Combined Sewer Overflow in Chicago

Chicago's combined sewer system serves as one of the most studied urban drainage infrastructures in the United States, and its overflow events carry direct consequences for public health, property integrity, and regulatory compliance across the city's 228 square miles. This page describes the structure and function of combined sewer overflows (CSOs) in Chicago, the regulatory framework governing them, the conditions that trigger overflow events, and the boundaries that determine when plumbing professionals, municipal agencies, or property owners bear responsibility for response and remediation.

Definition and scope

A combined sewer overflow occurs when a combined sewer system — a single pipe network that carries both sanitary sewage and stormwater runoff — receives more volume than its conveyance or treatment capacity can handle. The excess flow is discharged, either intentionally through designated outfall points or unintentionally through basement backflow and street flooding, before it reaches a wastewater treatment plant.

Chicago's combined sewer system covers approximately 375 square miles of the metropolitan service area managed by the Metropolitan Water Reclamation District of Greater Chicago (MWRD). The MWRD operates under a Long-Term Control Plan (LTCP) approved by the Illinois Environmental Protection Agency (IEPA) and the United States Environmental Protection Agency (EPA) under the authority of the Clean Water Act, 33 U.S.C. § 1251 et seq. (U.S. EPA CSO Policy).

Scope and geographic coverage: This page applies specifically to the City of Chicago and portions of Cook County served by the MWRD. Municipal systems in suburban communities with separate storm and sanitary sewers — including those in DuPage, Lake, and Will Counties — operate under different regulatory frameworks and are not covered here. Properties located within Chicago's city limits but connected to a suburban utility district should verify jurisdiction through the MWRD's service area maps.

How it works

Chicago's combined sewer network was largely built between 1855 and the mid-20th century, with trunk sewers designed to carry a blended flow of stormwater and sanitary sewage toward treatment facilities. Under dry-weather conditions, all flow is conveyed to one of the MWRD's seven water reclamation plants for treatment.

When rainfall intensity exceeds the system's hydraulic capacity, a structured overflow sequence occurs:

1. Inflow accumulation — Stormwater enters the combined system through street catch basins, roof drains connected to sanitary laterals, and foundation drain tiles.
2. Trunk sewer surcharge — As volume rises, trunk sewers pressurize, reducing the gradient available for tributary lateral flow.
3. Tunnel system engagement — The Tunnel and Reservoir Plan (TARP), also called the "Deep Tunnel," intercepts excess flow into deep-rock tunnels with a total storage capacity of approximately 2.3 billion gallons (MWRD TARP Overview).
4. Reservoir activation — When tunnel capacity is reached, flow is directed to surface reservoirs. The three TARP reservoirs — Thornton, McCook, and Majewski — provide an additional 20.4 billion gallons of combined storage capacity upon full completion (MWRD Reservoir Projects).
5. Permitted outfall discharge — Flow exceeding combined tunnel and reservoir capacity is discharged through NPDES-permitted outfall structures to the Chicago River system and, ultimately, toward the Illinois Waterway.
6. Basement backflow — When lateral sewers surcharge, flow can reverse through floor drains, toilets, and sinks within structures that lack backwater valves — a condition addressed in detail at Basement Flooding and Backflow Prevention Chicago.

The regulatory structure governing Chicago plumbing work connected to this system is documented at Regulatory Context for Chicago Plumbing, including Chicago Municipal Code requirements under Title 11, Chapter 4.

Common scenarios

Heavy precipitation events — Chicago receives an average of 36.9 inches of precipitation annually (NOAA Climate Normals, 1991–2020). Rainfall events exceeding roughly 0.5 inches per hour are sufficient to initiate surcharging in older, undersized combined trunk mains.

Spring snowmelt — Rapid snowmelt combined with frozen ground surface creates high inflow rates because soil infiltration is blocked, channeling melt directly into catch basins.

Illicit connections — Roof drains, sump pumps, and foundation drain tiles improperly connected to sanitary laterals increase dry-weather flow and accelerate surcharge during storms. The Chicago Department of Water Management (DWM) enforces disconnection requirements under the Sewer Discharge Ordinance.

Aging lateral condition — Deteriorated or root-infiltrated private sewer laterals allow groundwater infiltration, continuously adding to system load. This is distinct from CSO events but degrades capacity margins. Property owners bear responsibility for private laterals from the structure to the property line, a boundary described in the Chicago Sewer System Overview.

CSO vs. sanitary sewer overflow (SSO): CSOs involve permitted, regulated discharge points in combined systems. SSOs occur in separate sanitary-only systems and represent a distinct regulatory category with generally stricter discharge prohibitions under EPA enforcement. Chicago's combined system makes SSO-type events comparatively rare within city limits, but the distinction matters for permit compliance and liability classification.

Decision boundaries

Responsibility and authority over CSO-related conditions divide along several boundaries:

Public vs. private infrastructure — The MWRD and DWM hold jurisdiction over public mains, interceptors, and outfall structures. Property owners hold responsibility for private laterals, internal drain systems, and backflow prevention devices.

Permit-required work — Any modification to a sewer lateral, installation of a backwater valve, or connection alteration within Chicago requires permits issued by the Chicago Department of Buildings. Licensed plumbers operating under Illinois plumbing license requirements (Illinois Plumbing License Act, 225 ILCS 320) must perform this work. Licensing standards for contractors are covered at Chicago Plumbing Contractor Licensing.

Federal vs. local authority — EPA and IEPA regulate MWRD's NPDES permit and LTCP compliance. The City of Chicago and MWRD regulate connections, disconnections, and property-level infrastructure through municipal code and sewer use ordinances.

Emergency vs. routine conditions — Active backflow into an occupied structure triggers emergency response protocols separate from standard permitting timelines. The distinction between emergency and planned remediation work affects permit sequencing and inspection requirements, detailed at Chicago Department of Buildings Plumbing Process.

The full scope of Chicago plumbing infrastructure, including how CSO dynamics intersect with residential and commercial plumbing systems across the city, is covered through the Chicago Plumbing Authority index.

References

• U.S. EPA — Combined Sewer Overflows (CSOs)
• Metropolitan Water Reclamation District of Greater Chicago — TARP Overview
• Metropolitan Water Reclamation District — Reservoir Projects
• Illinois General Assembly — Illinois Plumbing License Act, 225 ILCS 320
• Clean Water Act, 33 U.S.C. § 1251 et seq.
• NOAA National Centers for Environmental Information — U.S. Climate Normals
• Chicago Department of Water Management
• Illinois Environmental Protection Agency — NPDES Program