Our latest survey gave readers the chance to explain why it’s important for them - or not - to specify grease interceptors

A few months ago, PM Engineer ran an extensive feature article on FOG (fats, oils and grease) and the importance of using grease interceptors to keep kitchen grease out of the municipal sanitary system.

Because the article was very well received, PME then decided to survey its readers about their design practices with grease interceptors. This article provides and analyzes the results of that survey. The survey addressed both types of grease interceptors, hydro-mechanical and gravity, and provided for some very interesting results.

Figure 1. Standard Used Most Often to Size Grease Interceptors

We sent questionnaires to 1,600 of our readers and 18% responded. In the area of marketing surveys, 18% is outstanding. Usually, a response of 3% is considered to be very successful. So, again, thank you for your responses and involvement in the industry.

Figure 2. Frequency of FOG Disposal System Specifications

What We Asked

This survey encompassed 19 questions [14 sliding scale and five “yes or no”] broken up into five sections. Section one focused on general standards and location; section two on type, size and related systems; section three on food-service establishments; section four on maintenance and section five on how food-service establishments eliminating trans fats has impacted designers.

The first question we asked was: When designing a drainage system, do you include a grease interceptor to capture fats, oils and grease? More than 97% of those responding gave an affirmative answer. The answer could have easily been 100%, but we suspected that some engineers do not get involved with drainage systems that have grease laden waste.

Figure 3. Preference For Size and/or Location for Grease Interceptor

For those who did include a grease interceptor, we wanted to know which standard they used most often to size interceptors (Figure 1). Based on our survey, the plumbing code is the most-preferred standard (42%), followed closely by local requirements (36%). There is a significant drop off after this, down to 13% for engineered design and 5% for Standard PDI G101 (by the Plumbing and Drainage Institute).

Just 2% of readers said they let the plumber decide which standard to use. And the same amount selected “other,” noting their preferences such as using EPA guides, manufacturer recommendations and flow requirements.

Figure 5b. Hydro-Mechanical Interceptor

According to our survey, a majority of readers prefer large outside interceptors. Question three addressed this topic specifically, by asking: What size and/or location do you prefer for your grease interceptors?

Most respondents (56%) selected large outside interceptor. Once again, there was a large drop off to the next preferred answer: small interceptor in the area of fixtures (18%). The next option, point of use at the fixture, drew a 14% response, followed by large centralized interceptors (10%). Just 2% selected “other” and said they simply size the interceptor per job application (small or large sink). This was somewhat of a shock since we expected the highest response to be smaller interceptors in the area of the fixture.

The next question regarded how often participants had to submit any special information to a jurisdiction to gain approval of a grease interceptor. Based on reader feedback, it appears this approval step is not required most of the time (53% no to 47% yes). The type of information that is submitted, according to our survey results, ranges from calculations to grease trap sizing formula on drives to venting requirements.

Figure 5c. Hydro-Mechanical Interceptor

Sizes and Systems

Section two focused on issues related to interceptor type, sizes and related systems. Question five asked: What type of interceptor do you prefer? The options were hydro-mechanical (HM) or gravity, and replies weighed heavily in favor of gravity (89%).

Question five also had more subquestions - two on HM interceptors and two on gravity interceptors. The first HM question asked designers about the type they specified most. A whopping 61% selected automatic (grease removal device). In descending order came manual (19%), semi-automatic (13%) and do not specify a type (7%).

Next, we asked which material type and flow rate they prefer for HM interceptors. Nearly one quarter (23%) said stainless steel - small to medium. Exactly 17% selected coated metal - small to medium (4 to 25 gpm), metal - medium to large (30 to 100 gpm) and thermoplastic - medium to large. A smaller percentage selected stainless-steel medium to large (13%) or thermoplastic - small to medium (3%). Ten percent had no preference.

Figure 5d. Gravity Interceptor

[Editor’s Note: A similar survey question was posted on www.pmengineer.com but drew somewhat different results. The breakdown was metal - medium to large (37%), thermoplastic - small to medium (23%), metal - small to medium (20%) and thermoplastic - medium to large (20%) .]

As for gravity units, first we wanted to know which material type readers preferred. Concrete is most popular (38%), followed by steel (25%), fiberglass (19%) and concrete with a specified coating or liner (17%). Also mentioned (1%) was cast iron.

Figure 5e. Gravity Interceptor

Next, we asked which size you tend to select most often. Interceptors from 600 to 1,500 gallons drew the largest response (42%), followed by units less than 600 gallons (31%) and those 1,501 to 3,000 gallons (23%). Only 4% said 3,001 to 5,000 gallons, and a few respondents said 5,001 or more gallons.

Our survey then asked about systems used in conjunction with interceptors. In question six (Figure 2), we wanted to know how often you specify a FOG disposal system. Two-thirds of readers said either occasionally (42%) or sometimes (23%). One-sixth, or 16%, said all the time. About the same amount (14%) said never, and 5% didn’t know.

To close section two, we wanted to know how commonplace it was for you to specify a grease recovery device (GRD). More than two-thirds said never (36%) or occasionally (34%), while 21% said sometimes. Those who specify GRDs all the time or didn’t know numbered just 5% and 4%, respectively.

Figure 7. Frequency of GRD Specification

Food-Service Establishments

Next came a question about food courts; specifically, how often do you allow multiple establishments to connect to a single interceptor? By a wide margin, respondents said “never” (39%). Interestingly, the next highest answer was don’t know with 24%. Rounding out the responses were occasionally (16%), sometimes (15%) and all the time (4%).

Figure 8. Frequency of Dealing With Food-Service Establishments That Require a Grease Interceptor


Maintenance is very important to the effectiveness of an interceptor so we included three basic questions. First was whether your specifications require or recommend maintenance after installation. The majority said yes (57%). While this is a high number, manufacturers of grease interceptors would prefer to see an answer of 100%.

Second, we wanted specifics on frequency of maintenance for hydro-mechanical interceptors. More than three-fourths of you said it depended on the job (78%). Others said every month (11%), every week (7%), every two weeks (2%) or every day (2%).

Figure 9. Frequency of Food Court Establishments Allowed to Connect to a Single Grease Interceptor

Next, we asked about frequency of maintenance for gravity interceptors. Once again, readers told us that the specific job determines maintenance requirements (55%). A much smaller percentage said every month (16%) or every quarter (14%). Just 8% said every 6 months, 5% said annually and 2% said when the separator is full.

It was good to see such high numbers for maintenance being required based on the job or installation. Most manufacturers recommend that every grease interceptor be evaluated for the particular installation.

Figure 11. Recommended Frequency That a Hydro-Mechanical Grease Interceptor be Cleaned

Final Question

There has been a lot of discussion in the mainstream media recently about the need for food-service establishments to cook healthier by eliminating the use of trans fats. PME was interested to know how much this trend has impacted its readers, so it addressed this topic in the last question of the survey.

We asked: If a facility has eliminated the use of trans fats, has this impacted your design of a grease interceptor? In no uncertain terms, readers told us NO! In fact, only 3% said they had been impacted.

Some manufacturers have expressed a concern that the reduction in trans fats may require an evaluation of the grease interceptor design. This is based on the fact that lighter oils are being used that may not be intercepted as well. It is clear from the survey that the engineering community does not agree with this conjecture.

Figure 12. Recommended Frequency That a Gravity Grease Interceptor be Cleaned

The impact on those who did reply yes was worth noting. According to reader comments, this shift has lead to specifying smaller and lower-capacity interceptors, as well as the need to install them in an area that can be cleaned easily. One respondent provided a very helpful bit of information. The person wrote: “Many schools are now using disposable plates and doing no frying. Mainly they’re just heating food, so, typically, just the pot sink requires a grease recovery device.”

Figure 14. Job Title or Classification

I’m sure you would agree that this was another fascinating survey. If you received the survey questionnaire and responded, our thanks for helping out. If you did not receive a questionnaire, keep an eye on your mail for the next survey. When you get it, take a few minutes to fill it out and send it in. You can help contribute to our next article.