For decades smaller point-of-use type grease interceptors were called grease traps. Many people wonder why the name changed to hydromechanical grease interceptors. Grease interceptor, grease trap, you say tomayto, I say tomahto - does it really matter? Actually, it does matter. A trap has a very specific function in a gravity drainage system and it isn't necessarily compatible with the primary function of a grease interceptor.
A grease interceptor is intended to separate free-floating fats, oils and grease (FOG) from wastewater discharges from food service establishments (FSE). A trap contains a water seal intended to prevent sewer gas from entering a building through the plumbing drainage system. If a grease interceptor does not have a water seal on its inlet then it cannot prevent sewer gases from entering the building through the plumbing drainage piping connected to the unit. This can and has happened, presenting a clear and present danger to human health and safety.
Let's look at how the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC) have evolved on this issue over time.
UPC
Through the year 1997, the UPC mandated that grease traps have a 2" water seal (the minimum required for a fixture trap), allowed a maximum connection of four fixtures and allowed the grease trap to be used as a fixture trap for a single fixture provided that the distance between the fixture outlet and the grease trap did not exceed 4 feet and the vertical tailpipe or drain did not exceed 2-1/2 feet.
In 1994 the UPC added or clarified a restriction by stating that, "no fixture shall be double trapped," which created a conundrum. Since the code prohibited double trapping fixtures, and since a grease trap was allowed to serve up to 4 fixtures, if the grease trap is considered a "trap" owing to its 2" water seal, and it can serve as a fixture trap for a single fixture, and it can receive the discharge of up to 4 fixtures, and each of those fixtures must be individually trapped, and each of those fixtures is then routed to a grease trap, then all 4 fixtures are double trapped, which is a violation of the code.
The 2000 UPC eliminated some confusion by removing the language that allowed a grease trap to serve as a fixture trap as had been previously approved and eliminated the requirement for a minimum 2" water seal. Yet this really only added confusion since a grease trap would no longer be required to have a water or trap seal but it was still called a trap.
The 2006 UPC eliminated the term grease trap and introduced a new term, hydromechanical grease interceptor (HGI), to define passive grease interceptors. By eliminating the term grease trap and all references to it, and by not allowing a HGI to serve as a fixture trap for even a single fixture, the UPC eliminated the confusion that had existed over whether a grease trap was a trap. It's not.
IPC
Prior to 2006 the IPC defined grease interceptors and grease traps, which were distinguished from each other only in that a grease trap had a rated flow of 50 gpm or less while a grease interceptor had a rated flow exceeding 50 gpm. A grease trap intended to serve as a fixture trap in accordance with the manufacturer's installation instructions was permitted to serve as the trap for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the fixture outlet to the inlet of the interceptor did not exceed 30 inches and the developed length of the waste pipe from the furthest compartment outlet to the inlet of the interceptor did not exceed 60 inches (1002.1 Exception 3).
In 2006 the IPC removed the definition for grease trap and changed the definition of grease interceptor, removing any reference to rated flow, making it the term for a passive grease interceptor. The code language under 1002.1 Exception 3 from previous codes was not amended, retaining the term grease trap, a term which was no longer defined in the code.
The 2009 IPC removed all references to the term grease trap. The code language under 1002.1 Exception 3 was amended, changing the term grease trap to grease interceptor.
The 2012 IPC introduced a new definition of grease interceptor adding two new terms; hydromechanical and gravity. The term hydromechanical grease interceptor (HGI) was introduced as the new term for a passive grease interceptor while the term gravity introduced a liquid-volume retention-time based type of interceptor. Beyond simply defining the term, the code made no further reference to gravity grease interceptors. Section 1002.1 Exception 3 remained unchanged.
The 2015 IPC added another subcategory for grease interceptors called fats, oils and grease (FOG) disposal systems and added section 1003.3.6 governing gravity grease interceptors and the new FOG disposal systems. Section 1002.1 Exception 3 remains unchanged.
The problem I see with the IPC is in the misapplication of a HGI because the section doesn't require the interceptor to have a 2" water seal. Instead it relies on the manufacturer to stipulate that the interceptor is intended to be used as a fixture trap in compliance with the language of the section. That's dangerous to me. By dangerous of course I mean like wearing tuna-laced swim trunks and swimming in shark infested waters. The IPC could eliminate the confusion by simply requiring a 2" water seal for any interceptor approved for installation under section 1002.1 Exception 3.
The best policy, in my opinion, is to disallow all grease interceptors from being used as a fixture trap and instead to enforce the normal requirement that all fixtures be individually trapped and vented. The provides the best protection against the introduction of sewer gases into a building through the plumbing drainage system.