Editor's Note: PM Engineer has always sought to print the various opinions relating to the activities impacting codes and standards. For the past few years, the plumbing and mechanical engineering profession has been watching the development of new standards to regulate grease interceptors, grease recovery devices, and oil interceptors. Robert Tolar presents his views of the current standard activity, based on his participation in the standards development activity. PM Engineer welcomes consenting and opposing views to Mr. Tolar's article.

There seems to be an ever-growing trend toward new standards and certification testing for grease interceptors. I'm referring to the proposed ASME A112.14.3 Grease Interceptor Standard and the ASME A112.14.4 Grease Removal Devices Standard; ultimately a combined effort of the American Society of Mechanical Engineers (ASME) and the Plumbing and Drainage Institute (PDI). I'm hopeful that both proposed standards will be rejected since neither complies with any current pretreatment objective.

Grease Removal Efficiency Ratings

Sewer use bylaws vary, but typically limit total fat, oil and grease emissions (of animal or vegetable origin) to 150 mg/l or less. In order to achieve this municipal requirement the PDI's Standard PDI-G101 certification test (1 lb. of lard for every 10 gallons of water) would have to set its incremental and accumulated efficiency ratings on grease removal at 98.91%. Currently the incremental and accumulated efficiency ratings for grease removal are 80% and 90%, respectively.

All Standard PDI-G101 certification test procedures are performed under controlled conditions. As a result they are free from contaminants and are neither mechanically or chemically emulsified. Since the volume of water and lard used in the test is specified, a simple formula will reveal the proper efficiency ratings.

Example: 25gpm Grease Interceptor

The test interceptor is filled to its static water capacity. Then, 50 gallons of water and 5 lbs. of lard with a .876 specific gravity pass simultaneously through the interceptor, equaling its maximum flow rate. This can be expressed numerically:

50 gallons water at 8.3 lbs./gal.

5 lbs. Lard with a .876 sg (specific gravity)

8.3 lbs. water x .876 sg = 7.27 lbs./gal.

5 lbs. lard : 7.27 lbs./gal. = 0.687757909 gal.

0.687757909 gal. ÷ 50 gals. =

0.013755158 gal. x 1,000,000 ppm =

13,755 ppm

13,755 ppm - 150 ppm = 13,605 ppm

13,605 ppm ÷ 13,755 ppm = 98.91%

Manufacturers interested in having interceptors tested, rated and certified in conformance with Standard PDI-G101 may obtain certification from the PDI's test facility at Bodycote Industrial Testing Ltd. in St. Louis. Its basic test method is designed to simulate severe operational conditions (hence the 13,755 ppm lard and the vertical distance). Assuming Bodycote's test numbers are accurate, extrapolating ppm figures from the test chart is relatively simple. I used Lowe Engineering's Model 25 Hi-Boy certification test chart for clarification.

The 25 gpm Certification Test

The Model 25 Hi-Boy was designed specifically for use next to the three-compartment pot sink. Its stainless steel, self-cleaning tank is 36 inches long by 17 inches wide by 19 inches high. Static water capacity is 22.52 gallons with a retention time of 53.25 seconds at maximum rated flow. Test breakdown occurred after 19 equal increments with a cumulative total of 950 gallons water and 95.00 lbs. of lard. The results showed that 93.24 lbs. were retained and 1.76 lbs. was skimmed from the adjacent tank.

Whereas plumbing authorities might well be impressed with the lard retained (93.24 lbs.), the pretreatment officials will focus exclusively on the lard skimmed (1.76 lbs.). Obviously the most important function of all grease interceptors is to limit the amount of fat, oil and grease effluent.

To pass the 25 gpm Standard PDI-G101 certification test requires a grease holding capacity of 50 lbs. and a safety factor of 12.50 percent. Consequently, the test interceptor must complete 12 equal increments with a cumulative total of 600 gallons water and 60 lbs. of lard. Certainly, the Model 25 Hi-Boy was quite effective in passing the certification test; especially when tested at 10 feet vertical distance, 13,755PPM lard, and constant peak flow rate. Nevertheless, we must consider the pretreatment figures.

Incremental skim at breakdown:

0.28 lb. skim ÷ 7.27 lbs./gal. = 0.038514443 gal.

0.038514443 gal. ÷ 50 gals. = 0.000770289 gal.

x 1,000,000 ppm = 770.29 ppm

Accumulated skim at breakdown:

1.76 lbs. skim ÷ 7.27 lbs./gal. = 0.242090784 gal.

0.242090784 gal. ÷ 950 gals. = 0.000254832 gal.

x 1,000,000 ppm = 254.83 ppm

12 increments = 192.57 ppm-incremental

176.52 ppm-accumulated

5 increments = 247.59 ppm-incremental

143.05 ppm-accumulated

2 increments = 27.51 ppm-incremental

13.76 ppm-accumulated

Surely these figures are unacceptable by any pretreatment standard, but don't blame the manufacturer, blame the test. It only determines the grease holding capacity at conditions never encountered in real life, and ignores the most important factor--the effluent quality. Most assuredly the Model 25 Hi-Boy installed according to specifications will meet all current sewer use ordinances, and the grease holding capacity can be established by static water level. For example, its static water capacity is 22.52 gallons therefore its maximum gpm flow rate should not exceed 22.50 gallons. That is sufficient retention time to meet or exceed the grease holding requirements set forth by PDI.

The Standard PDI-G101 was first issued 50 years ago. Truman was President and restaurants still poured their fryer oils down the most convenient drain. Perhaps this explains testing at 13,755 ppm while the average fat, oil, and grease range is between 400 and 1000 ppm. Also, it is reasonable to assume that no manufacturer would certify their rating on a grease interceptor subjected to 10 feet vertical distance and its maximum gpm flow rate. Proper installation procedures would dictate a larger volume interceptor than anticipated by flow rate if a substantial vertical distance exists.

A Better Method

Lowe Engineering understood early on that the important function was to limit the amount of fat, oil and grease in the effluent. As a result, in 1987 they contracted the United States Testing Company (USTC) to evaluate effluent efficiencies. Testing was conducted at Lowe Engineering's headquarters in Lincoln Park, NJ, under the supervision of a USTC representative. The test set-up was similar to the Standard PDI-G101 certification test, i.e., 13,755 ppm lard, 10 feet vertical distance, and peak flow rate. The Model 35 Grease Recovery Unit was filled to its static water capacity. Then, 35 gallons of water and 3.5 lbs. of lard with a .876 specific gravity passed simultaneously through the interceptor, equaling its maximum flow rate. The effectiveness of the unit to remove the mixture of lard from water based upon this test procedure was 99.21%. Chemical analysis of the wastewater was as follows:

Wastewater Input = 13,755 ppm lard

Wastewater Output = 108 ppm lard

Note: The wastewater output of 108 ppm is equivalent to 0.4397 oz. of lard in 35 gallons of water.

Clearly the USTC test is more effective than the Standard PDI-G101 certification test in establishing effluent quality, but I'm not an advocate of either test. Grease holding capacity and effluent quality varies in each installation. No controlled certification test can assure the grease holding capacity or that the effluent loading will meet local sewer discharge requirements.

A Simple Solution

So what can be done? Well, certainly a step in the right direction would be a written standard requiring the static water level of each interceptor to be its maximum flow rate. Then, require that no flow control devices shall be used, this should compel manufacturers to properly size their interceptors. Flow control devices are installed to ensure that the flow rate does not exceed the interceptor's rated capacity or peak flow rate. Since the expected flow rate is always lower than the interceptor's rated capacity, why require flow controls? Each standard must provide for proper sizing to ensure conformance to fat, oil and grease effluent requirements; utilizing the recommended treatment times in Lowe Engineering's table provides an ample opportunity to meet those goals.

Is It Too Late?

Since most manufacturers have certified their interceptors to the Standard PDI-G101 certification test, expect a resounding "yes" vote for both proposed standards when balloted, regardless of its usefulness. If passed it will likely curtail competition since new construction is the province of the building code authorities, and the plumbing inspection department will almost certainly mandate its use. I am adamantly opposed to any proposal that stifles competition in the guise of a meaningless test.

Noting increasing surcharges, many restaurants are attempting to minimize releasing contaminants into the municipal sewer system. Most restaurants average 6 to 12 lbs. of fat, oil and grease daily. In real-life applications and installed according to specification, the Lowe Grease Recovery Unit's self-cleaning cycle is activated at least once a day. This begs the question, why require grease holding capacity testing for any automatic self-cleaning interceptor?

The Standard PDIG101 certification test can be salvaged, but the cure will likely result in many manufacturers crying foul. If grease holding capacity testing is as vital as PDI suggests, then let's expand their requirements to include passing the 150 ppm effluent limit--through 12 increments. Listed in Table 3 are the four most common models tested and the requirements necessary to pass, but don't "hold your breath" until the manufacturers embrace this suggestion.

Accummulated skim through 12 increments:

20 gpm = 0.5232 lb. lard = 49 lbs. lard added and 480 gallons of water

25 gpm = 0.654 lb. lard = 60 lbs. lard added and 600 gallons of water

35 gpm = 0.9156 lb. lard = 84 lbs. lard added and 840 gallons of water

50 gpm = 1.308 lbs. lard = 120 lbs. lard added and 1200 gallons of water

Unfortunatley, PDI does not reward surpassing the required 12 increments. Therefore, it seems excessive to go beyond the minimum needed. For example, the Model 25 Hi-Boy's grease holding capacity rating was 93.24 lbs. at breakdown, but its certificate rating is for only 50 lbs.

Perhaps, after 50 years it's time to update or replace the Standard PDI-G101 certification test. Surely, if a manufacturer wishes to test its interceptors to its requirements, by all means go for it, but don't coerce others by a written national standard. In my opinion, if passed, both proposed standards will be touted as the cure-all to efficiently meet all commercial kitchen effluent limits. When in reality, the performance guidelines considered passing for certification leave much to be desired. Testing and/or a written standard under these circumstances would seem futile. A reminder, grease interceptors are pretreatment devices--not plumbing fixtures--treat them accordingly.