Did you know that the term hydromechnical or hydro-mechanical if you prefer, is not unique to grease interceptors? For example, Hydro-mechanical drive-trains are a type of transmission used in a variety of different types of vehicles from small passenger hybrids to very large construction equipment.
I decided to look up the definition of hydromechanics, which is a noun that means "hydrodynamics" (seriously), which is a noun meaning, "the branch of fluid dynamics that deals with liquids, including hydrostatics and hydrokinetics." Hydromechanics and hydrodynamics are interchangeable terms that essentially mean the same thing. Hydromechanical is an adjective form of the term hydromechanics, which would be the same as the term hydrodynamical, if such a term existed, which apparently it does not. The dictionary definition of hydromechanical is, "relating to a branch of mechanics that deals with the equilibrium and motion of fluids and of solid bodies immersed in them."
The term hydromchanical grease interceptor was given a different definition in the 2006 edition of the Uniform Plumbing Code (UPC) when it was first introduced to the industry. Here's the definition in case you don't have it memorized, "A plumbing appurtenance or appliance that is installed in a sanitary drainage system to intercept nonpetroleum fats, oil and grease (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, or barriers in combination or separately..."
So, according to the UPC, a hydromechanical grease interceptor incorporates 'hydromechanical separation'. And, based on the definitions of hydromechanics, hydrodynamics and hydromechanical we can safely conclude that it somehow deals with the equilibrium and motion of fluids and of solid bodies immersed in them. Does that help?
Is the phrase intended to actually describe the way any particular grease interceptor actually works? I should hope not. At best both the term 'hydromechanical grease interceptor' and the phrase 'hydromechanical separation' only attempt to distinguish passive grease interceptors that are tested and rated for performance from those that are not, such as gravity grease interceptors.
The term hydromechanical grease interceptor doesn't describe how any particular grease interceptor actually works any more than the term 'hydromechanical drive-train' describes how any particular transmission actually works. Both terms are useful in distinguishing between different technologies but neither is intended to be more than that.
While all grease interceptors operate on the principle of gravity-differential separation, they don't all do the job the same way. Distinguishing between various designs or technologies is useful in plumbing codes as well as pretreatment programs. However, I think it is more important to leave the design and operation of any specific device up to the manufacturer. Instead plumbing codes and pretreatment programs should focus on approving devices based on testing and rating performance to nationally accepted standards.
We don't have to get hung up on the terms used when we have standards to rely on that ensure that any device that is approved will be tested and rated for performance. That's the best way to ensure the proper application of any type of grease interceptor regardless how it gets the job done.