Where does rainwater go?
Rainwater from roofs, roads and driveways is usually carried by surface water drainage. In many places, that water runs to the nearest river or brook.
Wastewater from sinks, showers, toilets and washing machines should travel through foul drainage for treatment. The two systems do different jobs.
When wastewater enters the surface water route, it can bypass treatment and pollute the river. That is why getting the connection right matters.