I would assume that he was referring to the "storm sewer
system" rather than the "sanitary sewer system." The
phrase "sewer system" is vague and should be read in context.

FYI, in our city (and many other areas) it was once perfectly
legal and accepted practice to tie basement perimeter drain
systems into the sanitary sewer system. During a heavy Spring
rain, it is often possible to observe a tremendous flow of water
from the perimeter drain system into the basement floor drain
and on to the sanitary sewer.

During some heavy storms I have removed the sewer cleanout
access and estimated the flow of water from my perimeter drains
to be equal to or greater than a garden hose at full blast. I feel
sorry for the folks who are downhill and downstream from us -
those sanitary systems do backup once every 5-10 years during
a Spring storm due to the overload from cross-connections which
are now illegal.

Gideon

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wrote
Is it legal to put rainwater into the city sewer system in
your area? In most parts of the country it isn't.