When a rain gutter and downspout send their discharge below grade to a pipe, it may appear that rainwater is draining into a sanitary sewer, but it really isn’t. In municipal areas with sewer systems, gutter (rainwater) discharge is normally routed into a storm sewer. This storm piping routes water to rivers and streams and is separate from the sanitary sewer system.
Municipal systems include a sanitary sewer system that routes toilet, shower and sink water to a sewage treatment plant. The flow of storm and sanitary sewer systems would never be combined unless a really old system is in place or there are problems with the system. In the old days before good sewage treatment, homes had combined sewers – but that is not common today.
When you live out in the country, your gutters may discharge below grade and be directed underground to the side of a hill or a lower spot. Rural areas don’t have storm sewers.
Here is a tip. Look at the curb and gutter in front of your house. If you see grates there, rainwater flows into a storm sewer system below the street.