I have a main sewer back up valve installed on i believe a 4" cast iron sewer line. The basement floor is cut open 12"X16"X20"deep. There is no cover on this opening. I have 8" of standing water in this opening. Is this normal? I have flushed both upstairs and basement toilets and ran all taps and i do not see any movement in the standing water. Thanks

The obvious first thing to do is try to determine the source of the water. It very well could just be groundwater seeping in, not raw sewage. If your measurements are close then you're talking about 6-7 gallons of water. Pump it as close to dry as you can and keep an eye on it while not running any water in the house. If, after an hour or so, no more water flows in, flush toilets and run household water and see if you have a leak somewhere in the pipe or the valve.

Good luck!

Thank you. It is not raw sewage as it is very clear water. I will use shop vac to empty and keep an eye on it