Calculating Grease Production for Optimal Interceptor Sizing

grease interceptor pump out from the outside

Fats, oils and grease (FOG) that go down the drain can create unpleasant odours, clog pipes and lead to costly sewer back-ups in your business. Don’t get caught in the trap! 

It is important to start thinking of your grease interceptor as an integral part of your business’ success. Without consistent, frequent maintenance, your grease interceptor may not only fail to do its job but can also accrue fines from the local government when it does. 

One of the most important tools to ensure sufficient grease management within a facility is to correctly size, install and maintain a grease interceptor. There are several methods in determining the correct size of grease interceptor for your application; the most common of which is a calculation of total flow rate. The equation for calculating the total rate of flow takes into account the volume of all fixtures connected to the grease interceptor and assigns a drain downtime (usually 1 minute). The result is the flow rate in gallons per minute (GPM) that your hydromechanical interceptor should support. While this calculation determines the maximum flow that an interceptor will face, the industry has voiced its concern that it is not enough information to make an accurate selection of equipment. 

The argument is that two restaurants with the same number and same size of fixtures can produce very different amounts of grease, yet they would have the same sized interceptor if size were based on flow rate. The end result in this scenario is that one restaurant may have to get its interceptor pumped out more frequently than the other. 

Selecting the right size grease interceptor

The American Society of Plumbing Engineers (ASPE) has recognized a need to create a formulation to account for the differences in grease production in different facilities and takes this into account when sizing and selecting an interceptor. 

Based on data collected from a 2009 study, average grease production values have been determined for 4 categories of grease-producers. These values, along with desired and required pump-out frequencies, can be used to determine the amount of grease an interceptor needs to hold for a specific job. 

The table below shows the grease production values for different restaurant types as published in the ASPE Plumbing Engineering Design Handbook (Volume 4, Chapter 8, 2016). To illustrate the application of these values, consider a high grease-producing restaurant serving 450 meals per day on average with a maintenance frequency of 30 days.

Table - example grease production values for restaurants
Source: ASPE (American Society of Plumbing Engineers) Plumbing & Design Handbook (Volume 4, Chapter 8, 2016)

Determining your cleaning interval

The second use of this equation is to determine a pump-out frequency for an existing interceptor. In this scenario, let’s assume our flow rate calculation came out to 95 GPM and we installed a 100 GPM unit that has been third-party tested and certified to hold 1058 lbs of grease. Let’s again say we are a high-volume, high grease-producing restaurant, with a grease production rate of 0.075 lbs per meal at 450 meals per day. This translates to 33.75 lbs of grease per day. If our interceptor can hold 1058 lbs, then it will reach capacity every 31.4 days. Thus, your pump-out frequency should be at 30 days. 

It is imperative to get familiar with your interceptor because the period between grease cleanouts differs for each type. The key takeaway with this is to always rely on experienced licensed professionals. Failure to act in accordance with this can result in mandatory penalties of up to $25,000 per day, per violation. 

Although oversizing an interceptor to stretch the amount of time between cleanouts seems like an attractive option, it will cost you in the long run. Chemical reactions occurring within the grease interceptor reduces the pH of its contents, which may result in the corrosion of porous material. The degradation of grease and food solids is known to produce high levels of hydrogen sulphide. This is where the “rotten-egg” odour associated with the interceptor comes from. Endura recommends you size your interceptor for a 30-day cleaning interval to safeguard your business.

For more information on maintenance tips, read Interceptor Maintenance: Why, When and How.

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