What is the 25% Rule with Grease Traps?

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Ever heard of the 25% rule? If you’re scratching your head, you’re not alone. Many FOG ordinances, enforced by City Water Authorities throughout the United States, make reference to the 25% rule in the context of grease trap maintenance. But what exactly does this rule mean?

What is the 25% Rule?

A grease trap plays a crucial role in capturing grease from kitchen wastewater during dishwashing and cooking processes. By regulating wastewater flow and allowing sufficient time for separation between grease and water, the trap effectively intercepts Fats, Oils and Grease (FOG). The separated grease, floating to the surface, remains confined in the tank, permitting only the “clean” water to enter the drains.

As grease separates, it forms three layers within the grease trap: surface grease, water in the middle, and solids at the bottom. While a properly sized grease trap can accommodate waste up to a certain point, it eventually reaches a saturation point where 25% of the contents, comprising surface grease and/or bottom solids, signals a loss of efficiency.

This marks a tipping point, beyond which the tank lacks sufficient volume to handle additional FOG. Consequently, the grease trap requires a complete emptying and refilling with clean water. Unfortunately, many kitchen operators struggle to determine when this critical 25% mark is reached.

Typical Grease Interceptor Diagram

How to Identify the 25% Mark in a Grease Trap:

  1. Calculation: Conduct a bit of detective work to estimate the trapped grease based on the number of covers or meals per day and the grease trap’s size. Seek assistance from your supplier for accurate calculations.
  2. Sludge Judge: Employ a device like the Sludge Judge, which measures grease and solids within the tank by taking a sample using a transparent cylinder.
  3. Inspection Window (Easiest Way) : Some grease traps, such as the Grease Guardian GGX Manual series, feature “globe” shaped windows at the 25% mark on the tank’s side. When the window displays a grease build-up, it’s a clear signal to empty the tank. See image below.
Grease Guardian Bio GGX Manual with Globe/Inspection Window

Strategies to Bypass the 25% Rule:

  • Regular Emptying: Commit to monthly emptying of grease traps to ensure compliance. In cases where the 25% rule cannot be checked, many city authorities mandate monthly emptying.
Grease Guardian Automatic Grease Removal Device (AGRD)

Being aware of when to empty a grease trap not only ensures compliance but also minimises issues such as odours and overflows associated with neglected grease traps.

Want to know more about Automatic Grease Removal Devices? Contact Us Today.

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