Here in Louisville we routinely encounter sewer lines that serve
a property nearby. They are usually at least seven feet deep,
obviously running or ran sewage, and the property lines have
been changed a couple times since they were installed. Often,
they were a carriage house or a mother-in-law house, and
are no longer seen as part of the property. That's why part of the
concern is valid, the fact that you don't know what it is isn't basis for
removal.
Having established that all the sewage is in one pipe, none in the
others, and not knowing why the other pipes are there is sufficient
cause for eliminating them, unless there's a vacant building nearby.
Even if there is, that's what new property service connections are for.
Perimeter drains are not supposed to go to the sewer. They are stormwater,
and should be directed otherwise. Part of that is to keep
stormwater from overwhelming a sewage treatment plant, another is to keep
sewage backups from filling perimeter drain areas and potentially delivering
sewage to the surface.
My opinion, worth what you paid for it.

CR wrote in message
...
Call the sewer department. In Tacoma they are in the process of relining
all
of the old sewer lines in the city. They are several years into this
process. They first come through with a camera through the main and locate
all of the side sewer hookups and then identify them so they can cut out
for them after they reline. They have very good records of the lines that
they have already done.

Two of the homes that I have owned in the northend of Tacoma have had
adjacent homes sewer lines running across my property. It would be strange
for them to run under your foundation however, unless there has been an
addition added to your home or your home is in an old neighborhood and was
built well after your neighbors. (50 yrs isn't that old in Tacoma) Check
to
see if there are records for an easement. I never knew about either one of
the sewers on my properties until years after I bought them. It is very
common it the older neighborhoods for homes to share a line on private
property before it gets to the main.

Under no means dead end them. In the last 50 years you can bet that they
have been smoke tested and are not storm water. Unless there is a trap or
a
very low spot in the line the smoke would have billowed out of your
downspouts. If they are a separate perimeter drain into the sanitary, then
so be it, leave them hooked to the sewer.
CR

wrote in message
ups.com...
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.