In The Trenches wrote:

I have a heavy duty basement sump pump which is submersed. The basement is
equipped with a toilet. The sump pumps vertically about 6' to send the waste
water to the city's sewer level. I have two 6" pipes going into the sump
vertically. One has a check valve to stop back flow and when the pump turns
on I can hear water moving through it but the other one I'm not sure about.
Any ideas? I thought maybe it was a vent stack but I have not idea why the
sump would need a vent.

Ah! Not a "sump" pump but rather a sewage pump (I assume).
These pumps are sealed to prevent sewer gas escape.
As the pump removes the water in the tank, air must be
fed in to replace the lost volume. That's the primary
reason for the vent connection.

Jim