I'm having the side sewer replaced in my 50 yr old home (Tacoma, WA).
On either side of the sewer line as it leaves the foundation, are two
other concrete pipes the same diameter, which join the middle pipe and
dump into the sewer main. We tested and all the wastewater from the
house comes out the middle pipe. What are the two other pipes designed
to do? It's impossible to tell what they lead to, upstream.