October 10, 2024
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.
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 1998 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.
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.
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