Difference Between Storm Drains and Sewers

Without storm drains, streets and parking lots would easily flood even during a light rain. But where does all that rainwater go? When you think "drain," you might automatically think "sewer." But not all drains lead to the water treatment plant!


The sanitary sewer is what you most likely think of when you hear the word "sewer." Your shower, toilet, kitchen sink, laundry tub, etc. all drain to the sanitary sewer, which takes the wastewater to a water treatment plant. There, the water is thoroughly treated and purified before being discharged to the nearest stream or body of water.

The storm system, completely separate from the sanitary sewer, is there to prevent flooding and is meant for rainwater and groundwater only. The drains you see on streets and parking lots are connected to the storm sewer system, which drains directly to the nearest stream or body of water. This water is not treated, so it's very important to never pour paint, solvent, oil, or anything else into storm drains, or else you risk poisoning fish, birds, and other wildlife.

These 2 systems are separate in order to avoid sewer overflows. The sanitary sewer system is not designed to handle the large amount of water that comes through the storm system during a big rainstorm - if all storm drains and sump pumps led to the sanitary sewer, the water treatment plant would easily become overwhelmed and cause a nasty backup. By keeping these systems separate, street flooding is minimized during storms, and sanitary sewers don't back up every time it rains. (For this reason, it is important that your sump pump discharges into the storm system and NOT the sanitary sewer.)

If you have drainage problems around your home, or if your sump pump is hooked up to the sanitary sewer, get it fixed before winter! Call Universal Plumbing at 586-459-0040. We can help!