Years ago it was pretty common to route sump pumps into the sanitary sewer systems. Think about it. What other sewer was available to pump into not that long ago for owners of properties with sump pumps? Basement floor drains are commonly hooked into the sanitary sewers as well and while current homes are built differently older homes wern't. I can't say for sure on this but I have noticed a lot of older homes had their gutters drain into what appears to be a sewer as well making me wonder were they hooked into the storm sewers or the sanitary sewers? Back in the day the sewers were simply a gravity fueled pipe to the river...how times have changed.