Newly replaced 3" house sewer keeps backing up

[deethomas]

Hi - 1st time posting here!

I replaced ~33' of 3" house sewer ABS DWV in a friend's house back in September. He has had repeated sewage back-up in this drain over the past three years. Once sewage exited his vent stack (~six years ago).

I installed a 1/4" pitch per foot on the new 3" drain, ran water through it for 1 hour, and it ran clear & effectively drained. My friend checked the drain about three weeks ago and there is a small quantity of sewage back up from the horizontal clean out.

A plumber drain-scoped the street-side/municipal drain two years ago and did not find any blockage/roots/etc. There is no backflow valve on the DWV.

Any insight/opinion/solutions appreciated.
THX.

FYI: The drain drops about 3-1/2' to exit through the wall to the street DWV; the street DWV is about 3' above the basement floor. There is a Y connection from a sewage sump pit connected about 18" above the street DWV and an air cheater/cheater vent from the sump pit.

 

[JohnD]

This is a UK site and our designs are not the same. Your pictures look strange to me. I don't understand the loop or the sump.

Our smallest soil pipe is 100mm (4 inch) in a single home, usually joining to 150mm or bigger in the street. In some cases there is a 4 inch pipe serving a small group of terraced houses.

 

[ree]

OP,
Your question and pics are difficult to understand.
The CNC has variations for various provinces due to climate, location and distance north.
The CNC national code for holding tanks and pumping is complicated, & requires a local plumbing contractor's attention.
Why not try a Canadian or local plumbing forum?