Grease interceptor and commercial sink (oops)

You would need a "vent" in place of the elbow on the trap's outlet to break the siphon.

hj said:

You would need a "vent" in place of the elbow on the trap's outlet to break the siphon.

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thanks for responding hj, are you talking about adding another vent after the grease trap before it ties into the sewer main? i thought about leaving it out because there's a vent down stream. i was able to look at it again and the vent on the sewer main is only about 3 1/2 feet away down stream of where the grease traps gonna be. space is a real issue i would put a vent before the flow control have the vent on the flow control and a vent after the grease trap, but with the space limited is there anything wrong with just using the vent on the flow control to vent the line before the grease trap? the design of the flow control tee that came with the trap takes a 2'' male adaptor out the top for the vent line.

There has got to be a link to proper grease interceptor installation on this site. Try searching in an effort to reduce repetition. More importantly is that you have an air gap and shouldn't have one at all.
You cannot expose waste water to the open air before or after a grease interceptor. No air gap. Every compartment should have a trap and every trap will have a vent. Vent on diffuser before interceptor, vent after interceptor.
Best and safe bet.

If my post sounded harsh, I apologize. I just wonder if you are an actual plumber doing the job and if you are licensed because this stuff should be accessible in your code book or you should have learned something about it during the apprenticeship. If you're not a plumber, then why are you doing commercial work like this with no idea of how to do it?

The vent before the grease trap does NOTHING to prevent siphonage. It MUST be after the trap and BEFORE the drain turns downward.

hj said:

The vent before the grease trap does NOTHING to prevent siphonage. It MUST be after the trap and BEFORE the drain turns downward.

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thank you again sir for your response, this is actually for a friends own personal building where he has gatherings from time to time. I don't think it will get much use. I'm just trying to help, we tend to help each other when we can. the trap he has is a little under sized for the size of the sinks, I told him I would have ran a totally different set up but all these thing were given to him and he wants to put a whole kitchen together cheap with odds and ends he accumulated over time. on a separate note the air gap is just to keep it from backing up into the sink if the sewer backs up the floor drain is not near the area where the trap is going. if not an air gap I would guess the only other preventive measure would be a backflow preventer in between the main sewer line and the interceptor. what are your thoughts on this? I think my friend has made a hobby out of putting things together with little cost. thanks again for your time.

hj said:

The vent before the grease trap does NOTHING to prevent siphonage. It MUST be after the trap and BEFORE the drain turns downward.

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Correct, the vent on the diffuser is for the diffuser only. By design, decreasing the size of the drain inside the diffuser to limit flow into the trap can cause what is commonly referred to as "vapor lock". The diffuser vent acts as a local vent to provide relief. The difference between the picture above and the installation is that each compartment should have a trap and each trap will have its own vent. Otherwise the grease vapors will vent up through the sinks.
When the vent for the diffuser is connected to the rest of the venting system, you would be venting the greasy,stinky air through the tees below the compartments and into the room.

Contrary to popular belief, grease interceptors do not siphon. The term referring to them as "traps" in relation to conventional fixture traps is misleading. The vent after the grease interceptor is to allow for venting of fouling gases from the system and allow for uniform flow through the interceptor itself. By design, when flow stops entering the interceptor the outlet will let and equal amount of water to leave and siphonage cannot occur.

I imagine this installation is in a commercial location yes? So why isn't a licensed plumber handling the installation?

I see the last comment was in 2014 and all of the illustrations show the three compartment sinks connected directly to the grease trap. The 2018 NC IPC says that each compartment in has to be run separately to an indirect waste with an air gap before it runs to the grease trap. Which plumbing code allowed the direct connections in 2014?