Grease Interceptors; 2009 UPC vs 2006 UPC

In the 2006 Uniform Plumbing Code the term 'Grease Trap' was replaced by a new term 'Hydromechanical Grease Interceptor' which is defined as:  "A plumbing appurtenance or appliance that is installed in a sanitary drainage system to intercept nonpetroleum fats, oil, and greases (FOG) from a wastewater discharge and is identified by flow rate, and separation and retention efficiency.  The design incorporates air entrainment, hydromechanical separation, interior baffling, and / or barriers in combination or separately..."  Hydromechanical Grease Interceptors are required to control the flow rate entering the device with either an external flow control or a built-in flow control (non-external), either directly connected or indirectly connected to the sewer system.  These devices are allowed a maximum of 720 drainage fixture units (DFU) according to Table 10-2 of the code.

The 2009 Uniform Plumbing Code revised the sizing requirements for Hydromechanical grease interceptors and eliminated the application of Drainage Fixture Units in sizing considerations.  In chapter 10 Table 10-2 was revised to size these devices by either using Gravity Flow Rates or Fixture Capacity.

2009 Uniform Plumbing Code, Table 10-2

There is some confusion about whether to use the One-Minute Drainage Period or the Two-Minute Drainage Period.  Lets clear that up.

Plumbing Engineers (specifiers) rely on the American Society of Plumbing Engineers (ASPE) for instruction and guidance on industry best practices for sizing plumbing piping systems.  In the Plumbing Engineering Design Handbook 2, chapter 1 Sanitary Drainage Systems, ASPE instructs engineers to size their gravity flow waste piping systems at half-full capacity to minimize the generation of pneumatic pressure fluctuations, to prevent self-siphonage of traps, and reduce noise, while providing enough flow to rapidly carry away the soiled water without clogging the pipes or leaving solids in the pipes.

Bearing in mind the instruction to engineers to design their gravity flow sanitary drainage systems at half-full capacity, we can easily see that the Two-Minute Drainage Period more closely reflects the actual flow rates the piping system was designed to deliver.  Thus when sizing an interceptor using the pipe size exiting the building you should use the Two-Minute Drainage Period.

When using Fixture Capacity the same rule applies.  Notice the total load in gallons in the example given is 49.9 gpm.  The One-Minute Drainage time assumes that the tail piece at the bottom of each compartment will allow the full contents of the sink to empty in one minute.  In reality this is not the case.  Most of the multi-compartment sinks either use a 1-1/2" or 2" tail piece.  The maximum flow rate of a 2" tail piece is 24 gpm.  Therefore you should use the Two-Minute Drainage Period which more closely reflects the actual flow rate coming from the fixture.

Whether it takes the sinks a minute or two to drain into the sanitary system makes little difference in the normal operation of a commercial kitchen.  The more important issue to consider when sizing grease interceptors is how much grease they should be expected to separate and store.  The plumbing code makes no allowance for determining this "FOG loading" and as such has done a poor job of correctly sizing grease interceptors.  For information on grease production sizing please see Schier Product's website at www.schierproducts.com.