Unbearable rotten egg smell

[Schemjo]

Hi all, I come here in dire need of help. I am not a plumber nor do I have any plumbing experience. I own a shop and can't get any help even after hiring 4 professional plumbers. I'm hovering around $10k in expenses trying to solve this hence my need for help.

My setup:

I have a 3 Bay sink and a mop sink on the same line that runs to an exterior grease trap. Each bay has its own p-trap as well as the mop sink. There's another p-trap outside. Also outside is the air vent that runs 2' above the roof.

The problem is whenever we drain a sink or run water in the mop sink we get the horrible rotten egg smell. The problem compounds because our air handler is in the same room so it distributes the smell into the dining area. We get NO smell if it's a windy day. I suspect it's a ventilation issue but each plumber said it's fine. I personally think as we drain the sinks the p-trap is also draining causing the smell to enter.

They've reconfigured the drainage several times, ran smoke tests, took down and redid the vent.

Is there anything I can test or even say to another plumber to prove or even disprove this? Any other suggestions or advice?

Edit: if I plug each sink and add water while draining a sink 90% I won't get a smell. This is why I think the p-trap is emptying because I'm essentially creating a p-trap.

Thank you.

 

[Reach4]

I would check for pressure or vacuum in the sewer pipes. It's the venting job to keep that close to atmospheric pressure. What if there is a vacuum? What if there is pressure?
The technique would be to get a clear piece of flexible plastic tubing maybe 5 ft long. Somebody on medical oxygen will throw away such tubing once per month. Most hardware stores would sell clear tubing.

Thrust end through the water in the toilet, and up the trap another maybe 8 inches. Then make a good blow into the other end to clear any water out that got in during insertion.
Put the end you were blowing into into a glass of water with maybe 4 or more inches of water. The water inside the tubing would normally rise to the level of the water in the glass. If there was a vacuum in the trap area of the toilet, the water in the tube would settle significantly lower. That tube in the glass forms an "open manometer", in case you want to look that up.
Then drain a different bowl. Watch the waterline in the tubing vs the waterline in the glass. Normally you would expect the difference to be less than an inch, I would think.

There are commercial manometers, but the tubing will be good for something that you don't do regularly. It is accurate.

I am not a plumber.