What is Air Entrainment?

Once in a while somebody makes a statement of "fact" that gives me pause and makes me want to write a blog post about it. Yes, I think about naming names, but in the end I'm really more concerned with correcting a misunderstanding.

Recently someone claimed that a manufacturer's grease interceptors are not hydromechanical grease interceptors (HGIs) because they do not incorporate air entrainment. A statement like that is a little inside baseball so I understand if you don't understand my initial reaction.

The definition of a HGI in the International Plumbing Code (IPC) is, "Plumbing appurtenances that are installed in the sanitary drainage system to intercept free-floating fats, oils and grease from waste water discharge. Continuous separation is accomplished by air entrainment, buoyancy and interior baffling."

The Uniform Plumbing Code (UPC) defines a HGI as, "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..."

Notice that both definitions include the phrase "air entrainment" as a defining characteristic of a HGI. But, what is air entrainment? According to Merriam-Webster's online dictionary entrain means, "to draw along with or after oneself, or to draw in and transport (as solid particles or gas) by the flow of a fluid." 

Without getting too scientific, may I point out that the molecular structure of water consists of two parts hydrogen and one part oxygen, which is to say that water cannot exist without air (normally).

There is a difference between natural air entrainment and deliberate air injection. The purpose of a vented external flow control is to inject additional air into the waste stream entering a grease interceptor to enhance separation efficiency. While many HGIs depend upon air injection for their performance, some do not. Are you expected to believe that air injection is the only solution to the efficient operation of a HGI? Then why bother testing an interceptor that does not utilize a vented external flow control? 

Side note: while PDI G101 mandates the use of an external vented flow control and air injection for certified interceptors, ASME A112.14.3 does not. Under the ASME standard an interceptor may be certified with a vented external flow control, an unvented external flow control, or a built-in flow control (integral). Regardless which type of flow control an interceptor incorporates they are all tested and rated to the same performance criteria.

Finally, consider that the purpose of the definition for HGIs is to distinguish them from gravity grease interceptors (GGIs). For some reason the industry refuses to abandon using residential septic tanks as commercial grease interceptors and since they are not tested and rated for performance we have to accept the argument that they must work efficiently based solely on retention time. Therefore it is essential to distinguish between interceptors that are tested and rated for performance from those that are not.

The point is that while there are definitions for both HGIs and GGIs in both national model plumbing codes, these definitions are only intended to distinguish between the two types of interceptors, but both are inadequate to describe how any particular type of interceptor actually works. All interceptors receive or "incorporate" air entrainment because it is impossible for them to receive waste water discharges that are "air free".

It's important for us all to understand how all grease interceptors actually work including what air entrainment is and the difference between that and air injection.