What is Air Entrainment?

October 18, 2016

Once in a while I hear something that makes my mouth fall open while a look of utter astonishment creeps over my face. Since I work from a home office most people would not be able to observe these visible signs of incredulity, but it might look something like:

Recently someone claimed that a certain 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 molecular structure of water consists of two parts hydrogen and one part oxygen, which is to say that water cannot exist without air. In a gravity drainage system the flow of the fluid into the drainage system also draws in more air and venting within the system adds even more air to ensure the system flows properly. Thus we can rightfully conclude that all grease interceptors receive waste water discharges with entrained air. 

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, like the Great Basin 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.

Don't get me started. Seriously.

The point is that while there are definitions for both HGIs and GGIs in both 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 and I appreciate the opportunity to clarify what air entrainment is and the difference between that and air injection.



  • Greg Williams

    Jan 30, 2017

    I agree with your conclusion but I disagree with the way you got there. As a minor point, water can exist without air. If you put water in a vacuum it boils and becomes water vapor but it does not breakdown into hydrogen and oxygen. Similarly, you could create water if you put hydrogen and oxygen into a vacuum. I expect you would also need a catalyst or some electricity and it would be dangerous (as in Kaboom!) but it is possible.

    My main point is. you mention that there is a difference between natural air entrainment and deliberate air injection your post does imply that the two work in a similar way because you say all interceptors incorporate air entrainment. At standard temperature and pressure water will contain ~0.023 g/kg air (http://www.engineeringtoolbox.com/air-solubility-water-d_639.html) but this can hardly be considered entrained. Entrained air is in the form of bubbles and these may or may not be present in flow into an interceptor without some kind of deliberate injection device. You need a lot of turbulence to entrain air.

    A more interesting question would be, does air entrainment actually improve performance? The model is usually dissolved air flotation (DAF) but the material being floated (microbial mats) and the quantity of bubbles are very different from what happens in your average HGL Air entrainment may not be relevant to interceptor performance. that would be a good thing to test.

    It does not make sense to take any performance claim for granted, they should all be tested.

  • Hunter Gordon

    Nov 06, 2016

    Great explaination.

  • Rich Tarr

    Oct 18, 2016

    Enjoyed the post. Great information for a new Schier representative. Keep them coming!!!

  • Richard Dalton

    Oct 18, 2016

    I agree, and it does happen more often than it should.

  • Jim Marco

    Oct 18, 2016

    Hi Ken! I learned something new today! Thanks!

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