What is the 25 rule for grease interceptors?

The 25% rule is a guideline utilized by various pretreatment authorities to ascertain when a grease interceptor reaches its full capacity. This rule indicates that the combined depth of the floating grease layer and the solids layer at the bottom should not surpass 25% of the interceptor's total liquid volume. To simplify, if your interceptor has a capacity of 1,000 gallons, then the grease layer and solids combined should equal 250 gallons, with each contributing 125 gallons.

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Methods for Determining Capacity

To accurately gauge this percentage, a core sampling technique is often recommended, employing devices like a Sludge Judge or Dipstick Pro. The device is lowered into the interceptor, a valve is closed, and the contents are extracted and measured. The grease sits atop while solids accumulate at the bottom, allowing for effective measurements of each layer. After measuring the overall depth, calculations reveal the percentages of grease and solids.

Origins of the 25% Rule

Research indicates that this rule emerged from the capacity assessments of smaller hydromechanical grease interceptors (HGIs), informed by the Plumbing and Drainage Institute (PDI). According to the PDI, the minimum rating for grease capacity in these interceptors is double the flow rate (e.g., a 20 GPM HGI should hold at least 40 pounds of grease to be certified). Measurements of these small units typically show grease storage at roughly 25% to 35% of capacity, leading to the conservative figure of 25% for maintenance considerations. PDI's publication, Guide to Grease Interceptors - Eliminating the Mystery, also supports this by noting that interceptors certified under PDI-G101 may necessitate maintenance when only 25% of their rated capacity is utilized.

Subsequently, jurisdictions like Honolulu and Orange County implemented FOG programs based on anecdotal data—often lacking scientific validation—leading to the widespread adoption of the 25% guideline across various regulatory frameworks. This rule is now prevalent in many FOG management programs, cultivated more by acceptance in society than by scientific legitimacy.

Challenges in Modern Applications

Given the firm establishment of the 25% rule nationwide, adapting to newer, more efficient technologies that high-efficiency, high-capacity HGIs offer can be challenging. Furthermore, GGIs lack performance data, leaving stakeholders unaware of when the 25% threshold may be reached in specific food service establishments.

Core Sampling of HGIs

Several manufacturers of HGIs do not adhere to the 25% rule, instead conducting rigorous testing for performance using the ASME A112.14.3 standard. Each of these interceptors possesses a maximum grease capacity determined by their flow rate, tank volume, and design. Reliable manufacturers should supply charts indicating how to measure their units appropriately.

The procedure for sampling an HGI parallels that of traditional gravity interceptors using tools like a Sludge Judge or Dipstick Pro. While measurements are taken similarly, HGIs are designed to retain more grease, with manufacturer guidelines specifying the expected grease layer height. For instance, the GB-250 can hold up to 22.5 inches of grease, outpacing the typical 25% rule since the entire unit measures 29.5 inches deep, allowing grease to constitute 75% of the total tank capacity. The GB-250 is also tested extensively, signifying a greater capacity than traditionally acknowledged.

Solids Capacity Considerations

The solids capacity for HGIs varies among manufacturers. For example, Schier specifies a minimum solids capacity of 20% in all exterior models. By utilizing the core sampling device, one can assess the solids height relative to the manufacturer's specifications. In this case, the GB-250 holds 20% solids, nearing 95% total capacity for grease and solids combined.

Discover further insights into the distinctions between HGIs and GGIs.

Dealing with the Other 75%

When pumping contractors service grease interceptors, it's essential to acknowledge that 75% of the effluent (brown grease) must be treated and dewatered, adding cost implications for owners, contractors, and wastewater plants. The pressure mounts on pumping contractors as certain wastewater facilities limit their intake of brown grease, leading private sectors to explore its value in producing by-products like fertilizers or biofuels. While these advancements are commendable, the necessity to treat 75% of the effluent often appears excessive, and HGIs yield significantly better efficiencies.

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In analyzing a jurisdiction with 1,000 food service establishments (FSEs), one can observe the discrepancy in the number of gallons when comparing GGI and HGI capacities. For instance, a 1,000-gallon GGI holds approximately 913 lbs of grease, whereas the GB-250 HGI can store up to 1,895 lbs as validated by ASME A112.14.3. Given that HGIs outperform traditional models by a factor of four to six times, the necessity for water during grease separation diminishes significantly, leading to less demand on water resources in locales where they are particularly precious. This reduction in water requirement allows all stakeholders—including pumping contractors and food service establishments—to benefit from innovative technology.

In conclusion, while the 25% rule has permeated grease interceptor regulations nationwide, it often neglects to represent the performance capabilities of more modern systems. Though many jurisdictions accept this guideline, evolving data supports the existence of efficient technologies that far exceed the 25% minimum. Embracing high-capacity HGIs enables substantial water savings, reduced operational costs, and a more sustainable approach to handling fats, oils, and grease (FOG) management.