FOG Interceptors

Managing FOG Interceptors -- FOG Interceptors are very useful in collecting FOG from kitchen wastewater, but they must be periodically serviced in order to be effective. FOG Interceptors are usually above ground or in ground tanks that allow water draining from a kitchen to slow down. When the wastewater slows, oil can float to the surface and solids can fall to the bottom as sludge. The wastewater between the top layer of FOG and the bottom layer of sludge can exit the FOG Interceptor and flow into the sewer system. FOG Interceptors are usually sized so that after two weeks to a month, they have collected as much FOG and sludge as they can hold. If they are not cleaned or properly sized, FOG will simply pass through the trap and into the sewer system.

FOG Interceptors must be cleaned by state certified companies specializing in FOG Interceptor service. Such companies can be found by looking under the approved hauler list on the Georgia FOG Alliance website ( Contract with a reputable service company to insure timely managing and maintaining of FOG Interceptors. FOG Interceptor cleaning cycles are determined by the Compliance Inspector.

FOG Interceptors size are based on the Uniform Plumbing Code standard method. Local building codes and wastewater treatment systems may have FOG Interceptor ordinances that require minimum sizes for FOG Interceptors, periodic cleaning, and inspections.

There are various designs of FOG Interceptors. FOG Interceptors can be designed according to plumbing code standards and DWM Code. Wastewater enters the influent side of the tank which is divided from the effluent side with a baffle located 2/3 of the way from the influent end. Water flows through the baffle via an opening about 1/3 of the distance down from the top of the tank. One vender sites a general rule of thumb for determining clean out frequency by measuring the grease layer thickness. The rule of thumb is that a FOG Interceptor needs cleaning when the floating grease mat exceeds 3/4" to 3" in depth or when the sludge layer exceeds several inches in depth. The grey water layer between the grease mat and the sludge layer should make up the majority of the depth of the FOG Interceptor. The Delkalb County Inspector shall determine the thickness and cleaning frequency during regular inspections.

DeKalb County cut sheet for 1000-2000 gallon FOG Interceptor.
(click to enlarge image)
1500 Gallon Pre-cast grease interceptor construction
1500 gallon Pre-cast finished.
Needs PVC sanitary 50 pound FOG Interceptor.
Tees and 90 degree elbow in baffle.
* DeKalb County requires that inside FOG Interceptors be
no less than 100 lbs., and outside interceptors
be no greater than 3200 gallon.
(click to enlarge image)

Other FOG Interceptor Tips

  • The plumber, contractor, or compliance inspector must inspect FOG interceptors to determine if they are installed correctly. One FOG Interceptor cleaning service has found that about 25% of all FOG Interceptors are installed backwards.
  • Waste food grinders generally should be installed ahead of a FOG Interceptor. Many restaurants have installed grinders to allow disposal of food waste in the drainage system.
  • Install strainers on sinks, dishwashers, and floor drains to prevent solids from entering FOG Interceptors.
  • Never dispose of fryer oil by pouring down the drain. Recycle FOG with a reputable vendor.
  • If a FOG Interceptor is undersized, it will not retain FOG. If no grease mat is forming, have the trap evaluated to determine if it is properly sized, properly installed, and meets building codes.
  • Never dispose of solid waste by placing in the drain.
  • FSE shall keep records of when and how much used oil is collected, when FOG Interceptors are inspected, when and who cleaned the FOG Interceptor, and where the material from the trap was taken. (Georgia Environmental Protection Division Commercial Waste Manifest Form).
  • Disposal of used fryer oil and residue from FOG Interceptor cleaning is regulated. Some landfills will not accept FOG. Recycling is the best method.
  • FOG Interceptors should be designed and located so that water leaving the trap has had an opportunity to cool, thus allowing FOG to solidify.
  • Septic Tanks can not be used in place of properly sized and designed FOG Interceptors in DeKalb County.