traps"

Grease traps separate fats, oils, and grease from wastewater using one of several methods:

Watch out: some municipalities permit only AGRUs in new installations, and no longer permit passive style grease traps such as the gravity traps described just below. Quoting:

New or replacement grease trap installations shall be of the Automatic Grease Recovery Unit (AGRU) type. Passive style grease trap installations shall not be permitted. Grease traps must have the NSF or Plumbing Drainage Institute certification. The minimum acceptable size is rated at 20 gpm / 40lbs. All grease traps must be installed in accordance with the manufacturer’s specifications, which include the flow restrictor and venting prior to the discharge entering the grease trap. - (Murfreesboro 2014)

Gravity grease trap, Atlantic Metalworks, formerly Prima Stainless Steel Supply www.primasupply.com

Shown at left: a gravity & baffle type grease trap produced by Atlantic Metal Works (formerly Prima Supply).

Hydromechanical grease trap illustration from BAPPGIllustrated at left is a hydromechanical grease interceptor [Bay Area Pollution Prevention Group BAPPG (2011) who have publised a series of helpful documents listed below.]

An excellent explanation of grease traps or interceptors is provided by a guidance document provided by the Carrolton Tx. government (2012) from which we quote:

[Grease interceptors or fat oil & grease FOG] traps use the physical principal that fats, oils and grease are lighter than water and will rise to the top of a water surface and heavy debris will fall to the bottom of the trap when the mixture is allowed to stand for a period of time in quiet conditions.

The longer the wastewater stays in the trap, the better the separation. But as the amount of retained grease and solids increase, the effective volume of the trap decreases, retention time decreases and physical separation decreases, resulting in pass through of solids, fats, oils and grease. To prevent this, grease traps and interceptors have to be regularly maintained by having a permitted liquid waste hauler remove both the top grease layer and the bottom solids.

FOG Grease Trap Size Requirements

The size required for a grease interceptor or grease trap is specified in the Uniform Plumbing Code (UPC). Typically local or municipal regulations indicate that grease trap size requirements depend of course on the volume of fats and oils passing into the drain system. Typically municipal codes regulating grease interceptor installation give general guidelines such as light or heavy food preparation, and additional capacity guidelines are given by the grease trap manufacturer's installation specifications (referred-to below). Typically the grease interceptor capacity required is specified as one of three categories:

Grease Trap Cleaning & Maintenance Requirements

The cleaning frequency depends on grease interceptor size or capacity, type, wastewater flow, and local regulations. For example, a community may specify that an AGRU must be "cleaned per the manufacturer's specifications", or that an existing (not permitted in new installations) passive style grease trap will be cleaned every two weeks, or at an interval specified by a local authority, or as follows:

If the FOG and food solid content of the grease trap is greater than 25%, then the grease trap must be cleaned every week, or as frequently as needed to prevent 25% of liquid capacity displaced by FOG and food solids (25 % Rule criterion).

Automatic Grease Recovery Unit (AGRU) style Grease Traps will be cleaned per the manufacturer’s recommendations, which typically includes FOG disposal and removal of food solids from the strainer basket on a daily basis and weekly maintenance of the skimming mechanism. - (Murfreesboro 2014)

Note: Additives, treatments, and enzymes are not required for grease traps and are prohibited in some municipalities. Quoting:

No additives (i.e. enzymes, bacteria, etc...) shall be added prior or directly to any chamber of the grease trap or any component of the plumbing connected to a grease control device. (Murfreesboro 2014) Also see SEPTIC TREATMENTS & CHEMICALS

Grease Trap Waste Disposal

The grease interceptor codes & specifications we reviewed were mostly silent on this topic, but those that did discuss grease disposal suggested bagging the grease or other trap or interceptor debris in heavy plastic garbage bags for disposal as food waste.

Speaking of disposing of grease interceptor waste reminds installers of the importance of making the grease interceptor easily accessible. A reader who asked us about installing a grease trap in a 25-foot deep manhole where a sump or ejector pump system was in use had perhaps not imagined the service person having to lug a 25 or 50 pound bag of grease waste up a ladder.

FOG Grease Trap Location Requirements

For a grease interceptor to work properly it must be located sufficiently far from the hot water source that the wastewater entering the trap will be cool enough that the trap can in fact do its job: trapping fats, oils and grease passing by. A grease trap must be accessible for regular inspection (through a wastewater sampling well) and cleaning.

The model code we cite below specifies the following:

Grease interceptors are to be installed at a distance of 8-10 meters from the last contributing fixture to allow for adequate cooling of the wastewater, while preventing grease solidification in the lines. Water temperatures must be less than 110 degrees Fahrenheit prior to entering grease trap.

Sample wells will have a 15” diameter access cover and a minimum 4” drop from inlet to outlet piping through the sampling well. Mechanical Grease Traps and Interceptors that are installed above ground must be equipped with an influent flow regulator and an effluent valve assembly that allows for sample collection. (Carrolton 2012)

Other Commercial Facility Plumbing Traps & Filters

Other commercial facilities are required to install FOG interceptors, debris and grit traps, or lint interceptors, such as automobile repair shops, car washes, and commercial laundries.

Automotive repair facilities are typically required to install a grit and oil trap and separator on their drain with a typical minimum capacity of 50 gallons for the first 100 sq. ft. of area drained (such as the garage floor) and an additional one cubic foot of capacity (7.5 gallons) per additional 100 sq .ft. of floor area.

Commercial laundries may be required to install suitable filters and traps also intended to protect the sewer system but designed to capture other clog-source materials such as lint traps. is specified in the Uniform Plumbing Code (UPC) Appendix H contains the formula for calculating a lint filter size for commercial facilities.

How are Grease Clogs Cleared?

In municipal and building plumbing systems when a grease-clogged drain line or sewer line are discovered to have been clogged by FOGs experts use high powerd vacuums or high pressure water to attempt to clear the drain. Simple mechanical drain cleaning that is used to clear more solid obstructions may be ineffective.

References & Sources for FOG Grease Interceptors & Traps, Codes, Specifications, Remedies

Grease Interceptor Suppliers

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GREASE TRAPS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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