Flood Preparedness
Ever wonder what happens after you flush? Follow a drop of water on its journey.
Step 1
Every time you flush the toilet or take a shower, that water leaves your house through a small pipe called a “lateral.”
Step 2
That pipe connects to the sewer under your street. Your city or village owns and takes care of this one.
Step 3
Your street’s sewer flows into a much bigger pipe owned by MWRD, called an “intercepting sewer.”
Step 4
MWRD’s big pipes carry the water to one of our treatment plants, where it’s cleaned before going back into rivers and lakes.
Below the Surface
Most sewer pipes in the Chicago area carry two things at once: the water from your drains AND the rain that falls on your street. These are called “combined sewers.” They were built a long time ago, before we had a way to clean sewage — back then, everything just flowed straight into the river!
When a big storm hits, more water can rush into the sewers than our treatment plants can clean all at once. When that happens, the extra water overflows straight into the river instead of backing up into homes. That’s called a “combined sewer overflow,” and it’s one reason rivers can get extra dirty right after a big storm.
To help with this, MWRD built a giant underground tunnel system called the Deep Tunnel (its official name is TARP). Think of it like a huge underground bathtub that can hold over 11.5 billion gallons of extra rainwater until the treatment plants are ready for it.
11.5 billion gallons is a lot of room, so the Deep Tunnel is almost never full. But in rare, really big storms, it can fill up. When that happens, sewers overflow into the river — just like they did before the Deep Tunnel was built.
Sometimes rain falls faster than the sewer pipes on your street can carry it away, so water backs up into basements or floods the street. As neighborhoods have grown, older sewer pipes often have to carry a lot more water than when they were first built. A damaged or blocked pipe from your house to the street (the “lateral”) can cause backups too — even when MWRD’s big pipes have plenty of room to spare.