Sanitary sewer drains are designed to drain wastewater and sewage from inside our homes. The sanitary sewer drains lead to the sanitary sewer system and ends up at a wastewater treatment plant. Unlike stormwater runoff from yards, streets, parking lots, etc, wastewater in this system is treated before being discharged into a natural water body.
Surface water runoff also called stormwater runoff flows across the top of yards, parking lots, streets, etc. In suburban areas, stormwater runoff enters a storm drain that takes water directly to the nearest water body. Water in a storm drain is untreated so it contains pollutants picked up by stormwater runoff and runs directly to the nearest creek or lake.