Backflow preventer placement

[toddmanqa]

https://imgur.com/a/AeLl5FE

Went on vacation for two weeks, had 24" of rain in one/two days, came home to a partially dried out, previously-flooded basement.

Not sure where the water came in. Currently removing plywood and drywall and insulation to get rid of mold, and to investigate cause.

One possibility I am considering is that I had a backup from the sewer line and it came up from the floor drain in the basement laundry room. If that turns out to be the case, I'd like to have a backflow preventer installed. Note: We have a combined stormwater and sewer system where I live. I do not live at the bottom of a hill. In fact, I'm pretty much at the top of the hill in my area.

The linked image shows the main trap rough in when the basement was finished. The large vertical pipe to the right is the trap vent, and it exits a few inches above the ground level on the front of the house.

The sewer pipe is 9-10 feet below the surface of the ground surrounding the house, so I don't believe it's feasible to install it outside. Thus, I think I need to install it in the basement.

Can I install the backflow preventer immediately after the main trap vent? That is, to the right of the main trap vent?

 

[Diehard]

Get rid of that main trap and the relief vent associated with it! They have been obsolete for some time now. They are not allowed by the plumbing codes in new installations.
You can then put your "sewer type backwater valve" in that area.
There are a few different manufacturers and types of sewer back water valves so I would ask the more experienced people to chime in on this.

You also have the option to modify your floor drain with a backwater device.

I assume there was no water use in your house while you were away on vacation.

Also, I assume the flooding took place before the installation of that PVC house trap I see in the picture ?

I have to assume that your statement regarding, "24" of rain in one/two days" is a typo.

 

[mike fiore]

Exactly what diehard said. cut out trap and vent and thats where you install backwater valve. New style also has cover to open for maintenance of check. also put a cleanout before exiting building for draincleaning in future