Industrial waste woes
By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.
Several years ago I worked at length with an IAPMO Task Group on revamping the 2006 UPC Chapter 10 on Grease Interceptors. We were successful in creating sizing charts that were rational, easy to use, and prevented the ridiculous over-sizing that often occurred using the old Appendix H.
Locally here in Los Angeles — and perhaps you have experienced something similar in the areas you design projects — the Industrial Waste department rewrote the Code, circumventing it, and increasing the size of grease interceptors. They did this in response to getting sued by the Heal the Bay Foundation for sewer spills, so they had good reason, but it was becoming clear after many grease interceptor tests that the over-sizing was not warranted — even for them.
Consequently, I wrote a letter to the IWMD highlighting the effect of their Code rewrite in an effort to bring grease interceptor sizing back into the realm of rationality. I will share the letter with you here in case you are experiencing similar frustrations locally, and I will keep you posted as to its effectiveness.
March 11, 2010
City of Los Angeles
Industrial Waste Management Division
2714 Media Center Drive
Los Angeles, CA 90065
Re: Grease Interceptor Sizing
To whom it may concern,
As you know, I participated on the 2005 IAPMO Task Group regarding grease interceptor sizing and was instrumental in revising Chapter 10 of the 2006 UPC/2007 CPC to what it reads today. Specifically, I created Tables 10-2 and 10-3 for grease interceptor sizing, which were approved after lengthy debate by the Task Group. The goal of the Task Group and associated Code change was to make the sizing of grease interceptors more rational, as interceptors had been drastically oversized for many years using Appendix H in previous editions of the Code. The oversized interceptors were found over time to produce hydrogen sulfide gas that was corroding sewer piping and creating dangerous health risks that resulted in the death of two men while cleaning an interceptor at a Las Vegas casino. This Code change was a great success because some Task Group members (the manufacturers) were not too keen about making interceptors smaller. However, eventually even they agreed with the sizing reflected in Tables 10-2 and 10-3. After this success, I was greatly disturbed to learn that the IWMD responded by creating its own sizing table (enclosed) that goes above and beyond the Code and results in, once again, dramatically oversized grease interceptors.
The philosophy behind Table 10-3 for sizing gravity grease interceptors (the primary concern of the IWMD) was to correlate Code dictated sanitary waste sizing criteria with a widely accepted standard grease waste retention time of 30 minutes. Code Table 7-3 establishes the waste loading factor in drainage fixture units (DFUs) for each type of fixture and Table 7-5 totalizes that loading with pipe capacity such that the drainage piping should not flow more than 50% full during peak conditions. These variables were all correlated in Table 10-3 to create a simple sizing procedure based on time-tested factors established by the Code — a document that is conservative by design.
I understand that the IWMD has been sued successfully by the Heal the Bay Foundation for sewer overflows that have polluted our waterways, and that this has necessitated the IWMD take a conservative approach to grease interceptors. However, if the IWMD follows the Code it is automatically taking a conservative approach since the Code, as already noted, is conservative by design. Instead, the IWMD has created a sizing table that often quadruples or quintuples the DFUs of the average kitchen, and this in turn creates larger grease interceptors. Let’s look at an example.
Consider a relatively small kitchen with the following list of fixtures and associated Code and IWMD fixture unit loading. Note that fixtures with indirect wastes (I.W.) are not included in the Code fixture unit loading since the floor sinks they drain to are included instead.
As this example illustrates, this small kitchen has over five times the DFU load using the IWMD chart versus Code. As a result, the interceptor required by Table 10-3 grows from a Code required size of 1,000 gallons to 1,500 gallons. This in and of itself is not a dramatic difference, but as the size of the kitchen grows so does the disparity of the grease interceptor size. A larger kitchen with 100 Code DFUs might have 500 IWMD DFUs, and the interceptor would increase from 1,500 gallons to 5,000 gallons. This is precisely the over-sizing the 2006 UPC revisions were intended to prevent.
There are several factors that contribute to the excesses of the IWMD sizing table. First, pot sinks, traditionally three compartments, are taken at three times the Code dictated value. This is unnecessary since the tailpieces of a pot sink manifold together into one 1.5" outlet. As such, it will drain at the same rate whether it has one, two, or three compartments — there is no need to differentiate.
Second, prep sinks with indirect wastes are added to the load in addition to the floor sink serving the prep sink. This is “double dipping” and artificially inflates the load as if there were two waste producing fixtures rather than one.
Third, general purpose “emergency” floor drains that do not receive water from fixtures or appliances are assigned three DFUs whereas the Code assigns zero DFUs.
Fourth, dishwashers are added to the load based on Hunter’s Curve. This is not the intent of the Code, as reflected in Table 7-4, where 1 DFU is assigned to 7.5 gpm. Per Code, a hard-piped 35 gpm dishwasher would equate to 6 DFUs per Table 7-4, whereas the IWMD calculates it to be 66 DFUs based on Hunter’s Curve.
Fifth, the IWMD insists that all fixtures within the kitchen drain through the grease interceptor, while the intent of the Code is that only grease laden fixtures should be connected to the interceptor. Hand sinks, drains for ice machines, and other grease-free kitchen drains do not normally drain to the interceptor and the IWMD’s insistence that they do so further inflates the size of the interceptor.
Sixth, the IWMD procedure does not provide for sizing interceptors installed to serve future “shell” restaurants where the fixture specifics are not known. The Code allows for sizing the interceptor based on the piping to the shell restaurant, and while the pipe size and associated interceptor would be adequate based on Code, the interceptor might be undersized based on IWMD criteria because it is not based on the Code.
Given all the aforementioned excesses associated with the IWMD’s grease interceptor sizing procedures I respectfully request that you strongly consider basing grease interceptor design on the time tested Code rather than your chart we engineers are forced to use. The resulting smaller grease interceptors will be safer for the maintenance companies, safer for the sewer agencies’ piping, less expensive, and easier to install. Further, since they will be easier to maintain it improves the likelihood that they will be maintained regularly as intended, further safeguarding the health of the public, which is the plumber’s credo.
Thank you for your consideration of this matter.
Sincerely,
Timothy Allinson, P.E.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters.