ok..i am confused as to your verbage...what is a spout?

couple of things to understand your system..
there is a drain line under the floor of the basement?
it carries the waste from the washer and floor drain?
to the sump pit ?
the sump pit ,,pumps liquid up and out, into the septic tank/field ?
the basement has washer and floor drain, draining into pit,
and what else?
in the pit...how far, from the bottom of the pit, is the bottom of drain pipe ?
does the pump, havea check valve installed?
where is it installed, on the vertical,,or horziontial plane?
what is the check valve...a swing ? or spring ?
answer these for me please..it will help me to figure out your situation...

or ill try and explain it better, though this is not my specialty. When we run washer, shower, sinks etc whatever instead of the water going out of the pipes into the septic tank, it comes back in the house through what i call a spout, or little pipe in the basement floor. I have a floor drain AND this little spout. Spout is about 6" high and goes into the basement floor. I assumed it was an overflow or backup pipe incase the septic got full or clogged, just my guess. When we run water in the kitchen sink, all the water (and you can tell cause its soapy water) comes up from that spout into the basement and then into my sump pump and outside. Im trying to figure out WHY the water is coming back into that spout and not into the septic tank outside. When I had my septic tank emptied, the guy had my run water and flush toliets and almost no water at all came into the septic tank? If there is not a clog, which roto router says there is not, then why isnt the water going into the tank. If it were not for the sump pump in the basement my house would be flooded everytime we take a shower. I hope this helps you understand a bit better. Im just trying to figure out what would prevent the water from going to the septic tank, if as i was told there is not a clog? Its confussing to me.

You don't mention your toilets, how well are they working?

Could the previous owner have plumbed the washing machine, showers, sinks, etc to a separate drywell and tried to only use the septic for toilet waste? If it is a separate system maybe it is full or plugged up. Or maybe their idea was just to pump the gray water out on the ground.

Would you say ALL of the sink water is coming up in the basement, or only a portion of it?

Try extending the standpipe higher above the floor with some hose or poly pipe and see what happens. Or try plugging it up temporarily and see if you get a backup,

The basement drains are too low to be drained out to the septic by gravity. The drains in the basement should be going to a sewage pit to be pumped out to the septic tank. This is a different pump than the sump pump. Sounds like the pump in that pit may not be functioning.

Another possibility is that the grey water(showers, sinks, washer) is routed past the septic direct tothe drain field. This would a different line than ths ewage line from the toilets. May only that line is damaged or clogged or even frozen.

would you answer some of my questions and post a picture
of the spout and floor drain and pit
i havea idea...

ok boys its call hydraulic curve the guy prob cleanrd out main stack to septic have him clean floor drain and then washing stand pipe should fix u problem for a couple years
good luck

a picture....worth a thousand words...a picture