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  • Ken Loucks

Where Did The 25% Rule Come From?

The 25% rule is used primarily by pretreatment authorities to determine when a grease interceptor is full. The rule states that the total depth of the floating grease layer plus the settle-able solids layer cannot exceed 25% of the total liquid depth of the interceptor. Determination is made by taking a core sample with something like a Sludge Judge or Dipstick Pro (pictured here). 

The device is lowered slowly through the fats, oils and grease (FOG) and solids layers of the interceptor then set aside to rest.  This allows the captured FOG to collect at the top of the sampler while the solids settle at the bottom for easy measurement. The total height of the water column in the sampler is taken and then If the combined FOG and solids layers are equal to or greater than 25% of the total water column then the interceptor is considered full.


For example, if the total water column is 48 inches and the FOG layer is six inches and the solids layer is six inches, use the following formula to determine whether the interceptor has reached its allowable capacity:


(6 + 6)/48 = 0.25 or 25%


While this methodology is widely used for both hyrdromechical (HGI) and gravity (GGI) type grease interceptors, it has no scientific justification when applied unilaterally. The reason why is rooted in how the rule was developed.


Honolulu appears to be where the rule was originally created during the late 1990s. James Baginski, then Regulatory Control Branch Head, Department of Environmental Services, developed the rule after analyzing the performance of tested and rated HGIs (known at the time as grease traps). These devices have published performance data including flow rate and grease storage capacity as well as detailed dimensions for various sizes. Baginski calculated the grease storage capacity as a percentage of the total volume for individual interceptors, which all fell within a range of 25 to 35 percent, regardless of manufacturer or flow rate. The jurisdiction chose 25% to be conservative and it became the rule for grease interceptor maintenance enforcement in their new FOG program.

The conclusions reached by Baginksi are further supported by the Plumbing and Drainage Institutes 1998 (R-2010) paper, Guide to Grease Interceptors - Eliminating the Mystery, in which they state that PDI-G101 certified interceptors may need maintenance when as little as 25% of their rated capacity has been reached.

Many of the jurisdictions in Orange County California use the 25% rule based on a recommendation in the 2003 Orange County FOG Control Study, which was not based on any science, but rather on a survey of FOG control programs around the US, a significant number of which were using the 25% rule or similar standard, such as maximum inches of accumulation of FOG or solids.

Older type HGIs that are tested and rated to the minimum requirements in PDI G101, ASME A112.14.3 or CSA B481 would certainly qualify to have the 25% rule applied to them. But, while many pretreatment inspectors claim the rule improves system performance, there is still no scientific or technical basis for applying it to GGIs. Moreover, applying the rule unilaterally has some negative consequences. For example, it discourages manufacturers from innovating designs to hold more grease and it negates the benefits a food service establishment would receive in longer and more affordable pump out frequencies that a higher capacity grease interceptor would provide.

In recent years’, some manufacturers such as Schier Products, Canplas and Thermaco, have developed grease interceptors that are tested and rated to have grease storage capacities well more than 50 percent with some models as high as 90 percent of total capacity. The solution for jurisdictions is to simply evaluate these high-capacity HGIs individually to determine their maximum capacity as a percentage of total liquid capacity. This can be greatly simplified by requiring the manufacturers to provide the information. As always, I recommend asking for the manufacturer’s certified test reports, including the incremental test data to confirm any claims of performance made by a manfucturer.

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