What is the 25 rule for grease interceptors?

The 25% rule is used by some pretreatment authorities to determine when a grease interceptor is full. It says that the total depth of the floating grease layer along with the solids layer at the bottom shall not exceed 25% of the total liquid volume of the interceptor. Simply put, if you have 1,000 gallons of capacity, the grease layer will equal 125 gallons and the solids will equal 125 gallons, giving you a total of 250 gallons of effluent.

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What is core sampling?

The best way to determine this percentage is by taking a core sample with a specific device like a Sludge Judge or Dipstick Pro. The device is slowly lowered into the tank, a valve is closed, and then the contents are raised up out of the interceptor and measured for conformance. The grease layer is located at the top and the solids at the bottom. A clean layer (sort of) of water between them allows for a measurement to be taken. Once the overall depth is measured, then the percentage of grease and solids can be calculated.
Where did this rule originate?

Research shows that the rule was based on the capacity of smaller hydromechanical grease interceptors (HGIs) along with information from the Plumbing and Drainage Institute (PDI). With regard to the interceptors, the minimum rating for HGIs in terms of grease capacity is doubling the flow rate (i.e. a 20 GPM HGI holds 40 pounds of grease in order to be certified). If we measure the depth of these smaller HGIs, the grease storage to the minimum standard is between 25% and 35%. So, as a conservative estimate, 25% was used to determine the capacity. PDI noted in in their paper, Guide to Grease Interceptors &#; Eliminating the Mystery, that interceptors certified to PDI-G101 may need maintenance when as little as 25% of the rate capacity has been achieved.

From there, jurisdictions such as Honolulu and Orange County created FOG programs based on non-factual information, sourced via hearsay, opinion and surveys, to determine grease capacity and then included Gravity Grease Interceptors (GGIs) as part of this 25% rule. The 25% rule now appears to be ubiquitous in FOG programs and ordinances, not because of any scientific or technical merit, but rather it seems to have its footing in the idea that &#;everyone else is doing it&#; so it must be right.

Since the 25% rule is entrenched across the country, it is difficult to be flexible to the newer, more efficient, and scientifically-proven technology of the high efficiency, high capacity HGIs of today. Additionally, GGIs have no performance data and none of the stakeholders know when this &#;25%&#; will be reached with any given food service establishment. 

How are HGIs core sampled?

A few HGI manufacturers test their products to failure using the ASME A112.14.3 standard for performance; they do not follow the 25% rule. Each of these interceptors has a maximum grease capacity based upon flow rate, tank volume, and design. The manufacturer of the product should provide information in the way of a chart to show how each unit should be measured.

When sampling an HGI, the process is the same as the traditional gravity unit. By using a device such as a Sludge Judge or Dipstick Pro, the core sample is taken to measure the contents of the interceptor. The difference being, the HGI is holding more grease by design and the measurements given by the manufacturer dictate the height of the grease layer. For instance, the GB-250 holds a maximum depth of 22.5&#; of grease. The overall depth of the unit is 29.5&#; which means that the grease layer is equal to 75% of the tank volume. This is way beyond the 25% rule. Again, the GB-250 is tested to failure so the certification shows that there is more capacity in HGIs.

For solids sampling of HGIs, it will vary by manufacturer. Schier builds in 20% solids capacity as a minimum for all exterior units. The core sampling device can measure each unit and the solids height can be compared against the manufacturer&#;s maximum requirements. The GB-250 holds 20% solids and this pushes the unit to 95% combined grease and solids capacity compared to the total volume.

See more information regarding the differences between HGIs and GGIs.

What about the other 75%?

When pumping contractors remove the contents of grease interceptors, it must be treated. 75% of the effluent (brown grease) needs to be dewatered and treated, which is an added expense to the owner, pumping contractor and wastewater plant. There is increasing pressure on pumping contractors, as wastewater plants don&#;t necessarily take the brown grease or they set limits on the amount that they can accept for treatment. Private industry is beginning to see the value of brown grease for bi-product production such as fertilizer, grid energy or even jet fuel which helps to lessen the burden on the treatment plants. While this is positive, the need to treat this 75% is wasteful and HGIs can provide much better efficiencies.  

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The chart below shows an example of a jurisdiction with 1,000 food service establishments (FSEs). If we take the number of interceptors and multiply it by the gallons of storage, you can see the total number of unused gallons and draw a comparison between the standard GGI and HGI. Grease capacities have been expressed in pounds by using 7.3 lbs/gallon, which is what grease typically weighs. We can predict that a 1,000 gallon gravity grease interceptor would hold 913 lbs of grease. The GB-250 HGI holds 1,895 pounds of grease as certified by ASME A112.14.3. Since HGI&#;s are 4-6 times more efficient, the volume of water needed for grease separation and storage is considerably less than the average GGI. In addition, the grease is much more concentrated and therefore is more desirable to the pumping contractor for use in other byproducts. This fact should be especially important in areas of the country where water resources are at a premium. By holding less water in the tank and using greater grease retention, all of the stakeholders benefit from the technology.

In summary, the 25% rule was adopted across the country for when a GGI is full. It does not consider the performance of the unit since we do not have any formal testing standards of GGIs for efficiency. While the rule is accepted by many jurisdictions, there is now better data of proven technology that allows for performance-tested products beyond 25%. Having HGIs account for 75% or greater grease and solids capacity saves water, decreases costs, and is a more environmentally friendly approach to FOG programs - creating sustainable, long lasting solutions.

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 s. 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 (R-) 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 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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