If you live in Chicago, your home is almost certainly connected to a combined sewer system. Most homeowners never hear the term until the basement floods during a heavy rain and someone explains why.

Understanding how Chicago’s sewers actually work helps you make smarter decisions about your home: whether to install a backwater valve, why your basement floods only during certain storms, and what kinds of upgrades actually pay off.

What Is a Combined Sewer System?

A combined sewer system is exactly what it sounds like: one pipe carrying two different things. Instead of separating household wastewater and stormwater into two networks, a combined sewer sends everything down a single pipe to a treatment facility.

Most of Chicago’s sewer infrastructure was built between the late 1800s and the mid-1900s, when combining the two flows was the standard approach. Today, around 75% of the city is still served by combined sewers. If your home is in Chicago, it’s a safe bet you’re on one unless you’ve specifically been told otherwise.

Storm Sewer vs. Sanitary Sewer: What’s the Difference?

A sanitary sewer carries only wastewater from homes and businesses. Toilets, showers, laundry — it all goes to a treatment plant before being released.

A storm sewer carries only rainwater and runoff. It flows (usually untreated) directly to the nearest river or lake.

A combined sewer mixes both into a single pipe. On a normal day, that pipe carries only sanitary flow and has plenty of capacity. During heavy rain, it has to handle both at once, and that’s where the trouble starts.

Advantages and Disadvantages of a Combined Sewer System

Combined sewers were the standard for a reason. They were cheaper to build, less disruptive to install, and they sent all flow to treatment instead of dumping stormwater directly into waterways. For a fast-growing city in the 1800s, it made sense.

The downsides show up when the weather doesn’t cooperate:

• Capacity limits during storms: A pipe sized for normal sewage can’t handle a heavy rainstorm on top of it.
• Combined sewer overflows: When the system is overwhelmed, the overflow gets released into rivers — a mix of stormwater and untreated sewage.
• Sewer backups in homes: When the main line is full, wastewater can flow backward into the lowest drains in the lowest homes.
• Aging infrastructure: Much of the network is over a century old and wasn’t sized for today’s storm intensity or population.

How a Combined Sewer Affects Your Home

Here’s what it actually means for you:

• Your basement is more vulnerable during heavy rain. If you have a basement floor drain, laundry tub, or basement bathroom, those fixtures connect to the same overloaded line as the rest of the block during a storm. When the city main can’t keep up, the pressure has nowhere to go but back up into your home. Learn more about waterproofing your home. 
• A backwater valve is one of the most effective protections you can add. It sits in your sewer line and closes automatically when it detects flow moving the wrong direction, physically blocking city sewage from entering your home. For many Chicago homeowners, it’s the single most cost-effective flood prevention upgrade available.
• Your sewer line condition matters more than it otherwise would. A cracked or root-intruded pipe is a problem in any home. In a combined system, where the city main routinely surges during storms, a weak pipe becomes a much bigger liability. Learn more about sewer repair. 

What to Do If You’re Worried About Your Setup

If you’ve had basement flooding before, or you’re not sure whether your home has any flood protection in place, the smartest first step is a camera inspection of your sewer. It tells you the condition of your line and whether you already have a backwater valve installed. From there, you can make informed decisions about what to upgrade.

Madden Sewer & Drain has been helping Chicago homeowners deal with the realities of the city’s combined sewer system for over 60 years. We’re family-owned, licensed, and bonded, with a 4.9-star rating across 300+ Google reviews. We’ll take a look, explain what we see, and help you figure out what makes sense for your home.

Call 773-588-7534 for a free estimate. We’ll help your home stay dry in the next storm.