Its a baffle. The poo comes in at the top and is forced down to the bottom of the pit. I
Thanks for the terminology....allowed me to find another thread
https://terrylove.com/forums/showthread.php?27922-Question-for-SewerRatz in which which the members in that thread called a "catch basin".
SewerRatz said:
The pit you are describing was a catch basin. The kitchen sinks, laundry, and floor drains as well as the storm water drained into the catch basin, there was a trap on the outlet pipe which headed back into the building sewer out front then into the city sewer system. Over the years people have repiped the sink line direct into the sewer and discontinued the catch basins, trouble was they had a hard time rerouting the storm water. So they turned it into a sump pit.
Based on what SewerRatz said, the kitchen sink, laundry, floor drains, and storm water drained into this pit, and this "square mass" which I can now call a baffle or trapped outlet as Smooky indicated, coincides with the above line "there was a trap on the outlet pipe which headed back into the building sewer". Makes sense as the kitchen is directly above this pit.
So I guess originally this was a pit for the kitchen, laundry, floor drains, and storm water which went to the building sewer via a trap/baffle at the outlet. Then someone came along and tied a basement bathroom into the inlet & threw an ejector pump in which pumped sewage up & out to a different building sewer line.
I am questioning if the baffle/trap is really just clogged up & rusted or if it may have been intentionally sealed/filled-in once they converted the "catch basin" into an ejector pit? Anyone have any additional info on what common things Chicagoans did with their catch basins?
Picture of one in the other thread: