sewer collapse question

Idle speculation: if the vacuum got the water started over the top of the P-trap, a siphon would be created that would not stop until all the water had been removed.

Or, until the trap is empty enough to allow air from above the trap to pass through. This would break the siphon, and any further water evacuation would be via venturi like mechanics. A strong enough air flow could possibly lift ripplets of water and take them away. Plus, the air flow could increase evaporation.

Also I suspect water in a trap is not an impenetrable barrier to gases.

I shouldn't say what my evidence is for that.

Oh, all right, I will. When I was in college I once saw a gadget the kids called a bong, I dunno if it has an engineering name. I'm not sure the usage is entirely legal.

It had a trap filled with smelly water, but differential pressure was sufficient to pull heated air through water and out the top.

I've also run into a case where the exhaust fan on a bathroom seemed to be able to pull sewer smell up through traps even though they were kept full. The door fit too tightly and/or the fan was too strong.

However it did not have an operable vent stack between the trap and the orifice to which you applied the necessary vacuum. If you recall otherwise it must have been some really good heated air

Sigh, and completely different settup. It's kind of the reverse of a P-trap. It's designed to let AIR pass through water. If it was designed like a P-trap with a blocked vent, you'd be sucking the water out until you finally got to the air.

It's certainly a U-trap though, and even in a plumbing P trap all the water is in the U.

Hmmm.......?

Why don't you think of the design of the two, how they operate and get back to us. For starters, a bong is not a U trap. It's not designed to block air/gas flow. It's designed to make air with burning smoke bubble up through water when pulled via a vacuum. So to say that because a bong passes smoke as it's designed to do somehow means that a P trap isn't effective at sealing out gases makes no sense.

The pressure at the bottom of an inch of water is ,036 psi

How much vacuum do you think it would take to pull a gas up through an inch of water (or less) in a trap?