Grease Interceptors

Why are grease interceptors required?

Grease interceptors are required by the Uniform Plumbing Code (UPC), as adopted by Duvall Municipal Code (DMC 10.01.100). Grease protection is an essential element for restaurants, cafes, catering facilities, commissaries, hotels, cafeterias, convenience stores, full-service grocery stores, schools, hospitals, and food manufacturing plants. Grease interceptors are installed on “gray” water drain lines and are designed to remove fats, oils and grease (FOG) from wastewater. The retained FOG should be regularly removed or pumped out of the interceptor and should be properly disposed of. The interceptor must be cleaned whenever 25% of any chamber becomes filled with FOG or other solids.


  • Hydromechanical Grease Interceptors (HGI) can be located inside or outside the facility and are required to have an approved type of vented flow restrictor. Flow restrictors slow the flow of water entering the grease interceptor. The total capacity of the fixtures discharging into an HGI, in gallons, shall not exceed two and one-half (2 ½) times the certified gallons-per-minute (gpm) flow rate of the interceptor.
  • Gravity Grease Interceptors (GGI) are generally installed in the ground outside the facility, upstream from the “black” water (sanitary waste) drain line and have a 500-gallon minimum capacity. In existing buildings like the downtown corridor, space for a GGI usually does not exist. Breaking down the fixtures into smaller groups and running them through an appropriately sized HGI is typically the best solution.
  • A Drainage Fixture Unit (DFU) is a unit of measure for the load-producing effects on a plumbing system from different kinds of plumbing fixtures. Things like produce prep sinks and hand washing sinks do not need to be connected to the grease device. If they are not plumbed into the device then they should not be included in the sizing calculation
Grease Interceptor

Fats, Oils and Grease

Information for food services establishments and commercial kitchens:

At Home:

Proper disposal of your cooking oil and other greases and fats may save you from a sewage backup in your home. Grease clogs cause about 40 percent of sewer blockages.

These items contribute to blockages and backups:

  • Butter and margarine
  • Cooking Oil
  • Food scraps with grease, butter or oil
  • Lard
  • Meat fat, grease and juices
  • Sauces that include grease, butter or oil
  • Shortening

Take these simple steps to stop clogs at home:

  • Pour cooking oils and grease in a container and dispose of it in the garbage
  • Place meat and greasy food scraps in the garbage not down the sink or garbage disposal
  • Wipe pots and dishes with a used paper towel or napkin to remove grease